Muscle Sympathetic Nerve Activity during Thermoneutral Head‐Out Water Immersion with and without Hyperoxia
Authors: Reed Emma L. , Worley Morgan L. , Sackett James , Bloomfield Adam C. , Johnson Blair 
Published: April 2020

Muscle sympathetic nerve activity (MSNA) is typically reduced while breathing hyperoxic gas and leads to reductions in heart rate, blood pressure and ventilation during dry conditions. Evidence also indicates that MSNA may be blunted during 10 min of thermoneutral water immersion. However, the effects of the combination of hyperoxia and water immersion on MSNA remain unknown.PURPOSEWe tested the hypotheses that MSNA will decrease during four hours of thermoneutral head‐out water immersion (HOWI) and that breathing hyperoxic gas will further attenuate MSNA compared to breathing room air.METHODSSix healthy men (age: 24±4 y) completed two randomized 4‐hour HOWI protocols in thermoneutral water (34.8±0.3°C) while breathing room air (21% O2; RA) or hyperoxic gas (100% O2; O2) before (PRE) and during HOWI. Intestinal temperature (Tc; ingestible telemetry pill), radial nerve MSNA (microneurography), heart rate (HR; ECG), mean arterial pressure (MAP; photoplethysmography), end tidal carbon dioxide tension (PETCO2; capnography) and middle cerebral artery blood velocity (MCAv; transcranial Doppler) were continuously recorded for 5 min before (PRE), 10 min into and at each hour of HOWI. MSNA was quantified as burst frequency and burst incidence. Values were analyzed as mean± standard deviation during PRE and the change from PRE (Δ) for each hour of HOWI.RESULTSThere were no differences between conditions at PRE for Tc (RA: 37.0±0.2 vs. O2: 37.1±0.2; P=0.20), burst frequency (RA: 15±3 vs. O2: 17±5 bursts/min; P=0.46), burst incidence (RA: 25±3 vs. O2: 30±10 bursts/100 heartbeats; P=0.22), HR (RA: 65±10 vs. O2: 57±6 bpm; P=0.22), MAP (RA: 94±9 vs. O2: 98±10 mmHg; P=0.46), or MCAv (RA: 59±17 vs. O2: 61±12 cm/s; P=0.70). PETCO2 was lower at PRE during the O2 condition compared to RA (RA: 43±2 vs. O2: 39±4 mmHg; P=0.04). During HOWI, Δburst frequency (interaction: P=0.05) decreased from PRE with O2 at Hour 1 (‐8±6 bursts/min; P=0.01) and Hour 4 (‐7±3 bursts/min P=0.05). ΔBurst incidence (interaction: P<0.01) decreased from PRE with O2 at Hour 1 (‐12±8 bursts/100 heartbeats; P=0.03). ΔBurst incidence decreased from PRE with RA at Hour 3 (−13±11 bursts/100 heartbeats; P=0.02). ΔMCAv (interaction: P=0.03) was different between conditions at Hour 1 of HOWI (RA: 4±8 vs. O2: −3±5 cm/s; P=0.06). ΔPETCO2 (interaction: P<0.01) was greater in RA versus O2 throughout HOWI. ΔPETCO2 increased from PRE and remained elevated in RA throughout HOWI (P<0.01) but did not change from PRE in O2 throughout HOWI (P>0.74). During HOWI, there was no effect on ΔTc (condition: P=0.58; time: P=0.07; interaction: P=0.86), ΔHR (condition: P=0.52; time: P=0.32; interaction: P=0.25) or ΔMAP (condition: P=0.95; time: P=0.05; interaction: P=0.95) for either condition.CONCLUSIONThese preliminary data indicate that MSNA is reduced during thermoneutral HOWI. However, the reduction in MSNA is not exacerbated while breathing a hyperoxic gas compared to room air. Additionally, PETCO2 during HOWI was lower in O2 vs. RA, which likely contributed to the lower ΔMCAv during O2. This suggests that breathing 100% O2 might reduce the risk of CO2 toxicity during diving.Support or Funding InformationSupported by ONR Director of Research Early Career Grant N‐000141712665