Age-related reductions in heart rate variability do not worsen during exposure to humid compared to dry heat: A secondary analysis

ABSTRACT

We conducted a secondary analysis to investigate whether age-related attenuations in heart rate variability (HRV) worsen during exposure to moderate, dry (36.5°C, 20% RH) or humid (36.5°C, 60% RH) heat conditions that resulted in greater body heat storage among older compared to young participants, and during humid compared to dry heat, regardless of age. Six HRV indices [heart rate (HR), coefficient of variation (CoV), detrended fluctuation analysis: α1, low frequency power, high frequency power, and low/high frequency ratio] were assessed in 10 young (21 ± 3 y) and 9 older (65 ± 5 y) adults for 15-min prior to (baseline), and at the end of a 120-min exposure to dry and humid heat while seated at rest. Our results demonstrated a condition (dry and humid) x time (baseline and end) interaction effect on HR (p = 0.047) such that HR gradually increased during humid heat exposure yet remained similar during dry heat exposure across groups. We also found an age-related attenuation in CoV at baseline for both the dry (young: 0.097 ± 0.023%; older: 0.054 ± 0.016%) and humid (young: 0.093 ± 0.034%; older: 0.056 ± 0.014%) heat conditions (p < 0.02). Those age-related attenuations in CoV, however, were not magnified throughout the exposure nor different between conditions (p > 0.05). While older adults stored more heat during a brief 120-min exposure to dry heat compared to their young counterparts, this was not paralleled by further age-related impairments in HRV even when body heat storage and cardiovascular strain were exacerbated by exposure to humid heat.

KEYWORDS:

• Aging
• autonomic nervous system
• parasympathetic nervous system
• sympathetic nervous system
• relative humidity
• heat stress

Author contributions

All experiments took place at the Human and Environmental Physiology Research Unit of the University of Ottawa. A.E.C. and G.P.K. conceived and designed the study. C.L.H., S.R.N., M.J.M., H.E.W B., and A.J.E.S. contributed to the acquisition, analysis, assembly, and interpretation of data. A.E.C. and A.D.F. completed the statistical analysis. A.E.C., S.R.N., M.J.M., and G.P.K. drafted the manuscript. All authors provided critical revisions of the manuscript, approved its final version, and are accountable for all aspects of the work. G. P. Kenny is supported by a University of Ottawa Research Chair. S. R. Notley is supported by a Postdoctoral Fellowship from the Human and Environmental Physiology Research Unit.

Disclosure statement

A.J.E.S. is a patent holder, Director and shareholder of Therapeutic Monitoring Systems (TMS) Inc., focused on commercialization of variability-derived clinical decision support tools developed in OHRI's Dynamical Analysis Laboratory. C.L.H. is a patent holder related to variability monitoring and physiological waveform analysis.

Abbreviations

CoV	=	Coefficient of variation
DFA α1	=	Detrended fluctuation analysis: α1
HF	=	High frequency power (0.15–0.4 Hz)
HR	=	Heart rate
HRV	=	Heart rate variability
LF	=	Low frequency power (0.04–0.15 Hz)
LF/HF	=	Low/high frequency power ratio
RH	=	Relative humidity

Additional information

Funding

This work was supported by the Canadian Institutes of Health Research under Grant [numbers 286363 and 39943].