CLINICAL ACID-BASE BALANCE DISTURBANCES

This section provides practical clinical examples to assist in understanding acid-base balance.

Metabolic Acidosis from Low Cardiac Output

A patient with coronary artery disease and an ejection fraction of 15% suffers myocardial perforation while undergoing balloon angioplasty in the cardiac catheterization laboratory (see Chapter 18 ). After resuscitation, intubation, insertion of an arterial line, and initiation of dopamine by infusion, the patient was transferred to the operating room for emergency thoracotomy. Blood pressure was 80/50 mm Hg, and the heart rate was 120 beats/min. During preparation for central venous cannulation, the blood pressure fell, followed by cardiac arrest. The patient underwent immediate thoracotomy. Blood gas determinations obtained at the time revealed pHa = 7.15, PaCO₂ = 35 mm Hg, BE = -15 mEq/L, bicarbonate = 12, and PaO₂ = 90 mm Hg—indicating a severe metabolic acidosis with mild respiratory alkalosis. After the administration of four units of packed red blood cells (PRBCs) and the administration of 88 mEq of sodium bicarbonate, repeat blood gas determinations revealed pHa = 7.14, PaCO₂ = 39 mm Hg, BE = -14 mEq/L, bicarbonate = 13, and PaO₂ = 95 mm Hg—still indicating a severe metabolic acidosis but with no respiratory alkalosis.

The first blood gas determination revealed a metabolic acidosis attributable to the low cardiac output with tissue ischemia and with slight hyperventilation. The second blood gas determination showed almost no benefit from the bicarbonate, probably because of continuing low cardiac output. Bicarbonate is converted to carbon dioxide, which may explain the slightly higher level of PaCO₂ .