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

Abstract

Objective. 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), andmean arterial (MAP) blood pressures, heart rate (HR), stroke volume (SV), andrespiratory rate (RR) were obtained in 50 healthy human subjects during baseline supine rest andexposure 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, andPETCO₂ decreased (p < 0.001). RR was not altered (p = 1.0). The decrease in PETCO₂ 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. PETCO₂ was correlated positively with SV (r² = 0.87). Conclusions. Although PETCO₂tracked decreases in SV in this human model of progressive central hypovolemia, reductions in PETCO₂were 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.

Key words:

• hemorrhagic shock
• lower body negative pressure
• cardiovascular collapse