Page 104 - BH-2-4

Brain & Heart RSA and breathing-specific heart rate

studies in dogs found a weaker f resp/f h-ΔHR’ relationship, increases, the number of heartbeats per breath decreases.
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likely due to the small sample size and large data variability. Hypothetically, if a subject’s f resp matched their f h, the
HR’ peak/f resp would be equal to one, and ΔHR’ would be zero.
What could explain the physiological mechanism
behind the f resp/f h-ΔHR’ correlation? If RSA is indeed a The observed stronger correlation (Figure 5) when f h
mechanism to improve pulmonary gas exchange, it is was represented by its peak (HR’ peak) rather than its trough
reasonable to expect that f resp and f h would combine to or mean values supported our hypothesis. HR’ peak is not
optimize RSA. In paralyzed dogs ventilated by phrenic influenced by parasympathetic regulation of f h, which,
nerve stimulation (diaphragm pacing) to maintain constant when present, introduces variability to both f h and HR’ trough.
ventilation, Hayano and Yasuma 18,19 manipulated f h to The average f h/f resp was approximately 5 beats/min
either remain constant or synchronize with lung inflation (Table 1), which is within the range observed in many
or deflation. These experimental conditions were designed mammals. In fact, the allometric functions of f h and f resp
to simulate, respectively, the absence of RSA, physiological in terrestrial mammals, spanning body weights from a
RSA, or “reversed” RSA. The authors found that blood gases few grams to several tons, show similar slopes and a ratio
were optimal when f h was increased during the inflation of about 4 beats/breath. This could suggest that 4 beats/
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phase of the breathing cycle. From these experiments on breath (which, with an average RSA of 20%, corresponds
anesthetized dogs, the authors proposed that the function to a 2.4:1.6 distribution of beats between inspiration
of RSA was to improve ventilation-perfusion matching by and expiration) represents an optimal compromise for
adjusting f h to the inspiratory airflow. The present finding coordinating the two convection systems — the ventilatory
of a tight f h/f resp-ΔHR’ relationship (Figure 5) extends and the cardiovascular — and for maximizing pulmonary
this concept to healthy humans, supporting Hayano and gas exchange efficiency.
Yasuma’s conclusions about RSA. 18,19 To appreciate the The results of this study clearly indicate that the larger
significance of the f h/f resp-ΔHR’ relationship, we must
consider the fundamental differences between pulmonary the mismatch between f h and f resp (i.e., the greater f h/f resp),
air and blood flow. the greater the RSA. This finding supports the hypothesis
that RSA may function to improve gas exchange. A large
At rest, cardiac output and pulmonary ventilation in RSA means that heartbeats predominantly occur during
adult humans are approximately equal, about 5 L/min. inspiration, keeping the blood flow high when the oxygen
However, the cardiovascular and respiratory systems content of air is highest. However, many subjects at rest do
are designed differently, which leads to contrasting flow not exhibit a large RSA, and some have a small RSA. Does
regimes. The parallel arrangement of the chest wall and this put them at a disadvantage? From the perspective of
lungs, along with the presence of a dead space, implies gas exchange, probably yes, but extra-pulmonary factors
that pulmonary ventilation must have a tidal volume at may require a more even distribution of heartbeats than a
least as large as the dead space, resulting in a relatively large RSA would allow. For example, excessive arrhythmia
lower f resp. Furthermore, the back-and-forth movement and the resultant irregular blood flow may not be desirable
of air through the tracheobronchial tree causes airflow to during muscle exercise, when a more even delivery of
be highly intermittent: It is zero at the start of inspiration, oxygen may take precedence over-optimizing pulmonary
peaks mid-inspiration, and returns to zero again at gas exchange. It would be valuable to compare RSA
end-inspiration when the air changes direction for the between resting and exercise conditions in individuals
expiration. In contrast, the cardiac pump is positioned with very different resting RSA values. One could predict
in series with the circulatory system, allowing for high f h that RSA might decrease during exercise, particularly in
with small stroke volumes and unidirectional blood flow subjects with high RSA at rest.
with minimal oscillation. Although airflow is inherently
intermittent, the higher the f resp, the less intermittent the 5. Conclusion
airflow becomes in relation to blood flow. The f h/f resp (also The breath-by-breath data on ΔHR’ from a large sample
known as the ratio between HR’ peak to f resp) can be seen as a revealed that the inspiratory-expiratory difference
proxy for the difference between cardiac flow and airflow. averaged approximately 8 beats/min or about 12% of the
A higher ratio implies greater disparity between the two mean f h. Variability in sympathetic control was a significant
flows, suggesting a higher demand for RSA. Conversely, a contributor to the inter-individual variability in RSA.
lower f h/f resp value may indicate less disparity between the Consequently, RSA should not be interpreted as an index
two flows and a lower need for RSA. The slightly better of parasympathetic control (or “vagal tone”), particularly
fit of the logarithmic function compared to the linear considering that f resp is a key determinant of RSA. Most
function (Figure 5) can be explained by the fact that as f esp importantly, f h/f resp showed an excellent correlation with
r

Volume X Issue X (2024) 7 doi: 10.36922/bh.3956