Easing children’s fears is the key to creating cooperation and obtaining accurate plethysmography test results.

Susan B. Blonshine, RRT, RPFT

Although quality control and test performance standards are consistent regardless of age, pediatric testing presents practitioners with a unique set of challenges. When conducting plethysmography in children, special consideration should be given to equipment selection, the environment, the technologist’s level of skill and training, and the child’s developmental age and test performance.

Plethysmography Equipment Selection

When selecting equipment, one must evaluate the accuracy of the flow-measuring device at low flows and volumes. The accuracy for volume should be ñ50 mL to a point less than 0.5 L. A visible real-time display should be available to encourage the child. The design of the mouthpiece and equipment should be flexible enough to use in children aged 4 to 18, as well as for those as small as 120 cm in height. The design may also include appealing colors or simulate an object familiar to children, such as a spaceship. It is important to assess whether the child can physically hold the required devices. The equipment may include visual and auditory cues for test performance. For example, a metronome helps children maintain the correct pant frequency. A visual cue–such as a light that appears and disappears at the correct intervals–may also be helpful. Overall, ease of use for children should be considered in equipment selection.

Environment and Technologist

Providing a pleasant and comfortable environment will promote patient cooperation. Creating such an environment is critical in obtaining valid test results in children. The atmosphere should be calm, quiet, and appealing to children. Freeing the area of all distractions is important. When possible, also free the area of any instruments, such as syringes and needles, associated with painful procedures.

Successful test results also depend greatly on the technologist. He/she must possess a desire to work with children, must be positive, enthusiastic, and friendly, and must be patient and capable of relating well to children. Enacting a training and competence-assessment program specific to pediatric testing is highly recommended. In fact, the Joint Commission on Accreditation of Healthcare Organizations requires documentation of age-specific training. The education program should address test methods, physiology, and pathology.

At a minimum, the technologist must have a good understanding of obstructive disorders (eg, asthma and cystic fibrosis) and restrictive disorders (eg, interstitial fibrosis) in pediatric populations. It is helpful to perform multiple tests with cooperative children who have normal pulmonary function before performing tests on actual patients. In pediatrics, attention to age-appropriate communication skills in instructing patients and in performing tests increases the ability to obtain valid test results.

Developmental Issues

The child’s developmental age will determine his/her cognitive learning stage. Other issues to consider include the child’s level of coordination and attention span, as well as parental presence during testing, which can be helpful with children but a hindrance with teenagers. The developmental issues concerned are related to language, hearing, and visual cues, as these will affect the technologist’s choice of communication strategies. Children aged 7 or younger are not able to reason logically or deductively, and they have no concept of time. Therefore, if you ask children this age to blow out for 6 seconds, they will not understand what you want.

Despite these limitations, it is sometimes possible–with patience and clear instructions–to obtain valid test results for children as young as age 4. On the other hand, many children are unable to perform tests reliably in a body plethysmograph until they are closer to age 7 or 8. The technologist must understand the developmental stage well enough to differentiate between inability to perform the tests and a lack of cooperation on the child’s part.

It is possible to have a child in this age range sit on a parent’s lap in the plethysmograph. In such cases, parents must be instructed to hold their breath during panting maneuvers; also, the parent’s body weight must be included with the child’s for correct calculations.

Obtaining maximum cooperation from children begins with understanding issues that may affect a child’s behavior. Children are generally positive and willing to cooperate; curious, but slightly timid; responsive to explanations; and eager to please adults. Always allow adequate time for the child to ask questions. It may not always be practical to work around the child’s schedule, but, when possible, determine the best time of day for the child, then check with the laboratory’s schedule. Performing a test during nap time may prevent the acquisition of results. Also, find out if the child is having any other tests that day that may affect his/her cooperation. In general, it is better to complete pulmonary function tests prior to painful procedures.

Plethysmography and Children

Pediatric Patient Instruction

The technologist should also allow time to explain the test to the child on a level that he/she can understand. Children are naturally curious, and explaining how the equipment works or relating it to something that the child is learning in school can be helpful. The procedure should always be demonstrated. After the demonstration, the intercom system used in the plethysmograph should be explained. For children, it can be compared to a walkie-talkie. A trial period in the plethysmograph, employed so the child can become used to the system, is also very helpful. Many systems have a door release available, but the technologist should use good judgment in explaining this to a child. In most cases, there is no need to mention it in the instructions.

The technologist may, as necessary, employ a variety of motivational techniques. Positive coaching is imperative throughout the test. Some children respond well to goal setting. The use (and type) of rewards is an organizational decision. Introducing an element of competition between the patient and the parent or a sibling is also effective. Regardless of the technique employed, the environment should always remain positive. The majority of pediatric patients tested have chronic diseases that increase the likelihood of their having to undergo tests on numerous occasions. The technologist should concentrate on developing positive relationships that will improve cooperation on subsequent visits.

Aside from the patient’s age, other issues to consider are the patient’s disease process and his/her reaction to medications. For example, a patient’s attention span or activity level may be affected by specific medications.

Plethysmography and Children

Application, Clinical Indications, and Contraindications

Static lung volumes, airway resistance, airway conductance, and lung-adjusted parameters are measured with the body plethysmograph. The lung-volume–adjusted airway resistance and airway conductance measurements are called specific resistance (Raw x volume) and specific conductance, reported in L/s/cm H2O/L, which is the reciprocal of Raw (1/Raw) divided by the lung volume at which the Raw measurement was made. Static lung volumes differentiate between restrictive and obstructive disease patterns, and assess the presence of hyperinflation.

Lung-volume–adjusted values are important parameters in assessing the effects of varying lung volumes on derived airway resistance values. For example, in the normal, healthy lung, increasing lung volume will correspond to a decrease in airway resistance. When instructed to pant, children with airflow limitation (asthma, chronic bronchitis, cystic fibrosis, or peripheral airway disease), with or without hyperinflation, will generally produce airway resistance measurements at lung volumes above the thoracic gas volume. The child naturally selects the higher lung volume because it is easier to pant at this level (due to decreased resistance). Although the airway resistance may be normal, the lung- volume–adjusted measurement (specific conductance) will be decreased; this is generally consistent with peripheral airway involvement.1,2

During airway-resistance measurements performed in a total-body plethysmograph, the child is instructed to pant at the lung volume at which he/she feels most comfortable, and the derived values are lung-volume adjusted (as specific airway resistance and/or conductance). Again, the child must feel comfortable during the testing process to allow acquisition of valid test results through maximum cooperation.

Lung-volume–adjusted values should be included in assessing the presence or degree of pulmonary dysfunction and the patient’s response to a provocative agent or bronchodilator. Airway resistance may be elevated in active asthma. Airway resistance is generally normal in peripheral airway diseases and may also be normal during asymptomatic asthma, although the specific conductance may be found to be reduced. Bronchodilator assessment should include an evaluation of airway resistance and specific conductance.

Clinical indications for body plethysmography include:

  • measurement of lung volumes to distinguish between restrictive and obstructive processes;
  • evaluation of obstructive lung diseases that may produce artifactual results using dilution-based evaluation methods;
  • measurement of lung volumes when repeated trials are required or when the patient is unable to perform multibreath tests;
  • evaluation of resistance to airflow;
  • determination of the patient’s response to bronchodilator administration;
  • determination of bronchial hyperreactivity in response to methacholine, histamine, and exercise; and
  • determination of the course of disease and of the patient’s response to medical intervention.

Relative contraindications for body plethysmography include mental confusion or physical limitations that prevent a child from entering the box or performing the test, claustrophobia, and oxygen therapy that cannot be discontinued.3

Plethysmography Test Performance

The method used to measure static lung volumes and airway mechanics in a whole-body plethysmograph should be consistent with published guidelines.3 Standardization of technique is important to decrease variability. The technologist must thoroughly explain the overall procedure so that the child knows exactly what to expect prior to entering the plethysmograph.

The child should be told about the importance of relaxing and breathing normally between measurements, and to pant, with cheeks supported, against a closed shutter when so instructed. The panting maneuver, whether performed with the shutter opened or closed, requires that the breaths be small (50 to 100 mL), quick (TGV [thoracic gas volume] at 1 Hz or at 1 breath per second and airway resistance at 1 Hz to 1.5 Hz), and uniform.

The panting maneuver should always be demonstrated to the child. Children can practice the maneuver by panting with their hand over their mouth to simulate a closed shutter. Simulating the shutter movement and allowing the child to hear it close is also very helpful. The maneuver should be practiced again in the box, with the door open. Reassure the child constantly to reduce any anxiety (which can alter resting lung volume) associated with the test or with sitting in the closed box. Stress that the test will take only a few minutes to perform and remind the child that there is an intercom to help him/her communicate. Also remind the child that you will not leave the room.

Reference Ranges

Selected reference ranges are based on the age and population being tested. The selected ranges should be verified by performing tests on 10 to 20 children who have normal lung function and who reflect the population tested by the laboratory. Male lung-function variables have been found to be discontinuous with age. There is a linear increase in standing height until puberty; at puberty, there is a sudden increase in height that is more marked than before puberty. A correction for varying thoracic sizes is needed to eliminate the effect of these changes. In females, a smooth curvilinear relationship between height and thoracic size is observed.4

Case Example

Asthma is one of the most common pediatric diseases evaluated with whole-body plethysmography. Data illustrating typical results obtained in a 10-year-old male with moderate obstructive disease are shown in Table 1.

A mild to moderate obstructive pattern is observed before bronchodilator administration. Lung volumes appear to be in the normal range. Most important, airway resistance is markedly increased, with a decrease in specific conductance. This is generally observed when central and peripheral airways are involved in the obstructive process. A patient with mild asthma may have apparently normal airway resistance and a decreased specific conductance if only the peripheral airways are involved in the process.

It is important to observe, in this example, the significant increase in forced expiratory volume in 1 second (FEV1) after bronchodilator administration. A significant increase, as defined by the American Thoracic Society, is an absolute increase of 200 mL and a 12% increase in forced vital capacity and/or FEV1.5 The airway resistance and specific conductance values return to normal after bronchodilator administration. This responsiveness to a bronchodilator and this complete reversal of the obstructive pattern are the hallmarks of asthma.

Plethysmography and Children: Conclusion

Successful testing among children is possible, but it requires extra skill on the part of the technologist. Patience, the ability to communicate well with children, and a desire to work with them are vital to successful testing. Providing a pleasant environment is also important in obtaining accurate results in this group. These children probably will need repeated testing during their lives. Taking steps now to build a positive relationship that grows with the child will bode well for the future.


Susan B. Blonshine, RRT, RPFT, is president of Technical Education Consultants, in Mason, Mich.


1. Niewoehner DE, Kleinerman J. Morphologic basis of pulmonary resistance in the human lung and the effects of aging. J Appl Physiol. 1974;36:412-418.

2. Van Brabandt H, Cauberghs M, Verbeken E, et al. Partitioning of pulmonary impedance in excised human and canine lungs. J Appl Physiol. 1983;55:1733-1742.

3. Shrake K, Blonshine S, Brown RA, Decker MJ, Ruppel GL, Wanger J. American Association for Respiratory Care Clinical Practice Guideline–Body Plethysmography. Respir Care. 1994;39:1184-1190.

4. Rosenthal M, Cramer D, Bain SH, Denison D, Bush A, Warner JO. Lung function in white children 4 to 19 years: II single breath analysis and plethysmography. Thorax. 1993;48:803-808.

5. American Thoracic Society. Lung function testing: selection of reference values and interpretative strategies. Am Rev Respir Dis. 1991;144:1202-1218.