Acid–base physiology and blood gas interpretation in the neonate

Acid–base balance describes the complex processes that work to maintain a stable extracellular pH for optimal cellular function. The process of balancing the production and neutralization of tissue acid provides a great challenge for newborn infants who have significantly higher rates of acid production than adults. Factors which help maintain a stable acid–base balance include buffers, the respiratory system and renal system. Both intracellular and extracellular buffers resist changes in pH; the most important extracellular buffer system for maintaining stable pH is the bicarbonate system. In response to acidosis or alkalosis the respiratory system balances the production of acid with the clearance of CO2 by altering respiratory rate. The kidneys provide a delayed but sustained response to pH imbalance by excreting either acidic or alkaline urine in response to systemic acidosis and alkalosis, respectively. In sick premature infants these systems have a limited capacity completely to control pH. This review discusses the physiology of the three systems which control acid–base balance. Patterns of clinical abnormalities in acid–base status and their underlying causes are discussed. We present a systematic approach to blood gas analysis and interpretation in newborn infants.