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Tachypnea (or "tachypnoea") (Greek: "rapid breathing") is the condition of rapid breathing. In adult humans at rest, any rate between 12–20 breaths per minute is normal and tachypnea is indicated by a ventilatory rate greater than 20 breaths per minute. Children have significantly higher resting ventilatory rates, which decline rapidly during the first three years of life and steadily until around 18 years.
Different sources produce different classifications for breathing terms.
Some describe tachypnea as any rapid breathing. Hyperventilation is then described as increased ventilation of the alveoli (which can occur through increased rate or depth of breathing, or both) where there is a smaller rise in metabolic carbon dioxide relative to this increase in ventilation. Hyperpnea, on the other hand, is defined as breathing more rapid and deep than breathing at rest.
Others give another classification: tachypnea is as any rapid breathing, hyperventilation is increased rate of breathing at rest, hyperpnea is an increase in breathing that is appropriately proportional to an increase in metabolic rate.
A third paradigm is: tachypnea is abnormally rapid respiration (though some may argue this is inaccurate as breathing differs from respiration), hyperventilation is increased rate or depth of respiration to abnormal levels causing decreased levels of blood carbon dioxide and hyperpnea is any increase in breathing rate or depth that is not normal.
Tachypnea may have physiological or pathological causes. Both of these categories would include large lists of individual causes. For example, physiological causes of tachypnea include exercise and labor during pregnancy. Amongst pathophysiological causes, tachypnea can be a symptom of carbon monoxide poisoning in which oxygen delivery to the tissues and organs is blocked causing hypoxia and direct cellular injury.