What is it?
Respiratory rate is the rate at which a complete breathing cycle occurs. While voluntary control can take over this, respiratory rate is an autonomic process controlled by the autonomic nervous system. This happens due to many inputs, including the brain’s respiratory center, which collects 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 corresponding to the breath cycle.
Correlations with health conditions
Respiratory rate is subject to change and may be an important vital sign to monitor. The two primary drivers of these changes 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 (increased acidity in the blood) and hypercapnia (a condition of abnormally elevated carbon dioxide levels in the blood), 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 or Acceptable Range
Breathing rate is individual-specific but can range from 12 to 20 breaths per minute (bpm). Within a particular individual, the 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 the respiratory rate to change significantly.
This combination of low variability but high responsiveness allows the respiratory rate to be a good indicator of acute problems. For example, the respiratory rate appears to be highly predictive of respiratory infection and responds before a typical diagnosis, which makes for an excellent biomarker for predicting the risk of respiratory infection.
In general, besides acute illnesses, the respiratory rate should remain relatively stable or trend downward with increased cardiorespiratory function.
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