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%.