Effects of Hypoxia on Heart Rate Variability in Healthy Individuals: A Systematic Review

Abstract

Background: Hypoxia is a physiological condition that may affect the cardiac autonomic modulation, which can be assessed by spontaneous fluctuations in heart rate, know as heart rate variability (HRV). Studies have reported reductions or maintenance of HRV in hypoxic situation presenting controversial effects. There is a knowledge gap in relation to changes in HRV during hypoxia.Objective: The aim of this study was to systematically review the effects of hypoxia on HRV in unacclimatized healthy adults at rest. Methods:This systematic review was performed according to PRISMA guidelines. Search terms used in MEDLINE, SCOPUS, LILACS and EUROPE PMC database were: "heart rate variability" OR "cardiac autonomic modulation" OR "cardiac autonomic regulation" AND NOT intermittent NOT sleep (hypoxia OR altitude). Records were filtered by species, age group and language. Results: At the end of the screening and eligibility, 13 manuscripts remained for qualitative synthesis. Discussion:The studies used different experimental protocols involving difference in barometric pressure, oxygen level, time of exposure to hypoxia and control of respiratory rate. Possibly the influence of these factors and also the interindividual variation to hypoxia may justify different responses in HRV. Conclusion:Based on the investigated studies, hypoxia has been capable of generating a decrease in HRV, either by reduction or maintenance of vagal modulation, or by sympathetic predominance or even the combination of these responses in healthy adults unacclimatized to hypoxia. This effect appears to be dependent on altitude level and barometric pressure. (Int J Cardiovasc Sci. 2017;30(3):251-261)

Cardiac Autonomic and Physiological Responses to Moderate- intensity Exercise in Hypoxia

“…At rest, a hypoxic stimulus increases sympathetic activity and reduces parasympathetic regulation [20], elevating HR and decreasing HRV [40]. In addition, during exercise HR is higher at any similar sub-maximal exercise intensity and decreased ad maximal intensities [11,26].…”

Sex Differences in Autonomic Cardiac Control and Oxygen Saturation Response to Short-Term Normobaric Hypoxia and Following Recovery: Effect of Aerobic Fitness

“…Due to the improvement in transportation, an increasing number of people can travel passively to high altitude for short visits, for instance by lift, car, and/or helicopter without previous training and/or acclimatization, and this may be a risk for acute mountain sickness (AMS) development ( 4 ). During the sojourn to high altitude, it is important to consider that a low oxygen environment represents an added stress to the body ( 5 ), and tolerability to high altitude exposure in human beings seems is highly variable ( 6 – 10 ). Both lower atmospheric pressure or low fraction of inspired oxygen (FiO 2 ) induces a progressive decline in arterial oxygen saturation (SpO 2 ), causing immediate compensatory responses in the pulmonary and cardiorespiratory systems to ensure the adequate supply of oxygen to vital tissues ( 11 ).…”

“…Ratio LF/HF is traditionally thought to be an index of sympathovagal balance ( 31 , 32 ). The hypoxia-induced increase in resting HR seems to be a result from a decrease in cardiac vagal activity and an increase in relative sympathetic activity ( 10 , 33 – 35 ). From a medical standpoint, it is well-known that long-term sympathetic predominance in autonomic cardiac regulation contributes to increasing risk of cardiovascular disease such as malignant arrhythmias ( 36 ), hypertension ( 37 ), and/or sudden cardiac death ( 38 ).…”

Vagal Threshold Determination during Incremental Stepwise Exercise in Normoxia and Normobaric Hypoxia

“…The primary purpose of this study was to compare changes in vagal activity during exercise with increasing intensity in normoxia and two simulated conditions of normobaric hypoxia. Previous studies were oriented mostly on assessing the T va level in normoxia [ 9 , 14 , 30 ] or vagal activity in various hypoxic conditions at rest (see review of Oliveira et al [ 19 ]) so the main research question was focused on possible shifts of selected physiological variables determined at T va level during incremental stepwise exercise.…”

“…Decreased SpO 2 level has been considered to be an important factor that influences the cardiovascular compensation response in hypoxic conditions [ 18 ]. As reviewed by Oliveira et al [ 19 ], hypoxia has been capable of generating a decrease in HRV primarily due to attenuation in vagal activity which is accompanied by relative increase in sympathoadrenal regulation. Consequentially, hypoxia leads to acceleration in the HR.…”