Today's Veterinary Practice

MAY-JUN 2013

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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| The PracTiTioner's acid–Base Primer: oBTaining & inTerPreTing Blood gases REVIEW OF DEFINITIONS • an ACID is a molecule that donates a hydrogen ion (h+) when a base molecule accepts one. • a BUFFER is a weak acid or base, which helps protect against large changes in ph. » The primary extracellular buffer is bicarbonate. » The intracellular buffers are phosphate, proteins, and hemoglobin. » Bone also acts as a buffer. • pH is the measure of acidity/alkalinity, and equal to the negative logarithm of h+ concentration. • ACIDEMIA is a blood ph < 7.35; AlKAlEMIA is a blood ph > 7.45. Table 1. normal Blood gas Values Arterial Values Venous Values 7.395 ± 0.03 7.352 ± 0.02 DOGS1 pH PO2 (mm Hg) 102.1 ± 6.8 55 ± 9.6 PCO2 (mm Hg) 36.8 ± 2.7 42.1 ± 4.4 HCO3 - (mmol/l) 21.4 ± 1.6 22.1 ± 2 BE (mmol/l) -1.8 ± 1.6 -2.1 ± 1.7 pH 7.34 ± 0.1 7.30 ± 0.08 PO2 (mm Hg) 102.9 ± 15 38.6 ± 11 CATS 2 PCO2 (mm Hg) 33.6 ± 7 41.8 ± 9 HCO3 - (mmol/l) 17.5 ± 3 19.4 ± 4 BE (mmol/l) -6.4 ± 5 -5.7 ± 5 • Patients with hypoxemia may become cyanotic, but this sign will not present until hypoxia is severe, and may only become evident just prior to death, making it an unreliable indicator of adequate oxygenation. • Respiratory rate and effort can be difficult to interpret as they are affected by many other factors, such as pain, excitement, fear, and metabolic derangements, and can be masked by sedation or anesthesia. If a patient is in respiratory distress, supplemental oxygen should be provided prior to obtaining samples for blood gas analysis. OBTAINING & HANDlING BlOOD GAS SAMPlES It is essential that blood gas samples be properly obtained and handled, particularly venous samples. Sample error can be introduced in a number of ways (see Potential Sample Errors, page 46). 44 Today's Veterinary Practice May/June 2013 • ACIDOSIS and AlKAlOSIS refer to the process causing a ph disturbance. Four basic types of acid–base disturbances have been classified by the traditional henderson–hasselbach approach: 1. Metabolic acidosis: a primary gain in acid or loss of base 2. Metabolic alkalosis: a primary gain in base or loss of acid 3. Respiratory acidosis: retention of co2 due to co2 production outpacing alveolar ventilation 4. Respiratory alkalosis: removal of co2 (by ventilation) outpacing co2 production • PaO2 is the partial pressure of oxygen dissolved in arterial blood. it is a measure of oxygenation, not ventilation. • PaCO2 is the partial pressure of carbon dioxide dissolved in arterial blood. it provides the best measure of a patient's ability to ventilate and determines whether respiratory acidosis or alkalosis is present. remember that co2 is approximately 20× more diffusible than o2, making it easier for a patient to maintain normal co2 concentrations in the presence of lung disease. • PvCO2 is the partial pressure of carbon dioxide dissolved in venous blood. When the sample has been obtained properly, it measures a patient's ability to ventilate, similar to Paco2. • BASE EXCESS/DEFICIT (BE): » reflects the metabolic portion of the acid–base balance, which takes into account all of the body's buffer systems » estimates how much base needs to be added or subtracted to achieve a normal ph at normal temperature » evaluates for metabolic acidosis or alkalosis. Venous Samples 1. Ideally, a venous sample should be taken from a central catheter (in the cranial or caudal vena cava) or by direct jugular venipuncture in order to obtain the best representation of the global acid–base and respiratory status. 2. Samples should be capped off to prevent exposure to air; then processed immediately. If the sample cannot be processed immediately, it should be placed on ice until evaluation can take place. 3. Samples can either be: • Processed immediately without anticoagulant • Drawn into a syringe that has been coated in heparin until they can be processed. 4. To prepare a sample with heparin, coat a 3-mL syringe with a small amount of liquid heparin; then draw air up to the syringe's 3-mL mark and forcibly expel the heparin several times. While most of the heparin will be removed from the syringe, enough heparin will remain to affect the measured ionized calcium, which will be unreliable.3

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