of Pulmonary Function in Infants and Preschool Children


Cystic fibrosis


Neonatal chronic lung disease


Postinfectious bronchiolitis obliterans


Recurrent bronchial obstruction that does not respond to the usual treatments


Central airway obstruction


Epidemiological and research studies




Measurement Conditions


Infants who are to be assessed through a functional respiratory study should fast for the previous 4 hours and be examined to rule out certain conditions in which the procedure is not advised, such as fever or the presence of secretions in the upper airway. The patient’s height and weight should be measured and the results should be compared with theoretical values. Prior to the procedure, 50–75 mg/kg of chloral hydrate is administered orally to induce a relaxed sleep, which in most cases lasts for 90–120 minutes. It is considered high-risk to administer this medication to patients with severe upper airway obstruction or those with some degree of hepatic, renal, or cardiac insufficiency.


The patient should be in a supine position during the study, with his or her head held straight in hyperextension. Small movements and flexing of the neck can alter the results and thus should be avoided. Arterial oxygen saturation and the heart rate are monitored continuously with an oximeter.


Methods


Measurement of Forced Expiratory Flow


Understandably, small children cannot perform maximum forced exhalations based on their total lung capacity for evaluation of the caliber of the peripheral airways. Different methods are used to prompt infants and small children to exhale forcibly. Deflation techniques, rapid thoracic compression at the tidal volume (RC-RTC), and rapid thoracoabdominal compression: RTC with preinsufflation are used to determine the magnitude of bronchial obstruction and variations in obstruction after administration of a bronchodilator or a bronchoconstrictor agent.


The deflation technique was the first to be developed, but it is no longer used, because it requires heavy sedation and intubation of the patient. Consequently, in this chapter, we discuss the other two methods.


Rapid Thoracoabdominal Compression Technique at Current Volume


Adler et al. (1978) and Taussig et al. (1982) described the rapid thoracoabdominal compression technique for determining the bronchial caliber in infants. It has been used to describe the normal growth and development of the airways in infancy, on the basis of which equations have been developed to establish the magnitude of bronchial obstruction, using a standard deviation score as a function of the sex and size of the subject. With this, the observed alterations in the functional response to the treatments applied in the respiratory clinic can be identified and evaluated.


The technique employs an inflatable vest inside another inextensible vest around the child’s chest and stomach, which goes from the armpits to the pubis, with the child’s arms outside both jackets. A reserve air tank connected to the inflatable jacket transmits air at the end of the inhalation, with sufficient pressure to produce rapid thoracoabdominal compression (Fig. 8.1). Expiratory flows are registered during forced exhalation by a pneumotachograph equipped with a facial mask, which hermetically covers the nose and mouth of the patient. The resulting flow/volume curve is graphed in real time on a computer screen. The curve characteristically presents a rapid rise in the expiratory flow until the peak expiratory flow (PEF) is reached, and then it gradually descends to a point below the one established as the functional residual capacity (FRC). The flow obtained at this point is termed VFRC. To determine the limitation in airflow, the maneuver is progressively repeated with more pressure on the jacket, ranging between +20 and +100 cm of water. The maximum flow at the FRC (VmaxFRC), which correlates with the airway caliber, is determined by applying the pressure that yields the highest VFRC values (Fig. 8.2).

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Fig. 8.1

Equipment used for rapid thoracoabdominal compression at tidal volume (calculation of the maximum flow at the functional residual capacity (VmaxFRC))


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Fig. 8.2

Partial forced flow/volume curve


While the highest pressures have no limit in relation to airflow in healthy infants, this is not the case in patients with bronchial obstruction, where much less pressure is required. Abrupt falls in airflow are attributed to glottic or pharyngeal closure, and they can be corrected according to the correct location in the neck. At least three acceptable and reproducible curves are obtained with maximum FRC values that do not vary by more than 10%, and VmaxFRC is the average of the three values. The curves obtained with this method are “partial,” given that they are based not on the total pulmonary capacity but, rather, on the end of the inspiration at the current volume.


Rapid Thoracoabdominal Compression Technique at a Volume with Previous Insufflation


A modification of the technique described above has recently been developed to obtain, in a noninvasive way, better forced expiratory curves. The equipment used is similar to that used for the other technique, with the addition of an insufflation system to take the patient close to his or her total pulmonary capacity immediately before the thoracoabdominal compression (Fig. 8.3).

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Fig. 8.3

Rapid thoracoabdominal compression with previous insufflation


The insufflation system consists of an external source of air, valves to automatically occlude the airway or a Y connector for manual insufflation, and a safety valve to control the insufflation pressure. The jacket reaches maximum pressure in 70–100 milliseconds. Given that there is a small delay between insufflation and compression, it is a common practice to activate the compression 10–100 milliseconds before releasing the occlusion of the airway. One of the main discrepancies among users has been the application of different insufflation pressures to reach higher volumes. However, there is a consensus in the application of 30 cm of water pressure for older infants and 20 cm of water pressure for newborns and premature babies. The inspiratory flow is established at 1.5 times the current flow. The real pressure at the level of the mouth is monitored in real time so that the operator can adjust the flows and optimize the pressure being administered. There are also disparate criteria in the ventilation pattern before the forced expiration. The pulmonary insufflation rate depends on the lung volume and flow magnitude, while the volume reached for a standard pressure is directly proportional to the infant’s level of respiratory compliance. Most authors suggest 2–5 successive insufflations until pressure and volume plateaus are reached.


The most relevant functional parameters are obtained on the basis of the “maximum” forced curve of volume/time and flow/volume (Fig. 8.4)—namely, the forced vital capacity (FVC), forced expiratory volume (FEV) in 0.5 seconds (FEV0.5), and forced expiratory flow at 25–75% of FVC (FEF25–75). Other parameters that can be considered are the expiratory flows at 50%, 75%, and 85% of FVC, and FEV0.4 in small children. The parameters that are reported are those that are obtained on the basis of the “best curve,” and at least two values that do not differ by more than 10% should be used, with the better value reported. Similarly, the results refer to theoretical values.

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Fig. 8.4

Maximum forced flow/volume (F-V) loop obtained by rapid thoracoabdominal compression with previous insufflation


Analysis of Breathing at Current Volume


Analysis of the curves obtained during current breathing is useful for functional respiratory assessment of infants, given that they yield greater variability and lower levels of specificity. However, there are different parameters that can be obtained from observing the flow curves/tidal volumes that allow for establishment of obstructive and low-compliance patterns (Fig. 8.5).

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Fig. 8.5

Flow/volume loop patterns


Simply by the airtight mask being placed on the patient’s face and attached to the pneumotachograph, the flow and volume signals are integrated to determine:


  1. (a)

    Parameters that are altered in pulmonary elastic retraction disorders: the rate between the peak expiratory flow and current volume (PEF/Vc)


     

  2. (b)

    Parameters that vary with obstructive pathologies: the percentages of volume and time to reach PEF

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Nov 7, 2020 | Posted by in Uncategorized | Comments Off on of Pulmonary Function in Infants and Preschool Children
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