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Anesth Analg 2006;103:800-808
© 2006 International Anesthesia Research Society
doi: 10.1213/01.ane.0000231890.95212.12

CARDIOVASCULAR ANESTHESIA

Section Editor:
Charles W. Hogue JrSection Editor Martin J. London

The Impact of Gender on In-Hospital Mortality and Morbidity After Isolated Aortic Valve Replacement

Andra Ibrahim Duncan, MD*, Jia Lin, MD, PhD*, Colleen G. Koch, MD, MS*, A. Marc Gillinov, MD{dagger}, Meng Xu, MS{ddagger}, and Norman J. Starr, MD*

From the Departments of *Cardiothoracic Anesthesia, {dagger}Cardiothoracic Surgery, and {ddagger}Quantitative Health Sciences, Cleveland Clinic Foundation, Cleveland, Ohio.

Address correspondence to Andra Ibrahim Duncan, MD, Staff, Department of Cardiothoracic Anesthesia, Cleveland Clinic Foundation, 9500 Euclid Avenue/ G30, Cleveland, OH 44195. Address e-mail to duncana{at}ccf.org.


   Abstract
Top
 Abstract
Introduction
 METHODS
 RESULTS
 DISCUSSION
 References
 
The objective of our retrospective investigation was to examine the influence of gender on in-hospital mortality and morbidity after isolated aortic valve replacement (AVR). Between January 1993 and June 2002, 2212 patients (782 females, 1430 males) underwent AVR. Propensity matching was used to adjust for numerous differences in baseline characteristics and perioperative variables between groups. Unadjusted in-hospital mortality was higher in females (27 [3.5%] females versus 23 [1.6%] males; P = 0.005). An analysis using 1:1 matching by propensity score did not find a significant difference in in-hospital mortality [OR (95% confidence intervals), 1.0 [0.4, 2.6]; P = 0.99) or overall morbidity (1.4 [0.7, 2.5]; P = 0.29) between groups. Further analyses, including classification of women and men into quintile groups by propensity scores and logistic regression models with propensity score adjustment, found that females were at increased risk for cardiac morbidity [OR (95% CI), 3.4 [1.1, 10.8]; P = 0.038), but not mortality (0.9 [0.3, 2.5]; P = 0.88) nor other morbidities. These results suggest that there is no greater than a 2.5-fold increase in risk for females compared with males undergoing AVR. Female gender, however, may impart increased risk for cardiac morbidity after AVR.


   Introduction
Top
 Abstract
 Introduction
METHODS
 RESULTS
 DISCUSSION
 References
 
Women undergoing coronary artery bypass grafting (CABG) are reported to be at increased risk for postoperative morbidity and mortality (1,2). This increase in risk in women is related, in part, to gender differences in preoperative risk profiles (3). Other explanations include smaller body size and/or smaller diameter of coronary arteries, which are associated with greater technical difficulties and risk for early graft closure (1,2,4). Others have suggested that referral bias may account for gender differences in mortality after CABG (5).

The performance of aortic valve replacement procedures (AVR) has increased because of growth in the size of the elderly population, greater awareness of valvular heart disease, and improvement in diagnostic tools. In addition, outcomes after cardiac surgery continue to improve. Thus, older patients and those with more advanced disease are being referred for AVR. To optimize care for the greater number of patients who will undergo this procedure, it is essential to investigate and define patient characteristics that may affect outcome after AVR. Reports investigating gender-related outcomes after AVR have yielded varying results, many of which are confounded by inclusion of concomitant procedures (6–9) or limited sample size (10). Moreover, the impact of gender on outcomes is often not the primary focus of these investigations. Furthermore, few investigations use propensity modeling, which allows one to adjust for a large number of confounding variables without the risk of over-fitting a model (such as with traditional logistic regression analysis). Finally, the impact of gender on development of cardiac, prolonged endotracheal intubation, and neurologic and renal morbidities after AVR is unclear. The objective of this investigation was to determine whether female gender was an independent risk factor for in-hospital mortality and multisystem morbidity after isolated AVR.


   METHODS
Top
 Abstract
 Introduction
 METHODS
RESULTS
 DISCUSSION
 References
 
The study population included 2212 patients (782 female, 1430 male) who underwent isolated AVR between January 1, 1993 and June 30, 2002 at the Cleveland Clinic. Patient data were obtained from the Cardiothoracic Anesthesia Patient Registry of the Department of Cardiothoracic Anesthesia and the Cardiovascular Information Registry of the Department of Thoracic and Cardiovascular Surgery at the Cleveland Clinic. Research use of these registries was approved by the IRB. Data for the Registry have been collected prospectively on all patients who have undergone cardiothoracic surgery in our institution. Medical and demographic data collected include information regarding preoperative comorbidities, intraoperative factors, and postoperative morbidity and mortality. Variables selected for this analysis are listed in Tables 1 and 2. Patients who received more than one AVR during this period were excluded from this analysis. Thus, each patient appeared only once in the data set.


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  		Table 1. Baseline and Operative Characteristics for the Unmatched Patient Population

 


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  		Table 2. Baseline and Operative Characteristics for Unmatched Patients

 
Outcome variables, which have been described previously (12), include 1) mortality (all-cause in-hospital mortality); 2) cardiac morbidity (combination of postoperative myocardial infarction and/or low cardiac output with a requirement for intra-aortic balloon pump, ventricular assist device, or extracorporeal membrane oxygenation). A diagnosis of postoperative myocardial infarction required specific findings on postoperative electrocardiogram (new Q waves ≥40 ms, R wave ≥25%), laboratory findings (creatine phosphokinase myocardial band ≥50 IU or aspartate aminotransferase ≥80 U/L) with documentation of myocardial infarction by a physician. Low cardiac output is defined as cardiac index <1.8 L/min/m2 despite adequate intravascular fluid replacement and high-dose inotropes for >4 h; 3) neurologic morbidity [new postoperative focal (aphasia, motor deficit, or hemiparesis based on clinical assessment and/or computed tomographic scan) or global neurologic deficit (diffuse encephalopathy with more than 24 h of severely altered mental status, and/or failure to regain consciousness postoperatively); 4) prolonged intubation (duration of endotracheal intubation >72 h); 5) renal morbidity [postoperative anuria or oliguria (urine output <400 mL/24 h) and/or institution of renal dialysis or ultrafiltration]; 6) infection morbidity (culture-proven pneumonia, mediastinitis, wound infection, or septicemia with associated clinical findings); and 7) overall morbidity (incidence of one or more of the above morbidities or mortality, since early death precludes observation of morbidity).

All available patients were included in this analysis (782 females, 1430 males). A power analysis found that inclusion of all 1430 males and a 2:1 ratio with females (715) could detect an odds ratio of 2.2 on mortality with 80% power assuming a mortality rate in males of 3.5% (13–15).

Univariate gender comparisons for preoperative demographics, comorbidity, laboratory values, and operative factors were made with {chi}2, Exact unconditional, and Wilcoxon's ranked sum test where appropriate. Before propensity matching, a parsimonious explanatory model was developed whereby variables found to be significantly associated with gender were identified. A propensity score was calculated for each patient from a logistic model that included 38 baseline and perioperative variables (Tables 1 and 2) (16,17). Patients missing any data elements were excluded from analysis. The C-statistic for the propensity logistic regression model was 0.95. Patients were matched 1:1 on propensity scores with greedy matching techniques (18). Morbidity and mortality outcomes were compared between matched pairs with {chi}2 and Exact unconditional tests where appropriate.

Because 550 of 782 (70%) females were unable to be 1:1 matched on propensity scores, we performed propensity quintile analysis on all 2212 patients. Patients were classified into five quintile groups according to propensity scores. In theory, variables included in the propensity score calculation will be balanced in each quintile. Outcome variables were compared between female and males in each quintile. Preoperative and intraoperative variables used in propensity score modeling are found in Tables 1 and 2. The inverse of preoperative creatinine and Log of cardiopulmonary bypass time were used instead of the original value to meet the linear relationship assumption.

Logistic regression models were used to compare female and male on outcome variables after propensity score adjustment. All results were analyzed with SAS 8.2 software (SAS Institute Inc., Cary, NC).


   RESULTS
Top
 Abstract
 Introduction
 METHODS
 RESULTS
DISCUSSION
 References
 
Risk Profiles
Tables 1 and 2 list gender distributions for baseline and operative variables for the patient population. There were significant gender differences in the preoperative profiles. Women were older with smaller body surface areas and more preoperative hypertension, heart failure, pulmonary hypertension, chronic obstructive pulmonary disease, diabetes mellitus, and a larger percentage of New York Heart Association (NYHA) Class III and IV than male patients. Females also had lower preoperative hematocrit, serum creatinine, albumin, and bilirubin levels and more often had isolated aortic stenosis, whereas isolated aortic regurgitation was more common in men. History of left ventricular dysfunction, prior cardiac surgery, vascular procedures, and smoking were more common in men. Regarding intraoperative factors, women had shorter cardiopulmonary bypass and aortic cross-clamp times, received more perioperative red blood cell and blood component transfusions, more bioprosthetic valves, and smaller valve sizes (measured by indexed orifice area, internal valve area, and labeled prosthesis size), and more aortic annular enlargement procedures. Allografts were implanted more often in men.

Outcomes and Gender
Overall in-hospital mortality was 2.3%, and cardiac, prolonged tracheal intubation, neurologic, renal, infection, and overall morbidities were 1.4%, 4.9%, 2.5%, 1.2%, 2.5%, and 7.9%, respectively. Univariate analysis for unmatched patients demonstrated that in-hospital mortality (27/782 [3.5%] versus 23/1430 [1.6%]; P = 0.005), cardiac (20 [2.6%] versus 12 [0.8%]; P = 0.001), prolonged tracheal intubation (50 [6.4%] versus 59 [4.1%]; P = 0.019), neurologic (27 [3.5%] versus 29 [2.0%]; P = 0.041), and overall morbidity (79 [10.1%] versus 95 [6.6%]; P = 0.004) were significantly higher in females compared with males, respectively. Postoperative renal (14 [1.8%] versus 13 [0.9%]; P = 0.071) and infection morbidities (25 [3.2%] versus 31 [2.2%]; P = 0.14) trended higher in females but were not statistically significant.

Propensity matching resulted in 232 matched female and male pairs with similar distribution of continuous and categorical baseline variables (Tables 3 and 4). No significant difference was found between matched male and female patients in in-hospital mortality, cardiac, prolonged tracheal intubation, renal, neurologic, infection, or overall morbidity (Table 5).


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  		Table 3. Baseline Characteristics and Perioperative Variables for the Propensity-Matched Pairs

 

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  		Table 5. Postoperative Morbidity and Mortality Outcomes for the Propensity-Matched Pairs

 


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  		Table 4. Baseline and Operative Variables for the Propensity-Matched Pairs

 

Matched females compared with unmatched females were younger, with larger body surface areas, more aortic insufficiency, and worse left ventricular function. Matched females were more likely to smoke and have previous cardiac surgery but less hypertension. Matched females also had higher hematocrits, serum creatinine, and bilirubin. Cardiopulmonary bypass and aortic cross-clamp times were longer in matched females. Matched females received larger implanted valve sizes, more allografts, less bioprosthetic valves, and less packed red blood cell transfusions. Baseline and operative characteristics are compared between matched and unmatched female patients in Online Appendices 1 and 2 (please see supplemental tables at www.anesthesia-analgesia.org)

The distribution of preoperative and intraoperative variables for quintile groups are summarized in Online Appendices 3 and 4 (please see supplemental tables at www.anesthesia-analgesia.org). Most preoperative and intraoperative variables were balanced between females and males across quintiles. Outcome comparisons among women and men within each quintile are listed in Online Appendix 5 (please see supplemental tables at www.anesthesia-analgesia.org). Mortality and neurologic morbidity outcomes were similar among women and men in all quintiles. A significant difference was found between males and females in 1 of 5 quintiles for cardiac (P = 0.006), renal (P = 0.024), prolonged tracheal intubation (P = 0.007) and overall morbidity (P = 0.003).

Males and females were compared by logistic regression models (Table 6). Females were found to have increased risk of cardiac morbidity [OR (95% CI), 3.4 (1.1,10.8); P = 0.038]. However, no significant gender-related differences were found for in-hospital mortality, prolonged tracheal intubation, renal, neurologic, infection, or overall morbidities.


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  		Table 6. Comparison Using Logistic Regression Models on Outcomes of Female Versus Male After Adjustment for Propensity Score and Body Surface Area

 


   DISCUSSION
Top
 Abstract
 Introduction
 METHODS
 RESULTS
 DISCUSSION
References
 
Women undergoing AVR are older and have markedly different preoperative risk profiles than male patients. Although unadjusted in-hospital mortality and multiple organ system morbidity rates after AVR are significantly higher in females, our investigation did not find more than a 2.5-fold increase in risk for females undergoing AVR. To exclude a smaller difference in risk, a much larger sample size is needed. For example, a sample size of 8000 patients undergoing isolated AVR is needed to exclude a 1.5-fold increase in risk for women. Female gender, however, may impart increased risk for cardiac morbidity after AVR.

Large variability in mortality rates has been reported after AVR surgery (Table 7). Overall, mortality rates have decreased over time. This is likely related to advancements in surgical techniques, improvements in perioperative care, and the use of cardioplegia (19). Several reports demonstrate higher mortality because of their focus on a specific subset of patients at increased risk because of advanced age (14), inclusion of concomitant procedures (7,20), or reoperations (21). Most reports include a combination of isolated AVR and AVR-CABG procedures. Fewer reports, including the present investigation, examine specifically isolated AVR.


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  		Table 7. A Comparison of Reported Gender-Related Outcomes After AVR

 

Many cardiac surgical procedures, notably CABG, have been examined for gender-associated risk. Unadjusted mortality rates for women after CABG have been reported to be higher than for men (2,22). However, adjustment for small body size and smaller diameter of coronary arteries in females decreases the difference in mortality (1,2,4). Smaller coronary arteries in females may pose greater technical difficulties during surgery or increase risk for early graft closure. In addition, older age and markedly different preoperative risk profiles contribute to gender-related differences in postoperative outcomes (3) and higher mortality for females (22). Indeed, when preoperative profiles of women and men undergoing CABG were matched on propensity scores, female gender was not associated with increased mortality (3). Referral bias resulting in older age and more advanced disease when presenting for CABG may explain the increased risk for adverse outcomes in females (5). Others have reported that risk factors affect men and women undergoing CABG differently and that women carry a higher mortality for each individual risk factor (23).

Issues similar to those associated with CABG procedures may also influence gender-related outcomes after AVR. For example, small body size has been proposed as a risk factor for adverse outcome in females undergoing AVR because of receiving smaller prosthetic valves (8,11), more frequent aortic annular enlargement procedures, (8) greater technical difficulty associated with smaller anatomy, or complications related to cardiopulmonary bypass, such as hemodilution or transfusion requirements. In addition, similar to females undergoing CABG, females who present for AVR are older and have vastly different preoperative risk profiles, which may adversely affect their outcome. Moreover, risk factors may affect males and females undergoing AVR differently. Further, referral bias may be present, resulting in older age (8) or advanced stage of disease (24) when AVR is performed, similar to females presenting for CABG. Our investigation found that females were older at the time of AVR, with more heart failure and worse NHYA classification, than men. Certainly, older age and more severe disease increase the risk for mortality (8,25).

Despite these gender-related issues, our investigation did not find more than a 2.5-fold increase in risk for mortality in females undergoing AVR. Although our analysis included more than 2000 patients, this sample size was not large enough to exclude a smaller difference in risk. Other reports of gender-related outcomes after AVR have yielded varying results (Table 7). Female gender was reported to be a risk factor for adverse outcome after performance of AVR with concomitant procedures (7,24) and for patients undergoing AVR-CABG (11,14,21, 26). In contrast, many reports of isolated AVR found no gender-related risk for mortality after isolated AVR (Analysis by the Society of Thoracic Surgeons Adult Cardiac Database provided by the Society of Thoracic Surgeons and the Duke Clinical Research Institute) (13,25,27). It is possible that females are at higher risk for AVR-CABG because of gender-related issues associated with CABG but not for AVR alone. Alternatively, relatively small sample sizes in these investigations may not have been able to detect a difference. Furthermore, the use of dissimilar determinants of outcome and different statistical modeling may have produced varying results.

The influence of gender on long-term outcomes after AVR is controversial. Worse long-term survival has been reported in both women (10) and men (19). Several issues associated with female gender are thought to increase risk for late-term mortality. For example, females often receive smaller-sized prosthetic valve sizes, which are associated with larger transvalvular gradients and less postoperative regression of left ventricular hypertrophy (28). Indeed, postoperative regression of left ventricular mass was less among females and those with smaller prosthetic valves (29), and small valve size was associated with worse long-term outcome (30). In contrast, others found that patients with smaller prosthetic valves developed normal left ventricular mass after AVR (31) and were not at higher risk of worse long-term outcome (32). Nevertheless, gender-related factors do affect the adaptive and recovery response of the left ventricle to pressure and volume overload. Greater improvement in ejection fraction was seen in females after AVR, although this improvement conveyed a similar survival benefit to both women and men (8). Preoperative diastolic dysfunction, which may be more common in women because of the greater degree of left ventricular hypertrophy seen in females, independently predicted late mortality after AVR (33).

Females may be at increased risk for cardiac morbidity. Gender differences in the pattern of left ventricular hypertrophy may explain this increase in risk. Females with aortic stenosis develop greater left ventricular wall thickness relative to chamber size, lower wall stress, and higher ejection fraction than males (34,35). Our observations of better preserved systolic function in females were consistent with known gender-related patterns of left ventricular hypertrophy. However, improved systolic function and greater left ventricular hypertrophy in females may be associated with suboptimal myocardial preservation, coronary flow abnormalities (36), or postoperative diastolic dysfunction (37), which may increase risk for cardiac morbidity.

The "gold standard" for comparing outcomes after cardiac surgery is a properly designed randomized controlled trial. However, "gender" is not a randomizable variable. Thus, a randomized controlled trial to determine gender's effect on outcome after AVR cannot be performed, and thus, multivariable analyses were developed to achieve balance of confounding variables in observational studies. The propensity score (16) is a balancing score, calculated by identifying factors predictive of being female. When patients are matched on propensity score, all measured risk factors are equal between groups, akin to a randomized, controlled trial. Direct comparisons can be made between groups with similar characteristics, except for gender. Thus, "apples-to-apples" comparisons can be achieved (38). Our investigation used propensity scores for patient matching, stratification, and multivariable adjustment. Matching by propensity score matched each female to one male with a similar propensity score. This eliminates problems of matching on multiple variables by compressing all variables into a single score. Although this method is well-validated, our sample size was significantly decreased (only 30% of females were matched). Consequently, propensity quintile analysis was performed, where propensity score was used to subdivide patients into quintiles. This resulted in groups with similar characteristics where outcomes could be compared within quintiles, and all patients could be included in the analysis. In addition, a multivariable analysis of outcome with propensity score adjustment was performed, where the propensity score adjusts the apparent influence of the comparison variable of interest for patient selection differences.

There are limitations related to our statistical modeling. Propensity methods are limited by potential for omitted or unmeasured variables that may bias the outcome results (16) and inextricable confounding (38), which occurs when treatment difference versus another difference cannot be distinguished, because they are inextricably intertwined. Clearly, propensity score analysis is not a substitute for a properly designed, randomized, controlled investigation. However, a propensity analysis is more versatile and more widely applicable than randomized trials because many factors, including gender, are not randomizable. Importantly, reliability of the results of all multivariable analyses depends on the fact that risk factors have a similar impact on males and females. If this impact differs between genders (22), propensity methodology and other multivariable analyses techniques may obscure a gender-related effect.

Our investigation has other limitations. As a result of its retrospective nature, the clinical data on the patients may be incomplete, and determinants of outcome may not have been captured. In addition, inherent bias may be associated with data analysis from a single academic institution. However, a large, consecutive series from a single institution involving patients undergoing the same operation may avoid many of the variables that occur in inter-institutional analysis. Methods for detecting morbidities in our investigation captured only the most severe complications and were insensitive to less severe outcomes. The possibility that risk factors may have a different impact on females than males was not evaluated in this investigation. Finally, evolution and advances in surgical techniques, valve technology, and patient care over the 10-year study period may have resulted in time-related bias in outcomes for patients treated early versus late during the period of study.

In summary, the risk profile of female patients undergoing AVR is significantly different from that of men. Although the unadjusted mortality and morbidity rate was higher in females, patient matching for baseline characteristics and comorbid conditions did not find female gender to impart more than a 2.5-fold increased risk for in-hospital mortality after AVR. Female gender may be associated with increased risk for cardiac morbidity.


   ACKNOWLEDGMENTS
 
The authors wish to acknowledge Eugene H. Blackstone, MD, Staff, Department of Thoracic and Cardiovascular Surgery and Quantitative Health Sciences, Cleveland Clinic Foundation, Cleveland, Ohio, for his expertise in statistical analysis, and Marvin J. Leventhal, MS, Department of Cardiothoracic Anesthesia, and the Department of Cardiothoracic Anesthesia Data Collection Group, Cleveland Clinic Foundation, Cleveland, Ohio, for data collection and retrieval.


   Footnotes
 
This article has supplementary material on the Web site: www.anesthesia-analgesia.org.

Accepted for publication May 23, 2006.

Supported, in part, by the Department of Cardiothoracic Anesthesia, Cleveland Clinic Foundation.


   References
Top
 Abstract
 Introduction
 METHODS
 RESULTS
 DISCUSSION
 References
 

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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press