Changes in arterial and mixed venous blood gases during untreated ventricular fibrillation and cardiopulmonary resuscitation

This investigation was designed to evaluate the changes in arterial and mixed venous acid-base conditions during untreated ventricular fibrillation and after institution of cardiopulmonary resuscitation (CPR). Fifty-two swine (weight: 25-40 kg) were studied after induction of ventricular fibrillation. In a subgroup of 10 animals, 10-min CPR trials were performed. Arterial and mixed venous blood gases were monitored at baseline, after 5 min of untreated ventricular fibrillation (nonintervention interval) and after 10 min of mechanical CPR. Standard CPR was performed at compression rates of 100/min with a 60% duty cycle. Arterial pH, Pco₂, and HCO₃ were unchanged when baseline values were compared with those obtained after 5 min of untreated ventricular fibrillation, while arterial Po₂ decreased from 81 to 69 torr. Mixed venous pH decreased from 7.41 to 7.35, Pco₂ increased from 43 to 48 torr, Po₂ decreased from 40 to 38 torr and HCO₃ decreased from 28 to 26 mEq/l (P < 0.05). Although these changes were statistically significant, many remain in the normal range. Both arterial and mixed venous pH and HCO₃ fell further after 9 min of CPR and Pco₂ increased (P < 0.05). Alterations in mixed venous pH and Pco₂ were more apparent than corresponding changes in arterial blood gas composition. We conclude that untreated cardiac arrest may be accompanied by normal arterial and mixed venous blood gas levels. Tissue acidosis is only revealed after tissue perfusion is restored and is most accurately reflected in the mixed venous blood gas composition. This apparent paradox provides insight into the relationship between tissue perfusion and arterial and mixed venous acid-base composition.