Central Chemosensitivity is Not Augmented during 60 min of Mild Hypercapnia in Healthy Adults

Abstract

Carbon dioxide (CO2) retention occurs during head out water immersion (HOWI) despite an augmented central chemosensitivity. To this end, it is unclear if an augmented central chemosensitivity during HOWI is due to CO2 retention alone (i.e., mild hypercapnia) or the combined effects of HOWI (i.e., CO2 retention, central hypervolemia, increased work of breathing, etc.).PurposeWe tested the hypothesis that central chemosensitivity is augmented during dry conditions while breathing hypercapnic gas to match the CO2 retention that occurs during HOWI.MethodsTwelve healthy subjects (age: 24±3 y, BMI: 25±3‐‐ kg/m2, 6 women) completed two experimental visits: a thermoneutral HOWI visit (HOWI) and a dry mild hypercapnia visit (Control+CO2). We measured ventilation (pneumotachometer) and expired CO2 tension (PETCO2; capnography), as an index of CO2 retention, throughout both visits. HOWI consisted of 60 min of thermoneutral (35.0±0.1°C) water immersion to the neck. During Control+CO2, small amounts of CO2 were added to the inspirate to match PETCO2 values that were obtained during HOWI. During both visits, central chemosensitivity was evaluated at baseline, 10 min, 30 min, and 60 min 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 minute ventilation vs. PETCO2 every 30 s throughout the test. Data were analyzed and reported as a change from baseline (mean±SD).ResultsThere were no statistical differences in PETCO2 (p=0.33), minute ventilation (p=0.99), or central chemosensitivity (p=0.59) between HOWI and Control+CO2 at baseline. PETCO2 increased from baseline during HOWI and Control+CO2 at 10 min (HOWI: 2±1 mmHg, p<0.01; Control+CO‐2: 2±2 mmHg, p<0.01), 30 min (HOWI: 1±2 mmHg, p<0.01; Control+CO‐2: 1±2 mmHg, p<0.01), and 60 min (HOWI: 2±2 mmHg, p<0.01; Control+CO2: 2±2 mmHg, p<0.01). The change in PETCO2 was not different between conditions at any time point (p≥0.52). Minute ventilation did not change from baseline during HOWI at any time point (p≥0.66), while it increased from baseline during Control+CO2 at 10 min (2.3±1.7 L/min, p<0.01), 30 min (2.0±2.2 L/min, p<0.01), and 60 min (1.6±1.6 L/min, p<0.01). Minute ventilation was greater during Control+CO2 vs. HOWI at 10 min (p<0.01), 30 min (p<0.01), and 60 min (p<0.01). During HOWI, central chemosensitivity increased from baseline at 10 min (0.32±0.34 L/min/mmHg, p=0.03), 30 min (0.35±0.38 L/min/mmHg, p=0.03), and 60 min (0.36±0.43 L/min/mmHg, p=0.03), while central chemosensitivity did not change during Control+CO2 at 10 min (−0.14±0.40 L/min/mmHg, p=0.74), 30 min (0.03±0.27 L/min/mmHg, p=0.99), and 60 min (0.01±0.47 L/min/mmHg, p=0.99). Central chemosensitivity was greater during HOWI vs. Control+CO2 at 10 min (p<0.01), 30 min (p=0.05), and 60 min (p=0.03).ConclusionThese data indicate that central chemosensitivity is not augmented during dry conditions while breathing hypercapnic gas to match the CO2 retention that occurs during HOWI. Thus, it appears that an augmented central chemosensitivity during HOWI is a function of the combined effects of HOWI and not solely CO2 retention.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.