Chapter 1 – Anesthesia Equipment and Physics – Flashcards

(B) Orifice flow occurs when gas flows through a region of severe constriction such as described in this question. Laminar flow occurs when gas flows down parallel-sided tubes at a rate less than critical velocity. When the gas flow exceeds the critical velocity, it becomes turbulent (Miller: Anesthesia, ed 6, pp 690-691; Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 224-225).

(C) During orifice flow, the resistance to gas flow is directly proportional to the density of the gas mixture. Substituting helium for nitrogen will decrease the density of the gas mixture, thereby decreasing the resistance to gas flow (as much as threefold) through the region of constriction (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 224-225; Miller: Anesthesia, ed 6, pp 690-691, 2539).

(C) Modern electronic blood pressure (BP) monitors are designed to interface with electromechanical transducer systems. These systems do not require extensive technical skill on the part of the anesthesia provider for accurate usage. A static zeroing of the system is built into most modern electronic monitors. Thus, after the zeroing procedure is accomplished, the system is ready for operation. The system should be zeroed with the reference point of the transducer at the approximate level of the aortic root, eliminating the effect of the fluid column of the system on arterial BP readings (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 275-278).

(C) Waste gas disposal systems, also called scavenging systems, are designed to decrease pollution of the OR by anesthetic gases. These scavenging systems can be passive (waste gases flow from the anesthesia machine to a ventilation system on their own) or active (anesthesia machine connected to a vacuum system then to the ventilation system). The amount of air from a venous gas embolism would not be enough to be detected in the disposal system. Positive pressure relief valves open if there is an obstruction between the anesthesia machine and the disposal system, which would then leak the gas into the OR. A leak in the soda lime canisters would also vent to the OR. Since most ventilator bellows are powered by oxygen, a leak in the bellows would not add air to the evacuation system. The negative pressure relief valve is used in active systems and will entrap room air if the pressure in the system is less than -0.5 cm H2O. (Miller: Anesthesia, 6th ed. pp 303-307; Stoelting: Basics of Anesthesia, ed 5, pp 198-199).

(D) The relationship between intra-alveolar pressure, surface tension, and the radius of alveoli is described by Laplace's law for a sphere, which states that the surface tension of the sphere is directly proportional to the radius of the sphere and pressure within the sphere. With regard to pulmonary alveoli, the mathematical expression of Laplace's law is as follows: T=½PR where T is the surface tension, P is the intra-alveolar pressure, and R is the radius of the alveolus. In pulmonary alveoli, surface tension is produced by a liquid film lining the alveoli. This occurs because the attractive forces between the molecules of the liquid film are much greater than the attractive forces between the liquid film and gas. Thus, the surface area of the liquid tends to become as small as possible, which could collapse the alveoli (Miller: Anesthesia, ed 6, pp 689-690).

(C) The World Health Organization requires that compressed-gas cylinders containing N2O for medical use be painted blue. Size "E" compressed-gas cylinders completely filled with N2O contain approximately 1590 L of gas (Stoelting: Basics of Anesthesia, ed 5, p 188).

(D) Many anesthesia machines have a check valve downstream from the rotameters and vaporizers but upstream from the oxygen flush valve. When the oxygen flush valve button is depressed and the Y-piece (which would be connected to the endotracheal tube [ETT] or the anesthesia mask) is occluded, the circuit will be filled and the needle on the airway pressure gauge will indicate positive pressure. The positive pressure reading will not fall, however, even in the presence of a leak in the low-pressure circuit of the anesthesia machine. If a check valve is present on the common gas outlet, the positive-pressure leak test can be dangerous and misleading. In 1993, the United States Food and Drug Administration (FDA) established the FDA Universal Negative Pressure Leak Test. With the machine master switch, the flow control valves and the vaporizers turned off, a suction bulb is attached to the common gas outlet and compressed until it is fully collapsed. If a leak is present the suction bulb will inflate. It was so named because it can be used to check all anesthesia machines regardless of whether they contain a check valve in the fresh gas outlet (Miller: Anesthesia, ed 6, pp 309-310).

(E) Check valves permit only unidirectional flow of gases. These valves prevent retrograde flow of gases from the anesthesia machine or the transfer of gas from a compressed-gas cylinder at high pressure into a container at a lower pressure. Thus, these unidirectional valves will allow an empty compressed-gas cylinder to be exchanged for a full one during operation of the anesthesia machine with minimal loss of gas. The adjustable pressure-limiting valve is a synonym for a pop-off valve. A fail-safe valve is a synonym for a pressure-sensor shutoff valve. The purpose of a fail-safe valve is to discontinue the flow of N2O if the O2 pressure within the anesthesia machine falls below 25 psi (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 46-47; Miller: Anesthesia, ed 6, p 276.)

(C) Boyle's law states that for a fixed mass of gas at constant temperature, the product of pressure and volume is constant. This concept can be used to estimate the volume of gas remaining in a compressed-gas cylinder by measuring the pressure within the cylinder (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 224).

(C) United States manufacturers require that all compressed-gas cylinders containing O2 for medical use be painted green. A compressed-gas cylinder completely filled with O2 has a pressure of approximately 2000 psi and contains approximately 625 L of gas. According to Boyle's law (see explanation to question 9) the volume of gas remaining in a closed container can be estimated by measuring the pressure within the container. Therefore, when the pressure gauge on a compressed-gas cylinder containing O2 shows a pressure of 1600 psi, the cylinder contains 500 L of O2. At a gas flow of 2 L/min, O2 could be delivered from the cylinder for approximately 250 minutes (Stoelting: Basics of Anesthesia ed 5, p 188).

(B) All of the choices listed in this question can potentially result in inadequate flow of O2 to the patient; however, given the description of the problem, no flow of O2 through the O2 rotameter is the correct choice. In a normally functioning rotameter, gas flows between the rim of the bobbin and the wall of the Thorpe tube, causing the bobbin to rotate. If the bobbin is rotating you can be certain that gas is flowing through the rotameter and that the bobbin is not stuck (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 40-42).

B

(C) It is important to zero the electromechanical transducer system with the reference point at the approximate level of the heart. This will eliminate the effect of the fluid column of the transducer system on the arterial BP reading of the system. In this question, the system was zeroed at the stopcock, which was located at the patient's wrist (approximate level of the ventricle). Blood pressure expressed by the arterial line will, therefore, be accurate, provided the distance between the patient's wrist and the stopcock remains 20 cm. Also see explanation to question 3 (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 276).

(C) O2 and N2O enter the anesthesia machine from a central supply source or compressed-gas cylinders at pressures as high as 2200 psi (oxygen) and 750 psi (N2O). First-stage pressure regulators reduce these pressures to approximately 45 psi. Before entering the rotameters, second-stage O2 pressure regulators further reduce the pressure to approximately 14 to 16 psi (see figure with answer to question 12) (Miller: Anesthesia, ed 6, pp 274-275).

(C) NIOSH sets guidelines and issues recommendations concerning the control of waste anesthetic gases. NIOSH mandates that the highest trace concentration of N2O contamination of the OR atmosphere should be less than 25 ppm. In dental facilities where N2O is used without volatile anesthetics, NIOSH permits up to 50 ppm (Miller: Anesthesia, ed 6, pp 303-304).

(C) Agent-specific vaporizers, such as the Sevotec (sevoflurane) vaporizer, are designed for each volatile anesthetic. However, volatile anesthetics with identical saturated vapor pressures could be used interchangeably with accurate delivery of the volatile anesthetic.

(A) The control mechanism of standard anesthesia ventilators, such as the Ohmeda 7000, uses compressed oxygen (100%) to compress the ventilator bellows and electrical power for the timing circuits (Miller: Anesthesia, ed 6, p 298).

(C) Five types of rotameter indicators are commonly used to indicate the flow of gases delivered from the anesthesia machine. As with all anesthesia equipment, proper understanding of their function is necessary for safe and proper use. All rotameter flow indicators should be read at the upper rim except ball floats, which should be read in the middle (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 40-43).

(B) The pressure gauge on a size "E" compressed-gas cylinder containing N2O shows 750 psi when it is full and will continue to register 750 psi until approximately three-fourths of the gas has left the cylinder. A full cylinder of N2O contains 1590 L. Therefore, when 400 L of gas remain in the cylinder, the pressure within the cylinder will begin to fall (Stoelting: Basics of Anesthesia, ed 5, p 188).

(B) In this question the reference point is the transducer, which is located 10 cm below the level of the patient's heart. Thus, there is an approximate 10 cm H2O fluid column from the level of the patient's heart to the transducer. This will cause the pressure reading from the transducer system to read approximately 7.5 mm Hg higher than a true arterial pressure of the patient. A 20-cm column of H2O will exert a pressure equal to 14.7 mm Hg. Also see explanations to questions 3 and 13 (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 275).

(D) Factors that influence the rate of laminar flow of a substance through a tube is described by the Hagen-Poiseuille law of friction. The mathematical expression of the Hagen-Poiseuille law of friction is as follows: ˙ (D P) V r = p 4 8 Lm where V . is the flow of the substance, r is the radius of the tube, ΔP is the pressure gradient down the tube, L is the length of the tube, and μ is the viscosity of the substance. Note that the rate of laminar flow is proportional to the radius of the tube to the fourth power. If the diameter of an intravenous catheter is doubled, flow would increase by a factor of 2 raised to the fourth power (i.e., a factor of 16) (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 225).

(D) The safe storage and handling of compressed-gas cylinders is of vital importance. Compressed-gas cylinders should not be stored in extremes of heat or cold, and they should be unwrapped when stored or when in use. Flames should not be used to detect the presence of a gas. Oily hands can lead to difficulty in handling of the cylinder, which can result in dropping the cylinder. This can cause damage to or rupture of the cylinder, which can lead to an explosion (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 8-11).

(A) Vaporizers can be categorized into variable-bypass and measured-flow vaporizers. Measured-flow vaporizers (nonconcentration calibrated vaporizers) include the copper kettle and Vernitrol vaporizer. With measuredflow vaporizers, the flow of oxygen is selected on a separate flowmeter to pass into the vaporizing chamber from which the anesthetic vapor emerges at its saturated vapor pressure. By contrast, in variable-bypass vaporizers, the total gas flow is split between a variable bypass and the vaporizer chamber containing the anesthetic agent. The ratio of these two flows is called the splitting ratio. The splitting ratio depends on the anesthetic agent, temperature, the chosen vapor concentration set to be delivered to the patient, and the saturated vapor pressure of the anesthetic (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 63).

(D) The contribution of the fresh gas flow from the anesthesia machine to the patient's Vt should be considered when setting the Vt of a mechanical ventilator. Because the ventilator pressure-relief valve is closed during inspiration, both the gas from the ventilator bellows and the fresh gas flow will be delivered to the patient breathing circuit. In this question, the fresh gas flow is 6 L/min or 100 mL/sec (6000 mL/60). Each breath lasts 6 sec (60 sec/10 breaths) with inspiration lasting 2 sec (I:E ratio = 1:2). Under these conditions, the Vt delivered to the patient by the mechanical ventilator will be augmented by approximately 200 mL. In some ventilators, such as the Ohmeda 7900, Vt is controlled for the fresh gas flow rate such that the delivered Vt is always the same as the dial setting (Morgan: Clinical Anesthesia ed 4, pp 82-84).

(C) Also see explanation to question 24. The ventilator rate is decreased from 10 to 6 breaths/min. Thus, each breath will last 10 seconds (60 sec/6 breaths) with inspiration lasting approximately 3.3 sec (I:E ratio = 1:2), i.e., 3.3 seconds times 100 mL/second. Under these conditions, the actual Vt delivered to the patient by the mechanical ventilator will be 830 mL (500 mL + 330 mL) (Morgan: Clinical Anesthesia, ed 4, pp 82-84).

(D) The saturated vapor pressures of halothane and isoflurane are very similar (approximately 240 mm Hg at room temperature) and therefore could be used interchangeably in agent-specific vaporizers (see explanation and table in explanation for question 16) (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 60-63; Stoelting: Basics of Anesthesia, ed 5, p 79).

Clinically significant concentrations of carbon monoxide can result from the interaction of desiccated absorbent, both soda lime and Baralyme. The resulting carboxyhemoglobin level can be as high as 30%. Many of the reported occurrences of carbon monoxide poisoning have been observed on Monday mornings. This is thought to be the case because the absorbent granules are the driest after disuse for two days, particularly if the oxygen flow has not been turned off completely. There are several factors that appear to predispose to the production of carbon monoxide: (1) degree of absorbent dryness (completely desiccated granules produce more carbon monoxide than hydrated granules); (2) use of Baralyme versus soda lime (provided that the water content is the same in both); (3) high concentrations of volatile anesthetic (more carbon monoxide is generated at higher volatile concentrations); (4) high temperatures (more carbon monoxide is generated at higher temperatures); and (5) type of volatile used: (*Given that MAC level used is the same for all volatiles. If the anesthesia machine has been left on all weekend, the absorbent should be changed before the machine is used again to avoid carbon monoxide production.) (Miller: Anesthesia, ed 6, pp 296-298).

(A) NIOSH mandates that the highest trace concentration of volatile anesthetic contamination of the OR atmosphere when administered in conjunction with N2O is 0.5 ppm (Miller: Anesthesia, 6 ed, pp 303-304).

(B) The O2 analyzer is the last line of defense against inadvertent delivery of hypoxic gas mixtures. It should be located in the inspiratory (not expiratory) limb of the patient breathing circuit to provide maximum safety. Because the O2 concentration in the fresh-gas supply line may be different from that of the patient breathing circuit, the O2 analyzer should not be located in the fresh-gas supply line (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 216-220). 30. (A) The ventilator pressure-relief valve (also called

(A) The ventilator pressure-relief valve (also called the spill valve) is pressure controlled via pilot tubing that communicates with the ventilator bellows chamber. As pressure within the bellows chamber increases during the inspiratory phase of the ventilator cycle, the pressure is transmitted via the pilot tubing to close the pressurerelief valve, thus making the patient breathing circuit "gastight." This valve should open during the expiratory phase of the ventilator cycle to allow the release of excess gas from the patient breathing circuit into the wastegas scavenging circuit after the bellows has fully expanded. If the ventilator pressure-relief valve were to stick in the closed position, there would be a rapid buildup of pressure within the circle system that would be readily transmitted to the patient. Barotrauma to the patient's lungs would result if this situation were to continue unrecognized (Eisenkraft: Potential for barotrauma or hypoventilation with the Drager AV-E ventilator. J Clin Anesth, 1:452-456, 1989; Morgan: Clinical Anesthesia, ed 4, pp 81-82).

(D) Vaporizer output can be affected by the composition of the carrier gas used to vaporize the volatile agent in the vaporizing chamber, especially when nitrous oxide is either initiated or discontinued. This observation can be explained by the solubility of nitrous oxide in the volatile agent. When nitrous oxide and oxygen enter the vaporizing chamber, a portion of the nitrous oxide dissolves in the liquid agent. Thus, the vaporizer output transiently decreases. Conversely, when nitrous oxide is withdrawn as part of the carrier gas, the nitrous oxide dissolved in the volatile agent comes out of solution, thereby transiently increases the vaporizer output (Miller: Anesthesia, ed 6, pp 286-288).

(E) The capnogram can provide a variety of information, such as verification of the presence of exhaled CO2 after tracheal intubation, estimation of the difference in Paco2 and Petco2, abnormalities of ventilation, and the presence of hypercapnia or hypocapnia. The four phases of the capnogram are inspiratory baseline, expiratory upstroke, expiratory plateau, and inspiratory downstroke. The shape of the capnogram can be used to recognize and diagnose a variety of potentially adverse circumstances. Under normal conditions, the inspiratory baseline should be 0, indicating that there is no rebreathing of CO2 with a normal functioning circle breathing system. If the inspiratory baseline is elevated above 0, there is rebreathing of CO2. If this occurs, the differential diagnosis should include an incompetent expiratory valve, exhausted CO2 absorbent, or gas channeling through the CO2 absorbent. However, the inspiratory baseline may be elevated when the inspiratory valve is incompetent (e.g., there may be a slanted inspiratory downstroke). The expiratory upstroke occurs when the fresh gas from the anatomic dead space is quickly replaced by CO2-rich alveolar gas. Under normal conditions the upstroke should be steep; however, it may become slanted during partial airway obstruction, if a sidestream analyzer is sampling gas too slowly, or if the response time of the capnograph is too slow for the patient's respiratory rate. Partial obstruction may be the result of an obstruction in the breathing system (e.g., by a kinked endotracheal tube) or in the patient's airway (e.g., the presence of chronic obstructive pulmonary disease or acute bronchospasm). The expiratory plateau is normally characterized by a slow but shallow progressive increase in CO2 concentration. This occurs because of imperfect matching of ventilation and perfusion in all lung units. Partial obstruction of gas flow either in the breathing system or in the patient's airways may cause a prolonged increase in the slope of the expiratory plateau, which may continue rising until the next inspiratory downstroke begins. The inspiratory downstroke is caused by the rapid influx of fresh gas, which washes the CO2 away from the CO2 sensing or sampling site. Under normal conditions the inspiratory downstroke is very steep. Causes of a slanted or blunted inspiratory downstroke include an incompetent inspiratory valve, slow mechanical inspiration, slow gas sampling, and partial CO2 rebreathing (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 240).

(B) Complications of tracheal intubation can be divided into those associated with direct laryngoscopy and intubation of the trachea, tracheal tube placement, and extubation of the trachea. The most frequent complication associated with direct laryngoscopy and tracheal intubation is dental trauma. If a tooth is dislodged and not found, radiographs of the chest and abdomen should be taken to determine whether the tooth has passed through the glottic opening into the lungs. Should dental trauma occur, immediate consultation with a dentist is indicated. Other complications of direct laryngoscopy and tracheal intubation include hypertension, tachycardia, cardiac dysrhythmias, and aspiration of gastric contents. The most common complication that occurs while the ETT is in place is inadvertent endobronchial intubation. Flexion, not extension, of the neck or change from the supine to the head-down position can shift the carina upward, which may convert a mid-tracheal tube placement into a bronchial intubation. Extension of the neck can cause cephalad displacement of the tube into the pharynx. Lateral rotation of the head can displace the distal end of the ETT approximately 0.7 cm away from the carina. Complications associated with extubation of the trachea can be immediate or delayed. The two most serious immediate complications associated with extubation of the trachea are laryngospasm and aspiration of gastric contents. Laryngospasm is most likely to occur in patients who are lightly anesthetized at the time of extubation. If laryngospasm occurs, positive-pressure mask-bag ventilation with 100% O2 and forward displacement of the mandible may be sufficient treatment. However, if laryngospasm persists, succinylcholine should be administered intravenously or intramuscularly. Pharyngitis is another frequent complication after extubation of the trachea. This complication occurs most commonly in females, presumably because of the thinner mucosal covering over the posterior vocal cords compared with males. This complication usually does not require treatment and spontaneously resolves in 48 to 72 hours. Delayed complications associated with extubation of the trachea include laryngeal ulcerations, tracheitis, tracheal stenosis, vocal cord paralysis, and arytenoid cartilage dislocation (Stoelting: Basics of Anesthesia, ed 5, pp 231-232).

(B) Gas leaving a compressed-gas cylinder is directed through a pressure-reducing valve, which lowers the pressure within the metal tubing of the anesthesia machine to 45 to 55 psi (Miller: Anesthesia, ed 6, p 276).

(C) There is considerable controversy regarding the role of bacterial contamination of anesthesia machines and equipment in cross-infection between patients. The incidence of postoperative pulmonary infection is not reduced by the use of sterile disposable anesthetic breathing circuits (as compared with the use of reusable circuits that are cleaned with basic hygienic techniques). Furthermore, inclusion of a bacterial filter in the anesthesia breathing circuit has no effect on the incidence of cross-infection. Clinically relevant concentrations of volatile anesthetics have no bacteriocidal or bacteriostatic effects. Low concentrations of volatile anesthetics, however, may inhibit viral replication. Shifts in humidity and temperature in the anesthesia breathing and scavenging circuits are the most important factors responsible for killing bacteria. In addition, high O2 concentration and metallic ions present in the anesthesia machine and other equipment have a significant lethal effect on bacteria. Acid-fast bacilli are the most resistant bacterial form to destruction. Nevertheless, there has been no case documenting transmission of tuberculosis via a contaminated anesthetic machine from one patient to another. When managing patients who can potentially cause cross-infection of other patients (e.g., patients with tuberculosis, pneumonia, or known viral infections, such as acquired immune deficiency syndrome [AIDS]) a disposable anesthetic breathing circuit should be used and nondisposable equipment should be disinfected with glutaraldehyde (Cidex). Sodium hypochlorite (bleach), which destroys the human immunodeficiency virus, should be used to disinfect nondisposable equipment, including laryngoscope blades, if patients with AIDS require anesthesia (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 100).

(B) The diameter-index safety system prevents incorrect connections of medical gas lines. This system consists of two concentric and specific bores in the body of one connection, which correspond to two concentric and specific shoulders on the nipple of the other connection (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 21, 30, 37).

(C) The amount of anesthetic vapor (mL) in effluent gas from a vaporizing chamber can be calculated using the following equation: VO=(CG×SVPanes)÷(Pb anes - SVP) where VO is the vapor output (mL) of effluent gas from the vaporizer, CG is the carrier gas flow (mL/min) into the vaporizing chamber, SVPanes is the saturated vapor pressure (mm Hg) of the anesthetic gas at room temperature, and Pb is the barometric pressure (mm Hg). In this question, fresh gas flow is 100 mL/min. 100 mL/min × 0.9 = 90 mL/min (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 61). VO =(100×360)÷(760-360) VO =36000/400 VO = 90 mL

(A) The output of the vaporizer will be lower at flow rates less than 250 mL/min because there is insufficient pressure to advance the molecules of the volatile agent upward. At extremely high carrier gas flow rates (>15 L/ min) there is insufficient mixing in the vaporizing chamber (Miller: Anesthesia, ed 6, p 286).

(C) Pulse oximeters estimate arterial hemoglobin saturation (Sao2) by measuring the amount of light transmitted through a pulsatile vascular tissue bed. Pulse oximeters measure the alternating current (AC) component of light absorbance at each of two wavelengths (660 and 940 nm) and then divide this measurement by the corresponding direct current component. Then the ratio (R) of the two absorbance measurements is determined by the following equation: R= AC 660/ DC660 AC 940/ DC940 Using an empirical calibration curve that relates arterial hemoglobin saturation to R, the actual arterial hemoglobin saturation is calculated. Based on the physical principles outlined above, the sources of error in Spo2 readings can be easily predicted. Pulse oximeters can function accurately when only two hemoglobin species, oxyhemoglobin and reduced hemoglobin, are present. If any light-absorbing species other than oxyhemoglobin and reduced hemoglobin are present, the pulse oximeter measurements will be inaccurate. Fetal hemoglobin has minimal effect on the accuracy of pulse oximetry, because the extinction coefficients for fetal hemoglobin at the two wavelengths used by pulse oximetry are very similar to the corresponding values for adult hemoglobin. In addition to abnormal hemoglobins, any substance present in the blood that absorbs light at either 660 or 940 nm, such as intravenous dyes used for diagnostic purposes, will affect the value of R, making accurate measurements of the pulse oximeter impossible. These dyes include methylene blue and indigo carmine. Methylene blue has the greatest effect on Sao2 measurements because the extinction coefficient is so similar to that of oxyhemoglobin (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 254-255).

(B) The mass spectrometer functions by separating the components of a stream of charged particles into a spectrum based on their mass-to-charge ratio. The amount of each ion at specific mass-to-charge ratios is then determined and expressed as the fractional composition of the original gas mixture. The charged particles are created and manipulated in a high vacuum to avoid interference by outside air and minimize random collisions among the ions and residual gases. An erroneous reading will be displayed by the mass spectrometer when a gas that is not detected by the collector plate system is present in the gas mixture to be analyzed. Helium, which has a mass charge ratio of 4, is not detected by standard mass spectrometers. Consequently, the standard gases (i.e., halothane, enflurane, isoflurane, oxygen, nitrous oxide, nitrogen, and carbon dioxide) will be summed to 100% as if helium were not present. All readings would be approximately twice their real values in the original gas mixture in the presence of 50% helium (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 203-205).

(E) Because halothane and isoflurane have similar saturated vapor pressures, the vaporizers for these volatile anesthetics could be used interchangeably with accurate delivery of the anesthetic concentration set by the vaporizer dial. If a sevoflurane vaporizer were filled with a volatile anesthetic that has a greater saturated vapor pressure than sevoflurane (e.g., halothane or isoflurane), a higher-than-expected concentration would be delivered from the vaporizer. If a halothane or isoflurane vaporizer were filled with a volatile anesthetic that had a lower saturated vapor pressure than halothane or isoflurane (e.g., sevoflurane, enflurane, or methoxyflurane), a lower-than-expected concentration would be delivered from the vaporizer (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 66-67).

(E) Gas density decreases with increasing altitude (i.e., the density of a gas is directly proportional to atmospheric pressure). Atmospheric pressure will influence the function of rotameters because the accurate function of rotameters is influenced by the physical properties of the gas, such as density and viscosity. The magnitude of this influence, however, depends on the rate of gas flow. At low gas flows, the pattern of gas flow is laminar. Atmospheric pressure will have little effect on the accurate function of rotameters at low gas flows because laminar gas flow is influenced by gas viscosity (which is minimally affected by atmospheric pressure) and not gas density. However, at high gas flows, the gas flow pattern is turbulent and is influenced by gas density (see explanation to question 2). At high altitudes (i.e., low atmospheric pressure), the gas flow through the rotameter will be greater than expected at high flows but accurate at low flows (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 38-43, 224-225).

(B) Pacemakers have a three to five letter code that describes the pacemaker type and function. Since the purpose of the pacemaker is to send electrical current to the heart, the first letter identifies the chamber(s) paced; A for atrial, V for ventricle and D for dual chamber (A+V). The second letter identifies the chamber where endogenous current is sensed; A,V, D, and O for none sensed. The third letter describes the response to sensing; O for none, I for inhibited, T for triggered and D for dual (I+T). The fourth letter describes programmability or rate modulation; O for none and R for rate modulation (i.e., faster heart rate with exercise). The fifth letter describes multisite pacing (more important in dilated heart chambers); A, V or D (A+V) or O. A VDD pacemaker is used for patients with AV node dysfunction but intact sinus node activity. (Miller: Anesthesia, ed 6, pp 1416-1418).

(A) Although controversial, it is thought that chronic exposure to low concentrations of volatile anesthetics may constitute a health hazard to OR personnel. Therefore, removal of trace concentrations of volatile anesthetic gases from the OR atmosphere with a scavenging system and steps to reduce and control gas leakage into the environment are required. High-pressure system leakage of volatile anesthetic gases into the OR atmosphere occurs when gas escapes from compressed-gas cylinders attached to the anesthetic machine (e.g., faulty yokes) or from tubing delivering these gases to the anesthesia machine from a central supply source. The most common cause of low-pressure leakage of anesthetic gases into the OR atmosphere is the escape of gases from sites located between the flowmeters of the anesthesia machine and the patient, such as a poor mask seal. The use of high gas flows in a circle system will not reduce trace gas contamination of the OR atmosphere. In fact, this could contribute to the contamination if there is a leak in the circle system (Miller: Anesthesia, ed 6, pp 3151-3153).

(A) Although all of the choices in this question can contribute as sources of contamination, leakage around the anesthesia face mask poses the greatest threat (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 128-129; Miller: Anesthesia, ed 6, pp 3151-3153).

(C) The amount of volatile anesthetic taken up by the patient in the first minute is equal to that amount taken up between the squares of any two consecutive minutes. Accordingly, 50 mL would be taken up between the 16th (4 × 4) and 25th (5 × 5) minute, and another 50 mL would be taken up between the 25th and 36th (6 × 6) minute (Miller: Anesthesia, ed 5, p 87).

(E) In evaluating SSEPs, one looks at both the amplitude or voltage of the recorded response wave as well as the latency (time measured from the stimulus to the onset or peak of the response wave). A decrease in amplitude (>50%) and/or an increase in latency (>10%) is usually clinically significant. These changes may reflect hypoperfusion, neural ischemia, temperature changes, or drug effects. All of the volatile anesthetics as well as barbiturates cause a decrease in amplitude as well as an increase in latency. Etomidate causes an increase in latency and an increase in amplitude. Midazolam decreases the amplitude but has little effect on latency. Opioids cause small and not clinically significant increases in latency and decrease in amplitude of the SSEPs. Muscle relaxants have no effect of the SSEP (Miller: Anesthesia, ed 6, pp 1525-1537; Stoelting: Basics of Anesthesia, ed 5, pp 312-314).

(E) Also see explanation to question 35. There is no evidence that the incidence of postoperative pulmonary infection is altered by the use of sterile disposable anesthesia breathing systems (compared with the use of reusable systems that are cleaned with basic hygienic techniques) or by the inclusion of a bacterial filter in the anesthesia breathing system (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 100).

(D) Vaporization of a liquid requires the transfer of heat from the objects in contact with the liquid (e.g., the metal cylinder and surrounding atmosphere). For this reason, at high gas flows, atmospheric water will condense as frost on the outside of compressed-gas cylinders (Stoelting: Basics of Anesthesia, ed 5, p 188).

(B) Pulmonary artery, esophageal, axillary, nasopharyngeal, and tympanic membrane temperature measurements correlate with central temperature in patients undergoing noncardiac surgery. Skin temperature does not reflect central temperature and does not warn adequately of malignant hyperthermia or excessive hypothermia (Miller: Anesthesia, ed 6, p 1591).

(D) Rotameters consist of a vertically positioned tapered tube that is smallest in diameter at the bottom (Thorpe tube). Gas enters at the bottom of the Thorpe tube and elevates a bobbin or float, which comes to rest when gravity on the float is balanced by the fall in pressure across the float. The rate of gas flow through the tube depends on the pressure drop along the length of the tube, the resistance to gas flow through the tube, and the physical properties (density and viscosity) of the gas. Because few gases have the same density and viscosity, rotameters cannot be used interchangeably (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 38-43).

(A) The critical velocity for helium is greater than that for nitrogen. For this reason, there is less work of breathing when helium is substituted for nitrogen (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 224- 225; Miller: Anesthesia, ed 6, pp 690-691).

(E) The Fio2 delivered to patients from low-flow systems (e.g., nasal prongs) is determined by the size of the O2 reservoir, the O2 flow, and the patient's breathing pattern. As a rule of thumb, assuming a normal breathing pattern, the Fio2 delivered by nasal prongs increases by approximately 0.04 for each L/min increase in O2 flow up to a maximal Fio2 of approximately 0.45 (at an O2 flow of 6 L/min). In general, the larger the patient's Vt or faster the respiratory rate, the lower the Fio2 for a given O2 flow (Miller: Anesthesia, ed 6, pp 2812-2813).

A

(C) The accurate function of dual-wavelength pulse oximeters is altered by nail polish. Because blue nail polish has a peak absorbance similar to that of adult deoxygenated hemoglobin (near 660 nm), blue nail polish has the greatest effect on the Spo2 reading. Nail polish causes an artifactual and fixed decrease in the Spo2 reading by these devices. Turning the finger probe 90 degrees and having the light shining sidewise through the finger is useful when there is nail polish on the patient's fingernails (Miller: Anesthesia, ed 6, pp 1448-1452).

(C) The minimum macroshock current required to elicit ventricular fibrillation is 50 to 100 mA (Brunner: Electricity, Safety, and the Patient, ed 1, pp 22-23; Miller: Anesthesia, ed 6, pp 3145-3146).

(D) The line isolation monitor alarms when grounding occurs in the OR or when the maximum current that a short circuit could cause exceeds 2 to 5 mA. The line isolation monitor is purely a monitor and does interrupt electrical current. Therefore, the line isolation monitor will not prevent microshock or macroshock (Brunner: Electricity, Safety, and the Patient, ed 1, p 304; Miller: Anesthesia, ed 6, pp 3140-3141).

(A) A scavenging system with a closed interface is one in which there is communication with the atmosphere through positive- and negative-pressure relief valves. The positive-pressure relief valve will prevent transmission of excessive pressure buildup to the patient breathing circuit, even if there is an obstruction distal to the interface or if the system is not connected to wall suction. However, obstruction of the transfer tubing from the patient breathing circuit to the scavenging circuit is proximal to the interface. This will isolate the patient breathing circuit from the positive-pressure relief valve of the scavenging system interface. Should this occur, barotrauma to the patient's lungs can result (Ehrenwerth: Anesthesia Equipment: Principles and Applications, pp 127-128).

(B) MAC for isoflurane is 1.15% of 1 atmosphere or 8.7 mm Hg. An isoflurane vaporizer set for 1.15% will use a splitting ratio of 1:39. For purposes of illustration, imagine 100 mL of oxygen passes through the vaporizing chamber and 3900 mL through bypass chamber. 100 mL × 240/(760 - 240) = 46.1 mL of isoflurane vapor (plus 100 mL oxygen) 46.1/(3900 + 100) = 46.1/4000 = 1.15% 1.15% × 760 mm Hg = 8.7 mm Hg (1 MAC) Consider now the same splitting ratio applied in Denver, Colo.: 100 mL × 240/(630 - 240) = 61.5 mL of isoflurane vapor (plus 100 mL oxygen) 61.5/(3900 + 100) = 61.5/4000 = 1.53% 1.53% × 630 mm Hg = 9.7 mm Hg (roughly 1.1 MAC)

(A) Automated noninvasive blood pressure (ANIBP) devices provide consistent and reliable arterial BP measurements. Variations in the cuff pressure resulting from arterial pulsations during cuff deflation are sensed by the device and are used to calculate mean arterial pressure. Then, values for systolic and diastolic pressures are derived from formulas that use the rate of change of the arterial pressure pulsations and the mean arterial pressure (oscillometric principle). This methodology provides accurate measurements of arterial BP in neonates, infants, children, and adults. The main advantage of ANIBP devices is that they free the anesthesia provider to perform other duties required for optimal anesthesia care. Additionally, these devices provide alarm systems to draw attention to extreme BP values and have the capacity to transfer data to automated trending devices or recorders. Improper use of these devices can lead to erroneous measurements and complications. The width of the BP cuff should be approximately 40% of the circumference of the patient's arm. If the width of the BP cuff is too narrow or if the BP cuff is wrapped too loosely around the arm, the BP measurement by the device will be falsely elevated. Frequent BP measurements can result in edema of the extremity distal to the cuff. For this reason, cycling of these devices should not be more frequent than every 1 to 3 minutes. Other complications associated with improper use of ANIBP devices include ulnar nerve paresthesia, superficial thrombophlebitis, and compartment syndrome. Fortunately, these complications are rare occurrences (Miller: Anesthesia, ed 6, pp 1269-1271; Stoelting: Basics of Anesthesia, ed 5, p 307).

(D) If the ventilator pressure-relief valve were to become incompetent, there would be a direct communication between the patient breathing circuit and the scavenging system circuit. This would result in delivery of part of the Vt during the inspiratory phase of the ventilator cycle directly to the scavenging system reservoir bag. Therefore, adequate positive-pressure ventilation may not be achieved and hypoventilation of the patient's lungs may result. Also see explanation to question 54 and accompanying figure (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 120).

(C) A size "E" compressed-gas cylinder completely filled with air contains 625 L and would show a pressure gauge reading of 2000 psi. Therefore, a cylinder with a pressure gauge reading of 1000 psi would be half-full, containing approximately 325 L of air. A half-full size "E" compressed-gas cylinder containing air could be used for approximately 30 minutes at a flow rate of 10 L/min (see definition of Boyle's law in explanation to question 9 and explanation and table from question 10) (Stoelting: Basics of Anesthesia, ed 5, p 188).

(E) Failure to oxygenate patients adequately is the leading cause of anesthesia-related morbidity and mortality. All of the choices listed in this question are potential causes of inadequate delivery of O2 to the patient; however, the most frequent cause is inadvertent disconnection of the O2 supply system from the patient (e.g., disconnection of the patient breathing circuit from the endotracheal tube) (Miller: Anesthesia, ed 6, p 300).

(A) The esophageal detector device (EDD) is essentially a bulb that is first compressed then attached to the ETT after the tube is inserted into the patient. The pressure generated is about negative 40 cm of water. If the ETT is placed in the esophagus, then the negative pressure will collapse the esophagus and the bulb will not inflate. If the ETT is in the trachea, then the air from the lung will enable the bulb to inflate (usually in a few seconds but at times may take more than 30 seconds). A syringe that has a negative pressure applied to it has also been used. Although initial studies were very positive about its use, more recent studies show that up to 30% of correctly placed ETTs in adults may be removed because the EDD suggested esophageal placement. Misleading results have been noted in patients with morbid obesity, late pregnancy, status asthmatics and when there is copious endotracheal secretion, where the trachea tends to collapse. Its use in children younger than 1 year of age showed poor sensitivity as well as poor specificity. Although a cardiac output is needed to get CO2 to the lungs for a CO2 gas analyzer to function, a cardiac output is not needed for an EDD (American Heart Association— Guidelines for CPR and ECC. Circulation Volume 112, Issue 24, pp IV-54, IV-150, IV-169, 2005; Miller: Anesthesia, ed 6, p 1648).

(D) The capnometer measures the CO2 concentration of respiratory gases. Today this is most commonly performed by infrared absorption using a sidestream gas sample. The sampling tube should be connected as close to the patient's airway as possible. The difference between the end-tidal CO2 (Etco2) and the arterial CO2 (Paco2) is typically 5-10 mm Hg and is due to alveolar dead space ventilation. Because non-perfused alveoli do not contribute to gas exchange, any condition that increases alveolar dead space ventilation (i.e., reduces pulmonary blood flow such as a pulmonary embolism or cardiac arrest) will increase dead space ventilation and the Etco2 to Paco2 difference. Conditions that increase pulmonary shunt result in minimal changes in the Paco2- Etco2 gradient. CO2 diffuses rapidly across the capillary-alveolar membrane (Barash: Clinical Anesthesia, ed 5, pp 670-671; Miller: Anesthesia, ed 6, pp 1455-1462).

(E) The last gas added to a gas mixture should always be O2. This arrangement is the safest because it assures that leaks proximal to the O2 inflow cannot result in delivery of a hypoxic gas mixture to the patient. With this arrangement (O2 added last), leaks distal to the O2 inflow will result in a decreased volume of gas, but the Fio2 of Anesthesia will not be reduced (Stoelting: Basics of Anesthesia, ed 5, pp 188-189).

(D) Most modern Datex-Ohmeda Tec or North American Dräger Vapor vaporizers (except desflurane) are variablebypass, flow-over vaporizers. This means that the gas that flows through the vaporizers is split into two parts depending upon the concentration selected. The gas either goes through the bypass chamber on the top of the vaporizer or the vaporizing chamber on the bottom of the vaporizer. If the vaporizer is "tipped" which might happen when a filled vaporizer is "switched out" or moved from one machine to another machine, part of the anesthetic liquid in the vaporizing chamber may get into the bypass chamber. This could result in a much higher concentration of gas than dialed. With the Datex-Ohmeda Tec 4 or the North American Drager Vapor 19.1 series it is recommended to flush the vaporizer at high flows with the vaporizer set at a low concentration until the output shows no excessive agent (this usually takes 20-30 minutes). The Drager Vapor 2000 series has a transport (T) dial setting. This setting isolates the bypass from the vaporizer chamber. The Aladin cassette vaporizer does not have a bypass flow chamber and has no "tipping" hazard (Miller: Anesthesia ed 6, pp 285-288).

(A) Accurate delivery of volatile anesthetic concentration is dependant upon filling the agent specific vaporizer with the appropriate (volatile) agent. Differences in anesthetic potencies further necessitate this requirement. Each agent-specific vaporizer utilizes a splitting ratio that determines the portion of the fresh gas that is directed through the vaporizing chamber versus that which travels through the bypass chamber. The table above shows the calculation (fraction) that when multiplied by the quantity of fresh gas traversing the vaporizing chamber (affluent fresh gas in mL/min) will yield the output (mL/min) of anesthetic vapor in the effluent gas. When this fraction is multiplied by 100 it equals the splitting ratio for 1% for the given volatile. For example, when the isoflurane vaporizer is set to deliver 1% isoflurane, one part of fresh gas passed through the vaporizing chamber while 47 parts travel through the bypass chamber. One can determine on inspection that when a less soluble volatile like sevoflurane (or enflurane for the sake of example) is placed into an isoflurane (or halothane) vaporizer, the output in volume percent will be less than expected. How much less can be determined by simply comparing their splitting ratios 27/47 or 0.6. Halothane and enflurane are no longer used in the United States, but old halothane and enflurane vaporizers can be (and are) used elsewhere in the world to accurately deliver isoflurane and sevoflurane respectively (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 67). 69. (C) Two percent of 4 L

(C) Two percent of 4 L/min would be 80 mL of isoflurane per minute. Since 1 mL of vapor produces 195 mL of gas or making the simplistic calculation of 195 × 150 mL = 29,250. It follows that 29,250/80 = 365 minutes or about 6 hours. Note that each mL of most volatiles will yield 200 mL vapor at 20° C. Thus 150 min × 200 mL/min = 30,000 min. It follows that 30,000 min/80 mL/min = 375 minutes or ≈ 6 hours. (Ehrenwerth: Anesthesia Equipment: Principles and Applications, p 60).

(C) The human ear can perceive sound in the range of 20 Hz to 20 kHz. Frequencies above 20 kHz, inaudible to humans, are ultrasonic frequencies (ultra = Latin for "beyond" or "on the far side of"). In regional anesthesia, ultrasound is used for imaging in the frequency range of 2.5 to 10 MHz. Wavelength is inversely proportional to frequency, i.e., λ = C/f (λ = wavelength, C = velocity of sound through tissue or 1540 m/sec, f = frequency). Wavelength in millimeters can be calculated by dividing 1.54 by the Doppler frequency in megahertz. Penetration into tissue is 200 to 400 times wavelength and resolution is twice the wavelength. Therefore, a frequency of 3 MHz (wavelength .51 mm) would have a resolution of 1 mm and a penetration of up to 100 - 200 mm (10-20 cm) whereas 10 MHz (wavelength 0.15 mm) corresponds to a resolution of 0.3 mm, but penetration depth of no more than 60-120 mm (6-12 cm) (Miller: Anesthesia, ed 6, p 1364).

(A) Microshock refers to electric shock in or near the heart. A current as low as 50 μA passing through the heart can produce ventricular fibrillation. Use of pacemaker electrodes, central venous catheters, pulmonary artery catheters and other devices in the heart make are necessary prerequisites for microshock. Because the line isolation monitor has a 2 milliamps (2000 μA) threshold for alarming, it will not protect against microshock (Miller Anesthesia, ed 6, page 3145).

(E) Intraoperative awareness or recall during general anesthesia is rare (overall incidence is 0.2%, for obstetrics 0.4%, for cardiac 1-1.5%) except for major trauma which has a reported incidence up to 43%. With the EEG, trends can be identified with changes in the depth of anesthesia, however the sensitivity and specificity of the available trends are such that none serve as a sole indicator of anesthesia depth. Although using the BIS monitor may reduce the risk of recall, it, like the other listed signs as well as patient movement, does not totally eliminate recall (Miller: Anesthesia, ed 6, pp 1230-1259).

(D) The minute ventilation is five liters (0.5 L per breath at 10 breaths per minute) and 2 liters per minute to drive the ventilator for a total O2 consumption of 7 liters per minute. A full oxygen "E" cylinder contains 625 liters. Ninety percent of the volume of the cylinder (≈ 560 L) can be delivered before the ventilator can no longer be driven. At a rate of 7 L/min, this supply would last about 80 minutes (Stoelting: Basics of Anesthesia, ed 5, page 188).

(C) After eliminating reversible causes of high peak airway pressures such as occlusion of the endotracheal tube, mainstem intubation, bronchospasm, etc., adjusting the ventilator can reduce the peak airway pressure. Increasing the inspiratory flow rate would cause the airway pressures to go up faster and would produce higher peak airway pressures. Taking the PEEP off would have no significant effect. Changing the I:E ratio from 1:3 to 1:2 will permit 8% (25% inspiratory time to 33% inspiratory time) more time for the Vt to be administered and would result in lower airway pressures. Decreasing the Vt to 300 and increasing the rate to 28 would give the same minute ventilation, but not the same alveolar ventilation. Recall that alveolar ventilation equals (frequency) times (Vt minus dead space); and since dead space is the same (about 2 mL/kg ideal weight) alveolar ventilation would be reduced, in this case to a dangerously low level. Another option is to change from volume cycled to pressure cycled ventilation, which produces a more constant pressure over time instead of the peaked pressures seen with fixed Vt ventilation. (Barash: Clinical Anesthesia, ed 5, pp 1484-1485; Miller: Anesthesia, ed 6, pp 2820-2822).

(D) The central hospital oxygen supply to the operating rooms is designed to give enough pressure and oxygen flow to run the three oxygen components of the anesthesia machine (patient fresh gas flow, the anesthesia ventilator and the oxygen flush valve). The oxygen flowmeter on the anesthesia machine is designed to run at an oxygen pressure of 50 psi and for emergency purposes the oxygen flush valve delivers 35 to 75 L/min of oxygen (Stoelting: Basics of Anesthesia, ed 5, pp 187-189).

(B) Within the respiratory system both laminar and turbulent flows exist. At low flow rates, the respiratory flow tends to be laminar, like a series of concentric tubes that slide over one another with the center tubes flowing faster than the more peripheral tubes. Laminar flow is usually inaudible and is dependent on gas viscosity. Turbulent flow tends to be faster flow, is audible and is dependent upon gas density. Gas density can be decreased by using a mixture of helium with oxygen. (Barash: Clinical Anesthesia, ed 5, pp 794-795, Miller: Anesthesia, ed 6, p 2539).

(B) Anesthesia machines have a high, intermediate and low pressure circuits. The high pressure circuit is from the oxygen cylinder to the oxygen pressure regulator (first stage regulator) which takes the oxygen pressure from a high of 2200 psi to 45 psi. The intermediate pressure circuit consists of the pipeline pressure of about 50 to 55 psi and goes to the second stage regulator, which then lowers the pressure to 14 to 26 psi (depending upon the machine). The low pressure circuit then consists of the flow tubes, vaporizer manifold, vaporizers and vaporizer check valve to the common gas outlet. The oxygen flush valve is in the intermediate pressure circuit and bypasses the low pressure circuit (Stoelting: Basics of Anesthesia, ed 5, p 187; Miller: Anesthesia, ed 6, pp 274-276).

(C) Two major problems should be noted in this case. The first obvious problem is the inspired oxygen concentration of 4%, a concentration that is not possible if the gases going to the machine are appropriate unless the oxygen analyzer is faulty. In this case, where both the oxygen analyzer and the mass spectrometer read 4%, the pipeline gas line supplying "oxygen" most likely contains something other than oxygen. Second, the oxygen line pressure is 65 psi. The pipeline pressures are normally around 50 to 55 psi, whereas the pressure from the oxygen cylinder, if the cylinder is turned on, is reduced to 45 psi. For the oxygen tank to deliver oxygen to the patient, the pipeline pressure needs to be less than 45 psi, which in this case would occur only when the pipeline is disconnected. Although we rarely think of problems with hospital gas lines, a survey of more than 200 hospitals showed about 33% had problems with the pipelines. Most common pipeline problems were low pressure, followed by high pressure and, very rarely, crossed gas lines. (Barash: Clinical Anesthesia, ed 5, pp 563-564, Miller: Anesthesia, ed 6, pp 274-276).

(D) There are many ways to monitor the electrical activity of the heart. The five-electrode system using one lead for each limb and the fifth lead for the precordium is commonly used in the operating suite. The precordial lead placed in the V5 position (anterior axillary line in the fifth intercostal space) gives the V5 tracing, which combined with the standard lead II are most common tracings used to look for myocardial ischemia (Barash: Clinical Anesthesia, ed 5, pp 889, 1539; Miller: Anesthesia, ed 6, pp 1392-1393).

(A) The DISS provides threaded, non-interchangeable connections for medical gas pipelines through the hospital as well as to the anesthesia machine. The Pin Index Safety System (PISS) has two metal pins located in different arrangements around the yoke on the back of anesthesia machines, with each arrangement for a specific gas cylinder. Vaporizers often have keyed fillers that attach to the bottle of anesthetic and the vaporizer. Vaporizers not equipped with keyed fillers occasionally have been misfilled with the wrong anesthetic liquid (Barash: Clinical Anesthesia, ed 5, p 563; Miller: Anesthesia, ed 6, pp 276 and 288).

(C) Calcium hydroxide lime does not contain the monovalent hydroxide bases that are present in soda lime (namely NaOH and KOH). Sevoflurane in the presence of NaOH or KOH is degraded to trace amounts of Compound A, which is nephrotoxic to rats at high concentrations. Soda lime normally contains about 13% to 15% water, but if the soda lime is desiccated (water content < 5% — which has occurred if the machine is not used for a while and the fresh gas flow is left on) and exposed to current volatile anesthetics (isoflurane, sevoflurane and especially desflurane), carbon monoxide can be produced. Neither Compound A nor carbon monoxide are formed when calcium hydroxide lime is used. With soda lime and calcium hydroxide lime the indicator dye changes from white to purple as the granules become exhausted; however, over time, exhausted soda may revert back to white. With calcium hydroxide lime the dye once changed does not revert to normal. The two major disadvantages of calcium hydroxide lime are the expense and the fact that its absorptive capacity is about half of soda lime (10.2 L of CO2/100 g of calcium hydroxide lime versus 26 L of CO2/100 g of soda lime) (Barash: Clinical Anesthesia, ed 5, pp 411-413; Miller: Anesthesia, ed 6, pp 296-298; Stoelting: Basics of Anesthesia, ed 5, pp 200-202).

82. (B) The aim of direct invasive monitoring is to give continuous arterial BPs that are similar to the intermittent noninvasive arterial BPs from a cuff, as well as to give a port for arterial blood samples. The displayed signal reflects the actual pressure as well as distortions from the measuring system (i.e., the catheter, tubing, stopcocks, amplifier). Although most of the time the signal is accurate, at times we see an underdamped or an overdamped signal. In an underdamped signal, as in this case, exaggerated readings are noted (widened pulse pressure). In an overdamped signal, readings are diminished (narrowed pulse pressure). Note however the mean BP tends to be accurate in both underdamped and overdamped signals (Miller: Anesthesia, ed 6, pp 1272-1279).

(E) Rebreathing of expired gases (e.g., stuck open expiratory or inspiratory valves), faulty removal of CO2 from the carbon dioxide absorber (e.g., exhausted CO2 absorber, channeling through a CO2 absorber or having the CO2 absorber bypassed — an option in some older anesthetic machines), or adding CO2 from a gas supply (rarely done with current anesthetic machines) can all increase inspired CO2. Absorption of CO2 during laparoscopic surgery when CO2 is used as the abdominal distending gas would increase absorption of CO2 but would not cause an increase in inspired CO2 (Miller: Anesthesia, ed 6, pp 1458-1461; Stoelting: Basics of Anesthesia, ed 5, pp 199-201, 314).

84. (B) 85. (A) 86. (D) Medical gas cylinders are color coded but may differ from one country to another. If there is a combination of two gases, the tank would have both corresponding colors, for example, a tank containing oxygen and helium would be green and brown. The only exception to the mixed gas color scheme is O2 and N2 in the proportion of 19.5% to 23.5% mixed with N2, which is solid yellow (air).