Best practice: Carrying out accurate spirometry testing - part 2
Author Rachel Booker, DN(Cert) HV, RGN, is independent specialist respiratory nurse/freelance medical writer.
Abstract Booker, R. (2007) Carrying out accurate spirometry testing: part 2. Nursing Times; 103: 45, 50–51.
Spirometers are precision instruments, calibrated to record the true volume of air exhaled into them. If their accuracy is not checked, errors will go undetected. Calibration checks must be carried out with all types of spirometer as a daily routine and a log should be kept for medico-legal purposes (Miller et al, 2005a). Additional calibration checks are needed if the ambient temperature changes during the day or the equipment is moved.
A calibration syringe injects a known volume of air through the spirometer. The spirometer should record within 3% of this known volume:
l +/-30ml for a 1-litre syringe;
l +/-90ml for a 3-litre syringe.
The calibration of some spirometers can be updated if it is outside these limits. Others must be returned to the manufacturer for recalibrating.
The spirometer should also be verified using a biological control – usually a member of staff without lung disease. The lung function values of the control are determined by daily spirometry, at the same time of day over a two-week period (at least 10 recordings) and calculation of:
The mean value for each spirometry parameter;
The normal range (+/-5% of the mean).
If the spirometer measures outside the normal range for that individual, it may need recalibrating. The spirometer and calibration syringe need regular servicing to ensure continued accuracy.
Infection transmission via spirometry equipment is rare (Miller et al, 2005b), but sensible precautions must be taken to protect patients and staff. Patients with active respiratory infection should not be tested unless it is essential for sound medical reasons.
Disposable mouthpieces, nose clips and filters should be used, and one-way mouthpieces reduce the risk of cross-infection via accidental inhalation through the spirometer. Handwashing before and after using spirometry equipment and between patients is one of the most effective methods of preventing infection. Disposable gloves should be worn when handling mouthpieces.
It is essential that equipment is disinfected and sterilised regularly according to the manufacturer’s instructions and local infection control policy. Keep a log of cleaning procedures and the date, time and details of patients tested on the equipment. This will enable risk assessment and the tracing of patients should a patient with active infection be inadvertently tested. Immunocompromised patients must be tested on newly disinfected equipment.
Spirometry can be performed opportunistically, but giving instructions and information about the test to help patients prepare is useful (Box 1).
Bronchodilators need to be withheld prior to diagnostic spirometry testing. This is not necessary for routine, follow-up spirometry as withholding bronchodilators can cause symptomatic patients unnecessary distress.
Spirometry is generally safe and there are no absolute contraindications but forced expiratory manoeuvres raise intracranial, intrathoracic and intra-abdominal pressures and some conditions may be aggravated by testing. It is important to evaluate the patient’s condition and, if in doubt, ask your local lung-function laboratory for advice.
When testing a patient, record her or his details and check for contraindications to testing. Ask about their compliance with instructions before attending. Record deviations so repeat tests can be carried out under the same conditions. Weigh and measure the patient accurately, without shoes. A proxy for height in a patient unable to stand, or with a dorsal spine deformity is the measurement across the back, from middle fingertip to middle fingertip, with the arms outstretched.
Forced expiratory manoeuvres can cause syncope and fainting. For safety, spirometry must always be performed with the patient sitting, well supported and upright in a chair with both feet flat on the ground. Instructions should be clear and, if possible, the technique should be demonstrated.
Relaxed respiratory manoeuvre (measure of vital capacity)
The relaxed expiratory manoeuvre is performed first. Use a nose clip to prevent air leak. Ensure the patient takes a maximum breath in and puts the mouthpiece far enough inside the mouth to prevent occlusion by the teeth and tongue. The lips must form an airtight seal. False teeth should be left in, unless they are very ill fitting. With minimal delay between inhalation and exhalation the patient should exhale, steadily, gently and completely, like a ‘big and complete sigh’.
Forced expiratory manoeuvre (measure of forced vital capacity)
The forced expiratory manoeuvre also requires a maximum breath in, appropriate positioning of the mouthpiece and minimum delay between inhalation and the start of exhalation. Absolute maximum effort must be made from the start and throughout the blow.
It is important to encourage the patient to keep blowing until their lungs are completely empty. If the spirometer displays a real-time graphic it will be possible to see when the volume/time graph has reached a plateau for one or two seconds, or when the flow/volume graph has merged with the horizontal axis (see Fig 1 and 2 in part one). Some modern, electronic spirometers sound when the patient has exhaled to forced vital capacity (FVC).
Spirometry is hard work and patients should have at least a minute between efforts to recover. Lack of adequate encouragement will result in inadequate spirometry and possible misdiagnosis.
Recognising poor technique
Spirometry technique takes practice and you need to be able to recognise and correct inadequacies. Common errors include:
Failing to make a maximum effort at the start of forced exhalation – a ‘slow start’;
Failing to exhale to FVC – an ‘abrupt stop’;
Taking an extra breath;
These errors produce typical patterns on the graphs and can be spotted by observing the patient throughout the procedure.
A slow start produces a typical ‘S’ shape to the volume/time graph. The flow/volume graph should always rise sharply to a point at peak expiratory flow. A slow rise and rounded peak indicates a slow start and a sub-maximal effort.
If the patient fails to blow to FVC the volume/time graph will not reach a plateau and there will be an abrupt ‘dropping off’ of the end of the flow/volume graph.
The volume/time graph should always be smooth and upward curving. An extra breath will result in an unexpected, rapid rise on part of the graph. An air leak, such as if the patient fails to make a good seal with their lips around the mouthpiece, will result in the graph ‘dipping’.
Coughing is a common problem during a forced manoeuvre, particularly in patients with bronchial hyper-reactivity and uncontrolled asthma. Allowing adequate rest between efforts may help.
To ensure results are reproducible you need to record a minimum of:
Three technically acceptable relaxed blows;
Three technically acceptable forced efforts.
The best two of both the relaxed and forced manoeuvres must be within 5% of each other. If necessary you can continue up to eight forced blows in order to achieve acceptability and reproducibility criteria, but no more.
Spirometry must be performed correctly on well-maintained equipment and the results should be accurately interpreted. Training is essential. Nurses responsible for spirometry testing must ensure they are competent for the task.
Miller, M.R. et al (2005a) ATS/ERS task force: standardisation of lung function testing: standardisation of spirometry. European Respiratory Journal; 26: 2, 319–338.
Miller, M.R. et al (2005b) ATS/ERS task force: standardisation of lung function testing: general considerations for lung function testing. European Respiratory Journal; 26: 1, 153–161.
This article has been double-blind peer-reviewed.