Heart Rate and Muscle Oxygenation during Hyperbaric Hyperoxia

Abstract

PURPOSE: Hyperbaric hyperoxia is known to increase oxygen concentration in arterial blood and tissue and also to decrease heart rate (HR). This study aimed to investigate the relation between muscle oxygenation and HR under the condition of hyperbaric hyperoxia (HBHO). METHODS: Three males and 4 females aged 27.1 ±4.7 yrs volunteered to participate in this investigation. Written informed consent was obtained from all subjects. They sat in the hyperbaric chamber for 60 min and breathed 1) air of normal pressure (1.0 ATA) for 10 min (baseline) and 2) 100% O2 of 1.3 ATA for 30 min. Between the two stages, the atmospheric pressure was gradually increased from 1.0 to 1.3 ATA for 10 min and decreased to 1.0 ATA for 10 min after the 2nd stage. Heart rate variability (HRV) and oxygenation of vastus lateralis were measured during each phase. Time domain (HR and SD of HRV) and frequency domain (LF/HF and HF/TOT, LF: low frequency power, HF: high frequency power, and TOT: total power) measures of HRV were calculated. The changes of oxyhemoglobin (deltaO2Hb), deoxyhemoglobin (deltaHHb) and total hemoglobin (deltacHb) as well as TOI (total oxygenation index: O2Hb/(O2Hb+HHb)) were evaluated using near-infrared spectrophotometry. Correlations between the changing rates for HRV measures and oxygenation related measures from baseline to HBHO were calculated. RESULTS: The decrease of HR and increase of TOI under HBHO were observed in all subjects. There was a significant positive correlation in the changing rates of HR and TOI. CONCLUSIONS: From the results of the present investigation, it can be said that lower oxygenation accompanied greater decrease of HR under HBHO. It might be due to individual differences in vasoconstriction which might occur in response to HBHO and which could induce the increase of venousreturn to the heart.