Subjects with a High Baseline Central Chemosensitivity Have a Delayed Onset of CO2 Retention during Thermoneutral Head Out Water Immersion
Authors: Sackett James R. , Schlader Zachary J. , O'Leary Morgan C. , Chapman Christopher L. , Johnson Blair D. 
Published: June 2021

Carbon dioxide (CO2) retention occurs during water immersion and increases the risk of CO2 toxicity. The central chemoreceptors primarily mediate the rise in ventilation during hypercapnia. It has been suggested that pre‐immersion baseline central chemosensitivity is related to the degree of CO2 retention during water immersion. Yet, it is unknown if subjects with a low baseline central chemosensitivity (LOW) experience greater CO2 retention than subjects with a high baseline central chemosensitivity (HIGH) during water immersion.PurposeWe tested the hypothesis that LOW would experience greater CO2 retention than HIGH during two hours of thermoneutral head out water immersion (HOWI).MethodsWe assessed central chemosensitivity in 26 subjects (age: 22±2 y, BMI: 24±3 kg/m2, 14 women) during thermoneutral (35±0°C) HOWI at baseline, 10 min, 60 min, 90 min, 120 min, and post. The partial pressure of end tidal CO2 (PETCO2; capnography) and total minute ventilation (pneumotachometer) were recorded continuously. Central chemosensitivity was evaluated via a rebreathing test where subjects rebreathed 7% CO2 and 93% O2 from a 10 L bag for 3.5 min. Central chemosensitivity was calculated as the slope of the linear regression line of total minute ventilation vs. PETCO2 every 30 s throughout the test. Following data collection, subjects were divided into tertiles (i.e., LOW [1.11±0.28 L/min/mmHg, n=9], NORMAL [1.60±0.12 L/min/mmHg, n=8], and HIGH [2.58±0.67 L./min/mmHg, n=9]) based on their baseline central chemosensitivity during HOWI. In order to create two distinct groups, data from subjects with a NORMAL baseline central chemosensitivity were discarded. Data are reported as a change from baseline (mean ± SD).ResultsPETCO2 increased from baseline in LOW at 10 min (2±3 mmHg, p<0.01), 60 min (3±3 mmHg, p<0.01), 90 min (3±3 mmHg, p<0.01), and 120 min (3±3 mmHg, p<0.01). PETCO2 increased from baseline in HIGH at 60 min (2±2 mmHg, p<0.01), 90 min (2±2 mmHg, p<0.01), and 120 min (3±2 mmHg, p<0.01). The change in PETCO2 was not different between LOW vs. HIGH at any time point (group main effect: p=0.25). The change in total minute ventilation did not differ over time (time main effect: p=0.20) or between groups (group main effect: p=0.64). Central chemosensitivity increased from baseline in LOW at 60 min (0.64±0.64 L/min/mmHg, p=0.04) and 120 min (0.64±0.71 L/min/mmHg, p=0.04). Central chemosensitivity increased from baseline in HIGH at 10 min (0.63±0.57 L/min/mmHg, p=0.02), 60 min (0.74±0.86 L/min/mmHg, p=0.01), 90 min (0.65±1.00 L/min/mmHg, p=0.02), and 120 min (0.72±1.10 L/min/mmHg, p=0.01). The change in central chemosensitivity was not different in LOW vs. HIGH at any time point (group main effect: p=0.85).ConclusionThese data indicate that LOW experience an earlier onset of CO2 retention than HIGH during water immersion. To this end, a low baseline central chemosensitivity might contribute to an earlier onset of CO2 retention during water immersion. However, the overall magnitude of CO2 retention did not differ between LOW vs. HIGH during water immersion.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.