Respiratory rate
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The respiration rate is the number of breaths a person takes per minute. The rate is usually measured when a person is at rest and simply involves counting the number of breaths for one minute by counting how many times the chest rises. Respiration rates may increase with fever, illness, and with other medical conditions. When checking respiration, it is important to also note whether a person has any difficulty breathing.
There is only limited research relating to monitoring respiratory rate, and these studies focused on issues such as the inaccuracy of respiratory rate measurement and respiratory rate as a marker for respiratory dysfunction.
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[edit] Measurement
Inaccuracies in respiratory measurement have been reported in the literature. One study compared respiratory rate counted using a 15 second count period, to a full minute, and found significant differences in the rates. Respiratory rates measurement in children under five years, for a 30 second or 60 second period, suggesting the 60 seconds resulted in the least variability. Another study found that rapid respiratory rates in babies, counted using a stethoscope, were 20% to 50% higher than those counted from beside the cot without the aid of the stethoscope.
[edit] Minute Volume
Minute volume is the volume of air which can be inhaled (inhaled minute volume) or exhaled (exhaled minute volume) from a person's lungs in one minute. This is normally registered when a person has a ventilator supporting the person's breathing due to a sickness or injury.
Minute volume is calculated by taking the tidal volume and multiplying the number of breaths per minute a person is taking. The higher the minute volume the more carbon dioxide (CO2) the person is releasing, the converse is the lower the minute volume the lower the amount of carbon dioxide the person is releasing. For example a person who is hyperventilating would have an increased minute volume.
A normal minute volume would be 5-8 liters per minute.
The symbol used to represent minute volume is a V with a dot overtop the V, or MV.
[edit] Diagnostic value
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.
[edit] Normal findings
Normal respiratory rate in a healthy adult is between 12 to 20 breaths per minute. (With such a slow rate, it is best to count the number of breaths over a full minute for accuracy.)
Respiratory rhythm should be regular.
Respiratory depth (as seen in the amplitude or excursion of chest wall movement) should also be the same between breaths.
Regular respiratory rate, rhythm, and depth are interrupted occasionally by a large breath (called a “sigh”) in normal subjects.
Normal respiratory rates, by age:
- Newborns: Average 44 breaths per minute
- Infants: 20-40 breaths per minute
- Preschool children: 20-30 breaths per minute
- Older children: 16-25 breaths per minute
- Adults: 12 to 20 breaths per minute
