Right Ventricular Function During Weaning From Respirator After Coronary Artery Bypass Grafting: Discussion
First, during PSV, the end-expiratory pressure remained constant at the PEEP of 5 cm H2O, but the BiPAP system switches between a low (Plo) and a high (Phi) airway pressure level of 5 and 10 cm H2O, respectively. Therefore spontaneous breathing with the BiPAP system was performed at two different pressure levels. Since the duration of Plo was 4 s and of Phi was 2 s, spontaneous breathing was performed in one third of time at the higher pressure level of 10 cm H2O. In comparison to PSV, this period of higher pressure level may cause an increase in afterload due to an increase in mean airway pressure. With another Phi.Plo ratio and a longer Phi period constituting 80 percent of time, Valentine et al reported a significantly higher mean airway pressure during APRV compared with PSV. They suggested this difference in airway pressure caused the increase in PVR during APRV. However, increasing levels of PEEP may contribute to an increase in RV volumes or a decrease in RVEF. http://buy-asthma-inhalers-online.com review This RV response was attributed to an increase in RV afterload or a decrease in RV contractility.2′ Accordingly, Jardin et al concluded that the impairment in RV function was the main consequence of PEEP, apparently by increasing afterload.
Second, each inspiration during PSV is supported by a demand flow up to the pressure level requested In contrast to this mechanically assisted spontaneous breathing, switching from Plo to Phi with the BiPAP system creates a mechanical lung inflation, ie, a controlled inspiration, similar to APRV, but not performed in PSV. This difference in inspiration may cause differences in hemodynamic response, because lung inflation by mechanical inspiration increased the RVEDV and RVESV. Jardin et al suggested an increase in RV afterload during inspiration to cause this enlargement in RV volumes. In experimentally impaired RV function in dogs, Lucking et al found a higher mean airway pressure during controlled ventilation compared with high-frequency ventilation, but they did not believe that this accounted for the hemodynamic differences between both ventilation modes. They speculated that the phasic changes of preload and afterload during controlled ventilation may contribute to the different hemodynamic response. Thus, we suggest that these differences in ventilatory support between the BiPAP system and PSV, ie, the period of breathing on a higher airway pressure level and the mechanically controlled inspiration with the BiPAP system, may contribute to the increased afterload compared with PSV.