Limitations of End-Tidal CO2 as an Early Indicator of Central Hypovolemia in Humans

This study tested the hypothesis that pulmonary end-tidal CO2 PETCO2 tracks reductions in central blood volume in human volunteers exposed to progressive central hypovolemia. Methods. Measurements of PETCO2 , systolic SBP, diastolic DBP, and mean arterial MAP blood pressures, heart rate HR, stroke volume SV, and respiratory rate RR were obtained in 50 healthy human subjects during baseline supine rest and exposure to progressive reductions of central blood volume produced by application of lower body negative pressure LBNP. Results. As increasing amounts of LBNP were applied, SBP, DBP, MAP, HR, SV, and PETCO 2 decreased p less than 0.001. RR was not altered p 1.0. The decrease in PETCO 2 did not begin to occur until 40 of maximal LBNP was applied. While PETCO2 decreased progressively thereafter, the range of baseline values 28.8 49.2 mmHg varied more than the reduction in PETCO2 elicited by maximal LBNP baseline 40.1 0.6 mmHg maximal LBNP 29.8 1.0 mmHg. The earliest significant alteration was observed in SV, which occurred at 20 of maximal LBNP. MAP did not decline significantly until 80 of maximal LBNP was reached. PETCO 2 was correlated positively with SV r2 0.87. Conclusions. Although PETCO2 tracked decreases in SV in this human model of progressive central hypovolemia, reductions in PETCO2 were small relative to the range of baseline values. Thus, monitoring such small reductions in PETCO2 as an early warning of imminent cardiovascular collapse during hemorrhage may not be clinically useful without monitors capable of providing continuous trending.