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J Thorac Cardiovasc Surg 2007;133:1012-1021
© 2007 The American Association for Thoracic Surgery

Surgery for Acquired Cardiovascular Disease

Association between indices of prosthesis internal orifice size and operative mortality after isolated aortic valve replacement

^a Departments of Surgery and Bioengineering, the University of Pennsylvania Health System, Philadelphia, Pa
^b Duke Clinical Research Institute, Durham, NC
^c Department of Surgery, University of Colorado, Denver, Colo
^d Department of Cardiovascular-Thoracic Surgery, Rush University, Chicago, Ill
^e Department of Surgery, University of Maryland Medical Center, Baltimore, Md
^f Department of Surgery, University of Florida, Jacksonville, Fla.

Read at the Eighty-fifth Annual Meeting of The American Association for Thoracic Surgery, San Francisco, Calif, April 10-13, 2005.

Received for publication April 8, 2005; revisions received November 1, 2006; accepted for publication November 16, 2006.

^_* Address for reprints: Charles R. Bridges, MD, ScD, Department of Surgery, the University of Pennsylvania Health System, Department of Surgery, 4 Silverstein, Hospital of the University of Pennsylvania, Philadelphia, PA 19104. (Email: cbridges{at}pahosp.com).

	Abstract

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Objectives: The appropriate index of prosthesis internal orifice size and its effect on operative mortality after aortic valve replacement are controversial. We examined the association between several relevant indices and patient size on operative mortality. Indices examined included projected in vivo effective orifice area and geometric orifice area, with patient size defined as body surface area.

Methods: A review of the Society of Thoracic Surgeons National Cardiac Database (2000-2004) yielded 48,722 patients who had isolated aortic valve replacement. This analysis is based on the cohort of 42,310 patients with the 8 most prevalent valve types with manufacturer’s labeled sizes 19 mm through 29 mm. Multivariable logistic regression models were employed to determine the effects of body surface area, effective orifice area, geometric orifice area, and selected derived indices (eg, effective orifice area/body surface area) on risk-adjusted operative mortality.

Results: In separate multivariable models, effective orifice area and geometric orifice area were both inversely correlated with operative mortality. However, an unanticipated finding was that with either effective orifice area or geometric orifice area held constant, body surface area was significantly and inversely correlated with operative mortality. When patients were stratified by effective orifice area, geometric orifice area, or manufacturer’s labeled valve size and type, elevations in body surface area were associated with a decrease rather than an increase in operative mortality.

Conclusions: Prostheses with small geometric orifice area or small effective orifice area are associated with increased operative mortality after isolated aortic valve replacement. Even for valves with small effective orifice area, however, mortality decreases as body surface area increases. With respect to operative mortality, therefore, our results do not support using arbitrary cutoff values of effective orifice area/body surface area to determine the valve to utilize in a given patient.

	Introduction

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Dr Bridges

Controversy exists regarding the importance of patient size and prosthesis internal orifice size on both early and late mortality after aortic valve replacement (AVR) surgery.^1-16 We can summarize the controversy as revolving around 4 key issues: (1) How do we define the most hemodynamically relevant index of aortic prosthesis internal orifice size? (2) However defined, is an appropriate, calculable, or measurable index of internal aortic prosthesis size an independent predictor of short-term or long-term morbidity and mortality after AVR? (3) What is the nature of the interaction between the defined index and patient size as predictors of operative mortality? (4) How should the conduct of AVR surgery be influenced—including the selection of an aortic prosthesis for a given patient—by considerations related to patient size and an appropriate index of prosthesis internal orifice size?

The term prosthesis–patient mismatch was first coined by Rahimtoola¹ and has been used extensively in the literature to describe the use of a prosthesis of a given type that is "too small" for a patient of a given size. He defined it to occur when "the effective prosthetic valve area, after insertion into the patient, is less than that of a normal human valve."¹ We now know that with the possible exception of aortic homografts and pulmonary autografts, all bioprosthetic and all mechanical valves are associated with some degree of mismatch by this definition.^3,16 As outlined above, controversy exists as to the most hemodynamically relevant index of prosthesis internal orifice size. It is well appreciated that a manufacturer’s labeled valve size (eg, 19 mm, 21 mm, etc) is not consistently defined across different manufacturers.^17,18 For any given manufacturer, the internal orifice diameter is always less than the manufacturer’s labeled size. Marked differences in internal orifice area (equivalent to geometric orifice area [GOA]) and external diameter exist by manufacturer for mechanical, stented, and stentless bioprostheses. For aortic homografts, the manufacturer’s labeled size is equal to the internal orifice diameter, and for the Toronto stentless porcine valve (St Jude Medical, Minneapolis, Minn) and the Medtronic Hall valve (Medtronic, Minneapolis, MN), it is equal to the external diameter of the valve; for most other mechanical valves, the external diameter is 3 to 5 mm larger than the manufacturer’s labeled size.^17,18

As a result of this confusing nomenclature, most studies that have attempted to relate prosthesis internal orifice size to long-term or short-term mortality have eschewed the use of manufacturer’s labeled valve size as a covariate as it is not a robust predictor (across different valve types and manufacturers) of the functional relationship between pressure gradient and flow for a given aortic valve prosthesis. Rather, indices of prosthesis internal orifice size such as the projected in vivo effective orifice area (EOA) or GOA have been utilized.^3-10,15 More recently, GOA/body surface area (BSA) has been proposed as an index of prosthesis internal orifice size.^3-5 Using GOA/BSA as an index of prosthesis internal orifice size, Blackstone and colleagues³ found that GOA/BSA was not a significant independent predictor of intermediate-term survival and that for mechanical valves but not for bioprosthetic valves, GOA/BSA was a significant but weak predictor of 30-day mortality. In contrast, Blais and colleagues⁶ found that EOA indexed by BSA (EOA/BSA) was a robust multivariable predictor of operative mortality and that patients with EOA/BSA <0.65 cm²/m² had an odds ratio (OR) of 11.4 (4.4-29.5) for operative mortality compared with those with EOA/BSA > 0.85 cm²/m². Furthermore, these authors argue that EOA (projected or estimated in vivo) rather than GOA is the most hemodynamically relevant preoperative index of prosthesis internal orifice size because it is more highly correlated with pressure gradients that occur during left ventricular ejection.^7,8,19 These considerations notwithstanding, others^3-5 have argued that because actual (rather than projected) in vivo EOA varies with exercise, aortic anatomy, and hemodynamics,^20-24 GOA is a more robust index of hemodynamically relevant prosthesis internal orifice size.

Here, in contrast to previous studies,^3-10,15 we do not decide a priori which index of internal prosthesis orifice size to utilize. Rather, we used multivariable models with data derived from the Society of Thoracic Surgeons (STS) National Cardiac Database to assess the significance of both projected in vivo EOA and GOA as independent predictors of operative mortality after isolated AVR. We also determine the independent predictive value of BSA as well as selected indices derived from rational combinations of EOA or GOA and BSA (eg, EOA/BSA) on operative mortality.

	Materials and Methods

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STS Database
The STS National Cardiac Database was established in 1989 to report surgical outcomes following cardiothoracic surgical procedures.²⁵ Sites enter patient data using uniform definitions (available online at http://www.sts.org) and certified software systems. Although participation in the STS database is voluntary, data completeness is high, with overall preoperative risk factors missing in fewer than 5% of submitted cases. The accuracy of submitted cases has been confirmed in independent comparison of hospital coronary artery bypass graft surgery (CABG) volume and mortality rates reported to the STS versus those reported to Centers for Medicare and Medicaid Services.²⁶

Patient Population
Patients in the STS adult cardiac database who had isolated aortic valve replacement surgery with a prosthetic valve from January 1, 2000 to December 31, 2004 were considered for inclusion in the study. Patients having concomitant procedures such as multiple-valve or CABG surgery were excluded, leaving 48,722 patients. The analysis was further restricted to 42,880 patients receiving 1 of the 8 most prevalent valve types in the database (mechanical prostheses: Carbomedics (Austin, Tex), Medtronic-Hall, St Jude Medical; stented bioprostheses: Carpentier–Edwards Pericardial (Edwards Life Sciences, Irvine, Calif), Hancock II Porcine (Medtronic), Medtronic Mosaic Porcine; stentless porcine prostheses: Medtronic, Toronto SPV) and the 6 most prevalent manufacturers’ labeled sizes (19 mm, 21 mm, 23 mm, 25 mm, 27 mm, 29 mm). We excluded the small minority of valves with even manufacturer’s labeled sizes from the analysis. Patients with status coded as "salvage" were excluded (n = 45) as well as subjects with missing data on 6 key variables (age, gender, BSA, body mass index [BMI], height, and weight), leaving a sample of 42,310 patients in 646 hospitals. Patients who received a valve for which EOA or GOA data were unavailable were excluded from multivariable models utilizing that variable. Patient characteristics are summarized in Table 1. Missing values are excluded.

Clinical End Point
The primary end point, operative mortality, is defined as death during the same hospitalization as surgery or after discharge but within 30 days of surgery. Additional data definitions are available at www.sts.org.

Multivariable Analyses
Several logistic regression models were developed to assess the association between surrogates of patient size (BSA) and prosthesis internal orifice size (EOA or GOA) and operative mortality while adjusting for covariates. In the first set of analyses, variables relating to patient size and prosthesis internal orifice size included: EOA, EOA², 1/BSA, 1/BSA², EOA/BSA, EOA²/BSA². We also performed parallel analyses by substituting GOA for EOA in the list of variables. The quadratic terms (EOA² or GOA²) allow the estimated relationship between prosthesis internal orifice size and mortality to be nonlinear on the log-odds scale. The ratio terms EOA/BSA and EOA²/BSA² (or GOA/BSA and GOA²/BSA²) allow the shape of the relationship between prosthesis internal orifice size and mortality to depend on the patient’s body size. Conversely, these ratio terms allow the shape of the relationship between BSA and mortality to depend on the valve’s estimated internal orifice size. We also fit these models without including the ratio terms to estimate the average association between internal orifice size and mortality averaging over body size. Additional variables in each model were: patient age (modeled as a 2-phase linear function with a change of slope at 75 years), diabetes, renal failure, dialysis, hypertension, cerebrovascular accident, infective endocarditis, chronic lung disease, peripheral vascular disease, reoperation, New York Heart Association (NYHA) class IV, pulmonary hypertension, myocardial ischemia within 3 weeks, status (elective, urgent, emergent), preoperative intra-aortic balloon pump, gender, ejection fraction less than 40, aortic stenosis, aortic insufficiency, congestive heart failure, cerebrovascular disease, arrhythmia, nonwhite race, obesity (BMI > 35 kg/m²), and year of surgery. An assumption of the model (made for simplicity) is that valve-related variables do not interact with patient factors other than BSA. We tested this assumption and found that it did not have a significant impact on the results. Substituting BSA for 1/BSA in the regression models also had little impact on the results.

Inferences about the association between indices of prosthesis internal orifice size and mortality are potentially confounded by several factors including: (1) unmeasured intrinsic valve characteristics peculiar to each valve model and (2) measurement error in the assessment of a valve’s projected EOA or GOA. Two different approaches were used to adjust for these sources of confounding. First, we fit the logistic regression models by including fixed-effect indicator variables for each valve model. This approach adjusts for any valve-related factors that might impact mortality through mechanisms unrelated to the valve’s internal orifice size. This is appropriate because the risk of mortality could potentially vary by valve model, even among valve models having the same internal orifice size. In these analyses, parameters describing the "effect" of EOA or GOA on mortality pertain to differences in manufacturers’ labeled valve size when the valve model is held constant. Second, we fit random effects logistic regression models that included random effects for each combination of manufacturer model and labeled size. The inclusion of random effects allows for an unobserved valve-level component specific to each combination of model and label size. These random effects capture any valve-level factors that were omitted from the model that might systematically increase or decrease the risk of mortality among patients receiving the same valve model and label size. In addition, this approach accounts for potential measurement error in the estimation of average EOA and GOA by subsuming the true EOA and GOA values into the random effects component. The results of both modeling approaches were similar with respect to both point estimates and confidence intervals (CIs). For brevity, only the fixed effect analyses are presented. These analyses were first performed using all patients in the study population and were subsequently repeated in the subpopulation of patients having BMI < 30 and in patients who received manufacturers’ labeled valve sizes 19 or 21.

We summarized the results of the multivariable models by presenting ORs along with their asymptotic 95% CIs. These ORs describe the relative increase or decrease in the estimated odds of mortality as a function of valve internal orifice size (EOA or GOA) compared with a reference size of EOA = 2.0 cm² or GOA = 5.0 cm². We also displayed the model results graphically, by plotting estimated mortality risk as a function of valve internal orifice size (EOA or GOA) and body size (BSA). For these plots, "risk-adjusted mortality" is defined as the predicted probability of mortality for a patient having the indicated values of EOA or GOA and BSA and a hypothetical baseline risk of mortality loosely corresponding to "average risk." The baseline risk was determined by requiring that the average risk-adjusted mortality rate (obtained by averaging across all of the observed values of EOA, GOA, and BSA in the study population) be equal to the observed mortality rate in the study population.

	Results

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Patient Characteristics
The characteristics of our patient population are summarized in Table 1. Fifty-eight percent of our patients were men, 89% were white, and most had a normal ejection fraction (72% were greater than 50%). Despite the relatively preserved left ventricular function, over 50% had significant heart failure symptoms (NYHA class III or IV). Hypertension was common (63%); roughly one third of the valves implanted were mechanical and two thirds were tissue valves.

Distribution of Valve Types
In Tables 2 and 3 we summarize the GOA and EOA values used and the manufacturers and types of valves included in the analysis as well as their respective frequencies in the data set. St Jude Mechanical prostheses and Carpentier Edwards pericardial prostheses together represent 70% of the valves in the data set.

View larger version (19K): [in this window] [in a new window]   Figure 1. Estimated risk-adjusted mortality rate as a function of BSA and EOA. BSA, body surface area; EOA, effective orifice area.

View larger version (9K): [in this window] [in a new window]   Figure 2. Estimated association between BSA and risk-adjusted mortality as a function of EOA. , EOA = 1.2 cm2; •, EOA = 1.4 cm2; , EOA = 1.6 cm2; , EOA = 1.8 cm2. BSA, body surface area; EOA, effective orifice area.

View larger version (9K): [in this window] [in a new window]   Figure 3. Estimated association between BSA and risk-adjusted mortality as a function of GOA. , GOA = 2.0 cm2; •, GOA = 3.0 cm2; , GOA = 4.0 cm2; , GOA = 5.0 cm2). BSA, body surface area; GOA, geometric orifice area.

	Discussion

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The notion of prosthesis–patient mismatch implies that for a fixed valve model and manufacturer’s labeled valve size, and holding all other factors constant, patients with larger BSA will have worse outcomes than patients with small BSA. In fact, we found no evidence of worse outcomes among patients with larger BSA after stratification by prosthesis internal orifice size as defined by EOA or GOA. On the contrary, larger BSA (ie, lower EOA/BSA) was generally associated with lower operative mortality regardless of the valve model and manufacturer’s labeled valve size inserted. This finding is consistent with the interpretation that unmeasured confounder variables that lead to a protective effect of increasing BSA appear to be more important determinants of operative mortality than what others have called "mismatch."

When BSA is held constant, we found that mortality appears to increase with decreasing values of EOA. Although it is also true that mortality increases as a function of EOA/BSA (treating BSA is a constant), we did not find compelling statistical evidence to support using EOA/BSA as a criterion for valve selection. In fact, because the ratio term EOA/BSA is nonsignificant in a model that includes EOA and 1/BSA as main effects, there is no evidence to conclude that the relationship between EOA and mortality depends on the patient’s BSA. Therefore, our data do not provide support for the convention of measuring EOA/BSA and using specific cutoffs in an effort to decrease operative mortality.

Taking a balanced interpretation of our results, it is important to note that our results also cannot exclude a significant causal mechanism related to hemodynamics ("mismatch") in a subset of patients. The lack of interaction between indices of prosthesis internal orifice size and BSA may be due to measurement error, the use of imperfect proxy variables, or unmeasured confounder variables. Nonetheless, it appears that the potential negative effects of poor hemodynamic performance of valves with small EOA or GOA may be overwhelmed by the operative mortality advantage of larger BSA. We can speculate that the apparent independent protective effect of increasing BSA may simply reflect relative ease of valve implantation in patients with larger aortic annuli and an attendant decrease in technically related complications. We did not find any clear cutoff values for EOA that are acceptable or unacceptable. Other than the dependence on EOA, we also did not find a consistent independent relationship of the type of valve by manufacturer on operative mortality. Our data neither refute nor confirm the contention^3-5 that GOA, because of its robustness, is preferable to projected in vivo EOA as a practical preoperative index of prosthesis internal orifice size.

	Conclusions

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Unlike our own analysis, Blais and colleagues⁶ included patients who had AVR combined with CABG. Because we limited ourselves to patients with isolated AVR, we cannot exclude the possibility that in patients with impaired left ventricular function³⁵ or coronary artery disease, prosthesis-patient mismatch may be a clinically important phenomenon, hypothetically as a result of concerns related to the energy supply-demand ratio. We would predict that this might occur more commonly in large patients (high energy demand) with small prostheses (greater myocardial energy loss with ejection) when combined with flow-limited oxygen delivery to the left ventricular myocardium due to both prolonged global ischemia and residual coronary ischemia after revascularization. Furthermore, our study provides no insight into the potential negative impact of aortic prostheses with small internal orifice area on left ventricular mass regression or on long-term mortality.^36,37

In the version of the STS software available during this study, the type of St Jude valve was not specified, and the EOA values used were those for standard (rather than high profile [HP] or Regent) models. The Regent model was not even available during the first 2 years of our study interval, and it likely was used in a very small proportion of our study group patients. In other studies,³ the HP model represented only 6% of St Jude mechanical valves. Therefore, we estimate that approximately 90% of the patients did in fact have the standard model St Jude valve. We repeated the analysis excluding all St Jude valves (Tables E5 and E6) and the conclusions did not change.

Earn CME credits at http://cme.ctsnetjournals.org

 

	Footnotes

 
The Society of Thoracic Surgeons through the Adult National Cardiac Database and the Duke Clinical Research Institute supported this work.

	References

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