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Muscle Sympathetic Nerve Activity during Thermoneutral Head‐Out Water Immersion with and without Hyperoxia
Author(s) -
Reed Emma L.,
Worley Morgan L.,
Sackett James,
Bloomfield Adam C.,
Johnson Blair
Publication year - 2020
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.2020.34.s1.06426
Subject(s) - microneurography , anesthesia , hyperoxia , heart rate , medicine , blood pressure , baroreflex , lung
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. PURPOSE We 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. METHODS Six 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% O 2 ; RA) or hyperoxic gas (100% O 2 ; O 2 ) before (PRE) and during HOWI. Intestinal temperature (T c ; ingestible telemetry pill), radial nerve MSNA (microneurography), heart rate (HR; ECG), mean arterial pressure (MAP; photoplethysmography), end tidal carbon dioxide tension (PETCO 2 ; 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. RESULTS There were no differences between conditions at PRE for T c (RA: 37.0±0.2 vs. O 2 : 37.1±0.2; P=0.20), burst frequency (RA: 15±3 vs. O 2 : 17±5 bursts/min; P=0.46), burst incidence (RA: 25±3 vs. O 2 : 30±10 bursts/100 heartbeats; P=0.22), HR (RA: 65±10 vs. O 2 : 57±6 bpm; P=0.22), MAP (RA: 94±9 vs. O 2 : 98±10 mmHg; P=0.46), or MCAv (RA: 59±17 vs. O 2 : 61±12 cm/s; P=0.70). PETCO 2 was lower at PRE during the O 2 condition compared to RA (RA: 43±2 vs. O 2 : 39±4 mmHg; P=0.04). During HOWI, Δburst frequency (interaction: P=0.05) decreased from PRE with O 2 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 O 2 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. O 2 : −3±5 cm/s; P=0.06). ΔPETCO 2 (interaction: P<0.01) was greater in RA versus O 2 throughout HOWI. ΔPETCO 2 increased from PRE and remained elevated in RA throughout HOWI (P<0.01) but did not change from PRE in O 2 throughout HOWI (P>0.74). During HOWI, there was no effect on ΔT c (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. CONCLUSION These 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, PETCO 2 during HOWI was lower in O 2 vs. RA, which likely contributed to the lower ΔMCAv during O 2 . This suggests that breathing 100% O 2 might reduce the risk of CO 2 toxicity during diving. Support or Funding Information Supported by ONR Director of Research Early Career Grant N‐000141712665