Novel insights into echocardiographic assessment of cardiac function following heart surgery

Assessment of cardiac function is a fundamental in everyday clinical decision making and is essential diagnostic tool for choosing therapy in patients with cardiac disease. Currently, echocardiography is the integral part in management of patients with different cardiac disease and the most established imaging tool in the assessment of cardiac function. Coronary artery bypass grafting (CABG) is an effective treatment in selected patients suffering from advanced coronary artery disease (CAD). Improvement in symptoms, functional status and cardiac function is often used to evaluate the success of CABG. Evaluation of cardiac function often is divided in assessment of systolic and diastolic function. However, systole and diastole are integrated and interconnected parts of cardiac cycle. Thus, a method for quantifying cardiac function which incorporates both systole and diastole is to be preferred. Myocardial performance index (MPI) assessed by pulsed-wave Doppler tissue imaging (PW-DTI) is combining systole and diastole, easy to acquire and is independent of cardiac geometry. The aim of this thesis is to evaluate the feasibility of MPI measured by PW-DTI in assessment of left and right ventricular performance in patients with CAD treated with CABG. In addition, to explore the role of B-type natriuretic peptide (BNP) in predicting long-term major adverse outcomes following CABG and exploring its association with MPI. Finally, this thesis aims to evaluate the impact of conventional aortic valve surgery in comparison to minimally invasive aortic valve surgery (MIAVR) on right ventricular function (RV) assessed by echocardiography.

Methods and Results: Study I. Forty six patients who were accepted for CABG were included. They all were investigated by dobutamine stress-echocardiography (DSE) prior to CABG and 3 month after CABG. Several methods for evaluation of left ventricular systolic and diastolic function had been applied, i.e. EF, longitudinal systolic and diastolic velocities as well as MPI. All the measurements were performed at rest and at peak DSE. The values from pre-CABG were compared to those after CABG. At baseline, MPI was prolonged both at rest (0.61 ±0.13) and at peak DSE (0.78±0.16). Accordingly, ejection fraction (EF) was also impaired at rest (42.7±8%) and at peak DSE (49.2±9). Similarly, wall-motion score index WMSI was impaired at rest (1.1±0.2) and at peak DSE (1.4±0.2). After CABG, MPI improved significantly both at rest (0.45±0.08; P < 0.001) and at peak DSE (0.56±0.1; P < 0.001). On the other hand, EF and WMSI did not improve at rest (43.7±8% and 1.1±0.2, respectively). However, at peak DSE an improvement of both EF (54.2±9; P < 0.05) and WMSI (1.1±0.16; P < 0.001) was observed.

Study II. The same patient cohort as in study I was used for analyzing the impact of CABG on RV function. Coronary angiography, DSE and exercise bicycle test were performed 1 month before and 3 months after CABG. Right ventricular index of myocardial performance (RIMP), right ventricular systolic velocity (RVS) and displacement (TAPSE) at the lateral tricuspid annulus were all assessed. The RIMP improved following CABG both at rest (0.45 ± 0.11 vs. 0.38 ± 0.08 CABG, P = 0.013) and during DSE (0.75 ± 0.23 vs. 0.49 ± 0.14, P < 0.001). Compared to baseline, TAPSE reduced substantially after CABG both at rest (23.9 ± 4.46 vs. 14.6 ± 3.67, P < 0.001) and during DSE (20.9 ± 4.16 vs 11.9 ± 3.60, P < 0.001). A significant decline in RVS was also observed following CABG both at rest (11.9 ± 2.40 vs. 8.5 ± 1.93, P < 0.001) and during DSE (15.6 ± 4.30 vs. 10.5 ± 3.21, P < 0.001). On contrary, compared to pre-CABG values exercise capacity improved significantly following CABG (128.4 ± 40.12W vs 142.1 ± 46.73 W, P = 0.014).

Study III was a predefined post hoc analysis of CMILE study (Cardiac Function after Minimally Invasive Aortic Valve Implantation including 40 patients with severe aortic stenosis and eligible for isolated aortic valve replacement. The patients were randomized 1:1 either to conventional aortic valve replacement (AVR) or minimally invasive aortic valve replacement (MIAVR). The impact of these two surgical techniques on right ventricular mechanics and contractility was evaluated by echocardiography. Compared to baseline RV strain rate (RV-LSR) was preserved after MIAVR (-1.52±0.5 vs -1.49±0.4 1/s, p=0.84) but declined following AVR (-1.67±0.3 vs -1.38±0.3 1/s, p<0.01). RV longitudinal strain (RV-LS) was deteriorated after AVR (˗27.4±2.9% vs ˗18.8±4.7%, p<0.001) and MIAVR (˗26.5±5.3% vs ˗20.7±4.5%, p<0.01). Peak systolic velocity of the lateral tricuspid annulus (RVS) declined by 18.8% in the MIAVR group (10.1±2.9 vs 8.2±1.4 cm/s, p<0.01) and 36.6% in the AVR group (9.3±2.1 vs 5.9±1.5 cm/s, p<0.01) when values from before surgery were compared to after surgery.

In Study IV, 99 patients with CAD who underwent CABG were evaluated by a biomarker BNP and Echocardiography. In a subpopulation of 40 patients DTI and MPI were obtained. Patients were followed-up for 5 years and during this period death, myocardial infarction, stroke and hospitalization due to heart failure were recorded. The role of postoperative BNP for predicting major outcomes was assessed and its association with MPI was determined. Seventeen patients experienced major adverse outcomes during the follow-up. Univariate analysis revealed that creatinine clearance (P<0.01), body mass index (BMI, P<0.01), postoperative BNP (P<0.001) and preoperative LV-MPI (P=0.04) were all significantly associated with major outcomes at follow-up. However, after correcting for cofactors in multivariate analysis only postoperative BNP (P=0.003) and BMI (P=0.025) were associated with major outcomes.

CONCLUSIONS: Myocardial performance index and right ventricular index of myocardial performance improved significantly following CABG in patients with CAD both at rest and peak DSE and appear to be a sensitive measure of myocardial function in patients with CAD. Postoperative BNP obtained in a stable clinical condition, 3 month after CABG is a predictive of major outcomes 5 years after CABG. Load-independent contractility is preserved following MIAVR but reduced following AVR. Load-dependent measures of myocardial function all declined following both MIAVR and AVR however, to a much lesser extent following MIAVR.

List of scientific papers

I. Hashemi N, Hedman A, Abdel Samad B, Brodin L-Å, Alam M. Feasibility of myocardial performance index for evaluation of left ventricular function during dobutamine Stress echocardiography before and after coronary artery bypass grafting. Echocardiography. 2014; 31: 989-995.
https://doi.org/10.1111/echo.12488

II. Hashemi N, Hedman A, Abdel Samad B, Brodin L-Å, Alam M. Improved right ventricular index of myocardial performance in the assessment of right ventricular function after coronary artery bypass grafting. Interactive Cardiovasc Thoracic Surgery. 2018; 26: 798-804.
https://doi.org/10.1093/icvts/ivx420

III. Hashemi N, Johnson J, Brodin L-Å, Gomes-Bernardes A, Sartipy U, Svenarud P, Dalén M, Bäck M, Alam M, Winter R. Right ventricular mechanics and contractility after aortic valve replacement surgery: a randomised study comparing minimally invasive versus conventional approach. Open Heart. 2018; 5(2):e000842.
https://doi.org/10.1136/openhrt-2018-000842

IV. Hashemi N, Brodin L-Å, Alam M, Abdel Samad B. Predictive value of B-type natriuretic peptide following coronary artery bypass grafting and its association with myocardial performance index. [Manuscript]