Central chemosensitivity is augmented during two hours 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 CO₂ retention during head out water immersion.

Abstract

Carbon dioxide (CO₂) retention occurs during water immersion. Therefore, we tested the hypothesis that central chemosensitivity to hypercapnia is blunted during two hours of thermoneutral head out water immersion (HOWI) in healthy young adults. Twenty-six participants (age: 22 ± 2 years, BMI: 24 ± 3 kg/m², 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 minutes, 60 minutes, 90 minutes, 120 minutes, and post HOWI and Control. End tidal CO₂ tension (PETCO₂), minute ventilation, blood pressure, and heart rate were recorded continuously. PETCO₂ increased from baseline throughout HOWI (peak increase at 120 minutes: 2 ± 2 mmHg; p < 0.001) and the change in PETCO₂ was greater throughout HOWI than Control (p < 0.001). The change in minute and alveolar ventilation was not different across time (p ≥ 0.173) or between conditions (p ≥ 0.052). Central chemosensitivity was greater than baseline throughout HOWI (peak increase: 0.74 ± 1.01 L/min/mmHg at 120 minutes; 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 vs. High during HOWI. Low demonstrated an increase in PETCO₂ starting at 10 minutes (2 ± 3 mmHg; p < 0.001), whereas High didn't exhibit an increase in PETCO₂ until 60 minutes (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.

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