Volume Dependence of Respiratory Impedance in Infants

Abstract

We previously studied low-frequency respiratory impedance (Zrs) data at an elevated lung volume to separate airway and tissue mechanical properties in normal infants (Am. I. Respir. Crit. Care Med. 1996; 154:161-166). The aim of the present study was to determine the volume dependence of the airway and tissue mechanics by extending Zrs measurements to lower lung volumes. Zrs spectra between 0.5 and 21 Hz were measured in supine sleeping infants (n = 8; 7 to 26 mo of age) at mean transrespiratory pressures (Ptr[mean]) of 20, 10, and 0 cm H2O, during periods of apnea induced by inflating the infants' lungs to a pressure of 20 cm H2O through a face mask. At each inflation pressure, a model containing airway resistance (Raw) and inertance (law) and tissue damping (G) and elastance (H) was fitted to Zrs data. At FRC, the values of Raw, law, G, and H were 20.6+/-4.9 (SD) cm H2O x s/L, 0.037+/-0.014 cm H2O x s2/L, 39.6+/-10.3 cm H2O/L, and 147+/-35 cm H2O/L, respectively. Increase of Ptr(mean) caused a monotonous decrease in Raw (42+/-7% of the value at FRC), while law remained constant. The tissue parameters were minimal at a Ptr(mean) of 10 cm H2O (68+/-10% and 78+/-6% in G and H, respectively) and significantly higher at both 0 and 20 cm H2O. Although Zrs measurements can be made in most infants at lung volumes as low as FRC, an inflation pressure of 20 cm H2O provides a higher success rate and is therefore a more suitable condition for general use.

Methacholine-induced volume dependence of respiratory resistance in preschool children

“…In infants, plethysmographic studies have shown variation in Raw during the respiratory cycle, alinearities of the pressure/flow relationship [29] and hysteresis of this curve on expiration in the presence of airways obstruction [30]. Recently, volume dependence of Raw estimated from model fitting of input Zrs data was demonstrated in eight infants during an induced apnoea [31]. The significant K3s observed in the current study are thus likely to be attributed to the airways.…”

Volume dependence of airway and tissue impedances in mice

“…In a number of species, including humans, respiratory mechanical properties are known to change with lung volume. Studies of the volume dependence of airways and lung tissues separately have shown that Raw decreases with increasing lung volume, whereas lung tissues become stiffer and tissue damping increases (15,18). Similar data are not available for mice but are needed to accurately interpret changes in lung function induced in chronic disease models.…”

Partitioning of Airway and Parenchymal Mechanics in Unsedated Newborn Infants

“…The ability to obtain technically satisfactory recordings during relatively short apneic intervals has several potential benefits: Although the Hering-Breuer inflation reflex is relatively strong in young infants (particularly those born preterm) (23), the higher respiratory rate and shorter expiratory time in such subjects means that the absolute duration of the apneic pause may be limited. In addition, it should facilitate future attempts to undertake physiologically relevant LFOT measurements at FRC, which have been precluded in older infants by the brevity of the expiratory pause after airway occlusions at low lung volumes (16). Likewise, measurements of tissue mechanics at the mean distending pressure during high-frequency oscillatory ventilation in the preterm lamb have identified the point at which optimal lung volume is achieved in the paralyzed preterm lamb (22).…”

“…The apparent negative correlation between tissue hysteresivity and length within this small and young infant cohort suggests that there may also be a change in the balance between viscous and elastic mechanical forces within the tissues during this alveolarization period of human lung development. Furthermore, hysteresivity would be expected to decrease with increasing lung volume, as collagen increasingly dominates lung mechanics with increasing lung volume (16,29,30). For the same reason, a reduction in hysteresivity with increasing body length is consistent with a maturational increase in the microscopic collagen content of the tissue.…”

“…This difficulty has been overcome recently in sedated infants aged from 1 mo to 2 yrs by employing the Hering-Breuer inflation reflex and hyperventilation to induce a short apneic interval (11), during which low-frequency (0.5-20 Hz) Z rs (11)(12)(13)(14)(15)(16)(17) is measured. The use of sedation is generally contraindicated for lung function tests in healthy, spontaneously breathing infants Ͻ44 wk postconceptional age (18).…”