Abstract
Temporal asymmetry is a peculiar aspect of heart period (HP) variability (HPV). HPV asymmetry (HPVA) is reduced with aging and pathology, but its origin is not fully elucidated. Given the impact of respiration on HPV resulting in the respiratory sinus arrhythmia (RSA) and the asymmetric shape of the respiratory pattern, a possible link between HPVA and RSA might be expected. In this study we tested the hypothesis that HPVA is significantly associated with RSA and asymmetry of the respiratory rhythm. We studied 42 middle-aged healthy (H) subjects, and 56 chronic heart failure (CHF) patients of whom 26 assigned to the New York Heart Association (NYHA) class II (CHF-II) and 30 to NYHA class III (CHF-III). Electrocardiogram and lung volume were monitored for 8 minutes during spontaneous breathing (SB) and controlled breathing (CB) at 15 breaths/minute. The ratio of inspiratory (INSP) to expiratory (EXP) phases, namely the I/E ratio, and RSA were calculated. HPVA was estimated as the percentage of negative HP variations, traditionally measured via the Porta’s index (PI). Departures of PI from 50% indicated HPVA and its significance was tested via surrogate data. We found that RSA increased during CB and I/E ratio was smaller than 1 in all groups and experimental conditions. In H subjects the PI was about 50% during SB and it increased significantly during CB. In both CHF-II and CHF-III groups the PI was about 50% during SB and remained unmodified during CB. The PI was uncorrelated with RSA and I/E ratio regardless of the experimental condition and group. Pooling together data of different experimental conditions did not affect conclusions. Therefore, we conclude that the HPVA cannot be explained by RSA and/or I/E ratio, thus representing a peculiar feature of the cardiac control that can be aroused in middle-aged H individuals via CB.
Highlights
Heart period (HP) varies on a beat-to-beat basis and this dynamic is denoted as HP variability (HPV)
In the present study we investigated the relation between HPV asymmetry (HPVA), as inferred from the percentage of negative HP changes, and respiratory sinus arrhythmia (RSA) in middle-aged H controls and in chronic heart failure (CHF) patients, both featuring physiological INSP to EXP (I/E) ratios
We found that HPVA was uncorrelated with the RSA even when pooling together data relevant to spontaneous breathing (SB) and controlled breathing (CB)
Summary
Heart period (HP) varies on a beat-to-beat basis and this dynamic is denoted as HP variability (HPV). Since the baroreflex exhibits an asymmetric behavior that leads to greater HP variations in response to arterial pressure rises than falls, it has been suggested that this peculiar property might explain HPVA [7]. This peripheral hypothesis does not exclude the concurrent action of additional influences. While keeping a physiological I/E ratio, it has been suggested [7] that, in presence of a negligible RSA (i.e. with limited HP variations at the respiratory rate), the PI is about 50% or even lower given that the INSP duration is shorter than the EXP one. If the magnitude of RSA is remarkable, a migration of PI toward 50% and above this value is expected [7], imposing a significant and positive correlation between PI and RSA
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