title: Cardiopulmonary and arterial baroreceptor unloading during passive hyperthermia does not contribute to hyperthermia-induced hyperventilation.
abstract: What is the central question of this study? Does baroreceptor unloading during passive hyperthermia contribute to increases in ventilation and decreases in end-tidal carbon dioxide during that exposure? What is the main finding and its importance? Hyperthermic hyperventilation is not mitigated by expanding central blood volume and reloading the cardiopulmonary baroreceptors via rapid saline infusion or by reloading the arterial baroreceptors via phenylephrine administration. The absence of a reduction in ventilation upon reloading the baroreceptors to pre-hyperthermic levels indicates that cardiopulmonary and arterial baroreceptor unloading with hyperthermia is unlikely to contribute to hyperthermic hyperventilation in humans. This study tested the hypothesis that baroreceptor unloading during passive hyperthermia contributes to increases in ventilation and decreases in end-tidal partial pressure of carbon dioxide (P ET ,CO2) during that exposure. Two protocols were performed, in which healthy subjects underwent passive hyperthermia (increasing intestinal temperature by ∼1.8°C) to cause a sustained increase in ventilation and reduction in P ET ,CO2. Upon attaining hyperthermic hyperventilation, in protocol 1 (n = 10; three females) a bolus (19 ± 2 ml kg(-1) ) of warm (∼38°C) isotonic saline was rapidly (5-10 min) infused intravenously to restore reductions in central venous pressure, whereas in protocol 2 (n = 11; five females) phenylephrine was infused intravenously (60-120 μg min(-1) ) to return mean arterial pressure to normothermic levels. In protocol 1, hyperthermia increased ventilation (by 2.2 ± 1.7 l min(-1) , P < 0.01), while reducing P ET ,CO2 (by 4 ± 3 mmHg, P = 0.04) and central venous pressure (by 5 ± 1 mmHg, P <0.01). Saline infusion increased central venous pressure by 5 ± 1 mmHg (P < 0.01), restoring it to normothermic values, but did not change ventilation or P ET ,CO2 (P > 0.05). In protocol 2, hyperthermia increased ventilation (by 5.0 ± 2.7 l min(-1) , P <0.01) and reduced P ET ,CO2 (by 5 ± 2 mmHg, P < 0.01) and mean arterial pressure (by 9 ± 7 mmHg, P <0.01). Phenylephrine infusion increased mean arterial pressure by 12 ± 3 mmHg (P < 0.01), restoring it to normothermic values, but did not change ventilation or P ET ,CO2 (P > 0.05). The absence of a reduction in ventilation upon reloading the cardiopulmonary and arterial baroreceptors to pre-hyperthermic levels indicates that baroreceptor unloading with hyperthermia is unlikely to contribute to hyperthermic hyperventilation in humans.
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas, TX, USA.|School of Sport, Exercise and Rehabilitation Sciences, University of Birmingham, Birmingham, UK.
authors: Rebekah A I
authors: RA
authors: Lucas
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas, TX, USA.|Department of Biology, University of Colorado at Colorado Springs, Colorado Springs, CO, USA.
authors: James
authors: J
authors: Pearson
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas, TX, USA.|Department of Exercise and Nutrition Sciences, University at Buffalo, Buffalo, NY, USA.
authors: Zachary J
authors: ZJ
authors: Schlader
authors: Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital of Dallas, TX, USA.
authors: Craig G
authors: CG
authors: Crandall
mesh_terms: D000328:Adult; D002245:Carbon Dioxide / Q000097:blood; D002496:Central Venous Pressure; D005260:Female; D005334:Fever / Q000503:physiopathology*; D006801:Humans; D006985:Hyperventilation / Q000209:etiology*; D008297:Male; D010313:Partial Pressure; D010656:Phenylephrine / Q000494:pharmacology; D011311:Pressoreceptors / Q000502:physiology*; D012119:Respiration; D012143:Respiratory Physiological Phenomena; D012881:Skin Temperature; D055815:Young Adult
chemical_list: D002245:Carbon Dioxide; D010656:Phenylephrine