To determine the association between regional strain changes and postoperative outcomes, all segments were categorized into normal (0), mild-to-moderate reduction (−1), and severe reduction (−2), and these segmental scores were then aggregated and correlated with outcomes. This scoring system was used so that the magnitude of decrement or change in regional ventricular function as measured by 3D strain could be quantified. This segmental strain scoring system is an adaptation of varying 3D strain measures to 2-dimensional (2D) visual regional wall motion scores traditionally used to quantify changes in regional ventricular function. The lower the segmental strain score for each strain measure, the greater the decrement in LV function. Lower preoperative regional strain scores were predictive of both LOS (Table 2) and 1-year event-free survival (Table 3). We also observed that the change in regional strain from preoperative to postoperative values was predictive of 1-year event-free survival for all types of strain (Table 3). However, neither postoperative strain nor the change from preoperative to postoperative strain values was predictive of LOS (Table 2).
Intraobserver and Interobserver Reliability
Thirty-two patients were analyzed for intrarater and interrater reliability. Intraobserver ICCs with 95% confidence intervals preoperatively and postoperatively indicated good or excellent reliability across all types of strain. Interobserver ICCs were lower than intraobserver ICCs both preoperatively and postoperatively across all types of strain; however, there was still moderately good agreement (Table 4).
In this study, we used 3D strain imaging to comprehensively characterize the changes in regional myocardial function and strain after cardiac surgery, which varied by surgical procedure and strain type. We found that differences in regional LV function, from presurgery to postsurgery, were associated with worsened 1-year event-free survival. These findings suggest that postoperative changes in myocardial function are heterogeneous in nature, dependent on specific surgical procedure, and influential on long-term outcome.
Three-dimensional speckle tracking echocardiography allows simultaneous measurement of strain in all myocardial planes, improving accuracy over traditional 2D measures.18 , 21 Previous studies have largely focused on global function and global strain parameters.8 , 22 , 23 While 3D strain can detect subclinical changes in global LV function, even before changes in ejection fraction,24–26 changes in regional LV function after cardiac surgery have not been previously fully characterized.27–29 We found that after all types of cardiac surgery, longitudinal strain had the least decrement in function, circumferential strain had the greatest, and both area and radial strains had intermediate reductions. Area strain is a relatively new modality that combines both longitudinal and circumferential strain and has been shown to correlate with ejection fraction and wall motion score index.6
The varying magnitude of reduction in different type of strains can be potentially related to unique LV myocardial fiber architecture and/or imbalances in coronary perfusion.30 , 31 In the LV myocardium, longitudinal fibers predominate in the subendocardium, and circumferential fibers predominate in the midmyocardial and subepicardial layers.32 The overall ratio of circumferential to longitudinal fibers within the myocardium is approximately 10:1.33 Therefore, circumferential strain may be more impacted18 , 34 and better able to detect decrements in regional function compared to longitudinal strain, which may partially explain our results. Furthermore, variability can be seen in coronary perfusion and myocardial protection across different regions of the heart during CPB.35 Cardioplegia solution has been found to be inconsistently delivered, with heterogeneous distribution in patients with valvular lesions, incomplete delivery to areas of myocardial ischemia or infarction, and increased distribution to the endocardium compared to the epicardium.36 Preferential subendocardial blood flow during CPB and inconsistent cooling has also been confirmed in several studies37–39 and may explain why longitudinal strain (subendocardial fibers) was less reduced, or possibly better preserved, in this study cohort compared to circumferential strain (midmyocardial and subepicardial fibers).
Our findings also demonstrate that the magnitude of regional dysfunction after cardiac surgery varied with the procedure performed. Patients undergoing MVR had the greatest number of segments with reduced function followed by patients undergoing CABG, while patients undergoing AVR had the fewest reduced segments. LV function has been shown to decrease after MVR and MitraClip (Abbott, Menlo Park, CA) implantation.38 , 40–42 Myocardial biopsies have demonstrated myofibrillar degeneration in patients with severe mitral regurgitation and normal systolic function.43 Therefore, it is likely that changing of loading conditions after MVR in patients with previously severe mitral regurgitation can “unmask” preexisting LV dysfunction, as demonstrated in our study by reductions in LV function measured by area, radial, and circumferential strains in nearly all segments except for the basal lateral wall.
We have demonstrated that patients undergoing AVR or CABG had fewer segments with reduced function compared to patients undergoing MVR. Recent work by Duncan et al22 found that global longitudinal strain did not change from the beginning to the end of surgery. While the study by Duncan et al22 was performed using transesophageal echocardiography in the setting of dynamic intraoperative variables, our study used 3D TTE under stable hemodynamic conditions. Their findings complement our results, which showed that area, circumferential, and radial strains identified segments with reduced function that were not detected by longitudinal strain. Indeed, a recent MRI study found that systolic dysfunction was distributed heterogeneously in patients with aortic stenosis, and circumferential and longitudinal strains detected different segments with functional impairment.44 It is possible that the LV remodeling that occurs in the presence of severe aortic stenosis may involve different fibers within the myocardium, as detected by different modalities of strain. Our findings suggest that the evaluation of function using only 3D longitudinal strain may miss significant changes.
In the CABG cohort, similar to our AVR cohort, there was notable sparing of the basal lateral and basal septal walls. Similarly, Søraas et al45 found the contractility of the basal and lateral segments to be unchanged after revascularization. Previous studies have primarily been focused on changes in LV septal function after CPB, with the finding of “paradoxical septal motion.46 , 47” Septal dysfunction appears to be a result of reduction in regional circumferential strain in the apical, midpapillary, and basal myocardial segments. Longitudinal strain was least reduced in septal regions. Notably, in addition to the septal abnormalities, our study also found significant decrements in circumferential strain in all regions except for the midpapillary inferolateral and basal inferoseptal, inferolateral, and anterolateral segments. Given that all of the patients in our CABG group had left anterior descending coronary artery disease, the observed changes in circumferential strain can be explained by the left anterior descending coronary distribution.
Of note, in all surgical subgroups, we found an increasing gradient in postoperative myocardial dysfunction from the LV base to the apex, with the smallest decrements in the ventricular base and the largest in the apex. During CPB, perfusion pressure may be reduced in distal vascular beds, leading to increased subendocardial ischemia. Coronary artery perfusion is related to vessel lumen area and perfusion pressure48–50; thus, it is likely that the distal apical and septal vascular beds have more compromised perfusion during CPB, in part, explaining our findings. The observed base-to-apex gradient in myocardial dysfunction could also be artefactual due to normal heterogeneity in regional myocardial strain or limitations of 3D echocardiography. However, we used differences in regional function compared to preoperative values, making baseline anatomic differences in strain an unlikely explanation. In addition, 3D echocardiography allows better tracking of speckles compared to 2D and has been shown to be correlated better with reference-standard MRI, reducing the likelihood of imaging-related artifacts.6 , 8 , 18 , 51 , 52
Relationship to Outcomes
ICU length of stay was chosen as the outcome reflective of the immediate perioperative period, whereas 1-year event-free survival was chosen to determine whether perioperative changes in function could be related to long-term adverse outcomes. Preoperative strain was correlated with both short-term (increased ICU length of stay) and long-term (decreased 1-year event-free survival) outcomes, although the change from presurgery to postsurgery was only correlated with decreased 1-year event-free survival. These results complement findings from our previous study, which demonstrated that preoperative strain was a predictor of acute postoperative outcomes.8 Other authors have also found a correlation between strain and immediate outcomes after cardiac surgery, including low cardiac output syndrome.53 These results suggest that preoperative function, rather than immediate postoperative function, is highly influential on clinical outcomes. Furthermore, we found that decreased preoperative strain of all types, as well as differences from preoperative to postoperative strain values of all types, were independent predictors of 1-year event-free survival after controlling for procedure type, CPB time, and APACHE II score. These results are supported by the findings of Swaminathan et al,13 who demonstrated that deteriorations in wall motion score after cardiac surgery increased the risk of death, myocardial infarction, and subsequent need for revascularization. The lack of correlation between change in presurgical and postsurgical strain with increased postoperative ICU length of stay may indicate that subclinical regional changes in function, although not immediately affecting postoperative outcomes, may have impacts on long-term postoperative function.
Interestingly, postoperative strain, by itself, was not correlated with either outcome. After surgery, cardiac disease has been ameliorated or corrected, thereby introducing new variables into the patient’s clinical condition. Thus, single postoperative values may not be reflective of long-term changes in ventricular function; rather, it is the change in baseline from preoperation to postoperation that determines outcome. Nevertheless, there may be unknown variables contributing to the postoperative period that we have not accounted for in our model. It is also possible that the time we chose to perform the postoperative assessment may be too late in the ICU stay to be predictive of outcome.
The number of abnormal segments identified by strain has been shown to be an independent predictor of all-cause mortality or hospitalization for heart failure.54 Because there is substantial variability in the magnitude of reduction of different types of strain, evaluation of LV myocardial function and the prediction of outcomes must be assessed via several strain modalities for a complete clinical picture.
This is a single-center study with a heterogeneous population. Therefore, our results may not be applicable across a wide patient population. Given the number of patients with images not suitable for analysis, our sample size may be underpowered to detect all differences in function from before to after surgery. Although our postoperative echocardiograms were performed when the patients were off pressors, extubated, and euvolemic, there could be slight differences in patients on postoperative day 2 compared to postoperative day 5, possibly explaining variability in results. In addition, our thresholds for normal, mild-to-moderately reduced, and severe reductions in strain were based on previous work on global LV function and have not been validated by other investigators; nevertheless, these thresholds are reasonable starting points for evaluating function and providing a meaningful link to well-established measures of cardiac function, such as ejection fraction. While 3D speckle tracking is associated with improvement in strain measurements, 3D strain results may not be comparable with 2D measurements; furthermore, the resultant large volume data sets and decreased frame rates may lead to decreased temporal resolution. Strain measurements among different vendors and software systems may not be comparable55; however, in this study, all echocardiographic images were acquired and software analysis was performed with 1 vendor.56 Finally, our study used TTE to obtain strain measures; transesophageal echocardiography measures may not be directly comparable and may produce different results.
The results of this study comprehensively characterize regional patterns of LV dysfunction after aortic valve, mitral valve, and coronary artery bypass surgeries and, importantly, demonstrate differential patterns of reductions in regional myocardial function after cardiac surgery. Regional changes in myocardial function were found to vary by surgical procedure and type of strain and are associated with adverse short- and long-term clinical outcomes. Therefore, 3D speckle tracking echocardiography may add value in assessing regional myocardial function after cardiac surgery.
Name: Kimberly Howard-Quijano, MD, MS.
Contribution: This author helped design the study, collect and analyze the data, and write the manuscript.
Name: Emily Methangkool, MD.
Contribution: This author helped analyze the data and write the manuscript.
Name: Jennifer C. Scovotti, MA.
Contribution: This author helped collect and analyze the data.
Name: Einat Mazor, RDCS.
Contribution: This author helped collect and analyze the data.
Name: Tristan R. Grogan, MS.
Contribution: This author helped analyze the data.
Name: Wolf B. Kratzert, MD.
Contribution: This author helped analyze the data and write the manuscript.
Name: Aman Mahajan, MD, PhD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
This manuscript was handled by: Nikolaos J. Skubas, MD, DSc, FACC, FASE.
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