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

End-tidal carbon dioxide tension (PET_CO2) 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_CO2 being an accurate surrogate for arterial carbon dioxide tension (Pa_CO2), the latter being the physiologically relevant variable. This study tested the hypothesis that PET_CO2 provides an accurate assessment of Pa_CO2 during the aforementioned conditions. Comparisons between these measures were made: 1) 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_CO2 (N = 7). PET_CO2 and Pa_CO2 decreased during heat stress (P < 0.001); however, there was no group difference between Pa_CO2 and PET_CO2 (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_CO2 and Pa_CO2 at three distinct thermal conditions were also compared using paired t-tests, revealing no difference between Pa_CO2 and PET_CO2 while normothermic (P = 0.14) and following a 1.0 ± 0.2°C (P = 0.21) and 1.4 ± 0.2°C (P = 0.28) increase in internal temperature. During LBNP while heat stressed, measures of PET_CO2 and Pa_CO2 were similar (P = 0.61). Likewise, during the end-tidal carbon dioxide clamping protocol, the increases in PET_CO2 (7.5 ± 2.8 mmHg) and Pa_CO2 (6.6 ± 3.4 mmHg) were similar (P = 0.31). These data indicate that mean PET_CO2 reflects mean Pa_CO2 during the evaluated conditions.