DISCUSSION
This prospective, observational study, evaluated the association between
non-systolic TEE metrics of left ventricular performance, assessed
during intraoperative TEE before CPB, and interval to hospital discharge
after CAB. We found that abnormal left ventricular performance during
diastole was: 1) associated with a significantly longer postoperative
hospital stay, 2) associated with a greater baseline comorbidity burden.
Still, multivariate analysis showed their 35-40% lower daily
probability of discharge (compared to subjects with normal performance
during diastole) was independent of baseline comorbidities, including
heart failure and myocardial infarction.; 3) dose-dependently related to
daily probability of discharge; and, 4) associated with numerous
complications after surgery, particularly heart failure and respiratory
insufficiency/failure. These complications did not fully account for the
relationship between diastolic dysfunction and daily probability of
discharge.
Compared to patients with normal performance during diastole, we
observed a 31% longer mean hospital stay in patients with diastolic
dysfunction. This does not appear to be an artifact driven by outliers,
given the nature of the nonparametric Kruskal Wallis evaluation
performed. The unadjusted ratio for daily probability of discharge in a
patient with diastolic dysfunction is 0.54. This suggests a 46% lower
daily probability of discharge if a patient has any degree of diastolic
dysfunction (vs normal performance during diastole). We have previously
demonstrated that, independent of systolic function, preoperative
TTE-measured diastolic dysfunction is associated with prolonged
hospitalization as part of a combined endpoint in patients undergoing
CAB, Aortic Valve replacement, or combined CAB and Aortic Valve
replacement [8]. Severe diastolic
dysfunction, determined by transmitral (E/A ratio, E wave deceleration
time) and pulmonary venous flow patterns obtained by pre-operative TTE,
has been shown to predict the occurrence of low output states within 30
days of CAB surgery [21]. The present
study demonstrates that, in patients undergoing CAB surgery alone who
have preserved systolic function, the presence of any degree of
diastolic dysfunction on baseline intraoperative TEE is associated with
prolonged post-CAB hospitalization as an isolated endpoint.
The graded association between abnormal non-systolic TEE metrics and
prolonged post-CAB hospital stay that we observed in this study mimics
the graded association between diastolic dysfunction and event-free five
year survival reported following CAB surgery
[14]. Both studies had comparable
patient comorbidity patterns and utilized similar echocardiographic
analysis of performance during diastole. In our study, the prolongation
in hospital stay persisted even after adjusting for all baseline
co-morbidities (including heart failure). This suggests that classic
co-morbidity focused pharmacotherapeutic interventions will fail to
modify hospital stay following CAB in patients with normal systolic
function but diastolic dysfunction in a manner similar to their failure
to modify long-term morbidity in patients with preserved systolic
function, diastolic dysfunction and a history of heart failure.
[22-26] .
The relevance of the E/e’ ratio as an indicator of left sided filling
pressures [15,
27-29] is one of great debate and
beyond the scope of our study. We note that, regardless of the E/e’
ratio, the presence of either E > 50 cm/sec or e’
< 10 cm/sec is associated with a lower daily probability of
discharge following CAB surgery. We found no interdependence of E
> 50 cm/sec and e’ < 10 cm/sec with regard to
daily probability of discharge, suggesting that these individual metrics
reflect distinct phenomenon. An E > 50 cm/sec indicates
that left atrial pressure is elevated in relation to LV diastolic
pressure, regardless of ease of LV relaxation. Similarly, a slow e’
suggests that early LV relaxation is impaired, regardless of LA:LV
pressure relationship [15]. The
existence of either one of these conditions (E > 50 cm/sec
or e’ < 10 cm/sec), regardless of the value of the other
parameter, is associated with a lower daily probability of discharge in
our study. The E/e’ ratio does not improve upon these associations.
There are several limitations to our prospective cohort study. First,
protocol adherence was inconsistent over the course of the study.
Limited feedback with respect to obstacles to adherence prohibits a true
understanding as to the breadth of the protocol’s utility. The small
study population restricts subgroup size, preventing utilization of a
more robust statistical analysis. Thus, while our LCA and the propensity
score analyses indicate that co-morbidities do not account for the
entire effect of baseline diastolic dysfunction on hospital stay, a
large number of our propensity score subjects did not have good matches.
The study population size may also obscure the identification of an
interaction between diastolic dysfunction and a perioperative morbidity.
Finally, the true value of E/e’ remains unclear. Although originally
defined as a noninvasive approach to evaluate left heart filling
pressures [30], this concept has been
challenged [29]. In our study, E/e’
does not add to the predictive capacity of E >50 cm/sec or
e’ < 10 cm/sec with regard to presence or absence of lower
daily probability of hospital discharge following CAB, but does show a
graded association with the degree to which daily probability of
hospital discharge is lowered. It has previously been shown to correlate
with long term risk of major cardiac adverse effects
[7]. In that study, as in ours, the
study population was patients undergoing CAB surgery with baseline LVEF
> 50%.