Minimally Invasive Mitral Valve Surgery Associated with Excellent Resource Utilization, Cost, and Outcomes

Abstract

Objectives

Minimally invasive mitral valve surgery (mini-MVR) has numerous associated benefits. However, many studies fail to include higher risk patients. We hypothesized that a minimally invasive approach in a representative cohort provides excellent outcomes with reduced resource utilization.

Methods

Mitral valve surgical records from 2011-2016 were paired with institutional financial records. Patients were stratified by approach and propensity-score matched to balance preoperative difference. The primary outcomes of interest were resource utilization including cost, discharge to a facility, and readmission.

Results

A total of 478 patients underwent mitral surgery (21% mini-MVR) and were balanced after matching (n=74 per group) with 18% of patients having non-degenerative mitral disease. Outcomes were excellent with similar rates of major morbidity (9.5% mini-MVR vs 10.8% conventional, p=0.78). Mini-MVR cases had lower rates of transfusion (11% vs 27%, p=0.01) and shorter ventilator times 3.7 vs 6.0 hrs (p<0.0001). Mean total hospital cost was equivalent ($49703 vs $54970, p=0.235) with mini-MVR having lower ancillary ($1645 vs $2652, p=0.001) and blood costs ($383 vs $1058, p=0.001). These savings were offset by longer surgical times (291 vs 234 min, p<0.0001) with higher surgical ($7645 vs $7293, p=0.0001) and implant costs ($1148 vs $748, p=0.03). Rates of discharge to a facility (9.6% vs 16.2%) and readmission (9.6% vs 4.1%) were not statistically different.

Conclusion

In a real world cohort, mini-MVR continues to demonstrate excellent results with a favorable resource utilization profile. Higher surgical and implant costs with mini-MVR are offset by decreased transfusions and ancillary needs leading to equivalent overall hospital cost.

INTRODUCTION

Minimally invasive mitral valve surgery (mini-MVR) has been around for decades, yet only a minority of surgeons perform the operations.[1, 2] In 2008, only a quarter of cardiac surgery centers reported performing mini-MVR, and of all mitral operations only 20% were by a minimally invasive approach. This low volume is not due to worse outcomes. Multiple studies have demonstrated equivalent or superior outcomes with mini-MVR.[3–8] The benefits reported include reduced blood loss, rates of transfusion, atrial fibrillation, shorter ICU, hospital stays and increased patient satisfaction. Moreover, the long-term durability appears equivalent to conventional mitral repair approaches.[9, 10] Knowing these results, the select few surgeons and institutions continue to advance mini-MVR with expansion outside of low-risk degenerative mitral disease. There are now studies examining mini-MVR for multiple valve surgery, low ejection fraction, obese, extreme elderly, and reoperative patients.[11–17]

Considering the excellent outcomes associated with mini-MVR, significant impediments must exist to prevent further widespread adoption. This certainly includes the learning curve and need for a physician champion.[18] However, resource utilization also is a large concern. Hospitals must purchase specialized equipment, train nurses and anesthetists, and be willing to dedicate additional operating room time. Several studies have examined resource utilization using mini-MVR for simple degenerative repairs.[19–21] As mini-MVR expands into higher risk and more complex surgeries, the studies should evaluate representative populations. Therefore, the purpose of this analysis was to examine the outcomes and resource utilization of mini-MVR and conventional approach in a representative mitral valve population.

METHODS

Patient and Cost Data

The University of Virginia Institutional Review Board approved this study with a waiver of patient consent due to its retrospective nature (Protocol # 19762). Patient records were extracted from an institutional Society of Thoracic Surgeons (STS) Adult Cardiac Surgery Database between January 2011 and December 2016. Inclusion criteria were mitral valve surgery with or without tricuspid valve repair, atrial fibrillation surgery (ablation, left atrial appendage ligation) or atrial septal defect repair. Of the 680 mitral surgeries performed in this time period, patients were excluded for other concomitant cardiac surgery (n=197) or emergent status (n=8) resulting in a cohort of 478 cases. Patients were stratified by approach into either conventional sternotomy or minimally invasive by right mini-thoracotomy. STS data captured operative metrics including time in the room as well as incision, cardiopulmonary bypass and cross clamp times. Standard STS definitions were utilized including operative mortality (in-hospital or 30-day) and major morbidity (permanent stroke, prolonged ventilation, reoperation for any reason, renal failure and deep sternal wound infection.[22]

Patient level STS data is merged with financial records obtained from the finance department. Every hospital charge is associated with a cost valuation that includes both direct and indirect costs. Direct costs include all operating costs such as supplies, salaries and equipment depreciation while indirect costs are typically hospital wide costs that are assigned to a department and recalculated quarterly. Both fixed and variable costs are incorporated, with variable costs statistically determined based on volume. The total costs (direct + indirect) of a department are then allocated to the services provided (by charge code) based on cost accounting studies performed every fiscal year by the cost accountant.

For each patient, the cost estimates for each charge code are categorized by International Classification of Disease version 9/10. Categorization buckets are based on those submitted with Uniform Billing (2004) forms. For this study they were organized by individual components of the hospital stay (e.g. operating room, intensive care, pharmacy, blood bank, etc.). Cost estimates are summed to calculate a total episode cost for the hospitalization. All cost data are presented as 2016 equivalent dollars with conversion using the market basket for the Center for Medicare and Medicaid Services Inpatient Prospective Payment System that captures medical related inflation.

Minimally Invasive Protocol

Patients are placed in a supine position with a bump under the right shoulder. The anesthetist places a right internal jugular vein central line with Swan-Ganz catheter, as well as a 16-gauge long sheath prepped into the field for percutaneous superior vena cava cannulation. The femoral artery and vein are access via a small incision for arterial and inferior vena cava cannulation. A 4cm right mini-thoracotomy in the 4^th interspace is made with visualization aided by a 5mm HD thoracoscope (Stryker Corp, Kalamazoo, MI). After cannulation and exposure, a long 14-gauge cardioplegia cannula is placed in the proximal aorta and the aorta is directly cross-clamped using a detachable cross-clamp (Glauber clamp, LivaNova PLC, London, England). Cardiac arrest is achieved with 1200-1500mL of antegrade Del Nido cardioplegia and redosed every 45-60 min with 500mL delivered antegrade only as needed. Del Nido solution is also used for valve testing to avoid washout of cardioplegia solution during valve repair. Surgical repair and replacement techniques are similar for both conventional and minimally invasive operations, although vary by surgeon preference with one surgeon performing all minimally invasive mitral valve operations. For both approach ablations were performed with cryotherapy and left atrial appendage ligation with either a clip device or suture ligation. Multi-modal analgesia includes liposomal bupivacaine for all thoracotomy incisions.

Statistical Analysis

Patients were matched in a 1:1 fashion for propensity to undergo minimally invasive approach. Demographics, comorbid conditions and mitral disease subtypes were considered in the logistic regression model with a lenient stepwise selection methodology (Supplemental Table 1). Matching was performed using a nearest neighbor greedy algorithm without replacement. Balance among matched pairs was assessed by standardized mean difference, with adequate matching defined as a maximum of 20% (Supplemental Figure 1). The distribution of matched pairs was also representative of the entire cohort as demonstrated in Supplemental Figure 2.

Categorical variables are presented as number (percent) while continuous variables as mean ± standard deviation, except for non-normally distributed data that is shown as median [interquartile range as 25^th, 75^th percentiles]. For the entire cohort, categorical variables were analyzed by Chi-Square test and continuous variables by Independent T-test or Mann-Whitney U-test as appropriate. Among the matched cohort, matched pairs were analyzed by Wilcoxon Signed-Rank test or paired t-test based on normality or McNemar’s test as appropriate. All analyses were performed using SAS Version 9.4 (SAS Institute, Cary, NC) with significance set to α<0.05.

RESULTS

Entire Unmatched Cohort

Of all patients who underwent mitral valve surgery, there were numerous baseline differences between those who had conventional compared to minimally invasive approaches (Supplemental Table 2). Patients who underwent surgery by a conventional approach had greater comorbid disease including diabetes, renal failure, chronic lung disease, cerebrovascular disease, coronary disease, and hypertension (all p<0.05). Additionally, the type of mitral disease differed with conventional approach more commonly used for patients with rheumatic, endocarditis, and ischemic mitral diseases, while minimally invasive approach had a higher proportion of degenerative disease (all p<0.05). Therefore, operative characteristics also differed by approach, with a higher rate of repair by mini-MVR and more atrial fibrillation surgery by sternotomy (all p<0.05; Supplemental Table 2).

Unadjusted results are shown in Supplemental Table 3. There was a lower rate of STS major morbidity with mini-MVR (9% vs 20%, p=0.0009), transfusion (9% vs 54%, p<0.0001), and prolonged ventilation (4% vs 15%, p=0.003). Resource utilization was lower nearly across the board for mini-MVR patients, except for readmissions, implant and supply costs. Mini-MVR patients had $19,776 lower mean hospital cost, in addition to shorter length of stay, time ventilated, fewer discharges to a facility, and less readmissions (all p<0.05).

Matched Baseline and Operative Characteristics

After matching, patient characteristics were well balanced and demonstrated no significant differences in demographics, comorbid disease, or mitral disease etiology (Table 1). The mean age was 62 and 43% were female. Importantly, there were no significant differences in mitral valve disease etiology, with 82% overall having degenerative mitral disease. Rates of mitral repair were not statistically different between mini-MVR and conventional approaches (80% vs 69%, p=0.10). The rate of concomitant tricuspid repair 9.5% overall and similar between groups.

Table 1.

Baseline and operative patient characteristics among matched patients

Baseline Characteristics	Mini (n = 74)	Conventional (n = 74)	p value
Age (y)	61.4 ± 15.2	61.9 ± 12.2	0.635
Female	32 (43.2%)	31 (41.9%)	0.881
CLD (moderate/severe)	6 (8.1%)	9 (12.2%)	0.500
Cerebrovascular disease	8 (10.8%)	7 (9.5%)	0.782
Diabetes	9 (12.2%)	13 (17.6%)	0.346
Dialysis dependent renal failure	0	1 (1.4%)	1.00
Hypertension	46 (62.2%)	43 (58.1%)	0.858
Coronary artery disease	7 (9.5%)	14 (18.9%)	0.127
Heart failure within 2 weeks	53 (71.6%)	51 (68.9%)	0.480
Atrial fibrillation	26 (35.1%)	33 (45.2%)	0.157
Ejection fraction (%)	60 [57-63]	60 [55-63]	0.532
MR (moderate/severe)	72 (97.3%)	72 (97.3%)	1.00
Mitral stenosis	6 (8.1%)	11 (14.9%)	0.166
TR (moderate/severe)	13 (17.6%)	19 (25.7%)	0.083
Ischemic mitral disease	2 (2.7%)	2 (2.7%)	1.00
Endocarditis mitral disease	4 (5.4%)	3 (4.1%)	0.655
Rheumatic mitral disease	7 (9.5%)	13 (17.6%)	0.058
Degenerative mitral disease	63 (85.1%)	59 (79.7%)	0.103
Operative Characteristics
Prior cardiac surgery	3 (4.1%)	6 (8.1%)	0.317
Elective	68 (91.9%)	67 (90.5%)	0.564
Mitral Repair	59 (79.7%)	51 (68.9%)	0.103
Neochord use	26 (44.2%)	9 (17.7%)	0.002
Tricuspid valve repair	6 (8.1%)	8 (10.8%)	0.564
Atrial fibrillation surgery	22 (29.7%)	34 (46.0%)	0.064
Left atrial appendage ligation	6 (8.1%)	29 (39.2%)	0.0001
Ablation	22 (29.7%)	25 (33.8%)	0.602
Cross clamp time (min)	101 [90-114]	77 [58-102]	<0.0001
CPB time (min)	155 [135-170]	108 [81-146]	<0.0001
Conversion	0	0	–

CLD = chronic lung disease; MR = mitral regurgitations; TR = tricuspid regurgitations; CPB = cardiopulmonary bypass

Most outcomes were similar between groups with one mortality in the conventional group (Table 2) and similar rates of major morbidity (9.5% vs 10.8%, p=0.78). However, mini-MVR was associated with significantly fewer transfusions (11% vs 27%, p=0.01).

Table 2.

Matched operative outcomes Mini Conventional

Operative Outcomes	Mini (n = 74)	Conventional (n = 74)	p value
Operative mortality	0 (0%)	1 (1.4%)	0.316
Major morbidity†	7 (9.5%)	8 (10.8%)	0.782
Permanent stroke	1 (1.4%)	0 (0%)	0.316
Cardiac arrest	0 (0%)	1 (1.4%)	0.316
Atrial fibrillation	15 (20.3%)	17 (23.0%)	0.670
Pneumonia	1 (1.4%)	2 (2.7%)	0.564
Prolonged ventilation	4 (5.4%)	6 (8.1%)	0.480
Renal failure requiring dialysis	1 (1.4%)	1 (1.4%)	1.00
Renal failure	1 (1.4%)	2 (2.7%)	0.564
Deep sternal wound infection	0	0	–
Transfusion	8 (10.8%)	20 (27.0%)	0.014
Transfusion (pRBC)	6 (8.1%)	12 (16.2%)	0.134
Reoperation for any reason	4 (5.4%)	4 (5.4%)	1.00
Reoperation for bleeding	2 (2.7%)	1 (1.4%)	0.564

^†Major morbidity includes: permanent stroke, cardiac arrest, renal failure, deep sternal wound infection, prolonged ventilation, reoperation for any reason

Resource utilization was largely balanced between groups with no significant difference in length of stay, discharges to a facility, or rates of readmission (Table 3). Mini-MVR was associated with significantly fewer hours ventilated (3.7 vs 6.0, p<0.0001). Operative times were longer for mini-MVR (291 vs 234 min, p<0.0001), and translated to marginally higher operative costs ($7645 vs $7293, p=0.85). However, mini-MVR was associated with significantly lower blood and ancillary costs (both p=0.001). With offsetting costs the total hospital costs were similar, as represented by the median ($40,237 vs 38,649) and mean total hospital costs ($49,703 vs $54,970). The distribution of costs are demonstrated in Figure 1.

Table 3.

Matched resource utilization Mini Conventional

Resource Utilization	Mini (n = 74)	Conventional (n = 74)	p value
Operative time (min)	291 [266-312]	234 [204-268]	<0.0001
Postoperative LOS (days)	5.5 [4-7]	5.5 [4-8]	0.443
ICU LOS (hours)	47.5 [24.5-73.7]	46.0 [24.7-102.5]	0.640
Time ventilated (hours)	3.7 [0-5.8]	6.0 [4.4-9.9]	<0.0001
Discharge to facility	7 (9.6%)	12 (16.2%)	0.225
Readmission	7 (9.6%)	3 (4.1%)	0.157
Total cost	$49703 ± 51501	$54970 ± 70078	0.235
Surgical costs	$7645 ± 3314	$7293 ± 4600	0.0001
Implant costs	$1148 ± 4014	$748 ± 2114	0.029
Accommodations costs	$16253 ± 24722	$19369 ± 27797	0.342
Pharmacy costs	$3314 ± 7345	$4669 ± 14511	0.635
Cardiac diagnostic costs	$1417 ± 1235	$1857 ± 1880	0.188
Radiology costs	$1258 ± 1714	$1289 ± 2256	0.720
Laboratory costs	$2621 ± 3510	$2968 ± 3368	0.343
Blood costs	$383 ± 1558	$1058 ± 3762	0.001
Dialysis costs	$188 ± 1605	$558 ± 3552	1.00
Supply costs	$13527 ± 5631	$12314 ± 6246	0.032
Ancillary services costs	$1645 ± 4466	$2652 ± 5937	0.001
Other costs	$283 ± 1611	305 ± 1417	0.672

LOS = length of stay; ICU = intensive care unit;

^*All times are presented as median (interquartile range); all cost data are presented as mean ± standard deviation; all p-values represent nonparametric paired analyses.

Figure 1.

Total hospital costs by approach with median and interquartile range.

To better understand the drivers of total hospital cost, multivariable linear regression calculated the independent costs associated with surgical approach, mitral repair versus replacement and addition of tricuspid and ablation procedures (Table 4). Neither minimally invasive approach (−$273, p=0.98) nor ablation (−1423, p=0.90) were significant predictors of total hospital cost, while mitral replacement ($37,966, p=0.001) and tricuspid repair ($40,428, p=0.024) were independently associated with total hospital cost.

Table 4.

Linear regression for total hospital cost Estimate 95% Confidence Limits p-value

	Estimate	95% Confidence	Limits	p-value
Mini-MVR	−$273	−19389	18843	0.978
Mitral repair	−$37966	−60361	−15571	0.001
Tricuspid repair	$40428	5506	75351	0.024
Ablation	−$1423	−23033	20186	0.897
Year	$13683	3573	23793	0.008

Mini-MVR = minimally invasive mitral valve surgery

DISCUSSION

This study assessed the comparative effectiveness of mini-MVR versus conventional approach on outcomes and resource utilization in a representative population that included non-degenerative mitral disease, atrial fibrillation surgery and tricuspid repairs. Outcomes were excellent in both groups with a low mortality and morbidity (0.7% and 10%, respectively). Mini-MVR was associated with fewer transfusions and shorter duration of mechanical ventilation, which translated into lower blood and ancillary costs. These cost savings are offset by longer operative times leading to higher surgical costs with mini-MVR. With offsetting costs the total hospital costs are equivalent for mini-MVR and conventional approach. Other resource utilization metrics including discharge to a facility and readmission are low in both groups.

Minimally invasive approaches to mitral valve surgery are currently expanding into higher risk and more heterogeneous patient populations. However, studies have mainly focused on repair of degenerative mitral disease.[19–21] Therefore, this study sought to analyze a representative cohort of patients to determine outcomes and resource utilization in this real world higher risk, more complex patient population. While this study failed to demonstrate superior morbidity and mortality results with mini-MVR, this was primarily due to excellent outcomes in both groups. Finding a difference when the mortality rate is <1% will prove difficult no matter the study sample size. The most significant finding was a 41% reduction in blood transfusions. These reductions are consistent with previous studies examining isolated mitral valve surgery, suggesting a strong correlation that persists in a higher risk cohort.[1, 2, 5, 6, 23]

Component cost analysis demonstrates lower transfusion rates resulting in lower blood costs, a $675 median savings for mini-MVR patients. Meanwhile, shorter ventilator times resulted in decreased ancillary costs, a $1,007 median savings. Meanwhile, longer operative times and specialized supplies led to $352 higher median surgical costs and $400 higher median implant costs. Although they marginally increased costs, the longer operative, cardiopulmonary bypass, and cross-clamp times did not translate into worse complication rates.

Looking at composite total hospital cost, the median total hospital cost was equivalent between groups although it is important to closely at the numbers. The high variance and skewed nature of the cost data resulted in the mean hospital cost being $4,788 lower in the mini-MVR group and the median $2,025 higher in the mini-MVR group. The multivariable analysis sheds some insight into these numbers where tricuspid surgery and mitral replacement were significant predictors of total hospital cost. Previous research has demonstrated higher costs with mitral replacement versus repair, and this study found $37,000 associated with mitral replacement.[21, 25] Tricuspid surgery was also a strong predictor of cost in our model and accounted for over $40,000 of total hospital cost. This may be in part due to collinearity with other risk factors as well as both being markers for a more complex patient at higher risk for complications, which is consistent with prior analyses.[25] Prior work has demonstrated that complications after surgery drive much of the cost variation and high cost outliers.[26–29] Finally, costs increased significantly per year and supports prior work demonstrating that changing practices may have improved outcomes, but these interventions are costly.[30]

This study is limited by its retrospective nature with inherent selection bias. This was limited using statistical methodology with careful propensity score matching. Additionally, while based on surgeon training the repair techniques should be similar there was not a protocol enforcing identical methods. As a single center study at an academic center with thoracic residents performing much of the operations, the generalizability may be limited. The cost conversions are estimates based on charges that may be influenced by the case mix of the hospital and does not include physician charges, thus may not represent true cost for each patient. However, both direct and indirect cost estimates for charges are routinely updated by a dedicated costing department providing high quality and accurately categorized estimates. Finally, the low sample size limits the power of the study, although significant correlations that were identified suggest sufficient power for the limited conclusions that were drawn.

As clinical practice adapts to the increasing risk of mitral valve patients, studies should follow suit and include a representative cohort of complex mitral disease and concomitant procedures. In this analysis, we demonstrate as patient complexity and risk increases, a minimally invasive approach continues to provide benefits such as reduced risk for transfusions and less time ventilated. These translate into lower associated costs with only marginally higher offsetting surgical costs that ultimately means equivalent total hospital cost. Importantly, drivers of cost are tricuspid valve surgery, mitral replacement, complications and changing practices over time. Minimally invasive mitral surgery provides patients with excellent outcomes and resource utilization even in higher risk, more complex patients.

Central Picture.

Mini-MVR operative setup with cannulation, port and retractor placement (72/90)

Video.

Download video file ^{(317.4MB, mp4)}

This is a video of a representative minimally invasive mitral valve repair.

Central Message.

In a real-world cohort, minimally invasive approach provides excellent outcomes and resource utilization. Character limit: 108/200

Perspective Statement.

Despite availability of minimally invasive approaches for decades, utilization remains low. While barriers include a learning curve and need for a physician champion, this study demonstrates cost and efficacy should not be concerns. Even in higher risk patients such as tricuspid repairs and non-degenerative mitral disease, mini-MVR provides excellent outcomes with equivalent resource utilization.

Abbreviations

CLD

chronic lung disease

CPB

cardiopulmonary bypass

ICU

intensive care unit

LOS

length of stay

mini-MVR

minimally invasive mitral valve surgery

MR

mitral regurgitations

STS

Society of Thoracic Surgeons

TR

tricuspid regurgitations