Respiratory Therapy: Formulas – Flashcards
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Charles' Law
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V1/T1 = V2/T2
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Gay-Lussac's Law
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(P1/T1 = P2/T2)
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Ideal Gas Law
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(P1 x V1)/T1 = (P2 x V2)/T2
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Partial pressure of oxygen in the atmosphere
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PiO2 = FiO2 x PB
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Write the formula for subtracting out the water vapor pressure at 37 degrees when fully saturated.
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H2O vapor saturated - Body humidity = 47mm Hg or 43.8mg/L
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Partial pressure of oxygen in the lungs
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PIO2 = FIO2 x PB
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Total Cycle Time (TCT)
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TCT = 60/RR
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I:E Ratio
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1: Te/Ti
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Inverse I:E Ratio
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Inspiratory time is more than expiratory time. Ti/Te :1
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Inspiratory Time (Ti) Expiratory Time (Te)
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Ti = TCT/# Te = TCT - Ti
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Static Compliance (Cs)
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Vt/Plat - PEEP Change in volume (tidal volume Vt)/Change in pressure (Plat)
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Dynamic Compliance (Cd)
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Vt/PIP - PEEP Change in volume (tidal volume Vt)/Change in pressure (PIP)
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Relative Humidity (%RH)
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%RH = (Absolute humidity)/(Humidity capacity) x 100
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Find the total flow of a venture mask system given the oxygen flow set on the flowmeter and the desired FI02.
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a. Add both parts of Air:O2 ratio together b. Multiply sum by the given oxygen flow set
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Oxygen Entrainment Ratio
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Air/O2 = (100 - Desired O2)/(Desired O2 - 21*) *Use 20% if desired is greater than 35%
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Minute Ventilation (Ve)
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Ve = RR x VT
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Peak Inspiratory Flow (Vi)
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Vi = Ve (3)
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Be able to tell whether the total flow of an air entrainment system is high enough to meet the patient's peak inspiratory flow.
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a. Find Peak Insp Flow b. Find Total flow of the system YES: If total flow is higher than the peak flow of the patient NO: If total flow is lower than the peak flow of the patient
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Tank duration of Flow *H Tank*
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Cylinder pressure (psi) x Cylinder factor/Flow rate of gas (L/min) H tank = 3.14
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Duration of Flow *E Tank*
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{Cylinder pressure (psi)} x {Cylinder factor}/{Flow rate of gas (L/min)} E tank = .28
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Ideal Body Weight Formula (IBW) *Pounds*
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Male: 106 + (height - 60 x 6) Female: 105 + (height - 60 x 5)
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Ideal Body Weight Formula (IBW) *Kilograms*
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Men: 50 + 2.3 x (Ht - 60) Women: 45.4 + 2.3 (Ht - 60)
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Convert a percent solution to mgs *1% solution*
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1% solution 1g = 1000mgs in 100ml 10mg/ml
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Find the mg/ml *On hand: 10% solution*
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10% 10g = 10000mg/100ml = 100mg/ml
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Find the mg/ml *1:10000*
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1g/10000ml 1000mg/10000ml 1mg/10ml 0.1mg/1ml So 0.1mg per 1ml
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Anatomic Deadspace
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1ml per pound of body weight
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Physiologic Deadspace (Bohr Equation) (Vd/Vt)
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Vd/Vt = PaCO2 - PeCo2/PaCO2 Physiologic VD = Anatomic VD + Alveolar VD
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Alveolar Ventilation (Va)
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Va = Ve - Vd Va = (Vt - Vd) (RR)
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% Predicted
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% Predicted = (Actual FVC/Predicted FVC) X 100
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% Change
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% Change = (Post value - Pre value)/ Pre value X 100
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Patient Effort
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FEV1 x 35
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Ideal Alveolar Gas Equation (PAO2)
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PAO2 = [PB - PH2O]FIO2 - PaCO2 (1.25)
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Fick's Law
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V (diffusion) gas = AD(P1 - P2)/T
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Oxygen content of arterial blood (CaO2)
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CaO2 = (Hb x 1.34 x SaO2) + (PaO2 x 0.003)
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Oxygen Content of Pulmonary Capillary Blood (CcO2)
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CcO2 = (Hb x 1.34) + (PAO2 x 0.003)
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Oxygen content of mixed venous blood (CvO2)
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CvO2 = (Hb x 1.34 x SvO2) + (PvO2 x 0.003)
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Pulmonary shunt (Qs/Qt)
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Qs/Qt = % Pulmonary shunt CcO2 - CaO2/CcO2 - CvO2
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Total oxygen delivery (DO2)
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DO2 = QT x (CaO2 x 10)
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Oxygen consumption (or Oxygen Uptake) (VO2)
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VO2 = QT (CaO2 - CvO2) x 10
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Oxygen Extraction Ratio (O2ER)
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ERO2 = (CaO2 - CVO2) / CaO2
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Dissolved Oxygen
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For each mmHg of PO2, 0.003 ml O2 will dissolve in 100 ml of blood Thus, a PaO2 of 100 = 0.3 ml 100 X 0.003 = 0.3 ml Written as 0.3 vol% Represents small percentage of total O2 transported.
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Hemoglobin *Carrying capacity and saturation*
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1.34ml of O2 per g% 15g% = 100% Hb sat 1.34 x 15 = 20.1 vol%
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Airway Resistance (Raw)
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Raw = PIP - PLAT/Flow Take L/min and divide by 60 to get L/sec
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FIO2 *Formula for desired PaO2*
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FIO2 = PaO2 Desired x FiO2 known/ PaO2 Desired PaO2 is always 80 Everything else is given
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Ve *Formula for desired PaCO2*
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Ve = Known PaCo2 x known Ve/Desired PaCo2 Desired PaCO2 is always 40 Everything else is given
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RR *Formula for desired PaCO2*
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RR = Known PaCO2 x known RR/Desired PaCo2 Desired PaCO2 is always 40 Everything else is given
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Vt *Formula for desired PaCO2*
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Vt = Known PaCo2 x known Vt (ml)/Desired PaCo2 Desired PaCO2 is always 40 Everything else is given
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Poiseuille's Law *Flow*
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Flow = Change in Pressure (P) x r^4
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Poiseuille's Law *Pressure*
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Change in Pressure (P) = Flow/r^4
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Time Constants (Tc)
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Tc = Raw x Cd
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