Chapter 24 – Principles of Acid-Base Balance: Fluid and Electrolyte Therapy – Flashcards
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What is molecular weight?
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sum of atomic weights of all elements in a chemical formula
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What is a millimole (mmol)
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molecular weight of a substance in mg, equals 1 mM
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What is milliequivalent (mEq)?
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weight of an element that combines or replaces 1 mg of hydrogen (H+)
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What is milliosmole (mOsm)?
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always contains 6.0 x 10^23 molecules and equals 1 mmol of a nondissociable substance
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What is milliequivalent per liter (mEq/l)?
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= mmol/l x valence
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Solutes exert an osmotic effect in solution that is dependent only on what?
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the number of particles in a solution and not on the molecular weight or valence
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What does osmolarity refer to?
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the number of osmoles per liter
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What does osmolality indicate?
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the number of osmoles per kg
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The effective osmolality, or tonicity, of a solution is related to what?
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the ability of a solute to attract water and to sustain an increase in osmotic pressure as a result of water movement
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What is the approximate percentage of total body water (TBW)?
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60%
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Total body water is broadly divided into what two types?
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intracellular and extracellular
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The extracellular fluid is divided into what four subcompartments?
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plasma volume, interstitial lymph fluid, transcellular fluid, and fluid present in dense connective tissue and bone
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Where is transcellular fluid found?
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in diverse locations including CSF, pleural cavity, GI tract, bladder, synovia, aqueous humor, and peritoneal cavity
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What is the most abundant and osmotically active ECF cation?
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Na+
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What is the most abundant ICF cation?
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K+
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Which ion is the major determinant of the resting cell membrane potential (-70 to -90 mV)?
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K+
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The critical distribution of water between the plasma and the interstitium is maintained by what?
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the colloidal osmotic pressure of plasma protein (oncotic pressure)
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What must occur to maintain electroneutrality?
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the number of cations in the ECF must equal the number of anions
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Calculation of the difference between the commonly measured cations and anions in ECF yields what?
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the anion gap
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The anion gap calculation can be useful in assessing what?
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the etiology of metabolic acidosis
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What is water turnover?
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the term used to describe input and output of body water over a given period of time
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Water turnover in mature dogs is approximated at what?
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40-60 ml/kg/day
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What are maintenance fluids defined as?
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as the volume of fluid required daily to maintain an animal in zero fluid balance - no net gain or loss of water
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What happens if ECF volume and renal perfusion decrease?
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renin will be secreted into the circulation
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What is thirst stimulated by?
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plasma hypertonicity and/or contracted ECF volume
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What is the rate limiting step in the renin-angiotensin system?
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conversion of angiotensinogen to angiotensin I
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What does angiotensin II stimulate?
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the zona glomerulosa of the adrenal cortex to secrete aldosterone which causes increased reabsorption of sodium from the distal nephron with excretion of K+ and H+
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In regards to normal water loss, what are sensible losses?
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water loss that occurs via urine, fecal water, and saliva
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In regards to normal water loss, what are insensible losses?
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water loss that occurs via evaporation from cutaneous and respiratory epithelia
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In a healthy animal, insensible losses account for TBW elimination of about what?
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15-30 ml/kg/day
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Metabolic rate is directly proportional to what?
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the ratio of body surface area to total volume
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Daily urinary water losses can be divided into what two groups?
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obligatory water loss and free water loss
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What is obligatory water loss?
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water eliminated in order to excrete the daily renal solute load (urea, Na+, K+, Ca++, Mg++, NH4+, Cl-, SO4^2-, PO4^3- and other cations/anions)
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What is free water loss?
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water excreted unaccompanied by solute
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Free water loss is controlled by what?
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vasopressin
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When does free water loss increase and decrease?
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it increases during relative water excess or hypotonicity and decreases during water deficit or hypertonicity
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What part of the nephron is responsive to vasopressin?
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the collecting ducts
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Renal reabsorption of sodium in the distal nephron is increased by what?
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aldosterone
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What are the three categories of dehydration?
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hypertonic, isotonic and hypotonic
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Causes of dehydration associated with pure water deficit include what?
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hypodypsia due to neurologic disease, diabetes insipidus, respiratory losses during exposure to elevated temperatures, fever and inadequate access to water
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What is third spacing?
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a term used to describe extravasation of fluid from the vascular compartment into extravascular spaces
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What has been included as a guideline for assessment of dehydration in neonatal calves?
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the measured gap between the eyeball and orbit
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What factors are used when assessing dehydration?
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extent of exophthalmos, skin-tent duration on neck, thorax, and upper and lower eyelids, heart rate, mean central venous pressure, peripheral (extremities) and core temperatures, PCV, and hemoglobin and plasma protein concentration
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What is sodium essential for?
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proper maintenance of membrane potentials, initiation of action potentials, and maintenance of acid-base balance
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What are the clinical signs of hypernatremia?
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neurologic deficits, thirst, anorexia, lethargy, vomiting and muscle weakness; dehydration may be present
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With hypernatremia, in the case of pure-water loss, what can the volume be replaced with?
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5% dextrose in water over a 48- to 72- hour period
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What is the treatment for hyponatremia?
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infusion with conventional crystalloid solutions
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When can cause hyperchloremia?
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fluid loss associated with small bowel diarrhea
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What can cause hypochloremia?
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fluid losses due to vomiting or excessive diuretic administration
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What is the treatment for chloride-responsive alkalosis?
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chloride-replete fluid such as normal saline
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What is the primary determinant of potassium secretion across renal tubular epithelial surfaces?
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aldosterone
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Most of the filtered potasium is reabsorbed where?
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in the proximal tubule
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Tubular secretion of potassium is aldosterone mediated and occurs where?
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in the distal nephron
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What does hypokalemia do to the resting membrane potential?
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makes it more negative thereby hyperpolarizing the cell
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What does hyperkalemia due to the cell
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makes it more positive causing hyperexcitability
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What can affect the distribution of potassium between the ICF and the ECF?
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changes in pH
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What is the rule of thumb regarding pH and serum potassium concentration?
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every 0.1 unit decrease in pH will be accompanied by a 0.6 mEq/l increase in serum potassium concentration
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What are some of the clinical signs of hyperkalemia?
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muscle weakness, twitching, and irritability
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How might ACE inhibitors cause hyperkalemia?
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by interfering with angiotensin II-mediated aldosterone secretion
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Emergency treatment is indicated if potassium rises and exceeds what?
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6.0-8.0 mEq/l
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What is the therapy for hyperkalemia?
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calcium gluconate
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What are the clinical signs of hypokalemia?
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weakness of skeletal and respiratory muscles and intestinal smooth muscle loss of tone
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What is feline hypokalemic polymyopathy syndrome often manifested as?
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ventroflexion of the head and a stiff, stilted gait
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Administration of intravenous potassium should not exceed a rate of what?
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0.5 mEq/kg/hr
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What is responsible for the maintenance of body pH?
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pulmonary and renal control of dissolved CO2 and bicarbonate concentrations
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Under normal physiological conditions, what is the ratio of HCO3- to dissolved CO2?
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20:1
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The respiratory component of acid-base regulation involves changes in what?
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respiratory rate and volume prompted by changes in PCO2
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The renal component of acid-base regulation involves what?
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selective absorption of bicarbonate and secretion of H+
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What happens during acidosis?
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more H+ are secreted while more K+, Na+ and HCO3- are retained
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What happens during alkalosis?
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K+ is secreted while more H+ and less Na+ and HCO3- are retained
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How do the kidney's regulate acid-base balance?
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by maintaining the appropriate HCO3- in the plasma
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Disorders of acid-base equilibrium can result from what?
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a primary disturbance in pulmonary regulation of the concentration of H2CO3 in body fluids via changes in alveolar ventilation and PCO2 levels, from metabolic changes in concentration of bicarbonate or from a combination of these mechanisms
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What is the best measure of respiratory disturbances?
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the partial pressure of CO2
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What does a blood gas analysis provide?
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three measured parameters (pH, PCO2, PO2) and two calculated values (actual bicarbonate and base excess)
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Actual bicarbonate values are useful in assessment of what?
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non-respiratory disorders
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What is the approximate normal anion gap in dogs and cats?
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13-24 mEq/l
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What is the anion gap often used to identify?
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causes of metabolic acidosis
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Increased anion gap often occurs in what?
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lactic acidosis, diabetic ketoacidosis, azotemic renal failure and poisoning
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A normal anion gap usually occurs in what?
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metabolic acidosis related to diarrhea, renal tubular acidosis, excessive use of carbonic anhydrase inhibitors or ammonium chloride administration and in iatrogenic expansion acidosis caused by excessive normal saline administration
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What is an acid-base disturbance?
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a primary disturbance with a secondary or compensatory response
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Metabolic acidosis may be characterized by what?
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a decrease in plasma HCO3- concentration, decreased pH, increased concentration of strong anions and decreased plasma sodium concentration associated with renal disease or diarrhea
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What are the clinical signs most associated with metabolic acidosis?
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hyperpnea and CNS depression
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Laboratory evaluation of blood and urine reveals what with a metabolic acidosis?
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a lowered urine and blood pH and a decreased serum HCO3-
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What is the most common acid-base disorder in dogs, cats and horses?
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metabolic acidosis
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What is the most common cause of normal anion gap (hyperchloremic) metabolic acidosis?
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loss of Na+ and HCO3- associated with diarrhea
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What is metabolic alkalosis characterized by?
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an excess of HCO3- caused by a deficit of H+ in the ECF
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What can metabolic alkalosis be caused by?
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excessive vomiting, excessive alkaline therapy or use of diuretics
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What are the clinical signs of metabolic alkalosis?
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depressed breathing (slow and shallow), nervous excitement, including tetany, and even convulsions and muscular hypertonicity
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Compensation for metabolic alkalosis requires what?
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the kidneys to excrete HCO3- and retain H+
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What does respiratory acidosis involve?
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retention of CO2 as a consequence of alveolar hypoventilation
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What does hypoventilation result in?
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CO2 retention, an excess of H2CO3 and thereby an excess of H+
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What is the compensatory mechanism for respiratory acidosis?
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for the kidneys to conserve HCO3- and excrete H+
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The most important treatment for respiratory acidosis is what?
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proper ventilation of the animal
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What is the purpose of fluid and electrolyte therapy?
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to correct dehydration or overhydration and electrolyte imbalance and/or acid-base imbalance
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What provides a rational basis for estimating patient fluid and electrolyte needs and monitoring treatment success?
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laboratory examination of the blood in combination with clinical signs
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What happens to specific gravity and urine volume during a state of dehydration?
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specific gravity increase and urine volume decrease
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If the specific gravity is unchanged or lowered and the animal shows clinical signs of dehydration, what does this typically mean?
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the kidneys are not functioning properly
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What are the three aims of volume replacement?
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correct existing deficits, satisfy maintenance needs and replace continuing loss
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What is the replacement volume equation?
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replacement volume = body weight (kg) x % dehydration
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When are maintenance fluids needed?
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when a patient does not voluntarily ingest sufficient food and water to replace normal losses occurring via urine, feces, respiratory tract and skin
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What is the rule of thumb for giving fluids for mature and immature animals?
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mature: 40-65 ml/kg/24 hour immature: 130 ml/kg/24 hour
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In less severe cases of dehydration, what should the infusion rate of fluids be?
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15-30 ml/kg/hr
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Large volumes of fluid administered too rapidly may overload the circulatory system, causing what?
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pulmonary edema and even death, especially in severely ill or toxic cases
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What is hypodermoclysis?
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subcutaneous administration of fluid
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What are the major categories of parenteral fluids?
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crystalloids, colloids, blood replacements and nutritional solutions
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What are the fluids going to be like that closely resemble the ECF?
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isotonic, high in sodium, low in potassium, and may be acidifying or alkalinizing
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What is the fluid of choice for treatment of diabetic ketoacidosis?
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normal saline
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What are the fluids of choice for metabolic alkalosis?
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normal saline and Ringer's solution
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What are the two types of colloids?
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natural and synthetic
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What are the natural colloids?
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whole blood, plasma, and albumin
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What are the synthetic colloids?
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dextran 40, dextran 70, hetastarch, pentastarch and oxypolygelatin
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Which type of fluid contain large particles and are retained within the vascular space more readily?
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colloids
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Initial tissue perfusion has been found to be better after volume expansion with what type of fluids?
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colloids or combination of crystalloids and colloids
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What are the indications for colloid use?
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perfusion deficits, hypooncotic states, deficiency of blood components, and diseases that lead to systemic inflammatory response syndrome (SIRS)
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What are the hallmarks of SIRS?
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alterations in temperature, heart rate, respiratory rate, PCO2 and white blood cell count
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What are the three major categories of synthetic colloids?
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dextrans, hydroxyethyl starches and gelatins
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What are dextrans prepared from?
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a macromolecular polysaccharide produced by bacterial fermentatino of sucrose
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What are hydroxyethyl starches derived from?
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plant amylopectin and are modified by hydroxyethylation to reduce hydrolysis by amylase
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What is the most commonly used hydroxyethyl starch?
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hetastarch (Hespan)
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What is the only approved plasma substitute in the U.S. for fluid resuscitation?
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oxypolygelatin (Vetaplasma) derived from bovine bone gelatin
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Hypertonic saline is used in the treatment of what?
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shock associated with hemorrhage, trauma, GDV, acute pancreatitis, burns and sepsis
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What does hypertonic saline do?
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draws interstitial and intracellular water away from edematous tissues and into the vasculature
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As hypertonic saline is short-lived, what should be combined for long-term resuscitation?
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crystalloids or colloids
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What are hemoglobin based oxygen carriers (HBOCs) useful for?
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replacing RBCs in anemic humans and animals
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What do hemoglobin based oxygen carriers contain?
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purified hemoglobin, removed from RBCs and suspended in solution
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What are hemoglobin based oxygen carriers developed from?
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animal or human hemoglobin
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Does oxyglobin need to be cross-matched?
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no, it does not have the potential to transmit disease
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What will grain overloading in ruminants cause?
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severe dehydration and metabolic acidosis
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What can provide benefit to calves that have hypernatremia and diarrhea?
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5% dextrose alone or with isotonic sodium bicarbonate
