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Brain & Heart RSA and breathing-specific heart rate

of peripheral and central inputs to the mechanistic basis between 0.15 and 0.45 Hz) relative to the low-frequency
of RSA. The results have been mixed, possibly because component (0.04 – 0.15 Hz) are often interpreted as
4-9
the relative importance of each factor may vary depending indicating changes in “vagal tone”. 24,25 This approach is
on the specific experimental or clinical conditions. What widely used in clinical settings for the rapid identification
10
is firmly established, however, is that the manifestation of RSA and has even been adopted as a predictor of cardiac
of RSA results from the temporary abolition of the vagal patient outcomes. 26-34 However, the assumption that the
output; this means that the f h values at end-inspiration high- and low-frequency components of the IBI power
solely depend on the sympathetic regulation of the heart. spectrum correspond to parasympathetic and sympathetic
Although the neurological basis of RSA is well control, respectively, has been repeatedly questioned,
documented, its physiological significance remains a especially since selective pharmacologic block studies have
subject of debate. Some have argued that the decrease in shown that these components do not always align with
8,19,21,35-37
f h during expiration lowers cardiac work during the low- autonomic influences on the heart. Importantly,
oxygen phase of the breathing cycle, or that RSA helps changes in the high-low frequency ratio of the IBI power
stabilize blood pressure or blood carbon dioxide levels. spectrum cannot measure the magnitude of RSA with
Others suggest that RSA might be an epiphenomenon accuracy.
with no meaningful physiological role. 11-13 The latter In contrast to power spectrum analysis, the breath-by-
hypothesis seems less likely, given that RSA is widespread breath calculation of the difference between the peak and
among mammals, ranging from small species such as mice trough of instantaneous HR (ΔHR’ = HR’ peak – HR’ trough)
and rats 14,15 to larger animals like dogs, sheep, horses, and provides a direct quantification of RSA magnitude. This
cows. 9,16,17 In studies involving paralyzed and artificially approach has been applied to analyze short periods of IBI
ventilated dogs, where stimulation of the cardiac nerves recordings in humans 4,6,23,38,39 and occasionally in other
was applied at various phases of the breathing cycle, the species. 9,15,16,40 Although more time-consuming than power
most favorable blood gas values occurred when f h was spectrum analysis, breath-by-breath computation of ΔHR’
increased during the inflation phase of the breathing cycle. offers a more accurate measurement of RSA.
These results prompted the hypothesis that RSA may The present work combined the data of several
improve ventilation-perfusion matching by adjusting f h to previous studies in which ΔHR’ was measured directly,
the inspiratory airflow. 18-21
breath-by-breath. The resulting database, which included
The magnitude of RSA has been shown to have over 600 subjects, allowed us to construct statistically
an inverse relationship with the breathing frequency meaningful correlations between ΔHR’ and the variables
(f resp). This phenomenon, revealed through direct thought to influence RSA magnitude, i.e., f h, f resp, HR’ peak,
breath-by-breath measures of RSA, 4,6,22,23 complicates and HR’ trough. The results indicated a highly statistically
the interpretation of RSA as a simple balance between significant correlation between f h/f resp and ΔHR’, strongly
parasympathetic and sympathetic cardiac control. The supporting the idea that RSA may play a role in improving
involvement of f resp in RSA could be seen as another the matching between quasi-continuous blood flow and
manifestation of RSA’s role in optimizing ventilation- intermittent airflow.
perfusion matching. The cardiorespiratory system is
designed such that the pulmonary airflow is intermittent, 2. Methods
while pulmonary blood flow is quasi-continuous. Thus, The data for the present work were collected between 2013
a higher f resp relative to f h should reduce the disparity and 2020 for research on the effects of chest wall distortion,
between the pulmonary air and blood flow regimens. mental tasks, and muscle exercises on RSA. 23,38,39,41 A
Based on this interpretation, one could hypothesize that re-examination of the original individual data files resulted
the magnitude of RSA is more strongly correlated with the in a sample of several hundred subjects, all of whom had
ratio of f h to f resp (f h/f resp, or the “breathing-specific HR”) RSA measured at rest using identical methodology and
than with f resp or f h alone. The aim of this work, therefore, analysis.
is to test the correlation between f h/f resp and RSA. This
required direct measurements of RSA amplitude in a 2.1. Subjects
large sample size of subjects to minimize the influence of The study population consisted of 635 undergraduate
the inter-subject variability. students (272 males and 363 females) aged 19 – 21 years. All
Most commonly, RSA is derived from the high- participants were non-smokers, free of cardiorespiratory
frequency band of the power spectrum of a series of IBI. disorders, and not undergoing any medical treatment. The
Changes in the high-frequency component (typically students were enrolled in a 1-year laboratory course in

Volume X Issue X (2024) 2 doi: 10.36922/bh.3956