THE RHYTHM OF YOUR HEART: A BRIEF INSIGHT INTO CARDIORESPIRATORY COUPLING
By Guadalupe Manrique
Our body works under the perfect synchrony of rhythms and oscillations that allow communication among organ systems. This interaction enables the body to perform essential functions to adjust to environmental changes and find balance in life. Of special interest is the functional relationship between the cardiovascular and respiratory systems. Both organ systems display oscillations from a range of sources including, heart contraction, waves in arterial blood pressure, breathing, among others. Hence, communication between these fluctuations leads to heart rate variability with consequent variability in breathing patterns and other cardiovascular measurements. This interaction is called cardiorespiratory coupling (CRC), a term used to describe the reciprocal interaction between the cardiovascular and respiratory systems that is essential for environmental adaptation.
CRC is the process in which cardiovascular activity is regulated by the respiratory system, and respiration is modulated by cardiovascular variables as well, particularly by heart rate. To better understand this relationship, it is important to highlight that the neuronal control of breathing and heart rate are functionally and anatomically linked. For example, the central respiratory pattern generator, a network of neurons that rhythmically control respiratory muscles, localizes in the brainstem. This area is situated in the posterior part of the brain and connects the cerebrum, cerebellum, and spinal cord.
Along with this network of respiratory neurons, other neurons localized in the same region are responsible for the central regulation of cardiovascular function. Instead of modulating respiratory muscles, these neurons regulate the sympathetic tone, which is the part of the autonomic nervous system that innervates the heart and blood vessels. Consequently, the brainstem is also responsible for modulating other critical functions such as heart rate and blood pressure. With that being said, CRC is a close interaction that synergizes cardiac and respiratory functions and is critical for survival. The best example of a CRC is respiratory sinus arrhythmia (RSA), a physiological phenomenon whereby heart rate increases during inspiration (breathing in) and decreases during expiration (breathing out). RSA occurs during both normal breathing and augmented breathing patterns (sighs, for example), and plays a role in reducing cardiac work while maintaining stable blood gas levels. In other words, this cardiorespiratory interaction stabilizes cardiovascular variables such as blood pressure and systemic blood flow. However, a reduction in RSA is a strong indicator of cardiovascular pathology. Generally, the presence of a respiratory dysfunction can negatively impact cardiovascular performance. Likewise, cardiovascular alterations are often associated with respiratory diseases.
One example of a disorder frequently associated with a decline in CRC is dysautonomia, a dysfunction of the autonomic nervous system which affects cardiovascular variables such as blood pressure and heart rate. As a consequence, there is an alteration in the respiratory system and the subsequent development of other cardiorespiratory dysautonomias. Among these, are apneas of prematurity, Sudden Infant Death Syndrome (SIDS), Obstructive Sleep Apnea, Familial Dysautonomia (FD), and Rett Syndrome. All these pathological conditions are related to alterations of CRC such as brainstem abnormalities, or sympathetic imbalance. For that reason, studying and understanding the close interaction between the cardiovascular and respiratory systems in terms of CRC is highly relevant. Basic and clinical research is essential to gather valuable information for the prevention, diagnosis, and treatment of cardiorespiratory diseases.
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