The Effect Of Different Ventilatory Pattern During Pre-oxygenation And Induction Of Anaesthesia Upon Duration Of Non-hypoxic Apnoea In Old Patients

Whatever the clinical preoperative evaluation,unexpected difficult airway may occur.Therefore, administration of 100% O2 has been a common practice for decades in order to increase the possible duration of nonhypoxic apnea, the so-called margin of safety. However, general anesthesia,even in the lung-healthy nonobese subject, causes an increase in intrapulmonary shunt, which may impair oxygenation. The magnitude of shunt is correlated with the formation of pulmonary atelectasis, which appears within minutes after induction of anesthesia. Using low FiO2 during administration of O2 prevents atelectasis formation during induction of general anesthesia; however, this technique is not recommended because it reduces the duration of nonhypoxic apnea. The aim of this study was therefore to evaluate the clinical benefit, in terms of duration of nonhypoxic apnea and alveolar to arterial oxygen gradient (A-aDO2), of four methods ventilation applied during the induction period of general anesthesia in old patients. After written informed consent,60 ASA physical statusâ… -â…¡patients aged 65-85 yr, scheduled for elective surgery were enrolled in this study and randomly assigned to 1 of 4 groups. In theâ…¡,â…¢andâ…£group, awake patients were administered 100% O2 through a continuous positive airway pressure (CPAP) device set at 6 cm H2O for 5 min,but in theâ… group,they breathed spontaneously without any CPAP. After induction of anesthesia, patients were mechanically ventilated with the four different methods of ventilation for another 5 min until tracheal intubation:Normal tidal volumes of 8ml/kg in theâ… group. Large tidal volumes of 12ml/kg in theâ…¡group.Extrinsic positive end expiratory pressure(PEEP) of 6cmH2O in theâ…¢group. Pressure control inverse ratio ventilation(IRV) in the IV group.After tracheal intubation, no pressure was applied to the airway and correct placement of the endotracheal tube was confirmed by fibroscopy.The endotracheal tube was left open to air at atmospheric pressure and The duration of apnoea until SpO2 reached 90% was measured. Arterial and mixed-venous blood gases were measured just before the beginning of the period of apnea and the arterial blood gases were measured again when SpO2 decreased to 92%.Result:Of the four groups PEEP(P<0.05) and IRV(P<0.05) significantly increased the duration of nonhypoxic apnoea. Just before apnea, PaO2 was significantly higher while PA-aO2 was significantly lower in the PEEP and IRV groups than in the other two groups. There was no difference for Qs/Qt in four groups.No difference was seen for PaCO2 at 92% SpO2.We conclude that that CPAP+PEEP and CPAP+IRV were most effective in increasing intraoperative PaO2 decreasing A-aDO2 and prolonging the duration of the nonhpoxic apnoea.