Discongruence Index　― Simple Indicator to Predict Prosthesis-Patient Mismatch After Transcatheter Aortic Valve Replacement ―

Abstract

Background: Prosthesis-patient mismatch (PPM) after transcatheter aortic valve replacement (TAVR) remains an important issue. The aim of this study was to assess the value of a new discongruence index, to predict PPM after TAVR.

Methods and Results: A total of 185 patients with severe aortic stenosis who underwent TAVR with the Edwards Sapien prosthesis or CoreValve Revalving system were included (Edwards valve, n=119; Core Valve Revalving system, n=66). Discongruence index was calculated pre-procedurally as the ratio of selected transcatheter valve size (mm) to body surface area (cm²). PPM was defined as effective orifice area (EOA) ≤0.85 cm²/m² on transthoracic echocardiography before hospital discharge. Mean age was 82±5 years and 72 patients (38.9%) were men. The overall incidence of post-TAVR PPM was 35.1% (n=65). Discongruence index correlated with post-TAVR indexed EOA (y=0.18+0.057x; P<0.001). On multivariate logistic regression analysis, discongruence index was the only independent predictor of post-TAVR PPM (OR, 0.15; 95% CI: 0.03–0.66; P=0.012), and the area under the receiver operating characteristic curve was 0.62 (95% CI: 0.54–0.70, P=0.003), with an optimal cut-off point of 15.02 (sensitivity, 86.2%; specificity, 72.5%; positive predictive value, 74.3%; negative predictive value, 83.4%).

Conclusions: The new discongruence index may be useful tool to predict PPM after TAVR.

Transcatheter aortic valve replacement (TAVR) is an established therapeutic option for patients with aortic stenosis considered to be at high or prohibitive surgical risk.^1–3 Valve prosthesis-patient mismatch (PPM) occurs when the effective orifice area (EOA) of the prosthetic valve is too small relative to body size.^4,5 Its principal consequence is the presence of high gradients through correctly functioning valves. Despite a lower incidence of PPM after TAVR compared with surgical replacement, it is still an important problem because it affects morbi-mortality.⁶ Thus, the implementation of preventive strategies to avoid post-TAVR PPM is necessary, particularly in high-risk patients (i.e., those with large body surface area [BSA]) or those susceptible to the effect of PPM, such as in the case of severe left ventricular (LV) hypertrophy, reduced LV ejection fraction (LVEF), and paradoxical low-flow severe aortic stenosis.⁷ The currently available options for post-TAVR PPM prevention, however, are limited, and it is crucial to carefully select prosthesis type and size. The aim of the current study was therefore to assess the value of a new discongruence index to predict PPM after TAVR.

Methods

Subjects

From March 2012 to November 2016 we included consecutive patients with symptomatic severe aortic stenosis who had successfully undergone TAVR with balloon expandable Edwards Sapien XT prostheses (Edwards Lifesciences, Irvine, CA, USA) or CoreValve Revalving system (CRS, Medtronic, Minneapolis, MN, USA) at the present center. Patients with severe symptomatic aortic stenosis were considered as candidates for TAVR if they had a logistic European System for Cardiac Operative Risk Evaluation score (EuroSCORE) >20%; if surgery was deemed to be of excessive risk due to significant comorbidities; or if other risk factors not captured by these scoring systems (e.g., porcelain aorta) were present. Eligibility for TAVR was established on the consensus of a local multidisciplinary heart team, including clinical cardiologists, interventional cardiologists and cardiac surgeons. An exclusion criterion was inadequate echocardiographic acoustic apical window. The new discongruence index was defined as selected transcatheter valve size (mm)/BSA (cm²). Transcatheter valve size refers to the external dimension from the manufacturer, not the internal geometric orifice area. BSA was calculated using the DuBois formula (BSA [cm²]=weight [kg]^0.425×height [cm]^0.725×71.84). The initial full sample consisted of 196 patients. Five patients were excluded because they did not undergo postoperative echocardiogram, and 6 due to the presence of poor acoustic window, resulting in a final sample of 185 patients. Transfemoral access was used in all patients. All patients gave written informed consent according to the approved protocol by the institutional review board.

Echocardiography

Transthoracic echocardiography was performed at baseline and before discharge by experienced sonographers using a commercially available ultrasound machine (iE33; Philips Healthcare, Amsterdam, The Netherlands; and Artida; Toshiba Medical Systems, Tokyo, Japan). A complete 2-D, color, pulsed and continuous-wave Doppler echocardiogram was performed according to the current guidelines.⁸ The severity of aortic stenosis was evaluated using the EOA obtained with the continuity equation, and the mean aortic gradient.⁹ LV volumes and EF were obtained using the Simpson biplane method. Specifically, all echocardiographic measurements were averaged from 5 consecutive beats in patients with atrial fibrillation. All echocardiographic parameters were calculated offline using Xcelera (Philips Health Care, Andover, MA, USA). All procedures were monitored on intraprocedural 3-D transesophageal echocardiography using an iE33 ultrasound system with a fully sampled matrix array transducer (X7-2t Live 3-D transducer, Philips Medical Systems, Andover, MA, USA). The prosthesis size was selected according to the average annulus diameter, area and perimeter, which were measured systematically on multislice computed tomography.¹⁰ These measurements were performed in compliance with currently available recommendations.^11–13 Post-TAVR paravalvular aortic regurgitation (AR) was evaluated using color Doppler and graded as mild (1/4), mild-moderate (2/4), moderate-severe (3/4) or severe (4/4) according to the sum of the cross-sectional vena contracta area on short-axis view. An integrative, semiquantitative approach was used to assess the severity of central, paravalvular, and total regurgitation.¹⁴

Definition of Post-TAVR PPM

The presence of PPM was assessed before hospital discharge. The aortic annulus measurement was performed in parasternal long-axis view.¹⁴ The diameter of the LV outflow tract (LVOT) was measured 5–10 mm from the aortic annulus in mid-systole. For LVOT measurements, pulsed-wave Doppler was used, and for transaortic measurements continuous-wave Doppler was used. The EOA was obtained from the continuity equation, and it was indexed to BSA. In patients with atrial fibrillation, 5 measurements were averaged. For the analysis, patients were divided into 2 groups according to the presence of PPM. PPM severity was graded using the indexed EOA, defining moderate as ≥0.65 and ≤0.85 cm²/m², and severe as <0.65 cm²/m².¹⁵

Statistical Analysis

Continuous variables are expressed as mean±SD. Categorical variables are presented as absolute number or percentage. For continuous variables, significant differences between groups were analyzed using Student’s t-test. Chi-squared test (when all expected cell counts were >5) or Fisher’s exact test (when any expected cell count was <5) was used to assess the differences in categorical variables. Linear regression analysis was used to assess the relationship between 2 continuous variables. Univariate and multivariate logistic regression analysis was performed to determine predictors of post-TAVR PPM. Receiver operating characteristic (ROC) curves were used to determine the predictive value of the different parameters. Box plots were created to illustrate the distribution of the post-TAVR indexed EOA in the 3 groups (no, moderate, or severe PPM). Differences were accepted as statistically significant at P<0.05. SPSS 18.0 (SPSS, Chicago, IL, USA) was used for the statistical analysis.

Results

Patient Data

One hundred and eighty-five consecutive patients who received the Edwards valve (n=119) or the CoreValve Revalving system (n=66) by transfemoral approach at the present institution were included. Patient clinical baseline characteristics are listed in Table 1. Mean age was 82±5 years and 72 patients (38.9%) were men. Median EOA at baseline was 0.61±0.17 cm² (indexed, 0.35±0.08 cm²/m²) and mean gradient was 49±19 mmHg. Mean aortic annulus diameter was 2.2±0.25 cm. Mean EF was 56±17%. A 23-mm prosthesis was used in 127 patients (68.6%); a 26-mm prosthesis in 36 (19.5%); a 29-mm prosthesis in 20 (10.8%); and a 31-mm prosthesis in 2 (1.1%). As expected, TAVR produced a significant improvement in EOA (1.7±0.45 cm², P<0.001), indexed EOA (0.99±0.28 cm²/m², P<0.001), and mean transaortic gradient (9.3±4.5 mmHg, P<0.001). On linear regression analysis, discongruence index correlated with post-TAVR indexed EOA (y=0.18+0.057x; P<0.001; Figure 1). After TAVR, there was a significant improvement in LVEF (from 56±17 to 63±11%, P<0.001). No AR was seen in 80 patients (43.2%); mild AR (1/4) in 51 (27.6%); mild-moderate AR (2/4) in 39 (21.1%); and moderate-severe AR (3/4) in 7 (3.8%). Severe AR (4/4) was not seen.

Table 1. Subject Clinical Characteristics vs. PPM After TAVR

Clinical characteristics	Overall (n=185)	No PPM (n=120)	PPM (n=65)	P-value
Age (years)	82.5±5.7	82.1±5	83.1±5	0.26
Gender (male)	72 (38.9)	47 (39.2)	25 (38.5)	0.92
Weight (kg)	71.3±12.2	69.2±12	75.3±10	0.03
Height (cm)	160.2±9.2	159.3±9	161±7	0.09
BSA (m2)	1.74±0.2	1.71±0.2	1.79±0.2	0.02
BMI (kg/m2)	27.8±4.3	27.2±4.3	28.8±4.1	0.04
Logistic EuroSCORE	17.1±9.9	17.5±10.1	16.21±9.5	0.39
Previous CAD	82 (44.3)	50 (41.7)	32 (49.2)	0.32
Hypertension	151 (81.6)	94 (78.3)	57 (87.7)	0.11
Hypercholesterolemia	110 (59.5)	72 (60.0)	38 (58.5)	0.83
Diabetes	56 (30.3)	39 (32.5)	17 (26.2)	0.37
Smoking	39 (21.1)	26 (21.7)	13 (20.0)	0.79
AF	61 (33.0)	40 (33.3)	21 (32.3)	0.88

Data given as mean±SD or n (%). AF, atrial fibrillation; BMI, body mass index; BSA, body surface area; CAD, coronary artery disease; EuroSCORE, European System for Cardiac Operative Risk Evaluation score; PPM, prosthesis-patient mismatch; TAVR, transcatheter aortic valve replacement.

Figure 1.

Linear regression plot of discongruence index vs. post-transcatheter aortic valve replacement (post-TAVR) indexed effective orifice area (EOA).

Post-TAVR PPM

The overall incidence of post-TAVR PPM was 35.1% (n=65), assessed before hospital discharge. Moderate PPM (indexed EOA 0.65–0.85 cm²/m²) was present in 54 patients (29.2%), and severe PPM (indexed EOA <0.65 cm²/m²) was seen in 11 patients (5.9%; Figure 2). Table 1 lists the baseline clinical characteristics according to PPM status. Larger body weight, BSA and body mass index (BMI) were associated with a higher incidence of PPM. Table 2 lists echocardiographic characteristics at baseline and post-TAVR according to PPM status. Patients with PPM had a significantly lower discongruence index (13.5±1.5 vs. 14.3±1.8, P=0.002). Nevertheless, there were no statistically significant differences in other variables, including valve type, prosthesis size, native annulus diameter, EOA, gradient, EF, or the presence of bulky calcifications. As expected, after TAVR, patients with PPM had a significantly higher peak gradient (21.80±11.4 vs. 15.65±5.8, P<0.001), mean gradient (5.7±0.71 vs. 3.2±0.29, P<0.001), and lower EOA (1.3±0.17 vs.1.9±0.40, P<0.001), and indexed EOA (0.72±0.07 vs. 1.14±0.25, P<0.001). Significant improvement in LVEF was noted after TAVR between patients with and without PPM but this was not significant (2.71±14.4% vs. 5.62±16.6%, P=0.26). Table 3 lists the univariate and multivariate predictors of post-TAVR PPM. On univariate analysis BSA (OR, 13.5; 95% CI: 2.9–23.1; P=0.02), body weight (OR, 1.04; 95% CI: 1.01–1.07; P=0.03), BMI (OR, 1.09; 95% CI: 1.01–1.17; P=0.04), baseline mean gradient (OR, 0.97; 95% CI: 0.95–0.99; P=0.02), interventricular septum thickness (OR, 1.25; 95% CI: 1.001–1.58; P=0.049), and discongruence index (OR, 0.75; 95% CI: 0.62–0.91; P=0.004) were significantly associated with post-TAVR AR. Multivariate logistic regression analysis was carried out including the following variables: BSA, baseline mean gradient, predilatation, interventricular septum thickness, baseline EF, and discongruence index. On this multivariate logistic regression analysis only discongruence index was an independent predictor of post-TAVR PPM (OR, 0.15; 95% CI: 0.03–0.66; P=0.012). The area under the ROC curve for discongruence index (Figure 3) was 0.62 (95% CI: 0.54–0.70, P=0.003), with an optimal cut-off point of 15.02 (sensitivity, 86.2%; specificity, 72.5%; positive predictive value, 74.3%; negative predictive value, 83.4%) to predict post-TAVR PPM.

Figure 2.

Distribution of prosthesis-patient mismatch (PPM) after transcatheter aortic valve replacement according to indexed effective orifice area (EOA). The box for each group represents IQR (25–75th percentile) and the line in the box is the median. Bottom and top whiskers indicate the 10th and 90th percentiles, respectively.

Table 2. Echocardiography Characteristics vs. PPM After TAVR

	Overall (n=185)	No PPM (n=120)	PPM (n=65)	P-value
Baseline
Peak gradient (mmHg)	81.4±25	83.7±27.7	77.4±22.2	0.12
Mean gradient (mmHg)	49±1	50±7	45±5	0.07
EOA (cm2)	0.61±0.17	0.60±0.14	0.63±0.17	0.20
Indexed EOA (cm2/m2)	0.35±0.08	0.35±0.08	0.35±0.09	0.59
Aortic annulus diameter (cm)	2.2±0.25	2.2±0.24	2.2±0.25	0.57
Large calcifications (>5 mm)	44 (23.8)	26 (33.8)	18 (35.3)	0.85
IVS thickness (mm)	15.5±3.8	14.2±3.6	17.1±3.6	0.03
Indexed LVM (g/m2)	148.7±3.8	140.3±45.6	158.9±59.2	0.33
LVEF (%)	56±17	57±14	61±13	0.11
After TAVR
Peak gradient (mmHg)	17.8±8.7	15.65±5.8	21.80±11.4	<0.001
Mean gradient (mmHg)	9.3±4.5	3.2±0.29	5.7±0.71	<0.001
EOA (cm2)	1.7±0.45	1.9±0.40	1.3±0.17	<0.001
Indexed EOA (cm2/m2)	0.99±0.28	1.14±0.25	0.72±0.07	<0.001
LVEF (%)	62.9±11	62.5±13	63.7±9	0.5
Prosthesis size (mm)	24.3±2.1	24.3±2.1	24.2±2.2	0.73
Prosthesis type (Edwards)	119 (64.3)	74 (61.6)	45 (69.2)	0.57
Discongruence index	14.1±1.7	14.3±1.8	13.5±1.5	0.002

Data given as mean±SD or n (%). EOA, effective orifice area; IVS, interventricular septum; LVEF, left ventricular ejection fraction; LVM, left ventricular mass. Other abbreviaions as in Table 1.

Table 3. Predictors of Post-TAVR PPM

	OR	95% CI	P-value
Univariate logistic regression analysis
Age (years)	1.03	0.97–1.08	0.26
Weight (kg)	1.04	1.01–1.07	0.03
Height (cm)	1.03	0.99–1.06	0.09
BSA (m2)	13.5	2.9–23.1	0.02
BMI (kg/m2)	1.09	1.01–1.17	0.04
Aortic annulus diameter (cm)	1.03	0.91–1.17	0.56
Baseline mean gradient (mmHg)	0.97	0.95–0.99	0.049
Baseline EOA (cm2)	4.4	0.44–44.7	0.20
Large calcifications (>5 mm)	1.053	0.54–2.02	0.87
Predilatation	1.88	0.92–3.87	0.083
IVS thickness (mm)	1.25	1.001–1.58	0.049
Indexed LVM (g/m2)	1.007	0.993–1.022	0.32
Baseline LVEF (%)	1.02	0.99–1.04	0.12
Prosthesis size (mm)	0.97	0.84–1.12	0.73
Prosthesis type	0.87	0.67–1.12	0.29
Discongruence index	0.75	0.62–0.91	0.004
Multivariate logistic regression analysis
BSA (m2)	9.5	0.94–17.4	0.11
Baseline mean gradient (mmHg)	1.03	0.87–1.22	0.67
Predilatation	3.07	0.12–8.46	0.23
IVS thickness (mm)	1.43	0.93–2.16	0.09
Baseline LVEF (%)	1.13	0.98–1.30	0.08
Discongruence index	0.15	0.03–0.66	0.012

Abbreviaions as in Tables 1,2.

Figure 3.

Receiver operating characteristic curve for prediction of post-transcatheter aortic valve replacement prosthesis-patient mismatch using the discongruence index.

Discussion

This study demonstrates that a simple index (discongruence index) is the best parameter to predict post-TAVR PPM. As far as we know, this subject has never been studied before. In the present study PPM was relatively common, occurring moderately in 29.2% and severely in 5.9% of the TAVR patients. These data are similar to those in previous studies, ranging from 22% to 40%, and are slightly lower than after conventional open aortic valve replacement.^15–17 Thus, PPM remains an important issue in the TAVR era and preventive strategies are needed, particularly in patients at high risk. Discongruence index correlated with post-TAVR indexed EOA, and the proposed cut-off point (15.02) had a high sensitivity (86.2%) to predict post-TAVR PPM. Taking this new index into consideration, prevention of PPM may be possible with the selection of larger transcatheter valve sizes in patients with discongruence index <15.

TAVR is now a widely accepted intervention for patients with severe aortic stenosis who are deemed to have prohibitive or high surgical risk, and it is included as such in the American and European treatment guidelines.^1–3 Optimization of patient selection and device implantation, however, is imperative to improve prognosis. Echocardiography plays an important role in measuring aortic annulus dimension in patients undergoing TAVR. This is important because it determines both eligibility for TAVR and selection of prosthesis type and size, and can be potentially important in preventing PPM. Generally, to minimize PPM and paravalvular regurgitation, it is suggested that the transcatheter valve be slightly larger than the aortic annulus.

PPM was first described by Rahimtoola in 1978,⁴ and occurs when the prosthetic valve is too small relative to the patient’s body size, causing a high transvalvular pressure gradient through a normally functioning prosthetic valve. PPM is defined as severe when the indexed EOA is <0.65 cm²/m² and moderate for indexed EOA 0.65–0.85 cm²/m².¹⁸ After conventional surgery, PPM has been described to range from 20% to 70%, being severe in 2–28% of patients.^15–17 Some patient characteristics have been shown to have a higher risk of PPM, such as advanced age, larger body size and aortic valve stenosis as the predominant lesion.¹⁹ Although the influence of PPM on short- and long-term survival is still controversial, there is considerable evidence that severe PPM has a detrimental effect on outcome.^20–22

Compared with surgical aortic valve replacement, TAVR has better hemodynamic performance, associated with a reduction in PPM.^6,15,23,24 On randomized comparison of patients from the PARTNER trial A cohort, TAVR was associated with lower prevalence of PPM (46% vs. 60%) and of severe PPM (20% vs. 28%), compared with surgical aortic valve replacement.⁶ The difference in severe PPM was particularly important in patients with small aortic annuli (<20 mm).^6,25 The lower incidence of PPM in the TAVR series compared with the surgical series may be partly explained by the distention of the aortic annulus, the absence of a sewing ring, and a thinner transcatheter stent frame, causing less obstruction to blood flow. Furthermore, surgical prosthesis size is usually smaller than that of TAVR prostheses. The Edwards SAPIEN valve is currently available in 20, 23, 26, and 29 mm. The CoreValve device is available in 23, 26, 29, and 31 mm. For surgical aortic valve replacement, prostheses of 19 or 21 mm are available, and the incidence of PPM increases with smaller prosthesis size.^23,26 Some authors suggest that TAVR may be contemplated in patients with a small aortic annulus, given that these patients have increased risk of PPM after surgical aortic valve replacement.^6,25

Despite a lower incidence, the concern about the occurrence of PPM after TAVR remains an important problem. A recent analysis from the PARTNER trial supported the relationship between severe PPM and increased mortality,⁶ pointing to the need to implement preventive strategies to avoid post-TAVR PPM.⁷ The currently available options for post-TAVR PPM prevention, however, are limited. With regard to TAVR, when additional surgical procedures (e.g., aortic root enlargement) are not available, it is crucial to carefully select the patients, prosthesis type and size. To avoid the occurrence of PPM, selection of larger prosthesis size and the improvement of valve design are needed. Nevertheless, excessive oversizing has to be balanced against the risk of aortic rupture. The current study has demonstrated that the new and easy-to-calculate discongruence index may be useful in the selection of appropriate prosthesis size, preventing PPM after TAVR.

Study Limitations

Some limitations of the present study must be mentioned. This was a retrospective, descriptive study in a single center. We included recipients of the Edwards valve or the CoreValve because this cohort reflected our clinical experience. The cut-off value and the diagnostic accuracy of the discongruence index in the prediction of PPM could differ depending on the type of valve used. This, however, was not the main purpose of this study, and the statistical power was not high enough to demonstrate that difference due to the limited cohort size. We think that this issue should be better explored in future studies that specifically analyze the differences between these 2 types of valves. A larger patient group might have improved the strength of the present study. Early echocardiography after a significant intervention may skew hemodynamic results due to a hyperdynamic state. It would be expected, however, that the stent frame dimensions and hemodynamics have stabilized before discharge. Furthermore, later echocardiography introduces the added confounder of potential valve deterioration. This study lacked clinical evaluation of the post-TAVR outcomes and the association with PPM.

Conclusions

Despite a lower incidence, PPM is relatively common in the TAVR era and it remains an important issue. Preventive strategies to avoid post-TAVR PPM are needed, particularly in patients at high risk. The new discongruence index may be a useful tool to predict PPM after TAVR. Preventing the occurrence of PPM after TAVR may be possible with the selection of larger valve sizes, taking this new index into consideration.

Disclosures

The authors declare no conflicts of interest.

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