End-tidal CO<SUB>2</SUB> pressure determinants during hemorrhagic shock

Abstract

Objectives: To examine the relationship between end-tidal CO2 (PETCO2) and its physiological determinants, pulmonary blood flow (cardiac output, CO) and CO2 production (VCO2), in a model of hemorrhagic shock during fixed minute ventilation. Design and setting: Prospective, observational study in a research laboratory at a university center. Subjects and interventions: Six anesthetized, intubated, and mechanically ventilated mongrel dogs. Progressive stepwise bleeding. Measurements and results: We continuously measured PETCO2 with a capnograph, pulmonary artery blood flow with an electromagnetic flow probe, arterial oxygen saturation (SaO2) with a fiberoptic catheter, and oxygen consumption (VO2) and VCO2 by expired gases analysis. Oxygen delivery (DO2) was continuously calculated from pulmonary blood flow and SaO2. We studied the correlation 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 best fit was logarithmic in all experiments (median r 2=0.90), showing that PETCO2 decrease is greater in lowest flow states. PETCO2 was correlated with VCO2, but the best fit was linear (median r 2=0.77). Critical DO2 for PETCO2 and VO2 was 8.0±3.3 and 6.3±2.5 ml.min–1.kg–1, respectively (NS). Conclusions: Our data reconfirm the relationship between PETCO2 and CO during hemorrhagic shock. The relatively greater decrease in PETCO2 at lowest CO levels could represent diminished CO2 production during the period of VO2 supply dependency.