Autonomic Cardiovascular Responses in Acclimatized Lowlanders on Prolonged Stay at High Altitude: A Longitudinal Follow Up Study

Abstract

Acute exposure to hypobaric hypoxia at high altitude is reported to cause sympathetic dominance that may contribute to the pathophysiology of high altitude illnesses. The effect of prolonged stay at high altitude on autonomic functions, however, remains to be explored. Thus, the present study aimed at investigating the effect of high altitude on autonomic neural control of cardiovascular responses by monitoring heart rate variability (HRV) during chronic hypobaric hypoxia. Baseline electrocardiography (ECG) data was acquired from the volunteers at mean sea level (MSL) (<250 m) in Rajasthan. Following induction of the study population to high altitude (4500–4800 m) in Ladakh region, ECG data was acquired from the volunteers after 6 months (ALL 6) and 18 months of induction (ALL 18). Out of 159 volunteers who underwent complete investigation during acquisition of baseline data, we have only included the data of 104 volunteers who constantly stayed at high altitude for 18 months to complete the final follow up after 18 months. HRV parameters, physiological indices and biochemical changes in serum were investigated. Our results show sympathetic hyperactivation along with compromise in parasympathetic activity in ALL 6 and ALL 18 when compared to baseline data. Reduction of sympathetic activity and increased parasympathetic response was however observed in ALL 18 when compared to ALL 6. Our findings suggest that autonomic response is regulated by two distinct mechanisms in the ALL 6 and ALL 18. While the autonomic alterations in the ALL 6 group could be attributed to increased sympathetic activity resulting from increased plasma catecholamine concentration, the sympathetic activity in ALL 18 group is associated with increased concentration of serum coronary risk factors and elevated homocysteine. These findings have important clinical implications in assessment of susceptibility to cardio-vascular risks in acclimatized lowlanders staying for prolonged duration at high altitude.

Modulations of Neuroendocrine Stress Responses During Confinement in Antarctica and the Role of Hypobaric Hypoxia

“…A former study of our research group in the same Antarctic environment already evidenced similar results with elevated catecholamine levels and assumed their association with stated immune alterations ( Feuerecker et al, 2014 ). Furthermore, several other studies in (simulated) high altitude investigated well the impact of hypobaric hypoxia on sympathoadrenal and adrenocortical stress responses and found a positive correlation ( Calbet, 2003 ; Simeoni et al, 2011 ; Dhar et al, 2014 ; Aliyev et al, 2017 ; Woods et al, 2017 ). Woods et al (2017) showed that simulated altitude under normobaric or hypobaric hypoxia (equivalent to 3,375 m) appears to induce similar effects in humans than genuine high altitude.…”

“…However, the question whether normobaric and hypobaric hypoxia elicit the same reactions remains contradictory ( Girard et al, 2012 ; Millet et al, 2012 ; Mounier and Brugniaux, 2012 ). Furthermore, to date, most studies investigated the consequences of acute or intermittent hypoxia ( Xie et al, 2001 ; Calbet, 2003 ; Lusina et al, 2006 ; Sander, 2016 ) but data and knowledge about the effect of prolonged and chronic exposure to hypoxic conditions are rather scarce ( Dhar et al, 2014 ).…”

Dynamics of the heart rate variability and oxygen saturation response to acute normobaric hypoxia within the first 10 min of exposure

“…The autonomic nervous system (ANS) plays a major role in regulating cardiovascular function (Aubert et al, 2003). To date, studies have reported ANS responses to hypoxia lasting several minutes (Bobyleva & Glazachev, 2007;Botek et al, 2015), hours (Wille et al, 2012), days (Bhaumik et al, 2013) and months (Dhar et al, 2014). However, there is a paucity of studies reporting effects of hypoxia on ANS using ultra-short, for example 1 min, recordings.…”

Parasympathetic withdrawal increases heart rate after 2 weeks at 3454 m altitude

“…Instead, spectral analysis of HR variability has been used and suggested that parasympathetic withdrawal persists even after 18 months at HA (Dhar et al . ). However, this finding should be interpreted with caution, since parasympathetic indices of HR variability may be influenced at HA by the concomitantly increased sympathetic activity and/or pulmonary ventilation (Chapleau & Sabharwal, ).…”

“…Furthermore, the limited insight derived from spectral analysis of HR variability supports that the observed withdrawal of parasympathetic activity is at least qualitatively representative for chronic hypoxia (Dhar et al . ). Whether potential changes in autonomic receptor function and/or density affect autonomic regulation of HR at a later point of hypoxic exposure, however, remains to be determined.…”