Carbon dioxide elimination during circulatory arrest

To learn modes of CO2 elimination during cardiac arrest, we continuously measured end-tidal CO2 concentration (ETCO2) in acutely arrested dogs with constant ventilation. A decrease in peak ETCO2 during cardiac arrest in each dog showed a washout biexponential function when graphed on semilog paper. The average half-times of each compartment were 19.2 +/- 3.1 (SD) sec for the fast compartment and 108.1 +/- 23.8 sec for the slow compartment; the fast compartment of the CO2 elimination curve suggested that CO2 was eliminated from the functional residual capacity, while the slow compartment indicated CO2 elimination from the pulmonary capillary blood and tissue stores. Neither pretreatment with sodium bicarbonate (1 mEq/kg iv) nor a 5-min cardiorespiratory arrest altered the mode of CO2 elimination. The ETCO2 also reflected the potential effects of external cardiac compressions on pulmonary blood flow, as previously reported. Besides mixed venous blood CO2 flowing back to the lungs by cardiac compressions, it should be noted that both alveoli and pulmonary capillary blood CO2 are also reflected in the ETCO2 during the first minute of CPR.