Oct. 25, 2022
There are 6 steps required to interpret blood gas results:
1. Determine If Sample Is Venous or Arterial
2. Assess Patient for Acidemia
3. Perform Additional Assessments for Acidosis
4. Perform Additional Assessments for Alkalosis
5. Assess Oxygenation
6. Determine Whether Compensatory Changes Have Occurred
Veterinary Blood Gas Analyzer
Either sample type can be used to evaluate overall acid–base status, with the exception of severe shock and post arrest situations, which may result in large discrepancies between arterial and venous samples. Poor tissue perfusion can result in sizeable increases in CO2 and secondary decreases in pH on the venous side despite low to normal CO2 on the arterial side.
ㆍAlthough information can be gained about ventilation from a venous sample, only an arterial sample can truly assess oxygenation.
ㆍIf unable to obtain an arterial sample, use:
ㆍPulse oximetry to measure oxygen saturation
ㆍVenous sample to evaluate acid–base status and estimate ventilation.
ㆍIf 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.
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.
ㆍRespiratory acidosis is present if PaCO2 > 45 mm Hg.
ㆍMetabolic acidosis is present if BE < -4 mmol/L (or HCO3– < 19 mmol/L).
ㆍRespiratory alkalosis is present if PaCO2 < 35 mm Hg
ㆍMetabolic alkalosis is present if BE > 2 mmol/L (or HCO3– > 25 mmol/L)
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%.
For example, if a primary metabolic acidosis is present, a compensatory respiratory alkalosis may also exist. Remember the rules of compensation:
ㆍA 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.
ㆍThe lungs can compensate quickly by adjusting minute ventilation in a matter of minutes.
ㆍThe kidneys compensate more slowly, with compensation beginning within a few hours and maximum compensation taking 4 to 5 days.
ㆍThe absence or presence and degree of compensation provides some information about the chronicity of the disturbance.
ㆍOvercompensation does not occur.
Interpretation of venous and arterial blood gases can be essential to treatment of many patients. Blood gas analyzers are becoming more common in veterinary practices and this analysis can aid in diagnosis and therapy for patients, indicating:
ㆍWhen fluid therapy is indicated
ㆍWhat fluid types are the best choices
ㆍIf sodium bicarbonate should be administered
ㆍWhen oxygen and mechanical ventilation are needed, including when the patient can be weaned off this support.
BE = base excess/deficit; CO2 = carbon dioxide; FiO2 = fractional inspired oxygen concentration; GI = gastrointestinal; HCO3– = bicarbonate; PaO2 = partial pressure of oxygen in arterial blood; PaCO2 = partial pressure of carbon dioxide in arterial blood