Metabolic Respiratory Disturbances

Acid–base disorders are not diagnoses by themselves. They are physiologic signals that often reveal failed ventilation, impaired renal acid handling, tissue hypoperfusion, toxin exposure, or a metabolic emergency. Arterial reference values are pH 7.35–7.45, PaCO2 4.7–6.0 kPa, and serum bicarbonate 22–26 mmol/L. Acidaemia and alkalaemia describe the measured pH. Acidosis and alkalosis describe the processes pushing pH down or up. A patient can have more than one process at once, so a near-normal pH does not exclude serious disease. The primary disorder is named for the component that changes first: - Metabolic acidosis: bicarbonate falls. - Metabolic alkalosis: bicarbonate rises. - Respiratory acidosis: PaCO2 rises from inadequate alveolar ventilation. - Respiratory alkalosis: PaCO2 falls from excessive alveolar ventilation. Compensation limits the pH change; it does not restore normal pH and does not reverse the cause. The lungs alter PaCO2 within minutes to hours. The kidneys need days to retain bicarbonate or excrete acid in response to a respiratory disorder. Thus, an acute rise in PaCO2 has little bicarbonate compensation, whereas chronic CO2 retention is accompanied by a substantially higher bicarbonate...