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.