Abstract
Objective: Hypoxic exposure can be used as a therapeutic tool by inducing various cardiovascular, neuromuscular, and metabolic adaptations. Hypoxic conditioning strategies have been evaluated in patients with chronic diseases using either sustained (SH) or intermittent (IH) hypoxic sessions. Whether hypoxic conditioning via SH or IH may induce different physiological responses remains to be elucidated.Methods: Fourteen healthy active subjects (7 females, age 25 ± 8 years, body mass index 21.5 ± 2.5 kg·m−2) performed two interventions in a single blind, randomized cross-over design, starting with either 3 x SH (48 h apart), or 3 x IH (48 h apart), separated by a 2 week washout period. SH sessions consisted of breathing a gas mixture with reduced inspiratory oxygen fraction (FiO2), continuously adjusted to reach arterial oxygen saturations (SpO2) of 70–80% for 1 h. IH sessions consisted of 5 min with reduced FiO2 (SpO2 = 70–80%), followed by 3-min normoxia, repeated seven times. During the first (S1) and third (S3) sessions of each hypoxic intervention, cardiorespiratory parameters, and muscle and pre-frontal cortex oxygenation (near infrared spectroscopy) were assessed continuously.Results: Minute ventilation increased significantly during IH sessions (+2 ± 2 L·min−1) while heart rate increased during both SH (+11 ± 4 bpm) and IH (+13 ± 5 bpm) sessions. Arterial blood pressure increased during all hypoxic sessions, although baseline normoxic systolic blood pressure was reduced from S1 to S3 in IH only (−8 ± 11 mmHg). Muscle oxygenation decreased significantly during S3 but not S1, for both hypoxic interventions (S3: SH −6 ± 5%, IH −3 ± 4%); pre-frontal oxygenation decreased in S1 and S3, and to a greater extent in SH vs. IH (−13 ± 3% vs. −6 ± 6%). Heart rate variability indices indicated a significantly larger increase in sympathetic activity in SH vs. IH (lower SDNN, PNN50, and RMSSD values in SH). From S1 to S3, further reduction in heart rate variability was observed in SH (SDNN, PNN50, and RMSSD reduction) while heart rate variability increased in IH (SDNN and RMSSD increase).Conclusions: These results showed significant differences in heart rate variability, blood pressure, and tissue oxygenation changes during short-term SH vs. IH conditioning interventions. Heart rate variability may provide useful information about the early adaptations induced by such intervention.
Highlights
Chronic hypoxia is known to be an aggravating factor in several cardiovascular and respiratory diseases, hypoxic exposure may be of significant benefits to one’s cardiorespiratory status as demonstrated in animal models (e.g., Béguin et al, 2005) and as reviewed recently by our group (Verges et al, 2015) and others (Almendros et al, 2014; Navarrete-Opazo and Mitchell, 2014; Mateika et al, 2015)
It is recognized that the consequences of hypoxic exposure depend on a dose–response relationship ranging from normoxia to deleterious severe hypoxia [intermittent inspiratory oxygen fraction (FiO2) < 0.08–0.10 and arterial oxygen saturation (SpO2)
Since we have previously shown that both muscle and cerebral deoxygenations display slower kinetics than arterial deoxygenation when FiO2 is reduced (Rupp et al, 2013), it can be suggested that 5-min hypoxic phases during intermittent hypoxia (IH) sessions did not allow maximal muscle and pre-frontal cortex deoxygenation compared to prolonged and sustained arterial oxygenation reduction characterizing sustained hypoxia (SH) sessions
Summary
Chronic hypoxia is known to be an aggravating factor in several cardiovascular and respiratory diseases, hypoxic exposure may be of significant benefits to one’s cardiorespiratory status as demonstrated in animal models (e.g., Béguin et al, 2005) and as reviewed recently by our group (Verges et al, 2015) and others (Almendros et al, 2014; Navarrete-Opazo and Mitchell, 2014; Mateika et al, 2015). Exposure to either normobaric or hypobaric hypoxia has historically been implemented to improve athletic performance through various physiological mechanisms, including increased hemoglobin mass and enhanced muscle capilarization and metabolic capacities (reviewed in Millet et al, 2010) From this perspective, and using methods previously employed in sport medicine, repeated hypoxic exposures, i.e., hypoxic conditioning, may be seen as a new preventive, and therapeutic strategy, especially for deconditioned individuals. Previous studies on hypoxic conditioning in healthy subjects and patients used two main types of hypoxic exposure, with repetitive sessions of either intermittent [IH, e.g., 3–7 min hypoxia and 3–5 min normoxia, repeated 3–10 times (Bernardi et al, 2001a; Burtscher et al, 2004; Lyamina et al, 2011; Zhang et al, 2014)] or sustained [SH, e.g., 1 h under sustained hypoxia (Rodríguez et al, 1999; Katayama et al, 2001, 2009; Lusina et al, 2006)] hypoxia Whether these two types of hypoxic conditioning may induce different adaptations needs to be elucidated in order to establish optimal hypoxic conditioning strategies
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