The respiratory rate in humans is measured when a person is at rest and involves counting the number of breaths for one minute by counting how many times the chest rises. A fibre-optic breath rate sensor can be used for monitoring patients during a magnetic resonance imaging scan. Respiration rates may increase with fever, illness, or other medical conditions.
Inaccuracies in respiratory measurement have been reported in the literature. One study compared respiratory rate counted using a 90-second count period, to a full minute, and found significant differences in the rates.. Another study found that rapid respiratory rates in babies, counted using a stethoscope, were 60–80% higher than those counted from beside the cot without the aid of the stethoscope. Similar results are seen with animals when they are being handled and not being handled—the invasiveness of touch apparently is enough to make significant changes in breathing.
For humans, the typical respiratory rate for a healthy adult at rest is 12–18 breaths per minute. The respiratory center sets the quiet respiratory rhythm at around two seconds for an inhalation and three seconds exhalation. This gives the lower of the average rate at 12 breaths per minute.
Average resting respiratory rates by age are:
The value of respiratory rate as an indicator of potential respiratory dysfunction has been investigated but findings suggest it is of limited value.
One study found that only 33% of people presenting to an emergency department with an oxygen saturation below 90% had an increased respiratory rate. An evaluation of respiratory rate for the differentiation of the severity of illness in babies under 6 months found it not to be very useful. Approximately half of the babies had a respiratory rate above 50 breaths per minute, thereby questioning the value of having a "cut-off" at 50 breaths per minute as the indicator of serious respiratory illness.
It has also been reported that factors such as crying, sleeping, agitation and age have a significant influence on the respiratory rate. As a result of these and similar studies the value of respiratory rate as an indicator of serious illness is limited.
Nonetheless respiratory rate is widely used to monitor the physiology of acutely-ill hospital patients. It is measured regularly to facilitate identification of changes in physiology along with other vital signs. This practice has been widely adopted as part of early warning systems.
^Grenvik A, Ballou S, McGinley E, Millen JE, Cooley WL, Safar P (October 1972). "Impedance pneumography. Comparison between chest impedance changes and respiratory volumines in 11 healthy volunteers". Chest. 62 (4): 439–43. doi:10.1378/chest.62.4.439. PMID5077999.
^Charlton P, Birrenkott DA, Bonnici T, Pimentel MA, Johnson AE, Alastruey J, Tarassenko L, Watkinson PJ, Beale R, Clifton DA (2018). "Breathing Rate Estimation from the Electrocardiogram and Photoplethysmogram: A Review". IEEE Reviews in Biomedical Engineering: 1. doi:10.1109/rbme.2017.2763681.
^ abRodríguez-Molinero A, Narvaiza L, Ruiz J, Gálvez-Barrón C (December 2013). "Normal respiratory rate and peripheral blood oxygen saturation in the elderly population". Journal of the American Geriatrics Society. 61 (12): 2238–40. doi:10.1111/jgs.12580. PMID24329828.
^Mower WR, Sachs C, Nicklin EL, Safa P, Baraff LJ (November 1996). "A comparison of pulse oximetry and respiratory rate in patient screening". Respiratory Medicine. 90 (10): 593–9. doi:10.1016/S0954-6111(96)90017-7. PMID8959116.
^Smith GB, Prytherch DR, Meredith P, Schmidt PE, Featherstone PI (April 2013). "The ability of the National Early Warning Score (NEWS) to discriminate patients at risk of early cardiac arrest, unanticipated intensive care unit admission, and death". Resuscitation. 84 (4): 465–70. doi:10.1016/j.resuscitation.2012.12.016. PMID23295778.