Ann Thorac Surg 2005;79:1915-1920
© 2005 The Society of Thoracic Surgeons
Original article: Cardiovascular
Small "Functional" Size after Mechanical Aortic Valve Replacement: No Risk in Young to Middle-Age Patients
Alfonso Penta de Peppo, MDa,
Jacob Zeitani, MDb,*,
Paolo Nardi, MDb,
Giuseppe Iaci, MDb,
Patrizio Polisca, MDb,
Ruggero De Paulis, MDb,
Luigi Chiariello, MDb
a Department of Cardiac Surgery, Second University of Naples, Naples, Italy
b Department of Cardiac Surgery, University of Rome Tor Vergata, Rome, Italy
Accepted for publication December 10, 2004.
* Address reprint requests to Dr Zeitani, Department of Cardiac Surgery, Università di Roma Tor Vergata, European Hospital, Via Portuense 700, Rome, 00149 Italy (E-mail: zeitani{at}hotmail.com).
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Abstract
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BACKGROUND: The impact of a valve prosthesis-patient size mismatch is still controversial. In most studies, the inclusion of a large proportion of poorly active old patients with low cardiac output requirements may be misleading, due to the close correlation between trans-prosthetic gradients and cardiac output. The aim of this study was to assess the impact of small "functional" prosthesis sizes in active young to middle-age patients.
METHODS: Eighty-three active patients with a mean age of 46 ± 8 years and a high health survey questionnaire score were followed for 80 ± 34 months after isolated aortic valve replacement with a mechanical prosthesis.
RESULTS: Patients with an indexed, Doppler derived, effective orifice area index less than 0.85 cm2/m2 (0.77 ± 0.1 cm2/m2) showed higher early trans-prosthetic gradients (peak, 34 ± 11 vs 26 ± 8 mm Hg; P = 0.001) than patients with a larger effective orifice area index. However, significant regression of the left ventricular mass index and improvement of the left ventricular ejection fraction were observed in both groups at follow-up (119.8 ± 26 vs 165.2 ± 38 g/m2 and 128.5 ± 25 vs 181.8 ± 5 0 g/m2; P < 0.001; 58 ± 6 vs 52 ± 11% and 58 ± 7 vs 53 ± 10%; P < 0.001), with no differences between groups (P = 0.4 and P = 0.7, respectively). At multiple linear regression, the final left ventricular mass index was positively related to the preoperative left ventricular mass index (P = 0.004) and was unaffected by the effective orifice area index (P = 0.4). Symptomatic improvement (New York Heart Association class 1.3 ± 0.4 vs 2.4 ± 0.8 and 1.2 ± 0.4 vs 2.2 ± 0.8; P < 0.001) and freedom from late cardiac death (93 ± 3% and 95 ± 6%) were comparable between groups (P = 0.6 and P = 0.7, respectively).
CONCLUSIONS: Our findings indicate that small "functional" prosthesis sizes with modern mechanical valves may not adversely affect outcomes of aortic valve replacement in young patients with high cardiac output requirements.
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Introduction
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Aortic valve replacement has become a standard procedure in valve disease by improving symptoms, survival, and accomplishing regression of left ventricular hypertrophy. However, it is well recognized that prosthetic valves present higher impedance to the left ventricular outflow tract than the native aortic valve [1]; therefore a prosthesis-patient mismatch, a definition first introduced by Rahimtoola [2], may occur if the effective prosthetic area is consistently less than that of a normal valve. Doppler echocardiographic measurement of the in vivo effective orifice area (EOA), indexed by the body surface area, or namely, the effective orifice area index (EOAi), provides a reliable estimation of the relation between prosthesis size and body size [3], and represents the individual "functional" prosthesis size. High trans-prosthetic gradients [4], persistent left ventricular hypertrophy [5], and reduced survival [6–7] have been reported in patients with small EOA indices. Nevertheless, clinical occurrence of mismatch with modern highly efficient mechanical valves is still being debated, as other studies on left ventricular mass (LVM) regression [8–9] and survival [8, 10] fail to confirm these findings. However, most observations have been made in a series of patients showing a wide range of ages, with a mean age often greater than 65 years. Indeed, trans-prosthetic pressure gradients correlate well with increasing stroke volumes as during exercise [11], which may therefore enhance the effects of a prosthesis-patient mismatch in physically active patients. Hence, in most series, variability of the impact of mismatch on postoperative outcomes might have been due to the low cardiac output requirements of inactive older patients.
The purpose of the present study was to assess the influence of the functional prosthesis sizes on trans-prosthetic pressure gradients, on the extent of LVM regression, and on survival in active young to middle-age patients with small EOA indices.
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Material and Methods
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Patients
Seven hundred six patients were operated on for first isolated aortic valve disease from September 1991 to February 2002; eighty-three patients aged less than 60 years (mean, 46 ± 8 years) who were undergoing valve replacement with a mechanical prosthesis for a stenotic or mixed valve lesion were selected for the investigation. Postoperatively, surviving patients attended the outpatient clinic regularly. Physical activity at follow-up was assessed by means of "physical functioning" and "vitality, energy, or fatigue" dimensions of the short form-36 (SF-36) health survey questionnaire to generate a response from 0 to 100 in each dimension [12], and this was compared with that of 78 older patients (mean age, 71 ± 4 years) who underwent isolated aortic valve replacement on the same dates at 60 years of age. The effective orifice area index was calculated for each patient from the projected prosthesis in vivo EOA, obtained from literature sources [13–18] (Table 1), divided by the body surface area, as indicated by Pibarot and colleagues [19]. This index was considered an expression of the individual functional prosthesis size; a possible mismatch was defined as an EOAi less than 0.85 cm2/m2, a generally accepted criteria for a prosthesis size-patient body size mismatch [20], and was classified as a grouping variable to divide the patients into two study groups as shown in Table 2. Due to the arbitrary definition of the 0.85 cutoff value, the EOAi was also considered as a continuous variable and influence of its entire range of values on the extent of LVM regression analyzed in a multivariate prediction model and as indicated in the data analysis section.
Surgical Technique
After midline sternotomy and heparin administration, cardiopulmonary bypass was established with mild systemic hypothermia; myocardial protection was achieved by tepid blood cardioplegia. The prostheses were implanted by using interrupted 2-0 polyester sutures, reinforced with subannular Teflon felt pledgets.
Echocardiography
M-mode, 2-dimensional, and Doppler echocardiographic studies were performed with a Sonos 2500 or Sonos 5500 machine (Hewlett-Pachard, Andover, MA), interfaced with a 2.5 MHz transducer. Standard views were obtained by only two sonographers, and measurements were made according to the recommendations of the American Society of Echocardiography preoperatively, just before discharge, and at the last follow-up. The LVM was calculated by using the American Society of Echocardiography cube formula (see Appendix).
Data Analysis
All statistical analyses were performed under the SPSS system for Windows, version 10.4 (SPSS, Inc, Chicago, IL). Comparisons of baseline variables between groups were performed by the t test, chi2 test, or the Fisher exact test, as appropriate. Two-way repeated measures of analysis of variance were used to assess the influence of time and EOAi on LVM index and left ventricular ejection fraction. The EOA was also considered as a continuous variable and was included, with several patient and prosthesis related factors, in a multiple linear regression model to identify predictors of the final LVM index at follow-up. The possible predictors analyzed were gender, age, pure native aortic valve stenosis, prosthesis size (manufacturer’s labeled size), expressions of prosthesis-patient size (the EOAi and the indexed geometric internal area), preoperative LVM index, preoperative left ventricular ejection fraction, operative aortic clamp time and cardiopulmonary bypass time, and early peak and mean trans-prosthetic gradients. Survival and freedom from cardiac death were computed by means of the Kaplan-Meier method; the log-rank test was used to compare estimates. Variables are presented as mean ± 1 standard deviation. A probability (P) value less than 0.05 was considered statistically significant.
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Results
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The mean follow up time was 80 ± 34 months (5 to 153 months). Scoring of the health survey questionnaire showed a more favorable physical state in the study patients compared with the older patients in the physical activity control group (physical functioning score, 80 ± 12 vs 53 ± 20; p < 0.001; vitality, energy, or fatigue score, 65 ± 12 vs 39 ± 20; p < 0.001). In the study patients, the group with EOA indices less than 0.85 cm2/m2 showed smaller manufacturer’s labeled prosthesis size and geometric internal orifice area index (Table 2); no other preoperative and operative differences were noted between groups. Types of prosthesis were the 49 Carbomedics Bileaflet (CarboMedics Inc, Austin, TX), the 13 St. Jude Standard (St. Jude Medical Inc, St. Paul, MN), the 11 Sorin Bicarbon (Sorin Biomedica, Saluggia, Italy), the 6 St. Jude HP (St. Jude Medical Inc), and the 4 ATS Open Pivot Bileaflet (ATS Medical Inc, Minneapolis, MN); they were equally distributed between groups (P = 0.07). No diameter enhanced prostheses were used. There was no operative mortality and patients were discharged from the hospital after 6 ± 2 days. Early peak and mean trans-prosthetic gradients were significantly higher in the EOAi < 0.85 cm2/m2 group (34 ± 10 vs 26 ± 8 mm Hg; P = 0.001, and 19 ± 8 vs 14 ± 5 mm Hg; P = 0.02, respectively). Marked regression of the LVM index and improvement of left ventricular ejection fraction were observed in both groups (P < 0.001), with no differences between groups (P = 0.4 and P = 0.7, respectively) (Table 3). At multivariate analysis, the final LVM index was closely related to the preoperative LVM index; no other factor, including the EOAi, predicted the final mass (Table 4).
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Table 3. Influence of Time and Effective Orifice Area Index on Study Variables (Two-Way Repeated-Measures of Analysis of Variance)
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Table 4. Prediction Model for the Final Left Ventricular Mass Index, Including the Effective Orifice Area Index as a Continuous Variable
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The New York Heart Association functional class improved significantly at follow-up (1.2 ± 0.4 vs 2.3 ± 0.8; P < 0.001), with no differences between groups (P = 0.6). Overall survival was 92 ± 3%. Causes of late cardiac death were sudden death in 2 patients and chronic heart failure in 1 patient; freedom from cardiac death was not different between the two groups (P = 0.7) (Fig 1).
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Fig 1. Kaplan-Meier freedom from cardiac death for patients with effective orifice area (EOA) indices  0.85 cm2/m2 and < 0.85 cm2/m2. (EOAi = effective orifice area index.)
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Comment
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The impact of age and, therefore, of a various level of physical activity on the extent of myocardial mass regression may be a frequent confounding factor in studies on the consequences of an aortic valve prosthesis-patient mismatch. Indeed, it has been shown that high stroke volumes, as during exercise, increase trans-prosthetic gradients in the noncompliant artificial valve prosthesis [1, 11]. Hence, the influence of a possible mismatch on the extent and pattern of regression of left ventricular hypertrophy and on clinical outcomes may be stronger in physically active young individuals than in older patients with low cardiac output requirements. Indeed, scoring of the "physical dimensions" of the health survey questionnaire in the present study group indicates a more favorable physical activity state compared with older patients. In this context, the wide range of the patients’ ages may have contributed in most investigations to the difference of findings, which makes it difficult to interpret the results. Although study groups in each series are usually comparable by age, the possibility that a various proportion of advanced age patients that may differently influence the impact of a prosthesis-patient mismatch remains. In particular, the reported mean age of patients is frequently between 60 and 70 years, with an often undefined proportion of patients greater than 70 years of age. For example, a significant relationship between the indexed EOA and the extent of mass regression was observed by Del Rizzo and colleagues [21] in patients with a mean age of 69 ± 10 years; González-Juanatey and colleagues [5] also reported impairment of myocardial mass regression 18 months after surgery in patients with small size prosthesis (19 and 21 mm), in study groups with a mean age of 59 years. Opposite observations on the impact of mismatch on mass regression have been reported in other series in which comparisons were made between groups showing mean ages from 62 to 65 [8], a mean age of 70 [22], and mean ages much greater than 70 years [9]. Advanced mean age of patients can also be noted in other studies examining the influence of mismatch on mortality [6, 8, 10] with different results. A large multi-institutional study [23] included 13,258 aortic valve replacements form nine representative sources to quantify the relationship between the indexed internal prosthetic orifice area (expressed by geometric, not functional, dimensions) and survival. The authors observed that indexed areas of less than 1.2 cm2/m2 were associated with a 1% to 2% increase in 30-day mortality, but did not reduce intermediate or long-term survival. Also, analysis for adjustment factors other than prosthesis-patient size showed the strong mortality effect of advanced age. Recently, Ruel and colleagues [24] investigated the outcome of congestive heart failure and mortality in 1,563 patients after aortic valve replacement, adopting the projected EOA reported in the literature to assess the functional prosthesis sizes; as in our study, they found that an indexed EOA of less than 0.85 cm2/m2 was not significantly associated with overall survival or heart failure, although a cutoff value of 0.80 cm2/m2 or less was a significant predictor of congestive heart failure only. Again, both symptoms of congestive heart failure and all causes of mortality were adversely affected by older age.
Therefore our criteria have been adopted in the attempt to better define these aspects avoiding influence of advanced age. The mean age of patients in the present series was 46 ± 8 years, and to our knowledge investigations on the impact of prosthesis-patient mismatch have not been conducted in similar or younger patients. In a previous study [25], we found that regression of myocardial mass 3 years after aortic valve replacement with a mechanical prosthesis was equivalent in small as well as in large prosthetic valve sizes; however, the mean age of patients in that series was greater than 60 years. Findings of the present study seem to confirm that the postoperative outcomes of a physically active young and middle-age population are unaffected by a small prosthesis functional size, at least at mid-term follow-up. However, it has to be considered that the majority of patients presented a potentially moderate to severe prosthesis-patient mismatch, given the average value of 0.77 ± 0.06 cm2/m2 in the group with small EOA indices; therefore, conclusions may not apply to the less frequent cases with severe degrees of mismatch. Nevertheless, we found that regression of the left ventricular mass was equivalent in patients with EOA indices less than 0.85 cm2/m2, the generally accepted criteria to define a prosthesis-patient mismatch in clinical practice [20], as in patients with larger orifice areas. Moreover, a multiple linear regression model including the entire range of EOA indices showed that the EOAi considered as a continuous variable could not predict the final LVM index, and therefore the extent of mass regression. This observation confirms findings of the analysis of variance in the two groups and may better analyze the entire range of values, in particular the values lower than 0.85 cm2/m2 in relation to the extent of mass regression. Of note, peak aortic valve pressure gradients are known to be positively correlated with the left ventricular myocardial mass [26], and therefore the higher peak trans-prosthetic gradients postoperatively observed in the group with smaller orifice areas would be expected to compromise the extent of mass regression. Nevertheless, our observation is consistent with that of Hanayama and colleagues [8] and supports their suggestion that residual abnormal gradients are clinically insignificant compared with the consistent relief of left ventricular overload or afterload, or both, after valve replacement. In line with the pattern of myocardial mass regression and postoperative improvement of symptoms, left ventricular ejection fraction and freedom from cardiac death were comparable between the two study groups.
Some potential limitations have to be considered. The study includes a relatively small cohort of young to middle age patients, only 23 (one third) with an EOAi less than 0.85 cm2/m2, and therefore negative results may be due to a type II error. Power of the study to detect differences in regression of LVM index between groups was nearly 50%. It has to be noted, however, that patients were selected according to several inclusion criteria and represent a particularly homogeneous group, about 12% of patients submitted to isolated aortic valve replacement in a 10-year period; this may enhance clinical significance of results. This aspect also seems to be confirmed by the large number of patients undergoing a first isolated aortic valve replacement (at least 1,700) that would be required, according to the inclusion criteria, to select a sample size of approximately 200 valve replacements needed to achieve 80% power. Nevertheless, the effective orifice area index was also considered as a continuous variable and was included with the other factors in a multiple linear regression model to better define its influence on the final left ventricular mass; this may also enhance significance of our results.
The functional prosthesis size was expressed by means of the Doppler EOA index, which is a clinically estimated measure and varies in different physiological conditions. However, use of the Doppler EOA index for evaluation of the individual prosthetic valve performance has been validated by Dumesnil and colleagues [3], and it has been widely applied to estimate the prosthesis-patient size and a possible mismatch. It could be argued that in our series no individual measurements were performed, but prosthetic EOAs were estimated according to published referent values for the various types of valve prosthesis. Nevertheless, this method has been well described by Pibarot and colleagues [19], who demonstrated that a prosthesis-patient mismatch can be accurately predicted by the preoperatively available projected EOA, obtained from literature sources and indexed by the body surface area of each patient. Indeed, a preoperative estimate of a possible prosthesis-patient mismatch is necessary for intraoperative decision making. The hemodynamic performance of mechanical valves implanted in our patients may vary; nevertheless, all were modern mechanical prostheses with expected minor dynamic differences. Moreover, the valve types were equally distributed in the two groups of patients and therefore this should have not influenced the comparisons between patients. Also, patients were not strictly homogeneous, including pure aortic valve stenosis and mixed valve lesions; however, patients with pure aortic regurgitation were not included in the analysis and types of valve lesion were equally distributed in the two groups.
In conclusion, findings of the present study would seem to suggest that the extent of regression of myocardial mass and clinical outcomes in physically active young and middle age patients after aortic valve replacement are not adversely affected by small EOA indices, despite the high cardiac output requirements of this population. Elevated early trans-prosthetic gradients are often observed in these patients, but do not seem to compromise outcomes.
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Appendix
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Left Ventricular Mass Calculation (American Society of Cardiology Cube Formula)
LVM = 0.000832 [(LVED + IVS + LVPWALL)3 - (LVED) 3] + 0.6 (IVS = interventricular septal wall thickness [expressed in cm]; LVED = left ventricular end-diastolic dimension [expressed in cm]; LVM = left ventricular mass [expressed in g]; LVPWALL = left ventricular posterior wall thickness [expressed in cm].)
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Abstract
Introduction
