It is well known that changes in end-tidal carbon dioxide partial pressure (PETCO₂) can reflect changes in cardiac output during cardiopulmonary resuscitation. The present study was performed to evaluate quantitative relationship between the changes in PETCO₂ and cardiac output in the acute hemorrahagic dogs.

Six anesthetized(isoflurane 1.0%), paralyzed, and mechanically constant ventilated dogs submitted to hemorrhage were studied. The dogs were hemorrhaged by progressive withdrawal of 50% of blood volume. After withdrawal of each 10% of blood volume, PETCO₂, arterial carbon dioxide partial pressure(PaCO₂), mean arterial pressure and cardiac output were measured.

After 40% blood loss, the percent decrease in PETCO₂ decreased significantly. The percent decrease in PETCO₂ correlated with the percent decrease in cardiac output(slope=0.33, r=0.7, P<0.001). The percent decrease in PETCO₂ correlated with the percent decrease in cardiac output(slope=0.35, r=0.55, P<0.05).

There is a linear correlation between the percent decrease in PETCO₂ and cardiac outpit with the ratio approximately 1:3 during acute hemorrhage in the constant tidal volume ventilation. The cause of PETCO₂ change induced by cardiac output might be change in PaCO₂. This finding suggests that PETCO₂ monitoring can easily detect a critical reduction in cardiac output when ventilationis constant.