End-tidal carbon dioxide during extremely low cardiac output

Study objective: A number of studies have shown that expired CO₂ concentration is closely related to cardiac output, but that cardiac output was not controlled as an independent variable. In addition, the partial pressure of end-tidal CO₂ (P_ETCO₂) during extremely low cardioac output has not been reported. The objective of the present study was to measure P_ETCO₂ during well-controlled, very low blood flow rates under conditions of constant minute ventilation. Design: Ten anesthetized, intubated, and mechanically ventilated swine (weight, 43 to 102 kg) were placed on two ventricular assist devices in order to control cardiac output. Minute ventilation was measured and kept constant. Ventricular assist device output (measured with an ultrasonic flow probe); P_ETCO₂; and aortic, pulmonary artery, and central venous pressures were recorded continously. Interventions: After electrical induction of ventricular fibrillation, pump output was decreased in steps. Measurements and main results: Cardiac index ranged from 0 to 5,371 mL/min/m²; 59% of P_ETCO₂ measurements were made at cardiac indexes of less than 1,313 mL/min/m² (30 mL/min/kg). The relationship of P_ETCO₂ levels to cardiac index was determined with linear regression analysis; P<.05 was statistically significant P_ETCO₂ correlated significantly with cardiac index (P<.0001). The best-fit line by least-squares analysis produced the equation: P_FTCO₂=4.98+0.012 [cardiac index] (r²=.82). Conclusion: Under conditions of constant minute ventilation, P_ETCO₂ correlated closely with cardiac index over a large range of blood flow rates, including extremely low rates.