ABSTRACT

Respiratory acidosis (primary hypercapnia) is initiated by an increase in the CO2 tension of the body fluids. As a consequence, carbonic acid concentration increases, increasing [Hþ]. At sea level, a PaCO2 higher than 44mmHg indicates the presence of respiratory acidosis. In young children (< 3 years old), during pregnancy, in subjects residing at high altitude, or in patients with metabolic acidosis, PaCO2 values < 44mmHg can be indicative of respiratory acidosis (see Chapters 21 and 22). A special case of respiratory acidosis is the presence of arterial eucapnia, or even arterial hypocapnia, coupled with venous and, therefore, tissue hypercapnia (1). This entity, termed pseudorespiratory alkalosis, can develop in patients with profound depression of cardiac function and pulmonary perfusion who have relative preservation of alveolar ventilation (2). Because body CO2 stores are increased, respiratory acidosis rather than respiratory alkalosis is present. Complicating acid-base disorders occur frequently in patients with respiratory acidosis (see Chapter 22). In this chapter, we review the pathophysiology, diagnosis, and management of primary hypercapnia.