Perioperative Acid-Base Balance

Blood gas analysis has been the most robust laboratory and bedside tool for identifying and monitoring critical illness over the past century. A key component of this process is consideration of the acid base status of the extracellular fluid. That is because the human body carefully controls the water, gas and electrolyte composition of its internal milieu and alterations result in abnormalities of in the hydrogen ion activity, represented by pH. All acid base abnormalities result from alterations in the relative activity of hydrogen and hydroxyl ions in body water, induced by changes in the local concentration of strong ions, weak acids and carbon dioxide. Traditionally acid base abnormalities can be divided into processes that acidity the extracellular fluid (acidosis - represented by a pH <7.35) and those that alkalinize it further (alkalosis - represented by a pH >7.5). Acid base abnormities can result from abnormalities of volatile acid (CO2 - respiratory disorders), mineral acids/bases and organic acids (metabolic disorders). Hence acid base abnormalities are classified as respiratory acidosis and alkalosis, acute and chronic, and metabolic acidosis and alkalosis. Mixed acid base pictures may occur in perioperative patients and, in particular in critical care. The body has a range of countermeasures (“compensation” and buffering) to restore pH to “normal” range (7.38 to 7.42). A variety of different approaches have been developed to interrogate the blood gas to determine if acid base abnormalities are present, the simplest of which involve just PaCO2 and bicarbonate (HCO3-). As [HCO3-] rises and falls roughly in proportion to hydrogen or hydroxyl ion gain, it is a useful tool in assessing acid base chemistry. The base excess approach utilizes the relative changes in [HCO3-], while controlling temperature and PaCO2, to demonstrate metabolic acidosis and alkalosis. By adding the valency of widely measured extracellular electrolytes, it is possible to demonstrate more precisely the cause of acid base abnormalities - hence the development of the anion gap (AG), the corrected AG, the delta gap, the base deficit gap and the strong ion gap (SIG). Although each of these “gaps” aid in the precision of diagnosis, previously unmeasured anions (L-Lactate and β-hydroxybutyrate) are now easily measured and should be incorporated into calculations. Respiratory acidosis, due to respiratory failure or under ventilation, and respiratory alkalosis, usually due to hyperventilation, are commonly seen in perioperative medicine. Abnormalities of mineral acids and bases principally arise from the relative concentrations of sodium and chloride and their dispersal in free water. The greatest impact, in perioperative medicine, on mineral acids and bases arises from intravenous fluids, nasogastric drainage and fluid loss from the kidneys and lower gastrointestinal tract. Organic acid accumulation is common in critical illness - these metabolic by products - lactate, keto acids and “renal acids” indicate severe underlying disease. Metabolic alkalosis is relatively common in critical care, and is usually caused by gastrointestinal loss of chloride and hypoalbuminemia. Acid base disorders are resolved by treating the underlying condition, correcting fluid and electrolyte abnormalities and allowing the body to repair itself. Specific therapies, such as sodium bicarbonate for metabolic acidosis, remain controversial.