End-tidal carbon dioxide tension reflects arterial carbon dioxide tension in the heat-stressed human with and without simulated hemorrhage

摘要

End-tidal carbon dioxide tension (PET_(CO_2)) is reduced during an orthostatic challenge, during heat stress, and during a combination of these two conditions. The importance of these changes is dependent on PET_(CO_2) being an accurate surrogate for arterial carbon dioxide tension (Pa_(CO_2)), the latter being the physiologically relevant variable. This study tested the hypothesis that PET_(CO_2) provides an accurate assessment of Pa_(CO_2) during the aforementioned conditions. Comparisons between these measures were made: 7) after two levels of heat stress (N = 11); 2) during combined heat stress and simulated hemorrhage [via lower-body negative pressure (LBNP), N = 8]; and 3) during an end-tidal clamping protocol to attenuate heat stress-induced reductions in PET_(CO_2) (N = 7). PET_(CO_2) and Pa_(CO_2) decreased during heat stress (P < 0.001); however, there was no group difference between Pa_(CO_2) and PET_(CO_2) (P = 0.36) nor was there a significant interaction between thermal condition and measurement technique (P = 0.06). To verify that this nonsignificant trend for the interaction was not due to a type II error, PET_(CO_2) and Pa_(CO_2) at three distinct thermal conditions were also compared using paired t-tests, revealing no difference between Pa_(CO_2) and PET_(CO_2) while normothermic (P = 0.14) and following a 1.0 +-0.2degC (P = 0.21) and 1.4 +- 0.2degC (P = 0.28) increase in internal temperature. During LBNP while heat stressed, measures of PET_(CO_2) and Pa_(CO_2) were similar (P = 0.61). Likewise, during the end-tidal carbon dioxide clamping protocol, the increases in PET_(CO_2)(7.5 +-2.8 mmHg) and Pa_(CO_2) (6.6 +- 3.4 mmHg) were similar (P = 0.31). These data indicate that mean PET_(CO_2) reflects mean Pa_(CO_2) during the evaluated conditions.