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acid–Base Primer | » Pulse oximetry to measure oxygen saturation » Venous sample to evaluate acid–base status and estimate ventilation. • If the patient is intubated, end tidal CO2 can also be used to estimate ventilation, but with severe pulmonary disease, end tidal CO2 can be much lower than PaCO2. 2. Assess Patient for Acidemia (pH < 7.35) or Alkalemia (pH > 7.45) If pH is within normal limits, the patient's body may have compensated for underlying disturbances or a mixed disturbance may be present. See Steps 3 and 4 to evaluate if metabolic or respiratory disturbances are present despite normal pH. 3. Perform Additional Assessments for Acidosis • Respiratory acidosis is present if PaCO2 > 45 mm Hg. • Metabolic acidosis is present if BE < -4 mmol/L (or HCO3- < 19 mmol/L). 4. Perform Additional Assessments for Alkalosis • Respiratory alkalosis is present if PaCO2 < 35 mm Hg • Metabolic alkalosis is present if BE > 2 mmol/L (or HCO3- > 25 mmol/L) For cats, substitute the reported normal values for PaCO2 and BE from Table 1 into steps 3 and 4. 5. Assess Oxygenation Normal PaO2 is 90 to 100 mm Hg. If the patient is on supplemental oxygen, PaO2 should equal approximately 5× the FiO2; the FiO2 of room air is 21%. These rules apply to the normal values listed in Table 1 for dogs. 6. Determine Whether Compensatory Changes Have Occurred For example, if a primary metabolic acidosis is present, a compensatory respiratory alkalosis may also exist. Remember the rules of compensation: • A change in the respiratory or metabolic component of the acid–base status normally induces an opposite compensatory response in an effort to normalize the pH. • The lungs can compensate quickly by adjusting minute ventilation in a matter of minutes. • The kidneys compensate more slowly, with compensation beginning within a few hours and maximum compensation taking 4 to 5 days. • The absence or presence and degree of compensation provides some information about the chronicity of the disturbance (Table 2). • Overcompensation does not occur. Other Useful Equations Another tool for interpreting lung function is the PaO2:FiO2 ratio, which allows arterial blood gases and Oxygen Solutions for Veterinarians Excellence through Experience AirSep is world renowned as a premier manufacturer of oxygen generators for veterinary medicine, as well as many other medical and industrial applications. Oxygen Generation Systems: • Reliable • Cost Effective • Easy to Install & Maintain For more information, call: (716) 691-0202 www.airsepcpd.com cpd@airsep.com May/June 2013 Today's Veterinary Practice 47