We recorded airflow, tidal volume, respiratory muscle electromyogram (EMG), and chest wall configuration in eight normal newborn infants to investigate the determination of end-expiratory lung volume (EEV). The expiratory flow-volume representation was nearly linear and EMG evidence of respiratory muscle activity was absent during the latter part of expiration in both supine and upright postures, consistent with passive expiration. Occasional breaths were associated with marked retardation of expiratory airflow (braking). During unobstructed apnea, expiration proceeded to the relaxation volume (Vr) with no change in slope of the flow-volume curve. During breathing, EEV was greater than Vr observed during apnea. We calculated the difference between EEV and Vr estimated by extrapolation of the linear portion of the expiratory flow-volume curve as 14.4 +/- 5.4 ml (supine) and 11.8 +/- 2.4 ml (upright). When infants were tilted from supine to upright, expiratory duration (TE) and the expiratory time constant (tau) increased significantly. Since the increases in tau and TE offset each other, the EEV-Vr difference was similar in both postures. We propose that while braking plays a major role in the early part of expiration, as long as the final portion of expiration is passive, the dynamic maintenance of EEV above Vr depends on the relative values of tau and TE. Expiratory braking mechanisms interact with the passive mechanical properties of the respiratory system to modulate the balance between tau and TE. These mechanisms provide a neonatal breathing strategy to maintain EEV above a low Vr until the chest wall stiffens with maturity.
To determine if regional differences exist in the activity of abdominal muscles during respiratory and nonrespiratory maneuvers, we studied four healthy subjects by comparing electromyographic (EMG) activity from surface electrodes placed lateral to rectus muscle, one pair on the upper abdomen and the other on the lower abdomen. In one subject EMG recordings were made from wires placed in various layers of the abdominal wall. Relative positions and changes in size of anatomic structures during maneuvers were determined from real-time ultrasonography of the abdominal wall. Expulsive or valsalva maneuvers evoked the same relative EMG activity in the upper and lower abdomen. In the resting supine posture no EMG activity was detectable; however, in the standing posture greater tonic EMG activity appeared in the lower abdomen. During rebreathing, phasic EMG activity during expiration was greater in the upper than in the lower abdomen in all subjects. Observations from ultrasonographic and electromyographic evaluations suggest that the control of abdominal muscles and their influence on respiratory mechanics are potentially more complex than has been suggested by previous reports.
To investigate the mechanism underlying the polyphasic airflow pattern of the equine species, we recorded airflow, tidal volum, rib cage and abdominal motion, and the sequence of activation of the diaphragm, intercostal, and abdominal muscles during quiet breathing in nine adult horses standing at rest. In addition, esophageal, abdominal, and transdiaphragmatic pressures were simultaneously recorded using balloon-tipped catheters. Analysis of tidal flow-volume loops showed that, unlike humans, the horse at rest breathes around, rather than from, the relaxed volume of the respiratory system (Vrx). Analysis of the pattern of electromyographic activities and changes in generated pressures during the breathing cycle indicate that the first part of expiration is passive, as in humans, with deflation toward Vrx, but subsequent abdominal activity is responsible for a second phase of expiration: active deflation to below Vrx. From this end-expiratory volume, passive inflation occurs toward Vrx, followed by a second phase of inspiration: active inflation to above Vrx, brought about by inspiratory muscle contraction. Under these conditions the abdominal muscles appear to share the principal pumping duties with the diaphragm. Adoption of this breathing strategy by the horse may relate to its peculiar thoracoabdominal anatomic arrangement and to its very low passive chest wall compliance. We conclude that there is a passive and active phase to both inspiration and expiration due to the coordinated action of the respiratory pump muscles responsible for the resting adult horse's biphasic inspiratory and expiratory airflow pattern. This unique breathing pattern perhaps represents a strategy of minimizing the high elastic work of breathing in this species, at least at resting breathing frequencies.
To investigate airflow regulation in newborn infants, we recorded airflow, volume, diaphragm (Di), and laryngeal electromyogram (EMG) during spontaneous breathing in eight supine unsedated sleeping full-term neonates. Using an esophageal catheter electrode, we recorded phasic respiratory activity consistent with that of the principal laryngeal abductors, the posterior cricoarytenoids (PCA). Sequential activation of PCA and Di preceded inspiration. PCA activity typically peaked early in inspiration followed by either a decrescendo or tonic EMG activity of variable amplitude during expiration. Expiratory airflow retardation, or braking, accompanied by expiratory prolongation and reduced ventilation, was commonly observed. In some subjects we observed a time interval between PCA onset and a sudden increase in expiratory airflow just before inspiration, suggesting that release of the brake involved an abrupt loss of antagonistic adductor activity. Our findings suggest that airflow in newborn infants is controlled throughout the breathing cycle by the coordinated action of the Di and the reciprocal action of PCA and laryngeal adductor activities. We conclude that braking mechanisms in infants interact with vagal reflex mechanisms that modulate respiratory cycle timing to influence both the dynamic maintenance of end-expiratory lung volume and ventilation.
To investigate the regulation of end-expiratory lung volume (EEV) in premature infants, we recorded airflow, tidal volume, diaphragm electromyogram (EMG), and chest wall displacement during sleep. In quiet sleep, EEV during breathing was 10.8 +/- 3.6 (SD) ml greater than the minimum volume reached during unobstructed apneas. In active sleep, no decrease in EEV was observed during 28 of 35 unobstructed apneas. Breaths during quiet sleep had a variable extent of expiratory airflow retardation (braking), and inspiratory interruption occurred at substantial expiratory flow rates. During active sleep, the expiratory flow-volume curve was nearly linear, proceeding nearly to the volume axis at zero flow, and diaphragm EMG activity terminated near the peak of mechanical inspiration. Expiratory duration (TE) and inspiratory duration (TI) were significantly shortened in quiet sleep vs. active sleep although tidal volume was not significantly different. In quiet sleep, diaphragmatic braking activity and shortened TE combined to maintain EEV during breathing substantially above relaxation volume. In active sleep, reduced expiratory braking and prolongation of TE resulted in an EEV that was close to relaxation volume. We conclude that breathing strategy to regulate EEV in premature infants appears to be strongly influenced by sleep state.
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