In the present study, increases in Pa02 and decreases in PIP were found after the initiation of PC-IRV These results are similar to those found in previous investigations* of PC-IRV The changes found in PaC02 and pH were minimal and probably could have been eliminated had the end-expiratory pressure been decreased, as was made possible by the improvement in oxygenation accompanying the use of PC-IRV. Because PC-IRV has been demonstrated to result in improved Pa02 and decreased PIP in patients with ARDS, it has been proposed as a useful ventilatory modality for severe respiratory failure. Although no clear improvement in patient outcome has yet been shown with the use of PC-IRV, several theoretic benefits are associated with its physiologic effects. Improved oxygenation as a result of PC-IRV in patients with ARDS will permit use of lower inspired concentrations of oxygen, minimizing pulmonary toxicity associated with high FIo2. High PIP is thought to contribute to barotrauma and pulmonary injury through the generation of elevated alveolar shear forces. Maintenance or improvement of oxygenation with PC-IRV at lower levels of PIP may decrease the incidence of these complications of mechanical ventilation. fully
Although several studies, including the present one, have found improvement in oxygenation associated with the use of PC-IRV the physiologic mechanism^) responsible for this effect is not well delineated. Increased functional residual capacity has been suggested as one reason for the effects of PEEP on oxygenation. A similar increase in FRC may occur with PC-IRV, coincident with the increase in mean airway pressures that often accompanies use of this modality. In one study, external end-expiratory volume was increased by an average of 1,200 ml with PC-IRV indicating that increased FRC probably is seen with PC-IRV.
An initially high inspiratory flow rate, followed by a rapidly decelerating flow pattern, and a lengthened expiratory time constant accompany the use of PC-IRV This rapidly decelerating flow pattern has been shown to produce an improvement in oxygenation in patients with severe respiratory failure and may play a role in the changes in Pa02 found with PC-IRV In addition, the long expiratory flow constant may also improve oxygenation through achieving a more homogenous distribution of ventilation to previously underventilated lung units in patients with severe respiratory failure.
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