Lung volumes and lung mechanics in anesthetized children

Sammanfattning: The thesis was intended to fill gaps in the knowledge regarding the normal development of lung volumes and lung mechanics in children, from young infancy to the mid-teens. In particular, data were previously lacking regarding pre-school children. The studies were done during anesthesia and muscle relaxation. Measurements: · Absolute lung volume was obtained with a tracer gas method (sulfur hexafluoride washout). · Relation between airway pressure and lung volume (pressure-volume relation of the respiratory system) was asssessed during a slow intermittently interrupted expiration from 30 to 0 cm H2O of airway pressure. · In a further study, attempts were made to separate P-V relations of the respiratory system into lung and chest wall components. This was done by analyzing esophageal pressure - that was taken to represent pleural pressure - in addition to airway pressure. · In a final study, a tracer gas was again used, now in an attempt to find the point during expiration, where significant airway closure occurred. Main findings: I. Absolute lung volumes, per kg body weight, were less in young infants than in older children. II. The most marked qualitative change (size factor eliminated) in the pressure-volume relation of the respiratory system occurred during infancy. III. In the supine position, esophageal pressure paradoxically remained positive as expiration continued towards low lung volumes and even increased in some instances. When an attempt was made, anyway, to separately assess the various contributions to "total elastance", the chest wall contribution was relatively minor (about 1/10th in infants). IV. The airway "closing phenomenon" occurred at a higher lung volume when the measurement maneuvre included a deep foregoing inspiration (to 30 rather than 20 cm H2O of airway pressure). Interpretation and possible clinical implications of findings: I. The lung is smaller, in relation to weight, in infants than in older subjects. Yet it is known from other studies that the rate of oxygen consumption at rest is greater. This suggests that infants will have reduced tolerance to stresses such as increased oxygen requirement (e.g. due to fever), apnea (e.g. during tracheal intubation), and restriction of lung capacity (e.g. resection of lung parenchyma, pneumonia, hydrothorax). II. The respiratory system of small infants has less elastic recoil than that of older subjects. This probably reflects a low elastin content in the lungs, as described by others. III. In the clinical assessment of mechanically ventilated infants, it is seldom worth the effort to separate P-V relations into lung and chest wall components - the simpler alternative of studying only respiratory system P-V relations will give an adequate picture of lung mechanics, if the infant is deeply sedated and temporarily relaxed. The same is probably true also for older children. IV. High insufflation pressures will recruit otherwise collapsed airways, that will close early during the subsequent expiration. The relevance of this finding to conditions prevailing during regular breathing (mechanical or spontaneous) is, at most, speculative.