If the ABG is pH 7.26, PCO2 66 torr, PO2 59 torr on the same ventilator settings, how would this ABG be interpreted? What treatment should be prescribed?
This patient has acidemia by definition because the pH is <7.35. To determine the origin of the acidosis, especially in a patient with COPD who may have an elevated PCO2 at baseline, it is necessary to measure the blood bicarbonate content. Caution should be exercised as the bicarbonate is usually calculated on ABG results but measured on the elec-trolyte panel. When the serum bicarbonate concentration is 24 mEq/L, elevation of the arterial PCO2 by 10 mmHg results in a fall in the pH of 0.08 pH units. Normal or high bicarbonate is suggestive of respiratory acidosis with the pH decrease being caused by the increased PCO2. In the case of this patient, an acute increase in PCO2 by 20 mmHg (e.g., from 46 to 66) would change the pH from 7.42 to 7.26. Decreased serum bicarbonate suggests that there is a metabolic acidosis due to organic acids, such as lactic acid. However, mixed syndromes are common. Calculating the anion gap (Na+ – [Cl− + HCO−3], normal 12 ± 4) can help differentiate the causes of metabolic acidosis.
Increasing the minute ventilation can treat the respira-tory acidosis, keeping in mind Equation (1). In this case, both the tidal volume (since the PIP is only 26 cm H2O) and respiratory rate can be increased. Experience with permis-sive hypercapnia shows that, even when the pH is allowed to decrease to values as low as 7.15, few adverse effects are seen except for obtundation (probably secondary to intra-cellular acidosis). Therefore, the risk of barotrauma should be carefully weighed before increasing mechanical ventilation parameters.
Metabolic acidosis requires treatment of the cause. The most common cause for metabolic acidosis in postop-erative patients is lactic acidosis due to hypoperfusion. Fluid resuscitation and optimization of hemodynamics usually results in spontaneous resolution of the acidosis. Intravenous sodium bicarbonate should be used only in cases of extreme acidosis (pH <7.10) with hemodynamic instability.