THE INFLUENCE OF DIFFERENT TYPES OF VENTI LATION ON OXYGENATION AND VENTILATION IN INFANTS DURING SHORT-LASTING ANESTHESIA - A STUDY UTILIZING TRANSCUTANEOUS PO2 AND PCO2 MONITORING

Monitoring of ventilation in infants is difficult and often not very r eliable. In this study, transcutaneous measurement of blood gas tensio ns was used to investigate the influence of four different modes of ve ntilation on oxygenation and ventilation in anaesthetized infants. Met hods. In a randomised study, transcutaneously measured PO2 (tc-PO2) an d PCO2 (tcPCO2) tensions were continuously registered in 42 ASA class I and II infants between 3 and 24 weeks of age undergoing minor surgic al procedures (inguinal hernia repair). Two breathing systems combined with different modes of ventilation were evaluated: manual ventilatio n with Kuhn's T-piece system and face mask (group A; n = 11) or endotr acheal tube (group B; n = 10); manual ventilation with paediatric circ uit system and face mask (group C; n = 11); and mechanical ventilation with paediatric circle system, endotracheal tube, and positive end-ex piratory pressure (PEEP) 3 cm H2O (group D; n = 10). Transcutaneous va lues were measured by a combined tcPO2/PCO2 electrode (E 5277, Radiome ter). Anaesthesia was maintained by controlled ventilation with N2O/O2 (67%/33%) and halothane 0.5-1.5 vol.%. Surgical and anaesthetic techn iques were standardized and the anaesthetist was blinded to the measur ed values. Results. Preoperative mean tcPO2 values while spontaneously breathing air ranged between 69 and 75 mmHg in all patients. During a naesthesia and controlled ventilation (FiO2 = 0.33), there was a signi ficant increase in tcPO2 (P < 0.01) in 3 groups: in groups A and D mea n tcPO2 increased to 90-100 mmHg and in group C to 110-120 mmHg. In co ntrast, tcPO2 in group B reached only 75-80 mmHg, which was not consid ered significant. Postoperatively, tcPO2 immediately reached baseline values in all patients (Fig. 2). Compared to preoperative values, the alveolar-tcPO2 difference (AtcDO2) significantly increased during anae sthesia in all groups (Fig. 3). The tcPCO2 measurements revealed marke d alveolar dysventilation, with hyperventilation supervening in groups A, B, and D; in group C, however, most (7 of 11) infants were normove ntilated (Fig. 4). Conclusions. Adverse effects of anaesthesia on pulm onary function in infants are caused by loss of the PEEP effect induce d by the physiological subglottic stenosis. Endotracheal intubation an d the increase in chest wall compliance during anaesthesia lead to a d ecrease in functional residual capacity (FRC) associated with prematur e airway closure and ventilation/perfusion mismatch. These pathophysio logical disturbances result in a marked increase in AaDO2 and low arte rial PO2 values despite high FiO2, as could be observed when intubated infants had been ventilated with a high-flow T-piece system (group B) . Mechanical ventilation with a paediatric circuit system and endotrac heal tube allows the use of low PEEP levels (group D), which may repla ce the lost subglottic function and partially restore the FRC. Ventila tion by mask does not disturb the functional subglottic stenosis, and the impairment of pulmonary function will depend solely on the decreas e in FRC caused by increased chest wall compliance (group A). If mask ventilation is combined with a paediatric circuit system (group C), th e pressure relief valve produces a low PEEP of 2 to 3 cm H2O, which ma y partially counteract the decrease in FRC. With regard to oxygenation , the paediatric circle system proved to be superior to the high-flow T-piece system independent of whether children were ventilated via a f ace mask or an endotracheal tube. The group-specific differences in de gree of dysventilation with manual ventilation show that the type of b reathing system is important with regard to the size of the tidal volu me delivered. Thus, tidal volumes will be unintentionally increased by the high fresh gas flow needed when a T-piece system is used. The low er flow and preadjusted pressure limit may prevent the delivery of exc essive tidal volumes with the paediatric circuit system. The high inci dence of dysventilation with mechanically controlled ventilation might be caused by the limited applicability of ventilation nomograms durin g anaesthesia for this age group.