End-tidal CO2 pressure determinants during hemorrhagic shock

Objectives: To examine the relationship between end-tidal CO2 (PETCO2) and its physiological determinants, pulmonary blood flow (cardiac output, CO) a nd CO, production (VCO,), in a model of hemorrhagic shock during fixed minu te ventilation. Design and setting: Prospective, observational study in a research laborato ry at a university center. Subjects and interventions: Six anesthetized, in tubated, and mechanically ventilated mongrel dogs. Progressive stepwise ble eding. Measurements and results: We continuously measured PETCO2 with a capnograph , pulmonary artery blood flow with an electromagnetic flow probe, arterial oxygen saturation (SaO(2)) with a fiberoptic catheter, and oxygen consumpti on (VO2) and VCO2 by expired gases analysis. Oxygen delivery (VO2) was cont inuously calculated from pulmonary blood flow and SaO(2). We studied the co rrelation of PETCO2 with CO and VCO2 in each individual experiment. We also calculated the critical point in the relationships PETCO2/DO2 and VO2/DO2 by the polynomial method. As expected, PETCO2 was correlated with CO. The b est fit was logarithmic in all experiments (median r(2) = 0.90), showing th at PETCO2 decrease is greater in lowest flow states. PETCO2 was correlated with VCO,, but the best fit was linear (median r(2) = 0.77). Critical DO2 f or PETCO2 and VO2 was 8.0 +/- 3.3 and 6.3 +/- 2.5 ml.min(-1) kg(-1), respec tively (NS). Conclusions: Our data reconfirm the relationship between PETCO2 and CO duri ng hemorrhagic shock. The relatively greater decrease in PETCO2 at lowest C O levels could represent diminished CO2 production during the period of VO2 supply dependency.