Health effects of intermittent hypoxic exposure in a sedentary population

The use of simulated altitude is frequently used in an attempt to enhance athletic performance. Similar techniques have also recently been used in a clinical population to improve cardiovascular risk. However, the effects of different frequencies of simulated altitude per week, follow up assessments and the effects of passive simulated altitude in addition to exercise (rather than exercising in hypoxia) has not been tested in a sedentary, middle-aged population. To simulate altitude, an intermittent hypoxic exposure (IHE) protocol was used whereby 5-min of hypoxia was alternated with 5-min of normoxia for 1 hour. Heart rate variability (HRV), i.e. the beat-to-beat fluctuations between R-peaks, cardiovascular fitness (VO2peak), systolic blood presssure (SBP), highly-sensitive C-Reactive protein (hs-CRP, indicative of systemic inflammation associated with athlersclerosis), high density lipoprotein, total cholesterol, and arterial stiffness were used to assess cardiovascular health during 3 IHE studies. In the first study, 4 IHE sessions/week for 4 weeks resulted in increased HRV compared to a placebo treatment (71.6 ± 52.5%, mean between-group change from the natural logarithm ± 90% confidence interval), indicating increased parasympathetic activity. The second study assessed the effects of IHE frequency on selected risk factors. Following the 5-week intervention, HRV (during paced breathing) increased in participants given both 2-3 (IHE3) and 5 (IHE5) IHE sessions per week (IHE5: 47.3 ± 41.6%; IHE3: 6.2 ± 6.9%), thereby confirming the changes in HRV reported in Study 1. In addition, haemoglobin concentration (2.7 ± 2.8%), time taken to complete the maximal fitness assessment (34.5 ± 36.8%), and maximal workload (14.4 ± 14.9%) were increased in IHE5 compared to the control group, while changes in hs-CRP (12.7 ± 48.7%) were trivial. None of these changes were seen in IHE3. There was an increased VO2peak (12.6 ± 9.3%) in IHE3 compared to the Control, but not IHE5. Due to the benefical effects on VO2peak and HRV, 2-3 IHE sessions/week were used together with 3 exercise sessions/week (IHE3+Ex), and compared to exercise only (Ex) in the third study. Immediately post intervention, 4- and 8-wk follow up measurements were taken. Both groups showed a tendency to decrease total cholesterol, improve arterial compliance (pulse wave velocity) and increase VO2peak and HRV, but these changes were unclear. High density lipoprotein (Post: 8.0 ± 8.0%; 8-wk: 10.0 ± 8.5%), SBP (Post: -3.4 ± 3.4%; -3.5 ± 3.7%) improved in IHE3+Ex more than Ex, and while VO2peak was qualitatively unclear at Post it was increased at the 4-wk (9.4 ± 8.0%) and 8-wk (7.9 ± 8.3) follow-up sessions. While differences in systemic arterial stiffness (augmentation index) was unclear between groups at Post, the augmentation index increased at 4- and 8-wk (4-wk: 11.8 ± 18.4%; 8-wk: 24.8 ± 19.7%) in the IHE3+Ex group compared to Ex. Overall, IHE appears to increase the parasympathetic contribution to the sympathovagal balance of the heart at rest in sedentary middle-aged participants, and may improve SBP and some fitness parameters, particularly with 4–5 IHE sessions/week. While the short-term effects of IHE on arterial health are promising, more research on the long-term effects of IHE treatments is needed before IHE can conclusively be considered ‘safe’.
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