Metabolic acidosis is a common acid-base disturbance characterized by a primary reduction in the serum concentration of bicarbonate (HCO3–), which lowers blood pH and triggers a compensatory decrease in the arterial partial pressure of carbon dioxide (PaCO2). This condition can manifest as part of a complex acid-base disorder, particularly in critically ill patients, where multiple types of acidosis may coexist and contribute to the overall acidemia. The term acidemia refers to the state in which the blood pH is below normal (less than 7.35), indicating increased acidity in the bloodstream. Metabolic acidosis specifically pertains to an imbalance in bicarbonate levels and signifies an underlying pathological process that requires intervention to prevent severe outcomes, including organ dysfunction, morbidity, and mortality.
Acute metabolic acidosis is a frequent condition in hospital settings, especially among seriously ill patients. For instance, studies have shown that approximately 64% of patients in intensive care units (ICUs) in the United States exhibit metabolic acidosis at some point during their stay. In contrast, chronic metabolic acidosis, while less prevalent, tends to be associated with long-term conditions such as chronic kidney disease (CKD). Data from the National Health and Nutrition Examination Survey (NHANES III) revealed that only 1.9% of the general population had serum bicarbonate levels below 22 mmol/L. However, the incidence of chronic metabolic acidosis increases in patients with reduced kidney function, particularly those with an estimated glomerular filtration rate (eGFR) between 15 and 29 ml/min/1.73m². As the prevalence of CKD rises with the aging population, chronic metabolic acidosis is also expected to become more frequent.
Respiratory Compensation
The fall in pH stimulates the peripheral and central chemoreceptors, and minute ventilation rises within minutes; when the acidosis is severe the breathing becomes deep and labored (Kussmaul respiration). The expected PaCO2 is given by Winter's formula, PaCO2 = 1.5 x HCO3– + 8 (plus or minus 2), so a bicarbonate of 12 mEq/L should be matched by a PaCO2 near 26 mmHg. The response is complete within 12 to 24 hours, it is only partial (pH never returns fully to normal), and a PaCO2 that is higher than predicted means a coexisting respiratory acidosis, which is common when an anesthetized or sedated patient cannot increase ventilation on their own. Under general anesthesia the provider must supply this compensation by raising minute ventilation.
