ANAESTHESIA AND THE VENTILATORY SYSTEM IN INFANTS AND YOUNG CHILDREN

Abstract

It is not possible in a single review article to cover all aspects of anaesthesia and the respiratory system in infants and young children. We shaJl concentrate, therefore, on those matters which are of particular relevance to the anaesthetist, and those in which there have been recent developments. We are also aware of the increasing numbers of ex-premature infants presenting for surgery, who appear to be particularly vulnerable to the adverse effects of general anaesthesia. The increased susceptibility of infants and young children to respiratory infection is well documented and results in part from the limited respiratory reserve in this age group. Oxygen consumption is high, decreasing gradually from around 7 ml kg1 min1 at birth to the adult value of 3-4 ml kg1 min1. The outward recoil of the chest wall in infants and young children is low (the pressure-volume curve is shifted), although the inward recoil of the lungs is similar to that of the young adult [2,33, 109]. Negative intrathoracic pressure, upon which the patency of small airways mainly depends, is less negative in absolute terms so that, assuming the vertical pleural pressure gradient down the lung remains unchanged, the tendency to airway closure during tidal breathing may increase [59]. As functional residual capacity (FRC) is the lung volume at which the outward recoil of the chest wall exactly balances the inward recoil of the lungs (fig. 1), the shift of the pressure—volume curve of the chest wall without corresponding change in lung recoil leads to a reduction in FRC unless it is maintained by other mechanisms [61]. There is evidence that mechanisms such as laryngeal braking during expiration and active diaphragmatic and intercostal expiratory tone may have roles in maintenance of lung volume in the awake state, although not necessarily during anaesthesia [62, 63, 136]. The geometry of the rib cage changes during the first 2 years of life, with the gradual development of the bucket handle configuration of the ribs seen in adult life (fig. 2) [106]. The more horizontal configuration of the infant's rib cage limits the potential for thoracic expansion. In addition, the ventilatory pump (rib cage, diaphragm, abdominal and accessory muscles) is thought to be less efficient in young children because of instability of KEY WORDS Anaesthesia: paedwtric. Lung. Airway. the chest wall and liability of the diaphragm to fatigue as a result of the relative paucity of type I muscle fibres. All these factors increase the susceptibility of infants, not only to respiratory infection, but also to