🩺 Peak Flow Measurement

Peak Flow Measurement (Peak Expiratory Flow Rate - PEFR) is a bedside test that measures the maximum speed of expiration. It is primarily used to monitor and help diagnose asthma by assessing airway obstruction and its variability. It is a simple, cost-effective tool that relies heavily on patient technique and effort. Results are compared to predicted normals based on height and age to assess the severity of respiratory conditions.

Diagnosis of asthma, particularly looking for diurnal variation (morning vs. evening dips) or 'occupational' asthma (work vs. home readings). Monitoring the severity of an acute asthma exacerbation. Assessing the response to treatment, such as a trial of steroids or bronchodilators. Long-term management of chronic asthma to provide an 'action plan' for the patient based on their personal best. Differentiating between obstructive and restrictive lung disease patterns (though spirometry is definitive).

A recent pneumothorax (within the last few weeks) due to the risk of recurrence during forced expiration. Recent thoracic, abdominal, or eye surgery (due to increased pressure). Current myocardial infarction or unstable angina. Presence of a large thoracic or abdominal aneurysm. Severe haemoptysis of unknown origin. It should be delayed if the patient is in significant respiratory distress where the effort of the test would be detrimental.

Peak Flow Meter (standard or low-range). Disposable one-way mouthpieces. Peak flow diary or chart. Patient's predicted values chart (based on age, height, and sex). Hand sanitiser. Alcohol wipes for the device (if shared).

Introduce yourself and explain the purpose of the test. Ensure the patient is comfortable and preferably standing. Attach a clean mouthpiece to the peak flow meter. Ensure the pointer is at the bottom of the scale (zero). Ask the patient to take as deep a breath as possible to fill their lungs completely. Tell them to place the mouthpiece in their mouth, seal their lips around it, and blow out as hard and as fast as they can (short, sharp blow). Note the reading. Reset the pointer and repeat the process twice more. Record all three readings and select the highest of the three as the final result. Compare this result with the patient’s predicted or best-recorded value.

Measurements are compared against the patient's 'personal best' or a predicted value based on age, sex, and height (using the EU scale). A significant diurnal variation (more than 20% difference between morning and evening readings) is suggestive of asthma. In acute asthma, readings are categorised: >75% of best/predicted (moderate), 50-75% (moderate/approaching severe), 33-50% (severe), and <33% (life-threatening). A 'best of three' approach ensures the reading reflects maximum effort. Low peak flow without variability may suggest COPD rather than asthma.

Using a 'lazy' technique where the patient does not take a full inspiration or does not exhale with maximal effort, leading to falsely low readings. Failing to ensure the pointer is reset to zero before each attempt. Blockage of the device by the patient's tongue or teeth during the blow. Not allowing for adequate rest between blows, particularly in patients with active bronchospasm. Recording only one attempt rather than the best of three, which may not reflect the patient's true ventilatory capacity.

Ensure the patient is standing up if possible, as this allows for maximal lung expansion. Demonstrate the technique yourself using a spare mouthpiece so the patient understands the required effort ('a sharp, short, huff'). Remind the patient to keep their lips tightly sealed around the mouthpiece. Always reset the marker to zero yourself to ensure accuracy. If the patient is struggling to understand 'maximal effort', use the analogy of 'blowing out all the candles on a cake in one go'. Record the three readings and clearly state you are taking the highest value.

Aligned with MLA 'Clinical Skills' (Diagnostic): Peak expiratory flow rate. Understanding the use of the EU scale versus the old Wright scale is important. Students must be able to calculate a patient's 'percentage of predicted' or 'percentage of personal best' to triage the severity of asthma. The importance of peak flow diaries in chronic management is a recurring theme in primary care questions.