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Regional Left Ventricular Myocardial Dysfunction After Cardiac Surgery Characterized by 3-Dimensional Strain

Howard-Quijano, Kimberly, MD, MS*; Methangkool, Emily, MD*; Scovotti, Jennifer C., MA*; Mazor, Einat, RDCS*; Grogan, Tristan R., MS; Kratzert, Wolf B., MD*; Mahajan, Aman, MD, PhD*

doi: 10.1213/ANE.0000000000003785
Cardiovascular and Thoracic Anesthesiology: Original Clinical Research Report
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BACKGROUND: Three-dimensional (3D) strain is an echocardiographic modality that can characterize left ventricular (LV) function with greater accuracy than ejection fraction. While decreases in global strain have been used to predict outcomes after cardiac surgery, changes in regional 3D longitudinal, circumferential, radial, and area strain have not been well described. The primary aim of this study was to define differential patterns in regional LV dysfunction after cardiac surgery using 3D speckle tracking strain imaging. Our secondary aim was to investigate whether changes in regional strain can predict postoperative outcomes, including length of intensive care unit stay and 1-year event-free survival.

METHODS: In this prospective clinical study, demographic, operative, echocardiographic, and clinical outcome data were collected on 182 patients undergoing aortic valve replacement, mitral valve repair or replacement, coronary artery bypass graft, and combined cardiac surgery. Three-dimensional transthoracic echocardiograms were performed preoperatively and on the second to fourth postoperative day. Blinded analysis was performed for LV regional longitudinal, circumferential, radial, and area strain in the 17-segment model.

RESULTS: Regional 3D longitudinal, circumferential, radial, and area strains were associated with differential patterns of myocardial dysfunction, depending on the surgical procedure performed and strain measure. Patients undergoing mitral valve repair or replacement had reduced function in the majority of myocardial segments, followed by coronary artery bypass graft, while patients undergoing aortic valve replacement had reduced function localized only to apical segments. After all types of cardiac surgery, segmental function in apical segments was reduced to a greater extent as compared to basal segments. Greater decrements in regional function were seen in circumferential and area strain, while smaller decrements were observed in longitudinal strain in all surgical patients. Both preoperative regional strain and change in regional strain preoperatively to postoperatively were correlated with reduced 1-year event-free survival, while postoperative strain was not predictive of outcomes. Only preoperative strain values were predictive of intensive care unit length of stay.

CONCLUSIONS: Changes in regional myocardial function, measured by 3D strain, varied by surgical procedure and strain type. 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, depending on the surgical procedure, and that these changes may have long-term impacts on outcome. Therefore, 3D regional strain may be used to identify patients at risk for worsened postoperative outcomes, allowing early interventions to mitigate risk.

From the Departments of *Anesthesiology and Perioperative Medicine

Medicine Statistics Core, David Geffen School of Medicine, University of California at Los Angeles Health System, Los Angeles, California.

Published ahead of print 8 August 2018.

K. Howard-Quijano and A. Mahajan are currently affiliated with the Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.

Accepted for publication August 8, 2018.

Funding: This study received support from intramural department funds and University of California at Los Angeles, Clinical and Translational Science Institute Grant Number UL1TR000124. A.M. is supported by National Heart, Lung, and Blood Institute and National Institute of Health Research Project Grant (R01) HL084261, Bethesda, MD. K.H.-Q. is supported by the Foundation for Anesthesia Education and Research Mentored Research Training Grant.

The authors declare no conflicts of interest.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website.

Reprints will not be available from the authors.

Address correspondence to Aman Mahajan MD, PhD, Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA. Address e-mail to amahajan@pitt.edu.

KEY POINTS

  • Question: Are there differential patterns in regional left ventricular dysfunction as detected by 3-dimensional speckle tracking longitudinal, circumferential, radial, and area strain, after cardiac surgery? Were these changes correlated with clinical outcomes?
  • Findings: Reduction in regional strain varied depending on type of strain and surgical procedure; both preoperative reduction in strain and change in strain from presurgery to postsurgery were correlated with 1-year event-free survival.
  • Meaning: Regional myocardial function as characterized by 3-dimensional strain is altered after cardiac surgery; the magnitude and pattern of changes in strain depend on the surgical procedure and are predictive of adverse clinical outcomes.

Decreases in left ventricular (LV) systolic function may occur after cardiac surgery and can impact postoperative morbidity and mortality.1 , 2 Myocardial strain or deformation has been validated as an accurate and reliable measure of LV function in many clinical conditions.3–7 We have shown recently that global LV function decreases significantly after all types of cardiac surgery and that preoperative global 3-dimensional (3D) strain can predict worsened postoperative outcomes.8

Three-dimensional speckle tracking echocardiography, in addition to measuring global strain and cardiac function, has been shown to accurately measure changes in regional LV function when compared to cardiac magnetic resonance imaging (MRI).9 Previous studies in heart transplant, ventricular tachycardia, and postmyocardial infarction patients suggest distinct patterns of changes in regional strain, which may be independent predictors of outcomes.10–12 Ventricular wall motion and regional myocardial function changes are associated with increased mortality; however, the regional changes in myocardial function after different cardiac surgical procedures have not been well characterized.13 Therefore, 3D strain imaging may provide comprehensive assessment of changes in regional myocardial function after cardiac surgery and may be predictive of postoperative outcomes.

The primary aim of this study was to evaluate changes in regional LV function after cardiac surgery using perioperative transthoracic echocardiography (TTE) 3D strain imaging. We hypothesized that there will be differential patterns of regional myocardial dysfunction after aortic valve, mitral valve, coronary artery bypass graft (CABG), and combined cardiac surgery as detected by 3D strain. Our secondary aim was to investigate whether changes in regional LV function identified by longitudinal, radial, circumferential, and area 3D strain were associated with worsened postoperative outcomes, including length of intensive care unit (ICU) stay and 1-year event-free survival.

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METHODS

After institutional review board approval and informed consent, we evaluated adult patients scheduled for elective cardiac surgery with cardiopulmonary bypass (CPB) who were prospectively enrolled in our previously published study on the predictive role of preoperative 3D strain imaging on outcomes after cardiac surgery8 (Supplemental Digital Content, Appendix A, Figure 1, http://links.lww.com/AA/C570). This is a supplementary analysis of changes in regional LV function after cardiac surgery in patients from that cohort undergoing aortic valve replacement (AVR) for aortic stenosis, mitral valve repair or replacement (MVR) for mitral regurgitation, CABG for coronary artery disease, and combined surgery (CABG and valve, multiple valve, or AVR and aortic surgery). Only patients in the combined group received >1 intervention. Exclusion criteria were arrhythmias, congenital heart disease, emergency surgery, and poor TTE imaging windows.

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Anesthetic and Surgical Management

All patients underwent placement of standard American Society of Anesthesiologists monitors, as well as arterial lines, central lines, pulmonary artery catheters, and transesophageal echocardiography probes. Anesthesia was induced with intravenous fentanyl, lidocaine, propofol, and rocuronium and maintained using inhaled isoflurane, intravenous rocuronium, and fentanyl in all patients. The surgical approach involved either a full midline sternotomy or a minimally invasive half-sternotomy. Cannulae for CPB were placed in a standard fashion, and antegrade with or without retrograde cardioplegia was administered. Heparin and protamine were given according to standard protocols. For weaning from CPB, epinephrine infusion was administered in patients with cardiac index <2.0 L/min/m2 and vasopressin infusion was administered for vasoplegia with systemic vascular resistance <800 dynes·s/cm5. Norepinephrine infusion was initiated for vasoplegia resistant to vasopressin. After surgery, all patients were admitted to the cardiac surgery ICU for postoperative management.

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Clinical Data Collection and Outcome Measures

Preoperative (European System for Cardiac Operative Risk Evaluation II), intraoperative (procedure, CPB time, cardioplegia type, antegrade, and/or retrograde cardioplegia delivery), and postoperative data were collected. Definitions of all data variables collected were per the Adult Cardiac Surgery Database Training Manual, version 2.73.14 Severity of disease score on admission to the cardiac ICU, Acute Physiologic Assessment and Chronic Health Evaluation II (APACHE II) Scoring System, was calculated as described previously.15 Postoperative outcome variables were length of ICU stay (LOS) and 1-year event-free survival. Major adverse cardiac events per the Society of Thoracic Surgeons database were cardiac arrhythmias (ventricular tachycardia, ventricular fibrillation, and asystole), multiple organ failure, ventricular assist device (balloon pump or extracorporeal membrane oxygenation), and reoperation. Mortality was defined as all-cause 30-day or 1-year mortality and was ascertained from electronic hospital records and telephone follow-up. One-year event-free survival was defined as percentage of patients free from any major adverse cardiac event or all-cause mortality <1 year after the date of study enrollment.16 ICU length of stay was chosen as an outcome measure reflective of the duration of time required for a patient to be stabilized in the acute postoperative period after cardiac surgery including duration of mechanical ventilation, resolution of postoperative heart failure, and treatment of operative complications such as bleeding. In addition, 1-year event-free survival was chosen as an outcome measure to determine whether perioperative changes in function could be reflective of long-term adverse outcomes.

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Echocardiographic Analysis

Each patient underwent standardized 3D TTE performed by the same experienced sonographer (E.M.). Transthoracic echocardiograms were performed preoperatively in awake, spontaneously ventilating patients on the day of surgery and postoperatively on the second to fourth day after surgery, once participants were extubated and chest tubes were discontinued. Three-dimensional full volume data sets for strain and ejection fraction were obtained, with acquisition of multibeat images at frame rates ranging from 26 to 50 Hz. Data analysis was performed offline by 1 of 2 experienced echocardiographers (K.H.-Q., E.M.) blinded to patient identifiers, surgery type, and preoperative or postoperative study using the 4D Auto LVQ offline EchoPAC system (GE Healthcare, Buckinghamshire, United Kingdom). The full volume data set was displayed as orthogonal 4-chamber, 2-chamber, and short-axis images. Semiautomated endocardial surface detection software outlined endocardial borders and tracked the endocardium in all frames and automatically calculated strain. If the endocardial border tracking appeared inadequate, manual adjustments of automatic tracing were made (necessary in <10% of images) and the sequence analysis repeated. Software output included calculation of global and segmental peak systolic area, circumferential, longitudinal, and radial strain. Area strain is a relatively new modality, unique to 3D echocardiography, that measures changes in endocardial surface area throughout the cardiac cycle, thus representing changes in myocardial deformation in both longitudinal and circumferential planes; area strain has also been shown to correlate with ejection fraction and wall motion score index.6 Individual LV myocardial segments with reductions in strain were evaluated by type of strain and type of surgical procedure, with results displayed using the standard bullseye format of LV anatomy.

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Sample Size and Power Calculation

Sample size for this supplementary analysis was based on the power calculation of the primary study, for 80% power at α = .05 to detect an odds ratio of 2.5 between those with normal and reduced ventricular function for the primary outcome of major adverse event, assuming a baseline expected proportion of 30% in the reduced function group. These calculations yielded a sample size of 160 patients. Given this fixed sample size, we calculated that we were 90% powered to detect numerical differences in strain (preoperatively to postoperatively) of 6%, from baseline, for each surgical group (AVR/MVR/CABG/combined n = 45/45/46/23, respectively); for example, a numerical difference in strain of 6% would be a change from −10% to −4% from preoperatively to postoperatively. This is assuming a paired-samples t test using an average correlation as estimated from our data of 0.31 and average standard deviation of the differences to be 10.3. However, because this is a supplementary analysis of a previous study, there was no a priori power calculation to detect the association between regional score and outcomes.

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Statistical Analysis

Clinical variables were summarized using frequencies (percentages) and median (interquartile range) as appropriate unless otherwise noted. Differences in preoperative to postoperative strain values were examined using general linear mixed-effects models for each measure of strain (area, radial, longitudinal, circumferential) and each surgical group (AVR, MVR, CABG, combined). Segmental strain was reported using the 17-segment LV model.17 , 18 To determine which segments were significantly changing, pairwise contrasts from these models were extracted. Regional strain differences were then calculated by averaging mean differences in segmental strain across each anatomical region: basal (basal anterior, anteroseptal, inferior, inferoseptal, inferolateral, and anteroseptal), midpapillary (midpapillary anterior, anteroseptal, inferior, inferoseptal, inferolateral, and anteroseptal), and apical (apex, apical anterior, inferior, lateral, and septal) per standard LV segmental nomenclature.19

To assess the relationship between changes in regional strain and clinical outcomes (ICU length of stay and 1-year event-free survival), each regional segment, categorized by strain type and surgical procedure, was scored based on threshold values as listed in Supplemental Digital Content, Appendix B, Table 1, http://links.lww.com/AA/C570.8 Preoperative and postoperative scores were assigned for each myocardial segment as normal (0), mild−moderately reduced (−1), or severely reduced (−2) ventricular function. Differences in ventricular function for each segment from preoperative to postoperative were then scored using the following criteria: 0 for a decline in strain value <10%; −1 for a decline in strain value ≥10% and <30%; and −2 for a decline in strain value ≥30%. Patient scores were then summed for each region, strain type, and surgical procedure. Scores are presented as preoperative, postoperative, and the difference from presurgery to postsurgery. The association between these strain scores and (log) LOS was assessed using linear regression models correcting for covariates with demonstrated clinical relevance and/or significance in univariate analysis including APACHE score, CPB time, and surgical procedure type. Cardioplegia type and delivery were not found to be significant in univariate analysis and were therefore not included in multivariable models. The association between the same strain scores with 1-year event-free survival was assessed using Cox proportional hazards regression models, correcting for APACHE score, CPB time, and procedure type. Effects of strain on the outcomes of interest were represented by mean ratios (log LOS) and hazard ratios (Cox models) with 95% confidence intervals. Goodness-of-fit characteristics for these models was assessed using R 2 for the linear models and survival C-statistics for the Cox models.

Statistical analyses were run using R version 3.1.2 (R Foundation for Statistical Computing, Vienna, Austria), SPSS 24 (IBM Corp, Armonk, NY), and SAS version 9.4 (SAS Institute Inc, Cary, NC). Statistical significance was defined as P ≤ .05. This manuscript adheres to the Strengthening the Reporting of Observational Studies in Epidemiology statement.

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Reproducibility Analysis

Intraclass correlation coefficients (ICCs) were calculated on 32 randomly selected patients (20%) to assess the reliability and reproducibility of segmental strain measurement technology. This was done separately for each of the 4 measures of strain and for preoperative and postoperative time points. Intraobserver ICCs were calculated using measurements from the same observer and from the same set of patients. Interobserver ICCs were calculated from 2 different observers on the same set of patients. Medians and interquartile ranges were used to summarize the distribution of the 17 ICCs from each combination of factors. Intrarater and interrater reliabilities were determined using standard guidelines established by Cicchetti.20

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RESULTS

In total, 182 patients were enrolled consecutively in the primary study (Supplemental Digital Content, Appendix A, Figure 1, http://links.lww.com/AA/C570). Fifteen patients had echocardiograms unsuitable for speckle tracking strain imaging (10 for postoperative arrhythmias and 4 technically difficult studies), 4 patients refused postoperative TTE, 3 patients had procedures that did not require CPB, and 1 was lost to follow-up. Nine patients were paced after cardiac surgery; however, each patient was atrially paced with intact atrioventricular conduction and was therefore included in the final cohort. Data on 159 patients were analyzed, and 114 (72%) patients were followed for 1 year (Table 1).

Table 1

Table 1

Figure 1

Figure 1

Changes in regional longitudinal, circumferential, radial, and area strain were assessed before and after surgery in each subgroup: AVR, MVR, CABG, and combined. Results of individual LV myocardial segments with mean reductions in regional strain for each type of strain and surgical procedure are displayed using standard bullseye format of LV segmental anatomy in Figures 1–3. Individual preoperative and postoperative strain values by segment and strain type are presented in Supplemental Digital Content, Appendix B, Tables 2–5, http://links.lww.com/AA/C570.

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Changes in Regional LV Strain by Surgery Type

Aortic Valve Surgery.

In the AVR group, all types of strain were significantly reduced at the apex (Figure 1; raw values in Supplemental Digital Content, Appendix B, Table 2, http://links.lww.com/AA/C570). Area, circumferential, and radial strains showed statistically significant reductions in the apical segments, as well as in several of the midpapillary segments. These midpapillary reductions, however, were not detected by longitudinal strain; indeed, longitudinal strain tended to show the fewest reductions in segmental strain. In addition, in general, all types of strain, except circumferential, demonstrated sparing of reduction in LV basal function.

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Mitral Valve Surgery.

Figure 2

Figure 2

Patients in the MVR group experienced reductions in function in a majority of segments as measured by all types of strain (Figure 2; Supplemental Digital Content, raw values in Appendix B, Table 3, http://links.lww.com/AA/C570). All types of strain were significantly reduced at the true apex and apical segments. Area, circumferential, and radial strains measured reductions in function in the midpapillary and basal regions, with sparing only in the basal lateral segments. In contrast to the other types of strain, longitudinal strain measured the least number of segments with reduction in function, with sparing of all of the basal segments except for the basal anterior segment.

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Coronary Artery Bypass Surgery.

Figure 3

Figure 3

Patients in the CABG group had more heterogeneous changes in regional function (Figure 3; Supplemental Digital Content, raw values in Appendix B, Table 4, http://links.lww.com/AA/C570). All types of strain detected reductions in the apex and apical segments. Area, circumferential, and radial strains detected reductions in the midpapillary segments, with sparing of many of the basal segments. Similar to the patients undergoing AVR and MVR, longitudinal strain detected the fewest reduced segments.

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Combined Cardiac Surgery.

Patients in the combined surgical group received CABG and valve, multiple valve, or AVR and aortic surgery. Reductions in myocardial function were detected in the apex by all strain measures (Supplemental Digital Content, Appendix A, Figure 2, http://links.lww.com/AA/C570; Supplemental Digital Content, raw values in Appendix B, Table 5, http://links.lww.com/AA/C570). Significant reductions in strain were detected by area, circumferential, and radial strains in the apical and midpapillary regions. Similar to other surgical subtypes, there was basal sparing of the anterolateral, anteroseptal, and inferolateral segments; in addition, longitudinal strain demonstrated the fewest number of segments with reductions in strain.

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Changes in Strain From Base to Apex

Figure 4

Figure 4

After cardiac surgery of all types, the largest reduction in myocardial strain was seen in the LV apical segments, with less change observed in the midpapillary and basal segments. Therefore, to further investigate this pattern, data were combined for the entire surgical cohort according to LV anatomic region. When averaging mean differences in preoperative to postoperative strain in individual segments by anatomic region (basal, midpapillary, and apical), we found a base-to-apex difference such that the smallest reduction in strain was at the base and the greatest reduction in strain was observed in the LV apex (Figure 4). All strain measures showed this similar pattern, with the greatest reduction in myocardial function at the apex compared to the base.

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Changes in Strain and Postoperative Outcomes

Table 2

Table 2

Table 3

Table 3

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).

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Intraobserver and Interobserver Reliability

Table 4

Table 4

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).

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DISCUSSION

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

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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.

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LIMITATIONS

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.

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CONCLUSIONS

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.

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DISCLOSURES

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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