Summary of Key Points for OSCEs
Peak flow rate is measured using a simple peak flow meter that consists of a long cylindrical tube and a disposable mouth piece (Figure 64.1). As the patient exhales into the mouth piece, a piston is forced along the long axis of the peak flow meter. This is connected to a pointer that moves along the upper surface to give a reading on a calibrated scale.
Figure 64.1 A typical bedside peak flow meter, similar to ones you are likely to be given in the exam
Uses of Peak Flow Measurement
Peak flow measurement is useful in the diagnosis and monitoring of obstructive airway disease.
Diagnosis
Peak flow can be used to diagnose asthma by demonstrating ‘reversibility’. Following administration of a bronchodilator, for example via a salbutamol nebuliser or inhaler, an improvement of over 15% in peak flow rate indicates that the airway obstruction has a reversible component, in keeping with asthma.
If there is no convincing reversibility, the diagnosis is unlikely to be asthma but may well be chronic obstructive pulmonary disease (COPD). However, there are also patients with COPD or emphysema who demonstrate a degree of reversibility, so the differentiation in diagnosis between COPD or asthma is not always clear.
Monitoring
The trend in peak flow rate is more important than the actual value. Recording serial peak flow readings provides objective evidence of progress or response to treatment of the airway obstruction. In addition, diurnal variation is often seen in asthma, with early morning dips in the peak flow values. Patients will often keep a peak flow diary, which can be used to demonstrate this. If measurements are taken daily for monitoring purposes, they should be taken at the same time each day.
Interpretation of the Result
The best reading of three is taken so that the patient’s best effort is what is documented. This will often be the second reading. The greatest reading is compared either with the patient’s own best known reading or with a standardised chart that predicts what it should be according to height and gender (Figure 64.2). ‘Normal’ is considered to be a reading that is at least 80% of the predicted or best known value.
Figure 64.2 Peak expiratory flow rates for males and females – normal values. The graphs demonstrate predicted peak flow values for healthy males and females of different ages and weights. You only need to learn a few typical values for the exam, as well as how to interpret the graph.
Adapted by Clement Clarke for use with EN13826/EU scale peak flow meters from Nunn AJ, Gregg I, Br Med J 1898;298:1068–70
The peak expiratory flow rate (PEFR), expressed as a percentage of the best or predicted value, is a useful tool in establishing the severity of an asthma attack.
| PEFR expressed as % of best or predicted value | Severity of asthma attack |
| 80+ | Normal |
| 50–75+ | Moderate |
| 33–50 | Severe |
| <33 | Life-threatening |
Limitations of Peak Flow Measurement
Although peak flow measurement is a simple, inexpensive and easily performed technique, there are several drawbacks to its use, which must be considered:
- It depends significantly on the patient’s technique, cooperation and effort, so clear instructions, demonstration and encouragement are required. If patients do not put in their best possible effort, the results may not give a true reflection of their current respiratory state.
- The technique measures the PEFR but gives no indication of other markers of lung function, for example volume measurements.
- It does not assess the calibre of the smaller airways.
- It is unsuitable for use in children below 5 years of age as the airway resistance encountered in using a peak flow meter is too high, and it may be difficult to explain the technique to a small child and get their cooperation.
Spirometry
You will probably not be asked to demonstrate this in the exam, but you may be asked to interpret spirometry graphs as part of the station on PEFR.
Spirometry is performed in the laboratory by specially trained technical staff. Essentially, the patient exhales with maximum possible force through a mouth piece as rapidly as possible for as long as possible, after a maximum inspiration. This can be difficult and exhausting for patients, especially those with obstructive airway diseases since they have prolonged expiratory phases. The spirometer then produces a graph as shown in Figure 64.3.
Figure 64.3 Spirometry graphs for obstructive and restrictive lung disease. The graphs demonstrate the spirometry graphs you can expect to see for patients with obstructive and restrictive lung diseases, and show how the FEV1/FVC ratio changes
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