End-tidal Carbon Dioxide Tension Accurately Assesses Arterial Carbon Dioxide Concentration During Moderate Heat Stress

Abstract

PURPOSE: Whole-body heat stress reduces cerebral perfusion and cerebral vascular conductance. Reductions in arterial carbon dioxide concentration (PaCO2), as reflected by changes in end-tidal carbon dioxide tension (PETCO2), accompanying heat stress have been implicated in contributing to these cerebral vascular changes. While PETCO2 provides an accurate index of PaCO2 in a variety of normothermic settings, its accuracy as an index of PaCO2 during heat stress conditions remains unknown. The aim of this study was to test the hypothesis that PETCO2 is an accurate index of PaCO2, during heat stress with and without an orthostatic stress. METHODS: Internal temperature (telemetry pill), PaCO2 (radial artery catheter), and PETCO2 (capnograph) were assessed in 8 healthy individuals during four separate conditions performed sequentially; 1) normothermia (NT), 2) NT + ∼30 mmHg lower body negative pressure (NT + LBNP), 3) passive heat stress (HS), and 4) passive heat stress + ∼30 mmHg lower body negative pressure (HS + LBNP). RESULTS: Internal temperature was significantly elevated during HS from 37.0 0.3 to 38.1 ± 0.4 °C and was further elevated during HS + LBNP to 38.3 ± 0.4 (both P < 0.05 relative to NT). Two-way repeated measures analysis of variance revealed a significant main effect of condition (P < 0.01); however, there was not a significant main effect of measurement device (i.e. PETCO2 vs. PaCO2; P > 0.05) nor was there a significant interaction (see Table). CONCLUSION: These results indicate that PETCO2 provides an accurate index of PaCO2 during normothermic and moderate heat stress conditions with and without orthostatic stress. Supported by NIH Grants HL61388 & HL84072Values are mean ± SD *Significant differance relative to NT and NT + LBNP.