Rhythmic relationships between cardiac pump and ECG parameters as criteria for effectiveness of adaptive responses of cardiovascular system in acute hypoxia

BACKGROUND: Analysis of cardiovascular system variability in the frequency domain allows for the assessment of both neural and non-neural mechanisms of cardiac regulation. This analysis is challenging due to the multiple input and output mechanisms of cardiovascular and respiratory control.

AIM: The work aimed to investigate changes in spectral, phase, and coherence relationships, as well as the transfer function |gain(f)|, between oscillations of left ventricular end-diastolic pressure, specific peripheral vascular resistance, left ventricular stroke volume, and R–R intervals of the electrocardiogram during acute hypoxia at the dominant spectral component corresponding to the respiratory frequency.

METHODS: Effective (n = 17) and ineffective (n = 20) types of adaptation were determined based on the direction of changes in the heart pumping parameters during the transition from normoxia to hypoxia. Effective and ineffective types of adaptation were demonstrated in the same subject during different hypoxic tests, just as the same subjects could demonstrate either only effective or only ineffective adaptation.

RESULTS: Changes in gain(f) and cross-spectral power density in pairs of oscillatory processes were used to assess alterations in cardiac regulation (Frank–Starling mechanism, arterial–cardiac baroreflex, and others). For effective adaptation during hypoxia, a statistically significant (p < 0.05) increase in gain for end-diastolic pressure–stroke volume relationship was observed, whereas spectral power of this parameter did not change, reflecting inability to increase CO through heterometric myogenic autoregulation during hypoxia at rest. Under these conditions, increase in CO occurs mainly due to chronotropic effect. In pairs of oscillatory processes—end-diastolic pressure–specific peripheral resistance or specific peripheral resistance–R–R intervals—cross-spectral power density significantly decreased (p < 0.05) during effective adaptation. Reduction in amplitudes of these oscillatory pairs reflects adequate functioning of arterial–cardiac baroreflex in decreasing specific peripheral resistance during effective adaptation.

CONCLUSION: Quantitative changes in analyzed indices of cardiac pump function under acute hypoxia are determined by alterations in their rhythmic interactions, both among themselves and with oscillations of R–R intervals at dominant respiratory frequency, as well as by displacement of A_QRS (the maximum depolarization vector) in frontal plane. Identified patterns represent effectiveness of adaptation to severe hypoxia.