AJCN Tufts Nutrition Symposium, Boston & Online Sept 2009
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American Journal of Clinical Nutrition, Vol. 87, No. 6, 1656-1661, June 2008
© 2008 American Society for Nutrition

ORIGINAL RESEARCH COMMUNICATION

Preoperative unintended weight loss and low body mass index in relation to complications and length of stay after cardiac surgery1,2,3

Lenny MW van Venrooij, Rien de Vos, Mieke MMJ Borgmeijer-Hoelen, Cees Haaring and Bas AJM de Mol

1 From the Divisions of Dietetics (LMWV), Clinical Epidemiology and Biostatistics (RV), and Cardiac Thoracic Surgery (LMVV, MMMJB-H, and BAJMM), Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands, and the Trial Office, Division of Radiology, University Medical Center, Utrecht, the Netherlands (CH)

2 Supported by a grant from the Clinical Guidelines Committee, Academic Medical Center, University of Amsterdam.

3 Address reprint requests to LMW van Venrooij, Division of Cardio-thoracic Surgery, Room G4-228, Academic Medical Center, PO Box 22700, 1100 DE Amsterdam, The Netherlands. E-mail: l.m.vanvenrooij{at}amc.uva.nl.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: Several studies reported increased adverse outcomes after cardiac surgery in patients with low body mass index (BMI; in kg/m2). Little is known yet, however, about the effect of preoperative unintended weight loss (UWL) in cardiac surgery patients.

Objective: We explored the prevalence and effect of UWL in view of low BMI and vice versa adjusted for a validated set of preoperative risks, inflammatory activity, and duration of extracorporeal circulation on postoperative adverse outcome.

Design: A prospective cohort study was performed. Nutritional data of cardiac surgery patients were collected within 24 h of admission and linked to the standard postoperative complication registration database.

Results: The cohort consisted of 331 cases. Multivariate logistic regression analyses showed that preoperative UWL of ≥10% in the past 6 mo (≥10%UWLin6m) was associated with a prolonged length of stay in the hospital independent from low BMI [odds ratio (OR): 7.06; 95% CI: 1.78, 28.04]. Preoperative BMI ≤ 21.0 was associated with an increased incidence of postoperative infections and prolonged stay in the intensive care unit independent from ≥10%UWLin6m (OR: 4.62; 95% CI: 1.20, 17.82; and OR: 5.27; 95% CI: 1.28, 21.76, respectively). Preoperative undernutrition in cardiac surgery patients (≥10%UWLin6m or BMI ≤ 21.0 or both) was present in 9.1% of the study population (4.3% and 4.8%, respectively).

Conclusions: From this study, we recommend special attention for cardiac surgery patients with preoperative ≥10%UWLin6m or BMI ≤ 21.0 because both variables are independently related to adverse outcomes. Preoperative referral to a dietitian for further diagnostic assessment and nutritional treatment is strongly recommended.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The morbidity and mortality rates after cardiac surgery are known to vary with patient characteristics, preexisting comorbidities, inflammatory activity, and duration of extracorporeal circulation (1-7). An often overlooked preoperative comorbid condition in cardiac surgery is undernutrition. To quantify undernutrition (ie, disease-related or protein-energy malnutrition) unintended weight loss (UWL), low body mass index (BMI; in kg/m2), and hypoalbuminemia are currently the most frequently used variables (8). However, hypoalbuminemia is not specific for undernutrition because it is also influenced by catabolism and fluid state (9).

The chosen cutoffs of BMI to define undernutrition vary (10). For the general patient population a BMI < 18.5 is used (8), for patients with chronic obstructive pulmonary disease (COPD) a BMI cutoff < 21 was suggested (11), and in elderly patients a BMI cutoff < 24 was proposed (12). In cardiac surgery, preoperative BMI cutoffs < 20, < 21, and < 24 were reported to increase postoperative rates of morbidity and mortality independent from a wide range of preexisting comorbidities and inflammation-related factors such as preoperative hypoalbuminemia (5, 7, 9, 13-15). The role of the more inflammatory-specific variable C-reactive protein (CRP) is not yet studied in relation to a low BMI and increased incidence of postoperative adverse outcomes after cardiac surgery. This is also the case for patients with a history of UWL and decreased food intake before cardiac surgery. In general, patients are considered to have a severe risk on worse outcome when weight loss of >10–15% within 6 mo is present (8). In contrast to low BMI, the prevalence and the association of UWL in relation to adverse outcome in patients undergoing cardiac surgery is still to be investigated (16). Therefore, this study focused on the prevalence of preoperative UWL and low BMI in a population of cardiac surgery patients and the effect on adverse postoperative outcome independent from known risks factors such as a high operation risk score [European System for Cardiac Operation Risk Evaluation (EuroSCORE)], preoperative inflammatory activity (CRP, leukocytes, and albumin plasma concentrations), and duration of extracorporeal circulation. Specifically, we determined whether preoperative UWL was related to adverse outcome independent from low BMI and vice versa to determine whether in patients undergoing cardiac surgery both UWL and low BMI should be used to detect (ie, screen) undernutrition.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Patients and data collection
To assess the effect of preoperative UWL and low BMI on postoperative outcome, a prospective cohort study was performed. Included were all adult patients (≥18 y) undergoing coronary artery bypass grafting, heart valve surgery (primary as well as reoperative surgery), or both admitted to the ward of cardiothoracic surgery at the Academic Medical Center between February and September 2005. Preoperative nutritional data (body weight 1 and 6 mo before surgery, body length, and actual body weight) were collected within 24 h of hospital admission. Actual body weight was measured on admission. Body weight 1 and 6 mo before admission and body length were asked from the patient. Items of undernutrition were defined as 1) preoperative weight loss of ≥5% in the preceding month (≥5%UWLin1m), or 2) preoperative weight loss of ≥10% in the preceding 6 mo (≥10%UWLin6m), or 3) preoperative low BMI ≤ 18.50, or 4) preoperative low BMI ≤ 21.00, or 5) preoperative low BMI ≤ 23.50. The study was performed according to the responsible committee on human experimentation.

Data to describe patient-, cardiac-, and operation-related baseline characteristics were extracted from the standard electronic database. The database included the risk score based on the EuroSCORE, which is validated for Europe and North America (3, 17) (Appendix AGo). Age, sex, preoperative setting, and preexisting comorbidities and other risk factors as assessed by the EuroSCORE, subsequently called operation risk, were dichotomized as ≥65 y compared with <65 y, women compared with men, waiting at home compared with being hospitalized, and EuroSCORE ≥ 6 compared with < 6, respectively. A score of ≥6 defines a patient with high operation risk; a score of <6 refers to a patient with a low or medium operation risk (18). CRP, leukocyte, and albumin plasma concentrations on the day of admission or, if not available, the day of the preoperative outpatient visit as variables of an increased inflammatory condition were abstracted from the hospital information system. Increased inflammatory activity was dichotomized as CRP ≥ 5 mg/L or leukocytes ≥ 11 x 109/L. Apart from inflammatory activity, hypoalbuminemia was dichotomized as ≤39 g/L. Duration of the extracorporeal circulation were as follows: cardiopulmonary bypass (CPB) time and aortic cross-clamp (ACC) time were dichotomized at their 75th percentile (≥132 compared with <132 min and ≥94 compared with <94 min, respectively).


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  		APPENDIX A. Items considered in the European System for Cardiac Operation Risk Evaluation Patient-related factors

 
Outcome data were also extracted from the standard electronic database. Selected were the following 3 main outcome measures: 1) medical complications, containing organ failure, bleeding, and infection; 2) clinical-process outcomes [reintubation, prolonged length of stay in the intensive care unit (ICU), prolonged length of stay in the hospital, readmission]; and 3) operative mortality. Organ failure was aggregated from cardiac failure defined as creatine kinase myocardial band isoenzyme ≥ 100 µg/L or ischemic changes on electrocardiography, acute renal failure defined as postoperative serum creatinine ≥ 200 µmol/L with preoperative creatinine ≤ 150 µmol/L or dialysis, and neurologic failure defined as cerebrovascular accident or peripheral neuropathy. Bleeding was defined as abdominal bleeding or reoperation because of bleeding, and infection was defined as respiratory tract infection, urinary tract infection, mediastinitis, sternal wound infection, leg wound infection, and other infections such as phlebitis and rare cases of intraabdominal and dermatologic conditions. Clinical-process outcomes were defined as reintubation, prolonged postoperative ICU stay ≥ 48 h (readmission hours were excluded; patients who died were excluded), prolonged postoperative length of stay at the operating hospital ≥ 7 d (readmission days were excluded; patients who died were excluded), and cardiac surgery-related readmission to the operating hospital within 3 mo after surgery. Operative mortality was defined as mortality during hospital admission ≤30 d. All outcome data were registered at discharge from the cardiac surgery department.

Statistical analyses
Differences in the prevalence of individual undernutrition elements (≥5%UWLin1m, ≥10%UWLin6m, BMI ≤ 18.50, BMI ≤ 21.00, BMI ≤ 23.50) in relation to age, sex, preoperative setting, operation risk, inflammatory activity, hypoalbuminemia, CBP time, and ACC time were analyzed with the use of the chi-square test or, if necessary, the Fisher's exact test. To assess the relation between the individual items of undernutrition and the 3 main outcomes (medical complications, clinical-process outcomes, and operative mortality) odds ratios (ORs) with a 95% CI were calculated with the use of univariate logistic regression analyses for each nutritional item as independent variable and one of the outcomes as dependent variable. Then, to determine the independent effect of the undernutrition items on the 3 main outcome measures, adjusted for the potential confounding effects of age, sex, preoperative setting, operation risk, inflammatory activity, hypoalbuminemia, CBP time, and ACC time, multivariate logistic regression analysis was performed for those undernutrition items with a P value ≤ 0.20 in relation to the outcomes in univariate analyses. For those undernutrition items with a P value ≤ 0.05 after multivariate analysis, the presence of interaction between undernutrition and the other independent variables in the multivariate regression model was also assessed. An interaction term was judged to be significant at a P value ≤ 0.10. Finally, to describe the effect of UWL and low BMI independent from each other on the 3 main outcomes, taken into account the results of the prior multivariate analyses, UWL and low BMI were added together in the logistic regression models. Data elaboration and analysis were performed with the use of SPSS (version 14.0; SPSS Inc, Chicago, IL) statistical package for WINDOWS.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Between February and September 2005, 331 patients undergoing cardiac surgery were included. Preoperative baseline characteristics are summarized in Table 1Go. The prevalence of preoperative undernutrition in all study patients and in subgroups is shown in Table 2Go. The prevalence of undernutrition depends on the definition used, especially the chosen BMI cutoff; only 1 patient (0.3%) had BMI ≤ 18.5 compared with 18.7% of patients with BMI ≤ 23.5. The incidence of the postoperative outcome according to the previously defined 3 main outcomes is shown in Table 3Go.


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  		TABLE 1. Preoperative baseline characteristics of the study cohort1

 

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  		TABLE 2. Prevalence of preoperative undernutrition in all study patients and in subgroups1

 

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  		TABLE 3. Incidence of postoperative medical complications, clinical-process outcomes, and operative mortality1

 
Univariate analyses of the relation between undernutrition and medical complications (organ failure, bleeding, and infections) showed only a statistically significant relation between BMI ≤ 21.0 and postoperative infections (OR: 4.60; 95% CI: 1.29, 16.37) (see Table 4Go). Univariate analyses of the clinical-process outcomes (prolonged ICU stay and hospital stay) showed that BMI ≤ 21.0 and BMI ≤ 23.5 both were related to a prolonged postoperative stay in the ICU (OR: 4.99; 95% CI: 1.32, 18.89; and OR: 2.10; 95% CI: 1.13, 3.92, respectively). BMI ≤ 23.5 was also related to a prolonged stay in the hospital (OR: 2.20; 95% CI: 1.17, 4.14). The risk of a prolonged hospital stay was increased with ≥10%UWLin6m (OR: 4.70; 95% CI: 1.37, 16.13).


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  		TABLE 4. Univariate and multivariate logistic regression analyses of preoperative undernutrition and postoperative complications and length of stay1

 
The multivariate analyses that adjusted for age, sex, preoperative setting, operative risk, inflammatory activity, hypoalbuminemia, CPB time, and ACC time confirmed all associations related to adverse outcome that were shown in the univariate analyses except for BMI ≤ 23.5 (see Table 4Go). The association of ≥10%UWLin6m with a prolonged hospital stay became even stronger (OR: 7.50; 95% CI: 1.90, 29.60). The data did not support any interaction between undernutrition and the other independent variables in the multivariate models.

Finally, when ≥10%UWLin6m and low BMI were fitted together into the multivariate model, ≥10%UWLin6m was still associated with a prolonged hospital stay independent from BMI ≤ 23.5 (OR: 7.06; 95% CI: 1.78, 28.04). In addition, the other relations observed in the prior multivariate analyses persisted; BMI ≤ 21.0 was still significantly related to the occurrence of infections and a prolonged stay in the ICU (OR: 4.62; 95% CI: 1.20, 17.82; OR: 5.27; 95% CI: 1.28, 21.76, respectively) independent from ≥10%UWLin6m. In addition, treating BMI ≤ 21.0 as an independent risk of developing postoperative infections (Figure 1Go A) and prolonged ICU stay (Figure 1BGo) again confirmed that patients with BMI ≤ 21.0 faced the highest risk compared with the risks of the higher BMI subgroups. Preoperative undernutrition in cardiac surgery patients defined by ≥10%UWLin6m or BMI ≤ 21.0 or both was present in 9.1% of the study population (4.3% and 4.8%, respectively).


Figure 1
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  		FIGURE 1.. The relative odds ratios (ORs) of independent preoperative BMI subgroups in relation to postoperative infections (A) and to prolonged stay in the intensive care unit (B). Preoperative BMI ≤ 21.0 was set as the reference category (reference OR = 1). Adjusted for age, sex, preoperative setting, operations risk, inflammatory activity, hypoalbuminemia, cardiopulmonary bypass time, aortic cross-clamp time, and ≥10% unintended weight loss in the past 6 mo with the use of multivariate logistic regression analysis.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Preoperative ≥10%UWLin6m and BMI ≤ 21.0 were both present in {approx}5% of the patients. Both variables independently increase the risk of adverse outcome after cardiac surgery regardless of preexisting comorbidities, inflammatory activity, and duration of extracorporeal circulation.

In other areas in medicine, UWL was also associated with postoperative adverse outcomes (10, 19, 20). In patients with chronic heart failure, Anker et al (21) showed that UWL was significantly related to impaired survival at 8 mo of follow-up. We are not aware of studies that assess the effect of UWL on cardiac surgery outcome. In studies dealing with low BMI and cardiac surgery, similar associations between low BMI and postoperative outcome were found (13-15, 22). However, none of those studies corrected for UWL and preoperative inflammatory activity expressed as an increase of CRP, leukocytes, CPB time, and ACC time. This can be one plausible explanation as to why we found no significant associations after multivariate analyses in BMI ≤ 23.5 and postoperative outcome in contrast to some other studies (14, 15).

One of the possible mechanisms to explain the relation of preoperative UWL and low BMI on postoperative complications is less nutritional reserve by less fat-free body mass (FFM) and thereby less capacity to recover from surgery (23-26). In our study an increased inflammatory activity and hypoalbuminemia were related to UWL (especially in ≥5%UWLin1m) (see Table 2Go). It is well known that inflammatory activity leads to catabolism and anorexia and thereby to loss of FFM. That UWL was accompanied by an increased inflammatory activity and hypoalbuminemia reinforces the suggestion that UWL in our cohort indeed was accompanied by a decrease in FFM. Also after adjustment for inflammatory activity and hypoalbuminemia, the effect of ≥10%UWLin6m on a prolonged hospital stay remained significant. Thus, apart from preoperative inflammatory activity, UWL is an important determinant of the ability of the body to respond adequately to surgery. In addition, BMI ≤ 21.0 remained related to postoperative infections and prolonged ICU stay after multivariate analyses. For COPD patients, it was found that FFM-depleted patients had a lower BMI (20.5 ± 2.6) than did nondepleted COPD patients (25.1 ± 3.8) (11, 27). These data suggest that UWL and low BMI represent, at least partially, a lower FFM. Our study does not contain information about the exact body composition. However, the data represent a realistic and representative cross section of the day-to-day clinical practice (ie, screening variables).

In addition, obesity is commonly thought to be a risk factor for complications after cardiac surgery (13, 14). In our data risk of infections and prolonged length of stay in the ICU decreased in the higher BMI subgroup. The protectiveness of BMI subgroups compared with the subgroup BMI ≤ 21.0 on postoperative infections was present between BMI of 23.5 and ≤28.5 (see Figure 1AGo). Preoperative BMI subgroups with BMI between 26.0 and ≤36.0 were significantly protective on a prolonged ICU stay compared with the subgroup BMI ≤ 21.0 (see Figure 1BGo). However, the large 95% CIs for the higher BMI subgroups leave the possibility of a quadratic relation (ie, U-shaped) open between BMI and adverse outcome (28).

Our study also faced some limitations. Because weight history was self-reported by patients, some bias may be assumed because it is shown that underweight patients tend to overreport their body weight and overweight patients tend to underreport (29). This could have resulted in an underestimation of the prevalence of UWL in the overweight patients and an overestimation of the prevalence of UWL in the underweight patients (body weight on admission was measured). Bias is also possible for whether the weight loss was really unintentional; ie, not only fluid loss because of prior fluid retention resulting from heart failure. In addition, the absence of UWL or low BMI alike does not exclude low body composition indexes (11).

Our study suggests that both UWL and low BMI are relevant variables to detect (ie, screen) preoperative undernutrition in cardiac surgery irrespective of the exact knowledge of body composition. To start a nutritional intervention can only be decided after a more comprehensive diagnostic nutritional assessment of dietary intake and body composition of the individual patient (19, 30). In our institution, Tepaske et al (31) showed an effect of preoperative administration of immunonutrients on the reduction of postoperative infections in cardiac surgery patients with a high risk on infections. In addition, a nutritional intervention study may show whether, in the so-called undernourished patients, an immunointervention is effective and, if so, is significantly more effective than energy or protein interventions on the reduction of infections and length of stay after cardiac surgery. Especially, in patients with a low BMI, a preventive strategy by supplementary nutritional support should be considered with the focus on reinforcement of the immune status. Our study showed that a preoperative BMI ≤ 21.0 made patients susceptible to postoperative infections and a prolonged stay in the ICU. Preoperative ≥10%UWLin6m was indeed also a relevant factor for duration of recovery after cardiac surgery but seemed to be related to the general protein and energy requirements rather than to resistance against infections. Hypothetically, the methods to improve the nutritional condition may differ between patients with UWL and a low BMI.

Either at the outpatient clinic or at the time of the hospital admission, nurses may use a quick-and-easy questionnaire to establish undernutrition (32). When a screening tool without scoring a BMI ≤ 21.0 is used for preoperative assessment of cardiac surgery patients, approximately one-half of the undernourished cardiac surgery patients are missed. This is because, as shown by our study, one-half of the preoperative undernourished patients have UWL, and the other one-half have a low BMI, with minor overlap. The Malnutrition Universal Screening Tool recommended for the community by the European Society of Parenteral and Enteral Nutrition seems accurate and applicable for the preoperative assessment in cardiac surgery (33).

In our study the prevalence of both preoperative ≥10%UWLin6m and BMI ≤ 21.0 was rather low (4.3% and 4.8%, respectively) but still amounted for 9.1% of the cardiac surgical patients. One may question whether screening and intervention will be cost effective. However, the simplicity of the nutritional screening tool, the short duration of <5 min, and the fact that in a vulnerable group of patients, with relative simple means of costs and suffering, risk can be reduced make thereby "nutritional conditioning" an issue of risk control and patient safety. Waiting or planning time before surgery should be organized in such a way that the nutritional conditioning requirements can be met.

On the basis of this study, we recommend special attention for cardiac surgery patients with preoperative ≥10%UWLin6m or BMI ≤ 21.0 because both variables are independently related to adverse outcomes. Preoperative referral to a dietitian for further diagnostic assessment and nutritional treatment is strongly recommended.


   ACKNOWLEDGMENTS
 
We thank the nursing team of the department of Cardio-thoracic Surgery of the Academic Medical Center, University of Amsterdam, for their assistance in collecting of nutritional data. We also thank the dietitians of the Nutritional Team of the Academic Medical Center for their intellectual contribution, especially Mrs. Niki Doornink for presenting the early results of this study in Istanbul.

The author's responsibilities were as follows—LMWV: was responsible for the protocol, selection, analysis, and the writing of the manuscript; RV: participated at the methodologic development of the study protocol, reviewed the analysis, and coauthored the manuscript; CH: organized and linked the pre and postoperative databases; MMMJB-H and BAJMM: participated in the development of the protocol and reviewed the manuscript. None of the authors had a personal or financial conflict of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication September 18, 2007. Accepted for publication February 11, 2008.





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