Autonomic cardiovascular response to acute hypoxia and passive head-up tilting in humans

Abstract

Acute hypoxia may alter autonomic cardiovascular reflexes during orthostasis. Heart rate variability (HRV), arterial blood pressure (MAP), and respiratory sinus arrhythmia (RSA) were recorded during supine (SUP) and passive head up tilt (HUT) in eight healthy humans, spontaneously breathing either room air or 10% O₂ in N₂. In the time domain, heart rate increased and variability decreased with HUT in both trials, with no difference between trials. In the frequency domain, normalized low frequency HRV increased, and normalized high frequency HRV decreased with HUT in both trials, with no difference between trials. MAP was 74.9 (8.6) and 77.5 (11.7) mmHg when SUP in the room air and hypoxia trials, respectively. A significant increase in MAP occurred with HUT in the room air trial but not in the hypoxia trial. In both trials, end tidal CO₂ decreased with HUT, with no difference between trials. In the room air trial, end tidal O₂ increased with HUT, whereas during the hypoxia trial, end tidal O₂ decreased with HUT. The distribution of heart beats relative to the phase of ventilation (%HBIN and %HBOUT) was similar in both trials: the %HBIN was 43.5 (3.3) % and %HBOUT was 56.5 (4.2) % breathing room air when SUP, and 45.5 (3.0) and 54.5 (3.2) when hypoxic and SUP. For both trials, this distribution did not change with HUT. As both HRV and RSA showed similar responses to HUT when spontaneously breathing either room air or 10% O₂ in N₂, we suggest that autonomic cardiovascular reflexes are preserved during acute hypoxia.

Altered Autonomic Balance in Normal Tension Glaucoma

“…Highly significant decrease in value of RMSSD observed in NTG group (p<0.000) in comparison to controls. The above findings of time domain variables of HRV in the prospective study are in accordance with studies by Sharma et al and Brown et al [13,14]. Similarly, the HF variables (HF [nu]and HF [ms 2 ]) were significantly less (p<0.000), and LF/HF ratio was significantly high (p<0.000) in Group ІІ as compared to Group І.…”

Validity of Commonly Used Heart Rate Variability Markers of Autonomic Nervous System Function

“…The only purported sympathetic HRV marker that showed a statistically significant increase in response to both stressors was the LF spectral component when expressed as LF (nu). Statistically significant increases in LF power, when expressed in normalized units, have been reported in many previous studies for both the Stroop test [68] and head-up tilt [60-65, 76, 81, 82]. With regard to the absolute units of LF, a small number of studies have reported a significant increase in LF (ms 2 ) in response to orthostatic stress [62, 66, 76].…”

“…With regard to the absolute units of LF, a small number of studies have reported a significant increase in LF (ms 2 ) in response to orthostatic stress [62, 66, 76]. However, many studies found no statistically significant change [57, 64, 65, 79, 83, 84] and some even found a decrease in LF, when expressed in absolute units, in response to orthostatic stress [58, 60, 63, 85]. Our results appear to agree with the majority of findings where significant increases in LF (nu) were found in response to both stressors, and no statistically significant change in LF (ms 2 ) in response to either stressor was observed (see Table 1).…”

Statin restores cardiac autonomic response to acute hypoxia in hypercholesterolaemia

“…However, there are equivocal findings in the literature regarding the effects of hypoxia on sympathetic (LF) and parasympathetic components of HRV (HF) [27]. Some but not all studies observed increase in cardiac sympathetic modulation [27], which may be related to inherent limitations of HRV measurement and/or to different methodological approaches [5]. For example, in our study, LF-HRV increase in response to short-term hypoxic breathing was detected in untreated FH patients but not in control subjects.…”