Determinants of Aortic Pressure Variation During Positive-Pressure Ventilation in Man: Conclusion

14 Sep

We saw no difference between changes in EDA vs ESA within each subgroup. Because the absolute value of the end-systolic volume must be less than that of the end-diastolic volume, an equal decrease in both values might be interpreted differently if one used either the absolute volume or a percentage change. We used the absolute value of the area changes in our experiment, as based on previous studies to minimize this potentially confounding effect of proportionality.
We did not evaluate in our patients the effect of open or closed pericardium. We recorded the hemodynamic parameters when the chest was closed, open before and after bypass during closed pericardial conditions. We did not study the effect of a closed vs open pericardium because the clinical use of our observations, for the most part, is directed toward patients with a closed chest and pericardium. Pericardial constraint is a significant factor in relating positive-pressure ventilation and SAP. Pulsus paradoxus is a typical manifestation of an exaggerated increased pericardial pressure that would reduce venous return. In addition, changes in pericardial pressure will alter the pressure-volume relationship of the left and right ventricles, and systolic performance is augmented by an intact pericardium. Schertz and Pinsky observed, using an animal model, that LV ejection can enhance right ventricular stroke volume, but volume loading or the presence of an intact pericardium did not appreciably alter this interaction. natural asthma treatment

Interestingly, we observed in our study that the presence of an open or closed chest condition did not significantly change SA during a positive-pressure breath (Table 3).
Mean airway pressure was not measured; we gave a Vt, and this raised the peak airway pressure. We did not readjust the Vt during the study in order to keep the same mean airway pressure for each patient. We used a volume-cycled ventilator, not a pressure-cycled ventilator. Because lung compliance can change after a bypass, it is possible that for an identical volume, a variable transpulmonary pressure could be generated. This could alter the change in SAP variation. We preferred to use a defined volume for the simplicity of the test and also because previous studies were done in this fashion. Furthermore, we have previously demonstrated in an intact canine model that if Vt is held constant, then ITP increases by similar amounts, despite markedly changing lung compliance induced by oleic acid infusion. Thus, our ventilatory protocol was more likely to sustain a constant ITP variation across conditions than a protocol in which Vt was varied to maintain a common peak airway pressure during positive-pressure ventilation.
In summary, the degree of variation in SAP in patients during cardiac surgery cannot be explained by matched changes in LV area, and these results suggest that changes in SAP during ventilation cannot be used solely to assess the determinants of cardiovascular instability.