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Respiratory Rate: how its measured and why it matters for your health

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What is it?

Respiratory rate is the rate at which a full breathing cycle occurs, both inhalation and exhalation. While this is something that can be taken over by voluntary control, the majority of respiratory rate is an autonomic process, which occurs as a result of many inputs, including the respiratory center of the brain, which aggregates physiological sensory information throughout the body. These sensory inputs into the respiratory center of the brain include blood CO2, O2, and pH levels, lung stretch receptors, joint and muscle proprioceptors, other peripheral receptors, and additional information from higher brain centers that process emotion, speech, motor pathways, voluntary control, and more

How is it measured?

Respiratory rate can be measured through photoplethysmography (PPG) by measuring the baseline shifts that occur with breathing. The baselines move up and down in an oscillatory pattern, which corresponds to the breath cycle. 

Correlations with health conditions

Since basal respiration rates can be increased under times of increased ventilatory demand, typically requiring increased oxygen uptake or countering the buildup of carbon dioxide, respiratory rate may be a good vital sign to monitor.

The two major drivers of this response are lung complications and sympathetic stress response. Alterations to lung function, such as acute respiratory illnesses (pneumonia, upper respiratory tract infection, etc.), acute bronchoconstriction (such as asthma), and chronic illnesses (COPD, emphysema, pulmonary fibrosis, etc.) all can cause impaired gas exchange at the levels of the lung. This impaired gas exchange leads to acidosis and hypercapnea, both of which increase the respiratory rate through the respiratory control center. Through different mechanisms, sympathetic stress leads to increased respiratory rate, typically viewed as an anticipatory response to stress. 

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Normal values

Breathing rate is individual specific, but can range from 12 to 20 breaths per minute (brpm). Within a particular individual, breathing rate can stay relatively constant across days at basal levels (coefficient of variation ~ 5%). However, certain factors such as respiratory illness, high levels of fatigue, infection, and more can cause respiratory rate to change significantly. 

Tracking Trends

This combination of low variability but high responsiveness allows for respiratory rate to be a good indicator of acute problems. For example, respiratory rate appears to be highly predictive of respiratory infection and responds prior to a typical diagnosis, which makes for an excellent biomarker for risk of respiratory infection.

In general, aside from acute illnesses, respiratory rate should remain relatively stable or trend downwards with increased cardiorespiratory function.


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