Acid-base disorders in the PICU

An appropriate acid-base milieu is essential for normal cellular function of the human organism. Disturbances of the pH balance frequently occur in critically ill or injured children. These pertubations most often serve as a marker of an underlying disorder responsible for their occurrence, but acid-base disturbances may in themselves require monitoring and treatment in the PICU. Proper assessment and treatment of acid-base imbalances therefore requires an understanding of terminology and measurement, insight into buffer systems, and recognition of the compensatory interactions involved in maintaining a homeostatic balance. The terms acidosis and alkalosis refer to the mechanisms which result in a given acid-base disturbance. Primary acid-base disorders are further classified as either metabolic or respiratory. Metabolic contribution to acid-base homeostasis is based on the presence of strong anions and cations. Ion strength is based on the tendency of an ion to dissociate in aqueous solutions and tendency to combine with other ions. The concentration difference between the sum of all strong anions and strong cations is defined as the strong ion difference (SID). The anion gap, determined by presence or absence of unmeasured anions, helps guide understanding of the etiology of metabolic acidosis, one of the most common disturbances in the critically ill child. Hypercapnic acidosis impacts pH balance, but could have potential therapeutic effects in children with acute lung injury. Specific therapies, such as intravenous fluids and cardiopulmonary bypass, intrinsically affect acid-base balance, and their impact should be considered.