Difference in Acid-Base State between Venous and Arterial Blood during Cardiopulmonary Resuscitation

We investigated the acid-base condition of arterial and mixed venous blood during cardiopulmonary resuscitation in 16 critically ill patients who had arterial and pulmonary arterial catheters in place at the time of cardiac arrest. During cardiopulmonary resuscitation, the arterial blood pH averaged 7.41, whereas the average mixed venous blood pH was 7.15 (P<0.001). The mean arterial partial pressure of carbon dioxide (PCO₂) was 32 mm Hg, whereas the mixed venous PCO₂ was 74 mm Hg (P<0.001). In a subgroup of 13 patients in whom blood gases were measured before, as well as during, cardiac arrest, arterial pH, PCO₂, and bicarbonate were not significantly changed during arrest. However, mixed venous blood demonstrated striking decreases in pH (P<0.001) and increases in PCO₂ (P<0.004). We conclude that mixed venous blood most accurately reflects the acid-base state during cardiopulmonary resuscitation, especially the rapid increase in PCO₂. Arterial blood does not reflect the marked reduction in mixed venous (and therefore tissue) pH, and thus arterial blood gases may fail as appropriate guides for acid-base management in this emergency. (N Engl J Med 1986; 315:153–6.), DURING cardiopulmonary arrest and cardiopulmonary resuscitation, a critical reduction in tissue perfusion and the consequent failure to maintain oxygen delivery cause anaerobic metabolism and lactic acidosis.^1,2 Measurements of arterial blood gases may reflect, in addition to lactic acidosis, the effects of mechanical ventilation or the metabolic and respiratory derangements that preceded the arrest. The current standards for cardiopulmonary resuscitation include a recommendation that routine measurements of arterial blood gases be made to guide alkali therapy.³ During experimental studies of cardiopulmonary resuscitation in 53 mechanically ventilated pigs,⁴ we observed striking differences between the acid–base condition of mixed venous blood (right atrium,.