The most common acid-base disorder encountered in the postoperative period is alkalosis (27). Alkalosis is usually of no clinical significance and resolves spontaneously. Several etiologic factors may include hyperventilation associated with pain; posttraumatic transient hyperaldosteronism, which results in decreased renal bicarbonate excretion; nasogastric suction, which removes hydrogen ions; infusion of bicarbonate during blood transfusions in the form of citrate, which is converted to bicarbonates; administration of exogenous alkali; and use of diuretics. Alkalosis can be corrected with removal of the inciting cause and the correction of extracellular ﬂuid and potassium deficits (Table 22.3). Full correction usually can be safely achieved over 1 to 2 days.
Marked alkalosis, with serum pH higher than 7.55, can result in serious cardiac arrhythmias or central nervous system seizures. Myocardial excitability is particularly pronounced with concurrent hypokalemia. Under such conditions, ﬂuid and electrolyte replacement may not
Table 22.3 Causes of Metabolic Alkalosis
Disorder Source of Alkali Cause of Renal HCO Retention
Nasogastric suction Gastric mucosa ↓↓ECF, ↓K
Diuretics Renal epithelium ↓ECF, ↓K Respiratory acidosis ↓ECF, ↓K, ↑PCO2 and diuretics
Exogenous base NaHCO3,Na Coexisting disorder of ECF, citrate, Na lactate K, PaCO2 ↓ECF, extracellular ﬂuid depletion; ↓K, potassium depletion; ↑↑PCO2, carbon dioxide retention; NaHCO3, sodium bicarbonate; PaCO2, partial pressure of carbon dioxide, arterial.
be sufficient to correct the alkalosis rapidly. Acetazolamide (250 to 500 mg) can be given orally or intravenously two to four times daily to induce renal bicarbonate diuresis. Treatment with an acidifying agent rarely is necessary and should be reserved for acutely symptomatic patients (i.e., those with cardiac or central nervous system dysfunction) or for patients with advanced renal disease. Under such conditions, hydrogen chloride (5 to 10 mEq per hour of a 100-mmol solution) can be given via a central intravenous line. Ammonium chloride can be given orally or intravenously but should not be given to patients with hepatic disease.
Metabolic acidosis is less common than alkalosis during the postoperative period, but acidosis can be serious because of its effect on the cardiovascular system. Under conditions of acidosis, there are decreased myocardial contractility, a propensity for vasodilation of the peripheral vasculature leading to hypotension, and refractoriness of the fibrillating heart to defibrillation (27). These effects promote decompensation of the cardiovascular system and can hinder attempts at resuscitation.
Metabolic acidosis results from a decrease in serum bicarbonate levels caused by the consumption and replacement of bicarbonate by circulating acids or the replacement by other anions such as chloride. The proper workup includes a measurement of the anion gap:
Anion gap = (Na++ K+) − (CI− + HCO3 −) = 10 to 14 mEq/L (normal)
The anion gap is composed of circulating protein, sulfate, phosphate, citrate, and lactate (36).