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

Abstract

End-tidal carbon dioxide tension (PetCO2) 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 PetCO2 being an accurate surrogate for arterial carbon dioxide tension (PaCO2), the latter being the physiologically relevant variable. This study tested the hypothesis that PetCO2 provides an accurate assessment of PaCO2 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 PetCO2 ( N = 7). PetCO2 and PaCO2 decreased during heat stress ( P < 0.001); however, there was no group difference between PaCO2 and PetCO2 ( 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, PetCO2 and PaCO2 at three distinct thermal conditions were also compared using paired t-tests, revealing no difference between PaCO2 and PetCO2 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 PetCO2 and PaCO2 were similar ( P = 0.61). Likewise, during the end-tidal carbon dioxide clamping protocol, the increases in PetCO2 (7.5 ± 2.8 mmHg) and PaCO2 (6.6 ± 3.4 mmHg) were similar ( P = 0.31). These data indicate that mean PetCO2 reflects mean PaCO2 during the evaluated conditions.

Cerebral vasoreactivity: impact of heat stress and lower body negative pressure

“…It has been recently demonstrated that Pa CO 2 and PET CO 2 are similar over a range of hypocapnic and hypercapnic stimuli and respiratory rates [9]. PET CO 2 has also been shown to accurately reflect Pa CO 2 during whole body HS alone and during a simulated hemorrhage challenge [2]. Thus, in the current study PET CO 2 is an accurate index for Pa CO 2 .…”

Heat stress does not augment ventilatory responses to presyncopal limited lower body negative pressure

“…Specifically, while normothermic, during conditions where venous return is compromised (like LBNP), the reduction in overestimates the reduction in arterial CO 2 partial pressures (Immink et al 2006 a , b ; Serrador et al 2006). However, during LBNP while heat stressed, decreases in accurately track decreases in arterial CO 2 partial pressures (Brothers et al 2011). The net result is that there are similar values between thermal conditions during LBNP, even though ventilation is generally higher throughout this perturbation while heat stressed.…”

Heat stress exacerbates the reduction in middle cerebral artery blood velocity during prolonged self‐paced exercise

“…Pulmonary VO 2 , carbon dioxide elimination (VCO 2 ), VE, respiratory frequency (Rf), end‐tidal partial pressure of O 2 (PETO 2 ), and end‐tidal partial pressure of CO 2 (PETCO 2 ) were measured with the breath‐by‐breath cardiopulmonary system at 10%, 30%, 50%, 70%, 90%, and 100% of work completed. Given that direct measurements of PaCO 2 were note performed, PETCO 2 was used to estimate changes in PaCO 2 as it has been shown to accurately predict direct measurements of PaCO 2 at graded levels of hyper‐ and hypocapnia (Willie et al., ) and hyperthermia (Brothers et al., ; Nelson et al., ). Mean arterial pressure (MAP) was recorded manually by the same investigator using a sphygmomanometer (Gamma G5, Heine Optotechnik, Herrsching, Germany) and calculated as: DBP + 1/3 × (SBP − DBP), where DBP is diastolic blood pressure and SBP is systolic blood pressure.…”