Effect of inspiratory waveform on pulmonary ventilation and gas exchange

Abstract

The effect of square (constant) and decelerating inspiratory waveform on pulmonary ventilation and gas exchange were studied on 20 patients with apparently normal lungs, who were mechanically ventilated in volume controlled mode using Puritan Bennett 7200 respirator. The conventional ventilatory parameters like the tidal volume, respiratory rate, positive end expiratory pressure (PEEP), inspiratory to expiratory ratio (I/E ratio), inspiratory pause duration and inspired oxygen concentration (FIO2) were all kept constant throughout the 4-h study period. The square waveform was applied for the initial 2 h followed by decelerating waveform for the next 2 h. The parameters monitored every hour, for 4 h included peak, mean and plateau pressures; respiratory mechanics as resistance and static compliance. The arterial blood gases were sampled to measure the oxygen tension (PaO2), carbon dioxide tension (PaCO2), the hydrogen ion concentration (pH), the alveolar – arterial oxygen gradient A-a(DO2), the arterial – alveolar oxygen ratio (a/A ratio) and the shunt fraction (Qs/Qt) with the help of Slide rule assessment1 of venous admixture; haemodynamics like heart rate, non-invasive blood pressure, central venous pressure, arterial oxygen saturation, temperature and urine output. Statistical analysis was performed using one way ANOVA withF Decelerating flow pattern produced significant reduction of peak airway pressure and shunt fraction (Table 32).2 A significant improvement in oxygenation with increase in mean airway pressure2 and resistance was observed. There was near significant increase in the compliance2 and no significant change in the alveolar–arterial oxygen gradient, arterial carbon dioxide tension, acid–base status and haemodynamics when compared to the square flow pattern.