Central chemosensitivity is augmented during 2 h of thermoneutral head‐out water immersion in healthy men and women

New Findings What is the central question of the study? Is central chemosensitivity blunted during thermoneutral head‐out water immersion in healthy humans? What is the main finding and its importance? Central chemosensitivity is augmented during thermoneutral head‐out water immersion in healthy men and women. Thus, we suggest that the central chemoreceptors do not contribute to CO2 retention during head‐out water immersion. Abstract Carbon dioxide retention occurs during water immersion. Therefore, we tested the hypothesis that central chemosensitivity to hypercapnia is blunted during 2 h of thermoneutral head‐out water immersion (HOWI) in healthy young adults. Twenty‐six participants (age 22 ± 2 years; body mass index 24 ± 3 kg m−2; 14 women) participated in two experimental visits: a HOWI visit (HOWI) and a dry time‐control visit (Control). Central chemosensitivity was assessed via a rebreathing test at baseline, 10, 60, 90 and 120 min and after HOWI and Control. End‐tidal CO2 tension ( ), minute ventilation, blood pressure and heart rate were recorded continuously. The increased from baseline throughout HOWI (peak increase at 120 min 2 ± 2 mmHg; P < 0.001), and the change in was greater throughout HOWI than Control (P < 0.001). The change in minute and alveolar ventilation was not different throughout time (P ≥ 0.173) or between conditions (P ≥ 0.052). Central chemosensitivity was greater than at baseline throughout HOWI (peak increase 0.74 ± 1.01 l min−1 mmHg−1 at 120 min; P < 0.001), and the change in central chemosensitivity was greater throughout HOWI than Control (P ≤ 0.006). We also divided the cohort into tertiles based on baseline central chemosensitivity (i.e. Low, Intermediate and High) and compared Low versus High during HOWI. Low demonstrated an increase in starting at 10 min (2 ± 3 mmHg; P < 0.001), whereas High did not exhibit an increase in until 60 min (2 ± 2 mmHg; P = 0.018). These data indicate that CO2 retention occurs throughout HOWI despite augmented central chemosensitivity and that having a high baseline central chemosensitivity might delay the onset of CO2 retention.