The elderly are one of the fastest growing segments of American society. Individuals 65 years of age and older represent approximately 15% of the United States (US) population and they are projected to represent 26% by 2025.1 An epidemiologic parallel to the aging of society is an increasing rate of obesity in the general population, particularly among the elderly.2,3 Current estimates suggest an overall obesity prevalence rate of approximately 15% to 20% among the elderly.2 The relevance of aging and obesity to the morbidity and mortality affecting vascular surgery patients remains undefined.
Obesity is closely linked with many chronic diseases including diabetes mellitus (DM), hypertension, and dyslipidemia that are associated with the progress of arteriosclerotic vascular disease.4,5 Although obesity is a known independent risk factor for all-cause mortality in adults5 and is often considered a contributor to poor surgical outcomes,6,7 its role in contributing to the death of the elderly remains controversial.8 Certain confounders such as the greater prevalence of chronic diseases and the shortened life expectancy in the elderly make the direct attribution of increased mortality to obesity in this older age group problematic.1,9
Earlier studies on the effect of body mass index (BMI) on surgical outcomes often have had contradictory conclusions, and many were from single institutions with small sample sizes or were retrospective in design.10–14 Two recent reports assessing obesity in large numbers of patients using National Surgical Quality Improvement Program (NSQIP) data did not differentiate the effects of BMI on perioperative outcomes among older and younger adults,15,16 which is relevant because age is a known effect modifier in relating obesity to mortality. An effect modifier is a variable that changes the direction and/or strength of the relationship between an exposure and the outcome variable.17 For example, the relative risk of mortality with increasing BMI tends to decrease with age.17 Therefore, it becomes important to examine the elderly as a distinct group, especially in a setting where there is an appreciable postoperative risk of death. The operative hazards of vascular surgery provide an opportunity to assess the relevance of obesity in the aged.
The objective of this study was to determine whether there is a relation between 6 different BMI classes and 30-day morbidity and mortality of elderly patients undergoing vascular surgery. It was hypothesized that as the BMI increases, the risk of postoperative morbidity and mortality increases.
Clinical information on 25,337 patients older than 65 years who underwent a vascular surgical procedure was obtained from the 2005 to 2007 Participant Use Data File of the American College of Surgeons' National Surgical Quality Improvement Program. Data in this file provided risk-adjusted surgical outcome information from 186 hospitals in the US collected by NSQIP-trained clinical nurse specialists who reviewed these data using a standardized format with strict definitions. Patients were followed through their hospital course and after discharge for 30 days postoperatively. The methodology for collecting these data prospectively, including its accuracy and reproducibility, has been detailed in previous publications.18,19
Study patients underwent a variety of vascular operations, identified by the same Current Procedure Terminology codes used in an earlier report.16 Cardiac, urologic, orthopedic, obstetric, gynecologic, bariatric, and general surgical patients were excluded from the study. Also excluded were patients without records of their height or weight. BMI data were available in all study patients, representing the patient's weight in kilograms divided by the square of the height in meters (BMI = kg/m2). Patients were classified as: (1) underweight (BMI ≤18.5 kg/m2), (2) normal (BMI = 18.6–24.9 kg/m2), (3) overweight (BMI = 25–29.9 kg/m2), (4) obese class I (BMI = 30–34.9 kg/m2), (5) obese class II (BMI = 35–39.9 kg/m2), and (6) obese class III (BMI ≥40 kg/m2), as specified by the National Institutes of Health.20 Other important information such as preoperative functional independence, presence of “do not resuscitate” (DNR) orders, and prevalence of common chronic diseases, including hypertension, DM, pulmonary disease, cardiac disease, stroke, renal disorders, cancer, etc., were also examined. Operation complexity was measured using the work relative value units (RVUs) and total operation and anesthesia times. Work RVUs is a scale (0–95) to quantify the amount of work involved in a specific surgery based on preprocedural, intraprocedural, and postprocedural time; technical skill; physical effort; mental effort and judgment; and stress due to potential risk. It is the work portion of the Resource-Based Relative Value System adopted by Medicare to quantify the amount of work involved in each medical procedure. A score of 0 represents least complex and 95 represents most complex.15 The definitions for the perioperative variables are detailed in Appendix 1 (see Supplemental Digital Content 1, http://links.lww.com/AA/A196) and Appendix 2 (see Supplemental Digital Content 2, http://links.lww.com/AA/A197).
The primary outcome studied was death within 30 days of the vascular procedures. Secondary outcomes measured included common postoperative site-specific complications including superficial and deep wound infections, wound disruptions, graft failures, organ space infections, or bleeding requiring transfusion, as well as systemic complications including pneumonia, pulmonary embolism, delayed ventilator wean, unplanned reintubation, myocardial infarction, cardiac arrest, septic shock, postoperative stroke, reoperation, urinary tract infection, sepsis, deep venous thrombosis, and postoperative renal insufficiency not requiring dialysis. We then computed a composite morbidity variable defined as the occurrence of one or more of these postoperative complications and compared the frequency among BMI groups.
Analysis of data was performed using SPSS version 16.0 (SPSS Inc., Chicago, IL). Basic descriptive statistics including means, standard deviations, and percentages were calculated for demographic and anthropometric data. All continuous variables were examined for skewness and kurtosis. Prevalence of overweight and obesity was described as simple proportions. Pearson's χ2 (for categorical) and 1-way analysis of variance (ANOVA) (for continuous) variables were used to assess baseline clinical and perioperative differences between BMI categories. Pairwise comparisons using the normal weight BMI group as reference were performed using the Bonferroni multiple comparison of means method.
Before performing multiple logistic regression analyses, we examined the univariate predictors for multi-colinearity by first creating a correlation matrix and scanning for highly correlated variables (≥0.7). None of the variables had sufficiently high correlation indicating a low likelihood of significant colinearity. Multivariate analysis was performed using forward stepwise logistic regression to calculate the adjusted odds ratios for 30-day morbidity and all-cause mortality in the BMI categories with the normal group acting as reference. Variables were entered into the model on the basis of statistical significance and clinical relevance. A model fit was measured with the Hosmer-Lemeshow goodness-of-fit test.21 All reported P values were 2 sided and a P value of 0.05 was considered to be significant.
Demographic and medical information for the 25,337 patients studied are listed in Table 1. Certain characteristics of this group were notable. Obese class I, II, and III patients were significantly younger than normal weight or underweight patients (P < 0.001). Elderly female patients were significantly more likely to belong to the underweight class or obese class III, whereas male patients were more likely to be either overweight or in obese class I or II. Overweight and mildly obese class I elderly patients were more likely to be functionally independent. Underweight patients were significantly less likely to be functionally independent and more likely to have DNR orders.
The chronic diseases hypertension, DM, and congestive heart failure increased steadily across the BMI classes (ANOVA, P < 0.01). The highest frequency of a recent myocardial infarction or angina was in obese classes II and III. Underweight patients were more likely to have had a stroke, obstructive emphysema, disseminated cancer, recent weight loss, low serum albumin, or ascites.
Perioperative events, including mean preincision time, total anesthesia and operative times, and the primary procedure work RVUs were significantly different among the BMI classes (Table 2). In this regard, obese patients had significantly longer operative times than underweight and normal weight patients, despite their undergoing less-complex procedures (Bonferroni's pairwise comparisons, P = 0.02). Underweight patients underwent the most complex procedures and had the highest proportion of preoperative contaminated wounds. That group also had the highest transfusion rate, possibly a reflection of surgical complexity. The absolute number of red blood cell units transfused intraoperatively did not correlate with BMI class (data not shown), but was correlated with the complexity of the surgical procedure (r = 0.4; P < 0.001).
Of the 25,337 patients, there were 1009 deaths giving a mortality rate of 4% in this cohort of vascular surgery patients. Similarly, the composite postoperative morbidity rate was 28.5%. Analysis of the postoperative mortality and morbidity revealed important differences (Table 3). One-way ANOVA indicated a statistically significant difference for mortality among the BMI classes (F = 23.6, df = 5; P < 0.001). Overall crude mortality rate demonstrated a reverse-J distribution pattern (Fig. 1) with the highest mortality being 9.4% in the underweight group, decreasing to a nadir in the overweight and obese class I patients, with an increase in obese class II and III patients. The composite morbidity rate followed a pattern similar to the mortality curve (Fig. 1). Postoperative complications occurred most frequently in the underweight class with the frequency decreasing progressively to the obese class I group and then increasing in the higher BMI classes II and III. The hospital length of stay varied significantly by BMI classes (F = 21.9, df = 5; P < 0.001) and was similar to the variation in mortality rates.
The incidence of postoperative superficial and deep wound surgical site infections increased significantly across the BMI classes with the lowest frequency in underweight and the highest incidence in the obese class III patients. The incidences of postoperative respiratory complications, urinary tract infection, graft or prosthetic failure, and need for reoperation varied significantly by BMI class with the highest frequency of these complications occurring in the underweight group (P < 0.001).
Multivariate logistic regression defined a number of factors associated with 30-day mortality (Table 4). Adjusted mortality rate described a geometric pattern similar to that observed on univariate analysis (Fig. 2). The Hosmer-Lemeshow goodness-of-fit test for this model was not statistically significant indicating a good predictive model (χ2 = 8.2, df = 8; P = 0.42). A diagnosis of diabetes, chronic obstructive emphysema, or preoperative dyspnea was significantly associated with a 23%, 36%, and 52% higher risk of mortality, respectively. As might have been anticipated, patients with DNR orders had the highest risk for mortality. Interestingly, age and work RVUs had only a slight effect on the odds of mortality when controlled for other factors.
This investigation indicates that among elderly vascular surgery patients BMI class was a significant predictor of 30-day mortality, but the effect was nonlinear with a reversed J-curve response documenting the poorest outcomes in underweight, normal, and excessively obese patients. The distribution of composite postoperative morbidity followed a pattern similar to the mortality curve.
Obesity is a major public health issue in the US,1,2 and the 64% incidence of overweight and obese individuals in the present study of elderly vascular surgical patients was not unexpected. The US Department of Health and Human Services has recognized being overweight and obese as 1 of the 10 leading markers of poor health,22 and nearly a decade ago published a goal to reduce obesity rates in adults to 15% by 2010.23 This goal will not be met given the fact that approximately 66% of the country's adults are already either overweight or obese.24 Nevertheless, the projected health care cost of this epidemic25,26 demands that a focused effort to reduce obesity be pursued.
This study underscores the known relation of hypertension and diabetes to vascular disease with an increasing BMI.27,28 Contrary to earlier reports about obese vascular surgical patients,16 the current study demonstrated a strong association between high BMI and the prevalence of cardiac diseases, including heart failure, recent angina, and myocardial infarction. Obesity is a risk factor for heart failure and myocardial dysfunction independent of its role in hypertension, diabetes, and coronary heart disease.29 Because heart failure underlies a number of adverse cardiac events30,31 after surgery, it may account for a segment of the mortality in obese patients undergoing vascular operations.
Considerable interest surrounds the geometric nature of how BMI class is related to mortality. Epidemiologic studies have variously described J-shaped, U-shaped, monotonic, and linear relationships32,33; 2 recent surgical studies exhibited a reverse J-shaped relationship.15,16 This phenomenon is accompanied with an “obesity paradox” in which overweight and moderate obesity were associated with a lower, not higher, mortality.32 This finding was also evident among adults with heart failure and hypertension.34,35 The present study demonstrated a reverse J-shaped distribution in all-cause mortality with univariate and multivariate analyses (Figs. 1 and 2). It is possible that the relationship between obesity and mortality is substantially different between subjects in longitudinal studies and hospitalized patients.
Mechanisms underlying the obesity paradox remain speculative.34,36 Many have posited that adiposity may confer protection against various inflammatory mediators in heart failure patients by the production of “buffering” lipoproteins.37–39 Similar protection against the inflammatory response accompanying surgical trauma may be operational in obese class I and II patients, but is likely to be limited in the severely obese class III group. The relationship between BMI group and mortality in the present study is consistent with an earlier report in older hospitalized patients.40 These investigators, similar to us, demonstrated an increased death rate in the lowest BMI groups with only a slight increase in the highest BMI categories. It is conceivable that in elderly surgical patients, high BMI alone does not have a major prognostic implication for perioperative mortality. This in no way diminishes the observed negative impact of high BMI on longevity and quality of life in longitudinal studies of adult nonhospitalized patients.4,5,8
The DNR status in this study deserves note. It was included in a logistic regression model, unlike previous studies in vascular surgery patients.16 Patients in the current study with DNR orders had 3 times the odds of 30-day mortality compared with those without DNR orders, even after adjusting for variables such as age and disease severity.41 Clearly, those factors underlying a DNR order are important predictors of 30-day mortality in elderly vascular surgery patients. Similarly, the present study confirms the importance of preoperative functional status to postoperative outcome.42 Elderly patients who were functionally dependent before their operation had 3 times increased odds of mortality in the current study. Loss of functional independence is a good surrogate for overall physiologic reserve in the elderly as well as a predisposing factor for positive energy balance and obesity.43 Additionally, functionally dependent elderly patients are more likely to be frail and have sarcopenia (loss of muscle bulk), and these 2 conditions are associated with increased mortality.44 Because the current study is observational in nature, it cannot provide firm evidence for the contribution of sarcopenia and frailty to the observed outcome.
There are certain limitations in the current investigation that should be considered in interpreting the study data. Because one cannot exclude the possibility of unmeasured or unknown confounders influencing the associations observed in this study, the apparent effect of BMI on mortality in the elderly vascular surgery patient may not be a reflection of a direct causal relationship. In fact, indirect factors may be quite important. For example, an alternative reason for the obesity paradox may be that overweight patients develop symptomatic vascular disease earlier than lean patients, and are therefore operated on when their anatomic disease is less advanced. A similar observation in patients with heart failure39 suggests that their disease becomes symptomatic at an earlier stage compared with that affecting non-obese patients. Thus, obesity may simply unmask less-severe vascular disease at the time of its clinical presentation. It is also possible that obese patients may be more aggressively monitored and cared for in the perioperative period because of the general perception that they are at a higher risk for anesthetic and surgical complications. Another possible limitation is the disproportionate distribution of patients according to BMI class. However, our finding reflects the prevailing demographics of the elderly in the US.3 It is also conceivable that the sickest underweight or severely obese patients never made it to surgery. However, there is no way to determine this from a cross-sectional study.
In conclusion, this large cross-sectional study of elderly vascular surgery patients revealed an effect of BMI on morbidity and mortality that was nonlinear, in that morbidity and mortality were highest in the underweight and morbidly obese class III patients and lowest in overweight and obese class I and II individuals. In fact, patients having normal weight exhibited a greater mortality when undergoing vascular surgical procedures than did those who were overweight or in obese class I and II categories. This obesity paradox should be recognized when undertaking anesthetic management and surgical interventions for vascular disease. Additionally, BMI may be an important consideration in interpreting risk-adjusted surgical outcomes, which may be particularly important in pay-for-performance paradigms in elderly vascular surgery patients.
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