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 an d mixed venous acid-base conditions during untreated ventricular fibri llation and after institution of cardiopulmonary resuscitation (CPR). Fifty-two swine (weight: 25-40 kg) were studied after induction of ven tricular fibrillation. In a subgroup of 10 animals, 10-min CPR trials were performed. Arterial and mixed venous blood gases were monitored a t baseline, after 5 min of untreated ventricular fibrillation (noninte rvention interval) and after 10 min of mechanical CPR. Standard CPR wa s performed at compression rates of 100/min with a 60% duty cycle. Art erial pH, PCO2, and HCO3 were unchanged when baseline values were comp ared with those obtained after 5 min of untreated ventricular fibrilla tion, 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 d ecreased from 40 to 38 torr and HCO3 decreased from 28 to 26 mEq/l (P < 0.05). Although these changes were statistically significant, many r emain 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). Alterat ions 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 perfu sion 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 a cid-base composition.