4.2 Right ventricular geometry and afterload
Pressure-dimension index classifies patients according to the morphology and afterload of the RV. Previous observations have found that low RV systolic pressure (RVSP) and RV dilation identify a subset of patients with vulnerable RV that is more prone to have postoperative RVF (9,10,14), though high RV afterload also appears as a marker in other studies (15). In that sense, patients within the lower spectrum of PDI would be more likely to have an “exhausted” RV phenotype while those with higher PDI would be more likely to have “overburdened” (i.e., high RVSP) RVs. Contrary to our initial predictions, our findings indicated that patients with an “overburdened” RV have a much higher short-term mortality as compared to other tertiles, including those within the first tertile. A possible explanation of this finding is that the patients with a true exhausted RV phenotype might have been excluded as the analyzed data was obtained from an institutional registry and patients that were deemed to have a high risk of postoperative RV failure were not considered for LVAD implantation. Another possible explanation is that some patients with a low PDI might not have an exhausted RV phenotype (i.e. those with a low RV afterload and moderately dilated RV), thus creating a mixed group of patients with normal and dysfunctional RV’s. As such, in the first tertile neither visual assessment nor conventional echocardiographic findings suggested that RV dysfunction was as prevalent as could be expected from patients with a true exhausted RV phenotype. It can be assumed that those with a lower PDI that is found for the first tertile (x̄=1.94) could identify a group of patients that have a true exhausted RV and therefore at high risk for post-LVAD RV failure, though this assumption needs further studies to verify whether it could be useful in clinical practice.
In contrast, patients with an overburdened RV phenotype have an excess short-term mortality following LVAD implantation and therefore a high PDI can be a useful tool to identify those at high risk for post-LVAD mortality. These patients had a small RV with high PASP, high PVR and a lower RAP, but parameters associated with RV systolic function were preserved (Tables 1 and 2). Although evidence suggest that LVADs could reduce PVR in the long-term, a beneficial effect of unloading is less likely in the immediate postoperative period due to structural pulmonary remodelling, thus making these patients more susceptible to RVF (17,17). Moreover, a smaller RV cavity would be less likely to handle volume overload caused by an LVAD, which could be an additional factor that facilitates RVF (18). It should be emphasized that our data does not indicate an excess RVF in patients surviving to postoperative days 7 and 14 and the present study could not establish whether the cause of death was RVF in patients within the third tertile. It is equally difficult to suggest that a high PDI is not related to RVF given that there are no other reasons for PDI to predict mortality since this novel parameter only reflects alterations in RV. However, a cause-and-effect mechanism cannot be established with available data and as such, a high PDI should only be regarded as a marker of short-term mortality until more data becomes available.