Pulmonary Function Test in Infants And Children - Uses and Types

Introduction

A pulmonary function test is a diagnostic test used to evaluate lung function or measure any lung changes. It is also used to diagnose and follow certain chronic pulmonary diseases such as asthma, cystic fibrosis, and congenital or acquired airway malformations.

What is the Need for Sedation in an Infant before Pulmonary Function Test Is Performed?

A pulmonary test can be easily performed on a child of five years or more. But it can be quite a challenge for a child of fewer than five years. Very young children cannot cooperate or remain passive during lung function tests. So there is a need for the infant or a child to be sedated.

How Is the Pulmonary Function Test Performed in an Infant?

The procedure is done by sedating the infant with chloral hydrate, which has a minor effect on the respiratory pattern at a recommended dose of 50 to 90 mg/ kg body weight. The procedure is done in a supine position, with the neck and head extended slightly. The infant is wrapped with an inflatable vest around the thoracic cage.

An airtight mask is placed over the infant's nose and mouth, and this setup is connected to a computerized pneumograph. The pneumatic vest is inflated rapidly, thereby compressing the thorax. The functional residual capacity is determined from tidal breathing loops, and a maximum flow rate is obtained after thoracic compression. During testing, the infant is continuously monitored with the help of a pulse oximeter. Parents must be informed about the procedure, and consent must be taken before starting and sedating the child.

What Are the Uses of Pulmonary Function Tests?

Pulmonary function tests are used to:

• Diagnose and monitor chronic lung diseases such as asthma, chronic obstructive lung disease, and emphysema.

• To see whether the lung disease treatment is working.

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• To find the cause of breathing difficulty.

• To check lung function before surgery.

• Check for any exposure to chemicals or other substances that might have caused lung damage.

What Are the Different Types of Pulmonary Function Tests?

• Functional residual capacity.

  • Plethysmographic functional residual capacity.

  • Functional residual capacity using gas dilution method.

• Forced oscillation methods.

• Tidal breathing parameters.

• Passive mechanics of the respiratory system.

• Forced expiratory flow/volume loops.

Functional Residual Capacity:

Functional residual capacity is the volume of gas that remains in the lungs after a spontaneous exhalation and is the only static lung volume currently measured routinely. It can be measured using infants' whole body plethysmography or gas dilution technique.

Plethysmographic Functional Residual Capacity:

It provides rapid and reliable determination of functional residual capacity. During a stable end-expiratory level, the infants work against a closed shutter for a few seconds, and the pressure and volume are measured within the plethysmograph. The infant's lung volume is calculated using Boyle's law, which is pressure x volume = constant. This method detects lung hyperinflation, particularly in intrathoracic airway obstruction or obstructive lung disorders.

Functional Residual Capacity Using Gas Dilution Method:

It can be measured using a closed circuit dilution technique or an open circuit multiple breath washout techniques. Both methods measure lung volumes using inert tracer gas such as Helium (He), Nitrogen (N2), or Sulfur hexafluoride (SF6). The body does not absorb these gasses. The closed circuit He dilution technique is not used commonly as it only measures the functional residual capacity. The multiple breath washout technique not only measures the functional lung capacity but also provides various indices of uneven ventilation distribution if flow and gas concentration is measured simultaneously. A plethysmography measures all compressible intrathoracic gas; hence it is a more useful measure of hyperinflation than the gas dilution technique. Higher plethysmographic than gas dilution functional residual capacity indicates the presence of trapped gas and indirectly reflects peripheral airway obstruction.

Forced Oscillation Methods:

It is used to estimate airway and parenchymal mechanical properties. The measurements are performed during an induced Hering-Breuer reflex using low-frequency forced oscillation, and the airway and lung parenchymal resistance are calculated. A Hering-Breuer reflex is activated when there is an increase in the lung volume, which stimulates the pulmonary stretch receptors and then sends signals to the medulla by the vagus nerve, thus protecting it from overinflation of the lungs.

Tidal Breathing Parameters:

It is a simple and non-invasive technique to determine the breathing pattern. The expiratory and inspiratory flow volumes are recorded using a flow meter. Flow time volume time and flow volume tracings are displayed. Tidal volume, respiratory rate and time to peak expiratory, and the shape of the flow/volume loops are also observed. This method will tell us about lung mechanics and control of breathing. It is also helpful in confirming that the child is in quiet sleep.

Passive Mechanics of the Respiratory System:

This method is used to determine the resistance and compliance of the respiratory system. During late infancy, the chest wall is very compliant and directly reflects lung compliance.

Forced Expiratory Flow/Volume Loops:

Measuring the forced expiratory flow/volume loop directly helps determine the severity of bronchial obstruction. This technique is used to measure maximal flow at a given lung volume. Once a minimum effort has been exceeded, the maximum expiratory flow becomes independent of the effort applied. An inflatable bladder is placed over the chest and abdomen in a sleeping or sedated infant. A non-elastic vest is wrapped around the infant, and a bladder is inflated at the end of expiration, causing the infant to breathe out. Flow limitation is achieved by gradually increasing the pressure used to inflate the bladder.

Conclusion

A pulmonary function test in infants is most helpful when evaluating infants with specific symptoms or known respiratory disorders. It provides the opportunity for objective assessment of therapeutic intervention. It plays a crucial role in chronic lung diseases, where early intervention may be particularly important for long-term outcomes, such as cystic fibrosis. There is no universal test allowing the diagnosis, assessment of severity, and therapeutic response of an infant presenting with signs and symptoms of a respiratory disorder such as tachypnoea, wheezing, cough, chest deformity, or failure to thrive. Several techniques used in combination may be required to arrive at a reasonable explanation of the infant's respiratory problems.