Identifying lung function disturbances is paramount to the diagnosis of restrictive and obstructive lung conditions, assessment of condition severity and decision on course of treatment. Whole body plethysmography is a highly sensitive test used on its own or as an adjunct to conventional lung function tests for further investigation of lung mechanics. It allows for the measurement of total lung volume, assessment of airway resistance and also the test of bronchial hypersensitivity done with the use of histamine or metacholine. With that, reversibility can then be assessed through administering bronchodilators. Whole body plethysmography has the advantage of being non invasive and also not requiring the active cooperation of patients. This makes it well suitable for patients who cannot mount up the effort to carry out the conventional lung function tests such as young children and the elderly.
The scientific basis of measurement
A patient is enclosed in an airtight body size box whose door is sealed shut and they are asked to breathe through a pneumotachograph; a device with a mask covering both nose and mouth. It measures amount of airflow and its pressure. The plethysmograph itself has pressure transducers in its walls allowing for airway pressure to be deduced from changes in pressure across the mouthpiece relative to changes across the plethysmograph box. The patient takes normal breaths then a shutter is closed in the pneumotachograph at the end of normal expiration such that the patient makes respiratory effort against a closed shutter.
Plethysmography is based on the principle of Boyle's law, which states that pressure is inversely proportional to volume under constant temperature. As the patient tries to breathe in against the closed shutter; expansion of the lungs occurs and this increases lung volume, which consequently, decreases lung pressure. Because the box is sealed and airtight, the increase in lung volume means the volume in the box decreases creating an increased pressure in the box. This system therefore allows the pneumotachograph to measure alveolar pressure while the thoracic volume is measured indirectly by the change in pressure in the box as the patient expands and compresses their chest.
Whole body plethysmography therefore measures factors that indicate efficiency of lung mechanics such as; total lung volume, functional residual capacity, breathing frequency, tidal volume, peak inspiratory and expiratory flow, airway resistance etc.
If the patient has a lung condition, their measurements will differ from the reference ranges. A patient with an obstructive lung deficit such as asthma will have a high functional residual capacity due to the failure of the lungs to empty normally as a result of the high resistance of the airways, whilst a patient with a restrictive deficit will have a reduced total thoracic volume as well as reduced functional residual capacity due to the loss of elasticity and thus compliance of the lung parenchyma.
Indications for use of whole body plethysmography
The use of whole body plethysmography is indicated in the following cases:
Symptoms of lung disease such as, cough (+/-sputum), shortness of breath, and wheeze.
As an adjunct to spirometry and conventional lung function tests to help distinguish between obstructive and restrictive lung deficits. This is particularly because spirometry does not measure functional residual capacity, meaning that it cannot measure total lung volumes, something essential for the diagnosis of restrictive lung deficit. Spirometry can only show reduced FVC and FEV1 if the condition is restrictive therefore, plethysmography would be employed here to confirm the true diagnosis {{38 Thomas R. Gildea, Kevin McCarthy January 2009}}.
To carryout lung function tests on infants who are too young to comprehend and follow the rigorous instructions given in spirometry and also on sedated patients. However, few hospitals will probably have the facilities to carryout lung function tests in infants and sedated patients because of the expenses associated with remodelling adult equipment and also extra labour for things such as sedation which will be needed to accommodate the uncooperative patients. This will be discussed later under limitations and challenges of plethysmography.
To further investigate results of the gas dilution technique used to measure functional residual capacity where there is suspected obstructive lung deficit such as in severe airway obstruction or bullous emphysema. This is because the gas dilution technique can only measure residual volume in air spaces that are ungergoing gaseous exchange and therefore have a direct communication with the main bronchial tree. This means that the air in trapped parts of lung due to obstruction is not measured as it does not get to mix with the inhaled helium and cannot be included in the capacity inferred from the change in helium concentration exhaled {{42 Rodenstein, D.O. 1982}}.
To assess degree of airway resistance to airflow.
To assess degree of bronchial hypersensitivity through the administration of histamine or metacholine and also the response to bronchodilators to reverse the bronchospasm should it occur.
To carryout lung function tests in patients who cannot actively cooperate because much effort is required by the conventional spirometry test which is predominantly made up of forced inspiration followed by forced expiration for as long as patient can hold it. This is normally done 3 times for reliability but can also make the test less reliable because the patient gets tired with progressive breaths and thus underestimation of FVC and FEV1 can occur. This makes plethysmography a better test for patients who cannot mount up the effort to carry out the conventional lung function tests such as infants and the elderly {{34 Klug,B. 1996}}.
To monitor disease progression and assess the response to treatment.
The reliability of the test - sources of error
Sources of bias with body plethysmographyJust like any other physiological measurement test, whole body plethysmography is subject to bias, which invariably affects the reproducibility of the test.
Physiological variability
Patients will vary biologically therefore it is important to use their body size, race, gender, height and age to access the appropriate lung function reference values. A brief history is also important because it can reveal important things. It is also true that what is abnormal with regards to reference values may be normal for that particular person thus perhaps serial measurements may be necessary.
Measurement error
Systematic errors
Random errors
Predictable errors that can be avoided such as lack of careful calibration of the transducers & recording equipment, faulty equipment, thermal instability, leaking of pressure from the box and even operator bias; which is where the operator does not adequately coach or give instructions coherently to the patient, etc.
Unpredictable errors that cannot be avoided such as freezing of recording equipment or fidgety movements of patient, etc.
A study by {{44 Shore, S., Milic-Emili, J., Martin, J.G. 1982}} showed that in people with obstructive lung deficits, plethysmography may overestimate total lung capacity because when the patient tries to breathe against the closed shutter, the transmission of changes in alveolar pressure to the mouth is incomplete. This is because the extrathoracic airway (including the cheeks) has a degree of compliance, meaning that it can narrow. This causes the underestimation of alveolar pressure because less air flows out to be detected by the transducer in the pneumotachograph. A falsely high total lung capacity is therefore inferred in turn, from changes of pressure in the plethysmography box. The extent of overestimation depends on the elasticity of the extrathoracic airway and the degree of the obstructive deficit.
However, another study {{47 Shore, S.A., Huk, O., Mannix, S., Martin, J.G. 1983}} suggests that the above problem can be overcome if patients reduce frequency of panting during the time when the shutter is closed in the pneumotachograph. It showed that if patients exhibit high panting frequencies, alveolar pressure is conveyed less to the mouth therefore total lung capacity is falsely recorded as higher than it should be by the pressure across the box and this invalidates the results.
Another source of measurement error with body plethysmography is the challenge imposed by patients fearing the test. This could be claustrophobic adults or nervous children. The intercom communication with the operator allows the patient to be reassured and also the clear walls of the box make it less challenging.
Possible side effects or contraindications
Plethysmography is generally a non-invasive, indirect measurement method therefore is safe for most people. It does not have any known side effects but it can however be contraindicated in the following situations:
Severe claustrophobia, which triggers panic in the patient.
Patients with continuous long-term oxygen therapy that should not be stopped.
Patients with conditions that might be disturbed by the airtight pressure environment of the body box e.g. eardrum problems.
Disorientated patients or patients with uncontrolled muscular coordination. Such would be limiting to obeying the operator's instructions or adequately breathing against the closed shutter.
Given that the right precautions are employed, plethysmography is often uneventful but there is always a risk of infection spread if the equipment is not well sterilised and disinfected. The infection will spread to anyone who comes into contact through droplets and fluids left on any shared surfaces, particularly the facemasks of the pneumotachograph.
It can also be considered that should a patient stay in the body box for too long, they can become hypoxic or hypercapnic given that the box is airtight. Nevertheless, this rarely happens due to the fact that the test is does not go on for very long and also the body box is vented every so often. {{37 Susan Blonshine BS RPFT RRT, Mason MI, Catherine Foss BS RRT RPFT, Carl Mottram BA RRT RPFT, Rochester MN Gregg Ruppel MEd RRT RPFT, Jack Wanger MBA RRT RPFT May 2001}}
Sensitivity, specificity and predictive values
As mentioned earlier, body plethysmography has been shown to be a highly sensitive test in detection & monitoring of lung pathology and also assessment of airway response to bronchospasm treatment. The table below shows the results of a study carried out on two groups of non-sedated 2-5year old preschool children. One group was asthmatic and the other group was healthy controls. The study aimed to assess the repeatability of the Cold, dry air challenge (CACh) test and whether or not it is a feasible test to use in diagnosing asthma in children. One of the chief aims of the study was to also compare the sensitivity, specificity and predictive values of the lung function tests that were utilized and these are listed in the table:
Lung function test
Sensitivity (%)
Specificity (%)
Predictive value of +ve test (%)
Predictive value of -ve test (%)
Specific airway resistance as measured by whole body plethysmography (sRaw)
68
93
93
69
Respiratory resistance as measured with the interrupter technique (Rint)
32
97
92
52
Respiratory reactance at 5 Hz measured with impulse oscillation technique (Xrs5)
24
100
100
50
Respiratory resistance at 5 Hz measured with impulse oscillation technique (Rrs5)
19
97
88
47
Table 2: Sensitivity, Specificity and Predictive values of each lung function test using change > 3 standard deviation units after the Cold dry air challenge test (CACh) as the definition of airway hyperresponsiveness.
Adapted from {{51 Nielsen, K.G. 2000}}
Initially, for baseline measurements, all the tests employed demonstrated the expected; low lung function in the asthmatic children in comparison to the healthy controls. After administration of the isocapnic cold dry air, sRaw demonstrated the highest sensitivity, picking up 68% of the true positives. The results therefore show that all tests are capable of effectively assessing lung function but the best test was whole body plethysmography, which did the better job in distinguishing the asthmatics from the healthy controls.
The cost of the test
Buying price of an adult body plethysmograph is approx. £35000 with the infant one costing £55000. The machine will also have running costs, which include the purchase of disposable mouthpieces and facemasks, and also maintenance contracts to cover break down of the machine. Much money will also be spent in paying out the salaries of the specialist people who will be employed to operate the machines and report results. Infant body plethysmography will most probably require two people with even more specialised training in the field to work with each patient. This is even more expensive.
Value of the test
A test with high sensitivity has a higher chance of having many false +ves and similarly, a test with high specificity will have a higher chance of having many false -ves. This is just based on likelihood not absolute. This means that a test with high sensitivity is likely to be better at screening because it is better to have a false +ve result and assume the patient has a disease than to have a false -ve result and assume the patient doesn't have the disease. I say it is better to have a false +ve at screening because it is highly likely that the true test result will be picked up with further investigation whereas a false -ve will not be signed up for further investigation thus reducing the chances of true disease being discovered.
The reverse of the above is true, i.e. a test with high specificity is more likely to be a better diagnostic test.
The predictive values of plethysmography are high but cannot be used as measures of its strength as a diagnostic tests because they are subject to incidence of disease in a given population. This means that the same test will have different predictive values in a different population because of the difference in incidence of the particular disease in question.
Nevertheless, the value of plethysmography lies in diagnostics. Though it has a high sensitivity, it is not used as a screening test because it can only be done in specialist laboratories given the expense of the equipment. It also seems to have high specificity making it a good diagnostic tool in symptomatic patients and also a good monitoring tool in patients whose disease has been established and their disease progression and response to treatment requires close monitoring.
