Patients’ breathing test comes up short on accuracy, study finds

A routine test used to monitor patients’ breathing may be unreliable and putting them at risk, a study suggests. Incorrect results can mean clinical staff fail to spot how unwell a patient with respiratory problems is becoming, researchers say.

This widely used method, which counts breaths over a 30-second period, fails to take account of people’s irregular breathing patterns, the team says. The practice – key to assessing risk in many Covid-19 cases – could be improved by increasing the time of measurement to two minutes, the study concluded.

Counting breaths
A team from the University of Edinburgh focused on what is referred to as the respiratory rate, which is the measurement of the number of breaths a person takes in one minute. The rate is measured in all patients who arrive to hospital feeling unwell, as part of what is known as a warning score chart.

Most clinical staff believe that counting breaths taken over a 30-second period will give a reasonable measure of the respiratory rate, the researchers say.

Irregular breathing

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As breathing is not always regular, however, there can be a variation in the respiratory rate when measuring it over a short time period. The team analysed recordings of breathing in 25 hospital patients to determine how wide this variation can be. Each recording was made for between 30 minutes and an hour in patients with illnesses that included respiratory, cardiac, neurological and urinary conditions. Researchers sampled the recordings at random, many times, in the same way the way clinical staff might measure the breathing rate.

Large variation
It was found that there was a large variation in the respiration rate for each patient – more than half of the measurements differed by more than three breaths per minute. This change may sound small but, in 40 per cent of cases, the incorrect rate would have meant the warning score chart was wrong. The study is published in the journal ERJ Open Research.

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Dr Gordon Drummond, Honorary Clinical Senior Lecturer, from University of Edinburgh, said: “The lack of accuracy in measurement of respiration rate could have an impact on a patient’s treatment. We think accuracy would be improved by increasing the time of measurement to two minutes and using specialist equipment to measure respiratory rate.”

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University of Edinburgh

Journal Reference:

Current clinical methods of measurement of respiratory rate give imprecise values


Background: Respiratory rate is a basic clinical measurement used for illness assessment. Errors in measuring respiratory rate are attributed to observer and equipment problems. Previous studies commonly report rate differences ranging from 2 to 6 breaths·min−1 between observers.

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Methods: To study why repeated observations should vary so much, we conducted a virtual experiment, using continuous recordings of breathing from acutely ill patients. These records allowed each breathing cycle to be precisely timed.

We made repeated random measures of respiratory rate using different sample durations of 30, 60 and 120 s. We express the variation in these repeated rate measurements for the different sample durations as the interquartile range of the values obtained for each subject. We predicted what values would be found if a single measure, taken from any patient, were repeated and inspected boundary values of 12, 20 or 25 breaths·min−1, used by the UK National Early Warning Score, for possible mis-scoring.

Results: When the sample duration was nominally 30 s, the mean interquartile range of repeated estimates was 3.4 breaths·min−1. For the 60 s samples, the mean interquartile range was 3 breaths·min−1, and for the 120 s samples it was 2.5 breaths·min−1. Thus, repeat clinical counts of respiratory rate often differ by >3 breaths·min−1. For 30 s samples, up to 40% of National Early Warning Scores could be misclassified.

Conclusions: Early warning scores will be unreliable when short sample durations are used to measure respiratory rate. Precision improves with longer sample duration, but this may be impractical unless better measurement methods are used.

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