The Influence of Occlusion Time on Measuring Respiratory Resistance and Compliance in Infants with Bronchiolitis

Abstract

ABSTRACT. The passive, single-breath, flow-volume technique is a simple method for measuring the resistance (Rrs) and the compliance (Crs) of the respiratory system in infants. S o far, the potential influence of end inspiratory occlusion time on these measurements has not been investigated. We measured Rrs and Crs in 36 infants and toddlers with bronchiolitis; in each child, a spectrum of nine fixed occlusion times, ranging from 90 to 600 ms, was applied in random sequence. Increasing the duration of occlusions from 90 to 275 ms resulted in marked stepwise changes of measured Rrs and Crs; occlusions longer than 275 ms, however, produced highly reproducible measurements, as expressed 1 ) by minimal absolute differences between measured values at subsequent occlusion times and 2) by minimal percentage changes of measured values from one occlusion time to the next. There was no influence of age on the results; reproducible measurements were made in children as old a s 1.5 y. This suggests that, in infants with bronchiolitis, 1 ) occlusions between 300 and 450 ms might be ideal for obtaining reliable measurements, and 2) the age range for applying this method can be extended into the 2nd y of life. (Pediatv Res 33: 273-277, 1993) Abbreviations YE, expired volume VE, expiratory flow Pao, pressure measured a t airway opening Rrs, resistance of the respiratory system Crs, compliance of the respiratory system Trs, time constant of the respiratory system More than three decades ago, Comroe et al.(1) described a technique for measuring respiratory mechanics in animals by a sjngle passive deflation. Subsequently, McIlroy et al. (2) plotted VE against VE to obtain the time constant of this passive exhalation in anesthetized human subjects. , as well as LeSouef et al. (4,5), then adapted this "single-breath technique" for measuring passive respiratory mechanics in newborns and infants; in this age group, relaxation of the respiratory muscles is achieved by activating the Hering-Breuer reflex via an end-inspiratory occlusion of the airway. When the Pao reaches a plateau, muscle relaxation and pressure equilibration have occurred. The subsequent termination of the occlusion results in a passive exhalation, which is recorded in a flow-volume mode. From the linear portion of this plot, a Trs is calculated. Trs together with Pao allows for calculating the Rrs and Crs.This simple technique might develop into a routine method for assessing lung function in infants; as a prerequisite, however, several methodologic questions must be addressed. One problem that so far has not been sufficiently evaluated is the question of "optimum end-inspiratory occlusion time." Too-short occlusions do not result in complete muscle relaxation and pressure equilibration. As is evident from the lack of any linear portion of the expiratory flow-volume plot, too-long ones are disturbed by recommencing respiratory muscle activity.So far, most investigators have observed Pao visually and have released their manual occlusion when the tracing appeared f...

Assessment of the Single-Occlusion Technique for Measurements of Respiratory Mechanics and Respiratory Drive in Healthy Term Neonates Using a Commercially Available Computerized Pulmonary Function Testing System

“…Predetermined Tocc values of 0.3, 0.4 and 0.5 s, however, produced reliable results comparable to those in the automatically selected Tocc mode, with a significant trend of obtaining qualitatively better results with longer Tocc but without an effect on absolute values of Crs and Rrs. Though the Tocc requirements may be different in pathological conditions such as airway obstruction, Steinbrugger et al [13] in their study of infants with bronchiolitis found that Tocc between 0.3 and 0.45 s might be ideal for obtaining reliable results; this differs little from our results. In a recently published article by Gappa et al [12], the authors recommended holding occlusions for a minimum of 400 ms or a maximum of 1,500 ms to ensure that true pressure equilibration occurs when using an automated shutter.…”

“…To our knowledge, the influence of Tocc on respiratory mechanics in healthy neonates has not been reported in the literature. There are only two reports on this topic, but in these studies, measurements were performed on infants suffering from acute bronchiolitis and cystic fibrosis using Tocc varying from 90 to 1,200 ms [13,22]. They showed a tendency for Crs to decrease as Tocc increased but no effect on Rrs.…”

“…However, there is still little information about the appropriate length of occlusion time (Tocc) when applying the SOT. In two studies in infants with pulmonary disorders, there was a trend for Crs to become lower as the length of Tocc increased [12,13], confirming the need to standardize the length of Tocc. In a recently published article by Gappa et al [12], a minimum Tocc of 400 ms and a maximum of 1,500 ms were recommended when using the SOT with an automated shutter.…”

Effect of sleep stages on measurements of passive respiratory mechanics in infants with bronchiolitis

“…1 The apparatus used has been described in detail previously. 5 Briefly, it consists of a face mask with a pressure port, an electronically operated shutter system with a response time of 20 msec (Biegler, Mauerbach, Austria), a pneumotachygraph (Fleisch 0 or 1), and pressure transducers and amplifiers (Hottinger Baldwin Messtechnik, Darmstadt, Germany). The face mask was applied snugly to the face of the sleeping infant and sealed with a ring of silicon putty.…”

Assessment of passive respiratory mechanics in infants: doubleversussingle occlusion?

“…There is limited guidance in the existing literature regarding optimal duration of airway occlusions during measurements of passive mechanics [19]. While an experienced user with manual control of the shutter can usually judge very effectively when both equilibration and relaxation have occurred, it is far more difficult to develop suitable algorithms that will be robust enough to cope with the wide variety of breathing patterns encountered during infant lung function testing.…”