Respiratory muscle activities after birth in asphyxiated preterm lambs

Abstract

Laryngeal and pump muscle activities are important in the establishment and maintenance of functional residual capacity (FRC) after birth. The aim of this study was to determine the expiratory mechanisms by which laryngeal and diaphragmatic activities achieve the increments in FRC postnatally. Wire electrodes were placed in: the laryngeal abductor, a major laryngeal adductor, the inferior pharyngeal constrictor and the diaphragm of six fetal sheep. The lambs were delivered prematurely by cesarean section and a face mask with a pneumotachograph applied. A grunting respiratory pattern was characterized by severe expiratory airflow retardation, associated with laryngeal adductor activity. In grunting breaths, minimal volume loss at end-expiration and incremental increases in FRC occurred when the onset of diaphragmatic activity preceded the onset of laryngeal muscle activities associated with laryngeal opening. Thus the timing order of laryngeal and diaphragmatic muscle activities near end-expiration is a determinant of increments in FRC.

Laryngeal Muscle Activities with Cerebral Hypoxia–Ischemia in Newborn Lambs

“…Expiratory TA burst activity increased during the postocclusion tachypnea but was prominent during hypopnea when TE was prolonged and little or no expiratory airflow occurred (Figures 2-4). The increases in expiratory Pua during hypopnea (hypopnea, 12.3 Ϯ 4.3 cm H 2 O; baseline Pua, 1.98 Ϯ 1.13 cm H 2 O [p Ͻ 0.001]) were temporally associated with increased TA burst activity and severe retardation/absence of expiratory airflow (Figures 3 and 4), indicating that TA activity led to glottic closure (15). Only during hypopnea was the expiratory upper airway resistance increased (hypopnea, 0.011 Ϯ 0.008 cm H 2 O/L per second; baseline, 0.002 Ϯ 0.001 cm H 2 O/L per second [p Ͻ 0.001]).…”

“…Asphyxia complicates delivery in lambs (14,15,19 In utero , the lambs were in the high-voltage ECoG state: diaphragmatic and PCA activities absent and tonic TA activity present (17). The ECoG activity changed to low voltage before (two lambs) or during cesarean section delivery (Figure 1).…”

“…During grunting, the timing of expiratory airflow retardation has been related to increased trans-upper airway pressure (Pua) and to reciprocal activities of the laryngeal abductors and major adductors, namely the posterior cricoarytenoid (PCA), and thyroarytenoid (TA) muscles (14). An altered central coordination of the onsets of laryngeal and diaphragmatic muscle activities has been related to increments in end-expiratory lung volume (15). No paper has supported the suggestion that laryngeal closure in asphyxial apnea could maintain lung volume and thereby aid gas exchange (5).…”

Endotracheal Measurement of Thyroarytenoid Activity in Newborn Lambs

“…The nature and relationships between TA and PCA activities during eupnea and grunting have been quantitatively analyzed in lambs and demonstrated to be important determinants of airflow pattern [4,5]. In this study, TA activity was present in every lamb with an open-ended endotracheal tube in situ and was absent when not intubated.…”

“…To identify a possible physiological monitor of absolute lung volume, the breathing strategies adopted by the neonate to maintain end-expiratory lung volume (EEV) above functional residual capacity [2,3] were reviewed. Volume loss in expiration is limited by: shortening the duration of expiratory time; retarding (braking) expiratory flow with laryngeal adduction and post-inspiratory inspiratory diaphragmatic activity, and at end-expiration, inverting the normal relationship of the onsets of diaphragm and laryngeal abductor muscles such that diaphragmatic activity precedes that of laryngeal opening [4]. In neonatal respiratory distress with atelectasis laryngeal closure is pronounced [5] and produces audible grunting with generation of a positive airway pressure and improved oxygenation [5,6].…”