Abstract
Background:
Despite recommendations regarding the use of intraoperative transesophageal echocardiography (TEE), there is no randomized evidence to support its use in cardiac valve surgery. The purpose of this study was to compare the clinical outcomes of patients undergoing open cardiac valve repair or replacement surgery with and without TEE monitoring. We hypothesized that TEE monitoring would be associated with a lower 30-day mortality and shorter length of hospitalization.
Methods:
This observational retrospective cohort study used Medicare claims to test the association between perioperative TEE and 30-day, all-cause mortality and length of hospitalization among patients undergoing open cardiac valve repair or replacement surgery between January 1, 2010 and October 1, 2015. Baseline characteristics were defined by inpatient and outpatient claims. Medicare death records were used to ascertain 30-day mortality. Statistical analyses included regression models and propensity score matching.
Results:
219,238 patients underwent open cardiac valve surgery, of whom 85% received TEE. Patients who received TEE were significantly older and had greater comorbidity. After adjusting for patient demographics, clinical comorbidities, surgical characteristics, and hospital factors including annual surgical volume, the TEE group had a lower, adjusted odds of 30-day mortality (Odds Ratio (OR): 0.77 [95% CI: 0.73 – 0.82]; p<0.001) with no difference in length of hospitalization (<0.01% [95% CI: −0.61 – 0.62%]; p=0.99). Results were similar across all analyses, including a propensity score matched cohort.
Conclusions:
TEE monitoring in cardiac valve repair or replacement surgery was associated with lower 30-day, risk-adjusted, mortality without a significant increase in length of hospitalization. These findings support the use of TEE as routine practice in open cardiac valve repair or replacement surgery.
Keywords: Transesophageal Echocardiography, Cardiovascular Surgery, Health Services Research, Comparative Effectiveness Research, Outcomes Research
Introduction
Open cardiac valve surgery is a common treatment for advanced valve disease with cardiac valve procedures and accounts for one in four of the nearly 300,000 open cardiac surgeries performed in the US each year.[1] Transesophageal echocardiography (TEE) is an ultrasound-based, cardiac imaging modality used in a variety of cardiac surgeries for real-time confirmation of valve pathology,[2, 3] immediate assessment of the surgically repaired or replaced valve,[4, 5] identification of aortic atheromatous disease[6–8] and early diagnosis of cardiac failure after cardiopulmonary bypass.[2, 3, 9–11]
TEE is an American Heart Association (AHA) and American College of Cardiology (ACC) class I recommendation (level B evidence) in mitral valve repair surgery[3] and the surgical management of infective endocarditis.[3] Nonetheless, minimal comparative effectiveness research is available to quantify the potential clinical benefit of intraoperative TEE for other types of cardiac valve surgery.[3]
The purpose of this study was to compare 30-day mortality and length of hospitalization of patients undergoing open, cardiac valve repair or replacement surgery with vs without TEE monitoring. We hypothesized that TEE monitoring would be associated with a lower 30-day mortality and shorter length of hospitalization.
Methods
Data Source
The study used Medicare Part A (hospital) claims to identify all fee-for-service beneficiaries undergoing open cardiac valve repair or replacement surgery between January 1, 2010 and October 1, 2015. Cardiac valve surgeries were identified using International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM) codes. These data were then linked to Medicare Part B (physician) claims, which report Current Procedural Terminology (CPT) codes, which have been validated[12, 13] to identify echocardiography in both surgical[14] and nonsurgical populations.[15] Hospital-level information was obtained by merging the patient-level Medicare dataset with the American Hospital Association’s annual survey data between 2010 and 2016 matched to the year of the cardiac valve surgery.
Study Population
The study cohort consisted of all fee-for-service Medicare beneficiaries with a Part A (hospitalization) Medicare claim for cardiac valve surgery as defined by the ICD-9-CM procedure codes for tricuspid (3514; 3527–8), pulmonic (3513; 3525–6), aortic (3511; 3521–2), mitral (3512; 3523–4) or unspecified (3520; 3527–8; 3533; 3599) valve repair or replacement surgery.
Patients were excluded if they met any of the following criteria: (1) less than six months of continuous enrollment in Medicare prior to the index admission for cardiac valve surgery; (2) age < 65 years; (3) those with non-cardiac-surgery Diagnosis Related Group (DRG) codes, representing atypical recipients of the procedure or had procedural miscoding. (Appendix Table 1).
Exposure Variable
The primary exposure variable for this analysis was receipt of a TEE during the index hospitalization. We could not precisely identify whether a TEE was performed intraoperatively, thus we chose to include any hospitalization TEE claim as evidence of perioperative TEE receipt. Receipt of TEE was defined using Current Procedural Terminology (CPT) codes for TEE (93312–5; 93317–8; 93320–1; 93325) present in the Medicare Part B (physician) claims.[16, 17]
Dependent Variables (Outcomes)
The primary outcome of interest was all-cause 30-day mortality, defined using the Medicare Beneficiary Summary File. The death data in this file is generated by the Social Security Administration’s Death Master File and has been found to be a reliable indicator of mortality.[18]
The secondary outcome was length of hospitalization as reported on the institutional claim containing the cardiac surgery.
Independent Variables (Covariates)
Demographic data from Medicare included age, sex, race and year in which the surgery was performed. Surgery was categorized by operative valve (aortic, mitral, tricuspid or pulmonic), whether the surgical procedure was repair or replacement, and whether a simultaneous coronary artery bypass graft (CABG) surgery was performed. Medical data included 27 comorbidities identified by ICD-9-CM codes[19, 20] and present-on-admission indicators for the index admission and/or hospitalizations in the six months preceding the index admission for cardiac valve surgery. American Hospital Association survey data provided information on each hospital’s annual surgical volume, trauma level designation (1–4), Accreditation Council for Graduate Medical Education (ACGME) teaching status, and whether or not the hospital was a heart transplant center.
Statistical Analysis
Chi-square and t tests were used to evaluate baseline covariate associations between the two groups (TEE vs no TEE). Adjusted analyses consisted of multivariable logistic regression for 30-day mortality and linear regression for length of hospitalization. Because length of hospitalization was right-skewed this variable was log-transformed prior to fitting the regression analysis.[21] Post-regression, the resultant coefficients were appropriately transformed to provide estimates of the absolute percentage difference in length of hospitalization for each covariate in the model.[21] All statistical tests were two-sided and a p-value of <0.05 was set for statistical significance. Analyses used SAS 9.4 (SAS Institute, Cary, NC), and STATA 15.0 (StataCorp, College Station, TX).
Subgroup Analysis
TEE is an AHA/ACC class I recommendation in mitral valve repair surgery with established clinical outcomes benefit, but this is not the case for other valves.[3] Because of this, we pre-specified subgroup analyses that excluded mitral valve repair surgery (ICD-9-CM 3512) patients from the cohort.
Propensity Match Sub-Analysis
Because of baseline covariate differences between patients undergoing open cardiac valve surgery with vs without TEE, we chose to perform a propensity score matched sub-analysis to test the robustness of our primary analysis.[22] In our study, more patients were exposed (TEE recipients accounted for 85% of the cohort) than unexposed (non-recipients of TEE accounted for 15% of the cohort). Consequently, in order to ensure each exposed (TEE) subject had a matched control subject, we performed propensity score matching with replacement.[23, 24] In other words, a single subject in the control group could be used as a match for multiple subjects in the exposure group. The propensity score was calculated using logistic regression to determine the probability of TEE receipt as a function of observed patient demographics, clinical comorbidities, surgical characteristics, and hospital-level covariates (listed above). Subjects were matched with replacement and a caliper width of 0.2 standard deviation of the propensity score logit. This propensity score matching technique identifies subjects in the TEE recipient group for which there is no equivalent subject in the control group with a sufficiently similar propensity score and drops these subjects from the propensity score matched cohort. Covariate balance between the exposed (TEE recipients) and control (non-recipients of TEE) within the propensity score matched cohort was assessed using standardized differences.[25]
Because our propensity score match was performed with replacement, normalizing the weights of each control subject is necessary for appropriate variance estimation.[26] Therefore, the primary outcome of 30-day mortality in the propensity score matched sample was assessed by unconditional analysis.[27] That is, a direct comparison of the 30-day mortality outcome between the exposed (TEE recipients) and control (non-recipients of TEE) in the matched sample.[27] In the outcome analyses of the propensity score matched cohort, we used variance adjustment to account for the fact that control (non-recipients of TEE) subjects could be used as a match for multiple exposure (TEE recipient) subjects.[26]
Results
Following exclusions, our study cohort included 219,238 patients undergoing cardiac valve repair or replacement surgery. Among these patients 85% received a perioperative TEE, and 15% did not. Patients receiving TEE were older and had higher prevalence of arrhythmia, congestive heart failure, chronic pulmonary disease, diabetes, electrolyte abnormalities, hypertension, obesity, pulmonary hypertension, peripheral vascular disease, and renal dysfunction. Perioperative TEE was performed more frequently at hospitals performing at least 250 cardiac surgeries per year, hospitals with a heart or lung transplant designation, level I trauma hospitals, and centers with an ACGME program (Table 1).
Table 1:
Baseline Characteristics of Cohort
| Covariate | No TEE | TEE | P | |
|---|---|---|---|---|
| 32,483 (15) | 186,755 (85) | |||
| Age (years); mean (SD) | (74) (7) | 75 (7) | <0.001 | |
| Sex | Male | 19,584 (60) | 106,977 (57) | <0.001 |
| Female | 12,899 (40) | 79,778 (43) | ||
| Race/Ethnicity | Asian | 246 (0.8) | 1,490 (0.8) | <0.001 |
| Black | 1,318 (4.1) | 6,674 (3.6) | ||
| Hispanic | 409 (1.3) | 1,718 (0.9) | ||
| North American Native | 120 (0.4) | 596 (0.3) | ||
| Other | 482 (1.5) | 2,167 (1.2) | ||
| Unknown | 256 (0.8) | 1,000 (0.5) | ||
| White | 29,652 (91) | 173,110 (93) | ||
| Year | 2010 | 5,025 (15) | 31,566 (17) | <0.001 |
| 2011 | 5,548 (17) | 35,065 (19) | ||
| 2012 | 5,367 (17) | 32,187 (17) | ||
| 2013 | 5,711 (18) | 32,916 (18) | ||
| 2014 | 6,317 (20) | 31,673 (17) | ||
| 2015 | 4,461 (14) | 23,348 (13) | ||
| Trauma Center Level | 1 | 7,911 (24) | 51,433 (28) | <0.001 |
| 2 | 6,932 (21) | 40,349 (22) | ||
| 3 | 2,403 (7.4) | 11,778 (6.3) | ||
| 4 | 362 (1.1) | 663 (0.4) | ||
| Heart Transplant Center | 5,549 (17) | 44,330 (24) | <0.001 | |
| Lung Transplant Center | 3,912 (12) | 30,977 (17) | <0.001 | |
| ACGME Certified | 17,910 (55) | 112,634 (60) | <0.001 | |
| Annual surgical volume | < 250/year | 15,896 (49) | 78,652 (42) | <0.001 |
| ≥ 250/year | 10,812 (33) | 77,287 (41) | ||
| Unknown | 5,775 (18) | 30,816 (17) | ||
| AV repair | 675 (2.8) | 3,185 (1.7) | <0.001 | |
| AV replace | 26,877 (83) | 137,203 (73) | <0.001 | |
| MV repair | 2,540 (7.8) | 24,699 (13) | <0.001 | |
| MV replace | 3,500 (11) | 31,758 (17) | <0.001 | |
| PV repair | 3 (0.01) | 37 (0.02) | 0.19 | |
| PV replace | 19 (0.1) | 204 (0.1) | 0.01 | |
| TV repair | 632 (2.0) | 6,521 (3.5) | <0.001 | |
| TV replace | 132 (0.4) | 1,453 (0.8) | <0.001 | |
| Valve plus CABG | 13,932 (43) | 72,596 (39) | <0.001 | |
| Arrhythmia | 4,246 (13) | 33,196 (18) | <0.001 | |
| Congestive heart failure | 4,794 (15) | 35,400 (19) | <0.001 | |
| Chronic pulmonary disease | 3,413 (11) | 25,302 (14) | <0.001 | |
| Diabetes | 3,056 (9.4) | 20,573 (11) | <0.001 | |
| Electrolyte abnormality | 2,319 (7.1) | 17,346 (9.3) | <0.001 | |
| Hypertension | 7,008 (22) | 48,559 (26) | <0.001 | |
| Neurologic disease | 410 (1.3) | 3,112 (1.7) | <0.001 | |
| Obesity | 1,177 (3.6) | 8,128 (4.4) | <0.001 | |
| Pulmonary hypertension | 1,699 (5.2) | 13,829 (7.4) | <0.001 | |
| Peripheral vascular disease | 1,474 (4.5) | 10,035 (5.4) | <0.001 | |
| Renal disease | 2,079 (6.4) | 15,801 (8.6) | <0.001 | |
Abbreviations: TEE, transesophageal echocardiography; AV, aortic valve, MV, mitral valve, PV, pulmonic valve; TV, tricuspid valve; CABG, coronary artery bypass graft surgery.
Data are n (%) unless otherwise specified
Among the entire cohort, 9,730 patients died within 30-days (4.4%; [95% confidence interval (CI): 4.4 – 4.5%]). The mean length of hospitalization was 10.9 days (95% CI: 10.9 – 10.7). Compared to valve surgery performed without a TEE, surgery performed with a TEE was associated with a lower 30-day mortality (4.3% [95% CI: 4.2 – 4.4%] vs 5.2% [95% CI: 5.0 – 5.5%; p<0.001]), but a longer length of hospitalization (11.0 days [95% CI: 10.9 – 11.0] vs 10.6 days [95% CI: 10.5 – 10.7; p<0.001]).
Adjusted Regression Analysis: Overall
Following adjustment, among the overall cohort of 219,238 patients undergoing cardiac valve surgery, the TEE group demonstrated a lower adjusted odds for 30-day mortality (odds ratio[OR]: 0.77 [95% CI: 0.73 – 0.82]; p<0.001). TEE group did not demonstrate a significant increase in the length of hospitalization, with an absolute percentage increase of less than 0.01% (95% CI: −0.61 – 0.62%; p=0.99) (Table 2).
Table 2:
Adjusted Outcomes
| Mortality at 30 days | |||
|---|---|---|---|
| OR | 95% CI | P | |
| †Multivariable logistic regression overall (N=219,238) | |||
| TEE | 0.77 | 0.73 – 0.82 | <0.001 |
| MuInvariable logistic regression: excluding mitral valve repair (N = 191,999) | |||
| TEE | 0.79 | 0.74 – 0.83 | <0.001 |
| *MuInvariable logistic regression: Propensity score matched (with replacement) analysis: N=186,738 exposures (N = 186,738) matched to N=186,738 (N = 14,382 weighted) controls | |||
| TEE | 0.75 | 0.67 – 0.85 | <0.001 |
| Length of Hospitalization | |||
| Absolute % Change | 95% CI | P | |
| †Multiple linear regression: overall (N=219,238) | |||
| TEE | <0.01% | −0.61 – 0.62% | 0.99 |
| Multiple linear regression: excluding mitral valve repair (N=191,999) | |||
| TEE | −0.01% | −0.66 – 0.63% | 0.97 |
| Multiple linear regression: Propensity score matched (with replacement) analysis: N=186,738 exposures (N = 186,738) matched to N=186,738 (N = 14,382 weighted) controls | |||
| TEE | 0.47% | −1.01 – 1.96% | 0.53 |
Abbreviations: TEE, transesophageal echocardiography; OR, odds ratio; CI, confidence interval
Full regression models for primary analysis are found in Appendix Table 2 (multivariable logistic regression for 30-day mortality) and Table 3 (multiple linear regression for length of hospitalization).
Because the propensity match was performed with replacement, we performed an unconditional multivariable logistic regression analysis on the 30-day mortality outcome in order to account for the multiplicities as weighted in the matched controls.
Adjusted Regression Analysis: Without Mitral Repair
The results among the 191,999 patients undergoing cardiac valve surgery excluding the 27,239 patients undergoing mitral valve repair were consistent with the overall analysis. After adjustment, the TEE group demonstrated a lower adjusted odds of 30-day mortality (OR: 0.79 [95% CI: 0.74 – 0.83]; p<0.001) with no difference in length of hospitalization (−0.01% [95% CI: −0.66 – 0.63%]; p=0.97) (Table 2).
Propensity Match Analysis
The propensity score match (with replacement) compared 201,119 subjects; 186,738 with vs 14,382 without TEE receipt (with frequency weight adjustment: 186,738 TEE vs 186,738 control). A histogram of the weight of the matched controls may be found in the Appendix.Figure 1. Propensity score matching requires overlap of the propensity score itself between the exposure and control group prior to matching. The propensity score distribution of the exposure (TEE recipients) and control (non-recipients of TEE) groups both pre- and post-propensity score matching may be seen in Figure 1.a and Figure 1.b. There were 18,119 subjects that did not have a sufficiently similar propensity score and were therefore dropped from the propensity score matched sub-analysis.
Figure 1.a and 1.b:
Kernel density curves of propensity score (probability of TEE receipt) before (1.a; left) and after (1.b; right) the propensity score match. The propensity score for the exposure (i.e. TEE) is the solid blue line. The propensity score for the control (i.e. no TEE) group is the dashed green line.
Following propensity score matching, all covariates demonstrated a standardized percent bias less than 10%. The balance assessment by comparing the standardized percent bias for each covariates before and after the propensity score match may be found in the Appendix (Appendix.Figure 2). The results from the propensity score matched analysis were consistent with both the primary analysis and the analysis excluding mitral valve repair. After propensity score matching, by unconditional, multivariable logistic regression analysis the TEE group demonstrated a lower adjusted odds of 30-day mortality (OR: 0.75 [95% CI: 0.67 – 0.85]; p<0.001) with no difference in length of hospitalization 0.47% (95% CI: −1.01 – 1.96%; p=0.53); (Table 2).
Discussion
Among 219,238 Medicare beneficiaries undergoing open cardiac valve surgery in the US between January 1, 2010 and October 1, 2015, we observed a statistically significant lower adjusted odds of 30-day mortality among patients who received TEE without a difference in length of hospitalization. These findings were consistent across all analyses, including a propensity score matched sub-cohort.
Our work adds to previous observational research on TEE in cardiac valve surgery. In mitral valve repair surgery, TEE imaging has improved the surgical feasibility and long-term durability of a surgical repair.[28–30] In infective endocarditis, TEE is important in the surgical planning by detection of new diagnoses, [9, 10, 31] or by the early identification of surgical complications such as paravalvular regurgitation or malfunctioning leaflets.[9, 31, 32] Because of these diagnostic capabilities, TEE is an AHA/ACC class I recommendation (level B evidence) in mitral valve repair surgery and the surgical management of infective endocarditis.[3] However, TEE is a Class II recommendation (level B evidence) in all other types of cardiac surgery because its benefits remain unconfirmed.[3] In previous work, we have observed TEE use lower among US patients undergoing aortic valve replacement surgery compared to mitral valve repair or replacement surgery.[16] One possible explanation for this disparity is the relative lack of evidence to support TEE in aortic valve compared to mitral valve surgery.
While there is no randomized evidence to support TEE monitoring in open cardiac valve repair or replacement surgery, there is observational evidence that suggests TEE has the potential to reduce the morbidity associated with cardiac surgery. TEE offers confirmation of valve pathology,[2, 3, 33] immediate assessment of the surgically repaired or replaced valve,[4, 33] and assistance with surgical repair planning.[29] In other types of cardiac surgery, such as CABG surgery, TEE can pinpoint the anatomic location of severe aortic atheromatous disease,[6–8] provide an accurate assessment of volume status,[34] and aid in the early diagnosis of the most morbid complication associated with cardiopulmonary bypass; cardiac failure.[2, 3, 9–11] Despite these potential clinical benefits, there is a relative lack of comparative effectiveness work studying whether the use of TEE is associated with improved clinical outcomes, and therefore presents an important opportunity for impactful research
In the absence of randomized evidence, the current observational, retrospective, comparative-effectiveness study provides useful evidence of real-world outcomes. In contrast to prior observational studies,[9, 10] we aimed to determine if TEE monitoring was associated with a clinical outcomes benefit among patients undergoing repair or replacement surgery among any of the four cardiac valves. To accomplish this rigorously, we conducted three adjusted statistical analyses, including a propensity score matched analysis. All our analyses consistently demonstrated that TEE use was associated with improved 30-day mortality with no significant difference in the length of hospitalization, despite the TEE group having a greater burden of comorbid disease.
There are several possible explanations for this 30-day mortality benefit. First, the identification of significant paravalvular regurgitation immediately following surgical valve implantation by TEE could avoid a future, same-valve cardiac surgery during the hospitalization. Second, monitoring with TEE has the advantage of early identification of right or left ventricular dysfunction, accurate assessment of volume status, and appropriate selection and titration of inotropic or vasopressor medications based on real-time TEE imaging; potentially limiting complications such as acute kidney injury from high-dose vasopressors or prolonged mechanical ventilation from excessive volume resuscitation. Third, real-time TEE monitoring could identify systolic anterior motion (SAM) with left ventricular outflow tract obstruction. With TEE monitoring, SAM is easily medically managed by increasing afterload, increasing preload and decreasing heart rate.[35, 36] Without TEE monitoring, SAM could be mismanaged by escalating doses of inotropic agents, ultimately resulting in refractory hypotension and decreased cardiac output. Fourth, there is considerable value in obtaining echocardiographic images by TEE in the operating room to serve as a “baseline” for comparison with future transthoracic echocardiographic (TTE) studies. In the absence of intraoperative TEE imaging, the normal post-surgical inflammation surrounding the aortic root following an aortic valve replacement could be misconstrued as an aortic root abscess. Fifth, it is well established that cardiac surgery performed at high-volume, University hospitals have improved outcomes.[37–39] Moreover, it is likely that TEE is more frequently performed at these same high-volume, University hospitals. Even though our analyses adjusted for surgical volume, and other University hospital indicators, it is possible that the 30-day mortality benefit associated with TEE is related to unobserved, hospital-level factors.
It is important to interpret this study in the setting of several limitations. First, the observational, non-randomized, study design precludes proving a causal link between TEE and outcomes. While regression and propensity score matching adjust for observed differences in covariates, we were unable to adjust for unobserved variables that may have confounded the relationship between TEE receipt and mortality. However, because the TEE group demonstrated a greater burden of observed comorbid disease, it is likely that the burden of unobserved comorbid disease was higher in the TEE group as well. This suggests that the higher 30-day survival rate among TEE recipients cannot be attributed to TEE patients being healthier. Second, due to billing date inaccuracies associated with CPT codes, the TEE exposure was defined by having a claim for a TEE at any time within the index hospitalization for cardiac valve surgery. This could potentially overestimate the actual rate of intraoperative TEE monitoring. However since patients receiving a perioperative TEE outside of the operating room are likely to be doing so due to high perioperative risk and/or post-operative complications, our assignment method is in fact biased against a finding of improved outcomes for intraoperative TEE. Moreover, sensitivity analyses from our group’s previous work demonstrated unchanged results whether TEE was defined at the hospital-level or defined as “intraoperative” by having a TEE CPT billing date within one calendar day of the CT surgery CPT billing date. Third, as previously acknowledged, despite our adjustment for hospital-level factors such as annual surgical volume, transplant center designation, trauma center level, and ACGME certification, the 30-day mortality benefit we observed could have been related to unobserved differences in the hospitals that typically performed intraoperative TEE versus those that did not. Fourth, because this study was conducted using Medicare claims, these findings are not necessarily generalizable to patients undergoing surgery outside of the United States, or patients younger than 65 years.
Conclusions
This study demonstrated that TEE was associated with a lower 30-day, adjusted risk of mortality among patients undergoing open cardiac surgery on any valve. These findings support the routine use of TEE in open cardiac valve repair or replacement surgery.
Supplementary Material
Highlights.
There is lack of comparative effectiveness research investigating the clinical impact of transesophageal echocardiography in open cardiac valve surgery.
Transesophageal echocardiographic monitoring in open cardiac valve surgery is only a class II recommendation by the American Heart Association.
This study was a retrospective observational cohort investigation of 219,238 Medicare beneficiaries undergoing open cardiac valve surgery in the US between January 1, 2010 and October 1, 2015.
This study found that among Medicare beneficiaries undergoing open cardiac valve surgery, monitoring with transesophageal echocardiography was associated with lower 30-day, risk-adjusted, mortality without a significant increase in length of hospitalization.
Funding Statement:
This work was supported by a National Institutes of Health (NIH) T-32 Training Grant (5T32HL098054) to E. J. M.
Footnotes
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Conflict of Interest: All authors declare no financial conflicts of interest.
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