In fact, the lack of improvement in P&02 with PC-IRV may be more simply related to the decrease in Pv02 observed with this mode as a consequence of the reduction in CO. Indeed, reduced Pv02 may adversely affect arterial oxygenation in pulmonary disease with profound ventilation/perfusion inequalities, such as ARDS.
We observed a significant decrease in PaC02 with PC-IRV Such a finding is in accordance with the results of previous studies reporting a decrease in physiologic dead space induced by prolonged inspiratory time. This decrease in BaC02 with PC-IRV could permit a reduction in Vt in order to obtain the same level of PaC02 as in VCV. Such a reduction would probably lower the rise in mPaw observed in PC-IRV
In accordance with previous studies, PCV and expecially PC-IRV induced lower pPaw than VCV did. Such a finding is usually seen as beneficial in terms of barotrauma; however, studies in animals demonstrated that the occurrence of barotrauma is linked to lung volumes rather than to airway pressures per se. Therefore, peak static alveolar pressure (ie, Pplat) is probably more relevant than peak dynamic pressure (ie, pPaw) to assess the risk of barotrauma. We did not observe any decrease in Pplat with PCV and PC-IRV Therefore, the reduced risk of barotrauma consequent to the decreased pPaw observed in these ventilatory modes is questionable. On the other hand, as discussed earlier, PC-IRV may allow a reduction in Vt which might reduce barotrauma.
Our study especially addressed the hemodynamic consequences of the three studied ventilatory modes. The most striking finding was a significant reduction in Cl with PC-IRV Since PEEPt was maintained constant, the reduction in Cl with PC-IRV should be related to the increased mPaw with this mode. Indeed, mPaw as a reflection of mean alveolar pressure is known to be a major determinant of hemodynamic consequences of mechanical ventilation on cardiac function. In this way, human and animal studies comparing IRV to VCV reported either a decrease in Cl when mPaw increased with IRV or no change in Cl when mPaw was kept constant. The findings of Abraham and Yoshihara appear to be the sole exception to this rule, since they observed no significant change in Cl with PC-IRV The mPaw was not reported in this study, but PEEPt and minute ventilation were kept constant, so it is likely that mPaw increased in PC-IRV
As a consequence of the reduction in Cl, PC-IRV resulted in a decrease in Do2. In our patients, the decreased Do2 was not associated with a decreased Vo2. Such a Vos/Dos, independence was expected, since most of our patients had no oxygen debt, as attested by the normal blood lactate level; however, the deleterious effect of PC-IRV on Do2 must be emphasized, since Do2 was previuosly reported as an important prognostic factor in ARDS, and maximal improvement in Do2 is considered by many authors as a goal in the management of most critically ill patients.
In conclusion, our findings suggest that PC and PC-IRV have no more beneficial cardiorespiratory effects than VCV in patients with ARDS. The use of PC-IRV as an alternative mode in this setting should take into account its resultant effect on peripheral tissue oxygenation. Flexible Bronchoscopy