Comparative observational study on the deflation point characteristics in the pressure volume loop during application of end expiratory pressure of geriatric patients with that of young patients during general anaesthesia

The changes in the lung physiology of patients in the geriatric age group include a decrease in chest wall compliance, an increase in functional residual capacity with an increase in residual volume and reserve volume. As these changes necessitate the application of positive end-expiratory pressure (PEEP), it is important to know the ideal PEEP in this age group.  Intra-opeartive monitoring allows dynamic pressure-volume loop monitoring, which has been used to identify optimal positive end-expiratory pressure (PEEP). The LIP of the inflation limb has traditionally been accepted as the point at which significant recruitment occurs. Thus, the pressure at LIP was used as the optimal point to set PEEP. Rimensberger et al. (1999a) demonstrated in a rabbit model that significantly more alveolar units remained open with PEEP set above the UIP of the deflation limb compared with PEEP set at the LIP of the inflation limb. Albaiceta et al. (2004) showed in humans with CT that using the UIP of the deflation limb produced greater recruitment and aeration compared with using the LIP. Hence we planned to study if a safe PEEP can be provided in the geriatric age group using this principle. In patients fitting the criteria, after securing an endotracheal tube of appropriate size, the ventilation is provided by volume-controlled ventilation, with a tidal volume of 7 ml/kg of predicted body weight and a respiratory rate of 12 /minute. Inhalational anaesthetic is initiated to target a MAC of 1 at 1 l/min. The baseline spirometry loop is recorded after 10 minutes. Serial PEEP of 4, 7, 10 cm H2O are applied at 30 minute interval and then a PEEP of 4 cmH2O is applied at the end of 90 minutes. At the end of 30 minutes, the PV loops are recorded. At the end of surgery, patient trachea is extubated and patient is shifted to the post-anaesthesia care unit. The loops obtained are analysed for radius of curvature, angle of the tangent at the upper point of the deflation curve and the angle of the lower inflation line.