Unfortunately, because many variables can affect the relation between SAP and LV volume, it is still difficult to infer cardiovascular status from changes in SAP. The multiple determinants of SAP variation operating in all patients could explain the variability of findings in both the literature and in our data on this relation. Scharf et al, in their demonstration in an animal model, used intramyocardial radiopaque markers to assess sequential volume changes and aortic flow probe data to assess stroke volume, and they found that changes in SAP resulted from a combined interaction between decreases in aortic flow and transmitted increases in ITP. They demonstrated that the aortic flow could decrease as the SAP increased and that this difference was even more pronounced with faster respiratory rates (as seen in their Figure 2). We made similar observations in our validation group study.
The disparity in the interpretations of results obtained in these studies could be explained by the varied methods used for measuring LV volume and also by the effects of ventilation on LV area and SAP, which are not measured and recorded on-line.- In the study of Jardin et al, measurements were also done from a transthoracic approach, as opposed to a transesophageal view, from which image quality is significantly improved. Most of the studies on the effect of positive-pressure ventilation on LV volume and SAP were done in the closed chest condition, except for one in which four subjects had measurements made during the open chest condition. purchase birth control pills
Interestingly, in those open chest subjects, as in our study, minimal variation in SAP occurred during positive-pressure ventilation. Finally, although these workers felt that SAP variations, specifically increases in SAP, reflected a fluid-resuscitated heart-failure state, our data did not support this hypothesis, We saw no differences in the SAP response to positive-pressure ventilation between subjects when referenced either to LV EDA, as a measure of preload, or to fractional area of contraction, as a measure of contractility.