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Canadian Journal of Anesthesia 53:288-294 (2006)
© Canadian Anesthesiologists' Society, 2006
Cardiothoracic Anesthesia, Respiration and Airway

Obesity is a risk factor for failure of "fast track" extubation following coronary artery bypass surgery

[L’obésité est un facteur de risque d’échec de l’extubation «précoce» à la suite d’un pontage aortocoronarien]

Joel L. Parlow, MD FRCPC MSc, Richard Ahn, MD and Brian Milne, MD FRCPC MSc

From the Department of Anesthesiology, Queen’s University, Kingston, Ontario, Canada.

Address correspondence to: Dr. Joel Parlow, Department of Anesthesiology, Kingston General Hospital, 76 Stuart Street, Kingston, Ontario K7L 2V7, Canada. Phone: 613-548-7827; Fax: 613-548-1375. E-mail: parlowj{at}post.queensu.ca


   Abstract
TOP
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
Purpose: Obesity is a common comorbid condition among patients undergoing coronary bypass surgery. Previous studies have shown inconsistent results as to whether obese patients require prolonged ventilation after cardiac surgery. Fast track recovery strategies have become common to reduce the duration of ventilation and intensive care. The purpose of this study was to determine whether, in our practice, obesity affects post-operative ventilation time using a fast track recovery strategy.

Methods: A retrospective continuous quality improvement audit of 200 patient records was performed. Patients were divided into Obese and Non-obese groups using a definition for obesity of body mass index ≥ 30.0. Failure of fast track extubation was defined as intubation > six hours.

Results: Eighty-four (42.4%) of the patients audited met criteria for obesity. Although most patients successfully underwent fast track recovery, time to extubation was prolonged, and failure of fast track extubation was more common, in obese patients (63% Non-obese vs 46% Obese extubated in < two hours, 98% Non-obese and 85% Obese patients extubated in < six hours, P < 0.001). Increased body mass index, duration of operative time and postoperative serum creatinine were risk factors for failed fast track extubation among the obese patients. Postoperative blood loss through chest drains was reduced in obese patients.

Conclusions: While most obese patients can undergo a fast track recovery strategy following cardiac surgery, in our institution the incidence of failure of early extubation is significantly higher than in non-obese patients. Our experience may assist in resource planning for postcardiac surgery patients.


   Introduction
TOP
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
FAST track extubation (FTE) following cardiac surgery is commonly employed as a means to facilitate rapid recovery and discharge from an intensive care unit, thereby reducing costs of expensive resources.1 As part of a comprehensive recovery plan, FTE has been shown to reduce health care costs by almost 50% after cardiac surgery.2 Studies show that early extubation of elective cardiac surgery patients does not increase perioperative morbidity.2–4

Obesity is an independent risk factor for coronary artery disease, and is a common comorbid condition in coronary artery bypass graft (CABG) surgery patients.5–7 Obesity has been associated with an increase in complications of CABG, including sternal wound infection, sternal dehiscence, arrhythmias, and perioperative myocardial infarction.8–11 Other studies, however, have shown that the only complication rate which is increased in obesity is sternal wound infection,9,12 which may lead to an increased length of stay, higher hospital costs, and greater postoperative mortality within one year.13 While obesity has been shown to prolong postoperative ventilation in obese vs non-obese patients,14 one retrospective study of patients undergoing cardiac surgery, without using a "fast-track" recovery protocol, concluded that the extubation times are similar.15 It has not been determined whether obesity impedes fast-track extubation following CABG.

At our institution, we employ an FTE strategy for all patients undergoing elective CABG surgery, with an aim to wean and extubate the trachea of patients within the first few hours following surgery. The purpose of this retrospective continuous quality improvement audit was to determine whether obesity affects the duration of intubation following CABG surgery in patients scheduled for FTE management, and whether obesity should be considered a risk factor for delayed extubation.


   Materials and methods
TOP
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
Following approval of the Queen’s University Research Ethics Board, we audited a sample of 200 hospital records from those patients who underwent CABG surgery during a two-year period (2000–2001) at the Kingston General Hospital, a 452–bed tertiary care teaching hospital. The charts were selected by the Patient Records department as a random sample from the designated time period. No biasing criteria were given to the staff, who were blinded as to weight or body mass index (BMI) during chart selection. Investigators then utilized specific exclusion criteria (off-pump, minimally invasive or repeat CABG, need for intra-aortic balloon pump, or preexisting pulmonary disease) until the sample of 200 charts was obtained. Patients were managed in a dedicated cardiovascular intensive care unit (CVICU) under the supervision of a cardiac anesthesiologist. The overall goal of FTE was extubation as early as was practical (in general, within two hours following completion of surgery). Extubation criteria included being awake and appropriately responsive, hemodynamically stable, with minimal bleeding from chest drains, and body temperature greater than 36°C. Body weight was not specifically used as a criterion in determining fitness for tracheal extubation. For the purposes of this study, failure of FTE was defined as duration of intubation greater than six hours.

Patients were divided into Non-obese and Obese groups, using an obesity definition of BMI ≥ 30.0.16,17 Patient demographics, preoperative comorbid conditions, and surgical and postoperative data were analyzed. Hemoglobin, platelet count and serum creatinine were compared preoperatively, and every six hours for the first 24 postoperative hours. In addition, activated clotting time was compared preoperatively and on admission to the CVICU.

Data were analyzed using SPSS, version 12 (SPSS Inc, Chicago, IL, USA). Preoperative and intraoperative factors were compared using unpaired t test for parametric data, Mann-Whitney Rank Sum test for non-parametric data, and Chi-square tests for incidence data. Time to tracheal extubation was analyzed using univariate and multivariate (BMI, diabetes) analysis. Kaplan-Meier survival analysis was performed to further categorize BMI; extubation times were compared using Log Rank tests. Furthermore, multivariate logistic regression was used to identify potential risk factors among the obese patients failing early extubation. A P-value of < 0.05 was defined as statistically significant.


   Results
TOP
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Of the 200 sample records, two contained ambiguous information regarding extubation times. Of the remaining 198 charts, 84 patients (42.4%) had a BMI of ≥ 30 and were included in the Obese group, and 114 patients were included in the Non-obese group. One patient from each group died in hospital. Table IGo lists preoperative characteristics of the two patient groups. Obese patients had more than twice the incidence of diabetes mellitus (42.9 vs 20.2%, P = 0.001); there were otherwise no differences between groups with respect to preoperative characteristics.


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  		TABLE I Patient characteristics
 
Anesthesia consisted of propofol, rocuronium/pancuronium, isoflurane, and small doses of intraoperative opioids (sufentanil, fentanyl). Postoperatively, patients were sedated with propofol until preparation for tracheal extubation, and received morphine for analgesia.

Table IIGo lists pertinent operative factors for the two groups. There were no between-group differences with respect to the number of coronary grafts, duration of surgery, or mean cardiopulmonary bypass and aortic cross-clamp times. The proportion of procedures done by each of the surgeons and anesthesiologists was similar in the two groups. Intraoperative and postoperative opioid doses (indexed to BMI) were similar, as was mean body temperature on arrival in the CVICU. There was no difference in intraoperative fluid balance or estimated intraoperative blood loss. There was less blood loss from chest drains in the Obese group in the 12 hr following surgery (P = 0.029), with the greatest difference occurring in the first six-hour period (P = 0.006). However, hemoglobin concentrations on admission to the intensive care unit and on the first postoperative day were similar in the two groups, as were platelet counts, and activated clotting time values on arrival to the CVICU. Likewise, the rate of blood transfusion was not significantly different (26.3% Non-obese, 17.9% Obese; P = 0.22).


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  		TABLE II Surgical and recovery characteristics
 
Mean tracheal extubation time was 141 ± 183 min in the Non-obese group, and 338 ± 726 min in the Obese group (mean ± SD, P = 0.006; Figure 1Go). Overall, 56% of patients were extubated within two hours of arrival in the cardiac recovery unit, and 92% within six hours. Significantly more non-obese patients were extubated at these time periods (63% Non-obese, 46% Obese, P = 0.028 at two hours; 98% Non-obese, 85% Obese, P < 0.001 at six hours). One Non-obese and ten obese patients required tracheal intubation for longer than 12 hr (P = 0.002), and three patients in the Non-obese group and one in the Obese group required reintubation (ns). Using multivariate logistic regression, diabetes, independently of BMI, was not associated with extubation time. Despite differences in duration of intubation, hospital length of stay was similar between groups (Figure 2Go).


Figure 1
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  		FIGURE 1 Scatterplot of the time from end of surgery to tracheal extubation for all patients in the Non-obese (n = 114) and Obese (n = 84) groups (P = 0.002).

 

Figure 2
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  		FIGURE 2 Scatterplot of the number of days of hospital stay for all patients in the Non-obese (n = 114) and Obese (n = 84) groups (P = ns).

 
To differentiate possible effects of extremes of BMI, data were further categorized into four groups (BMI < 25.0, BMI 25.0–29.9, BMI 30.0–34.9 and BMI ≥ 35.0; Figure 3Go). Compared to patients with a BMI under 30, those with a BMI of 30–34.9 were 16 times [95% confidence interval (CI) 2.0, 132; P = 0.01] more likely to be intubated longer than six hours, while those with a BMI of > 35 were 35 times (95% CI 3.8, 316; P = 0.002) more likely. While small sample size resulted in extreme values for the upper limit of the 95% CI, the point estimates (odds ratio) are clinically significant, with a significant trend towards increasing time to extubation as BMI increases.


Figure 3
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  		FIGURE 3 Kaplan-Meier survival curves for patients sub-stratified into four body mass index (BMI) categories. The y-axis indicates the proportion of patients whose tracheas remained intubated at any given time.

 
Table IIIGo lists characteristics of the obese patients, grouped by successful (≤ six hours) or delayed (> six hours) early extubation. Independent predictors of failure of early extubation included BMI, total operative time (entry to operating room to transfer to CVICU), and elevated serum creatinine upon arrival in CVICU.


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  		TABLE III Characteristics of obese patients failing early extubation
 
Post hoc power analysis was carried out for time to tracheal extubation, utilizing the data obtained for group size, difference between means, and mean standard deviation of groups, yielding a power of 0.85 at an {alpha} level of 0.05.


   Discussion
TOP
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
Early tracheal extubation strategies have become common practice following cardiac surgery.1–3 This study was undertaken to determine whether, in daily practice, postoperative intubation is prolonged in obese patients following CABG surgery. Using time to tracheal extubation of six hours as falling within our "fast-track" criteria, only 2% of Non-obese vs 15% of Obese patients failed early extubation. Despite the higher number of obese patients requiring prolonged ventilation, hospital length of stay was not prolonged in this sample of obese patients, and in-hospital mortality was not different.

The implications of prolonged tracheal intubation, in the short term, mainly relate to cost of care. Earlier extubation theoretically results in a shorter stay in an expensive intensive care environment.1,2 In our institution, successful extubation also results in a shift from a 1:1 to 1:2 nursing ratio, which has the potential to reduce nursing costs. Fast track techniques have been shown to reduce costs and resource utilization in a number of centres.1–3 This study did not enable a true cost analysis, and thus conclusions cannot be made regarding differences in total patient cost of care. In actual practice, patients are not discharged from the intensive care environment until the morning following surgery, such that differences in time to extubation of even several hours may not necessarily reduce intensive care costs. Furthermore, nurses are usually scheduled by the shift, and thus a change to 1:2 nursing does not directly eliminate nursing costs. However, flexibility in nursing assignment might allow redistribution of nurses no longer required for 1:1 care. The higher incidence of failure of fast track tracheal extubation in obese patients at our institution may assist in resource planning.

Retrospective chart audits have both disadvantages and advantages. Medical charts may have incomplete or erroneous data recorded. The lack of strict extubation criteria, which could be included in a prospective study, may allow for bias in the decision to extubate particular patients. On the other hand, even a prospective study would not enable blinding of obesity from the caregivers. Furthermore, this study represents real practice at our institution, while meticulously controlled extubation criteria designed in a prospective trial would not reflect actual clinical care. The interpretation of results should take into account the fact that this was a single centre study, and the rate of obesity may not necessarily be generalized to other centres. Although the sample size of 198 is not large, it had sufficient statistical power to reach conclusions regarding tracheal extubation time. However, negative secondary endpoints must be interpreted with caution. Finally, we used a well accepted definition for obesity of BMI ≥ 30.0,16,17 although it was apparent that morbidly obese patients (BMI ≥ 35) have a higher likelihood of failing the early extubation strategy.

Obesity is a known risk factor for coronary disease, and obese patients comprised 42% of the sample population undergoing CABG surgery.5 Numerous studies have documented a prolongation of postoperative ventilation in obese patients following non-cardiac surgery, demonstrating significant impairment in indices of pulmonary function and chest wall mechanics.14,18 Obesity has been shown to be an independent preoperative risk factor for complications following CABG.8 However, one previous retrospective review concluded that there was no higher incidence of major morbidity or mortality in obese patients.15 Although no difference in time to tracheal extubation was demonstrated, this group did not employ a "fast track" recovery strategy (mean extubation time was 32 and 39 hr for Obese and Non-obese groups). Obesity has most strongly been associated with an increase in non-respiratory complications, mainly involving sternal wound infections and sternal dehiscence.9–13 These events have a definite effect on mortality and economic cost.12,13 The current study did not track long-term complications following CABG.

The higher incidence of diabetes mellitus in the Obese group is not unexpected.5,6 Diabetes has been shown to be an independent risk factor for complications of CABG surgery, which could theoretically prolong postoperative ventilation.8 In addition, blood glucose levels and insulin requirements may fluctuate widely in the perioperative period, potentially leading to delays in recovery time. Elevated serum creatinine following surgery constituted a risk factor for failed early extubation among obese patients. This may constitute a marker of more severe underlying disease, or alternately these laboratory results could have led to a bias in postoperative care. Previous studies have identified renal insufficiency as a risk factor for prolonged ventilation after cardiac surgery.19

In the current study, obese patients experienced less postoperative bleeding than the Non-obese group. This supports observations from previous studies, in which high BMI has been associated with a decrease in postoperative bleeding and reexploration for bleeding.9,10 No difference was found in our study with respect to measures of coagulation, although obesity has been consistently associated with a hypercoagulable state, which may not be revealed by the tested parameters.20,21 Alternately, there may be surgical explanations for the difference in bleeding, including a bias towards more cautious hemostasis.

In summary, obese patients required longer periods of postoperative intubation following CABG surgery, and a significant number of these patients failed our early extubation strategy. Morbid obesity, prolonged total operative time, and elevated postoperative serum creatinine were risk factors for failed early extubation strategy among the obese patients. However, mortality and total length of stay in hospital was similar to that of non-obese patients. The results of this study suggest that intensive care resources may require modification in obese patients undergoing "fast track" recovery following cardiac surgery.


   Acknowledgments
 
The authors thank Dr. Elizabeth vandenKerkhof PhD, for her valuable assistance in statistical analysis of the data.


   Footnotes
 
Accepted for publication July 5, 2005. Revision accepted September 15, 2005.


   References
TOP
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
1 Cheng DC, Wall C, Djaiani G, et al. Randomized assessment of resource use in fast-track cardiac surgery 1-year after hospital discharge. Anesthesiology 2003; 98: 651–7.[Medline]

2 Lee JH, Kim KH, vanHeeckeren DW, et al. Cost analysis of early extubation after coronary bypass surgery. Surgery 1996; 120: 611–9.[Medline]

3 Cheng DC, Karski J, Peniston C, et al. Early tracheal extubation after coronary artery bypass graft surgery reduces costs and improves resource use. A prospective, randomized, controlled trial. Anesthesiology 1996; 85: 1300–10.[Medline]

4 Reis J, Mota JC, Ponce P, Costa-Pereira A, Guerreiro M. Early extubation does not increase complication rates after coronary artery bypass graft surgery with cardiopulmonary bypass. Eur J Cardiothorac Surg 2002; 21: 1026–30.[Abstract/Free Full Text]

5 Caterson ID, Hubbard V, Bray GA, et al. Prevention conference VII. Obesity, a worldwide epidemic related to heart disease and stroke. Group III: worldwide comorbidities of obesity. Circulation 2004; 110: e476–83.[Free Full Text]

6 Aronne LJ. Classification of obesity and assessment of obesity-related health risks. Obesity Res 2002; 10: S105–15.[Medline]

7 World Health Organization. Obesity: preventing and managing the global epidemic. Report of a World Health Organization Consultation. Geneva, Switzerland: World Health Organization. WHO Obesity Technical Report Series, No. 894. 2000: 256.

8 Kurki TS, Kataja M. Preoperative prediction of post-operative morbidity in coronary artery bypass grafting. Ann Thorac Surg 1996; 61: 1740–5.[Abstract/Free Full Text]

9 Fasol R, Schindler M, Schumacher B, et al. The influence of obesity on perioperative morbidity: retrospective study of 502 aortocoronary bypass operations. Thorac Cardiovasc Surg 1992; 40: 126–9.[Medline]

10 Birkmeyer NJ, Charlesworth DC, Hernandez F, et al.. Obesity and risk of adverse outcomes associated with coronary artery bypass surgery. Northern New England Cardiovascular Disease Study Group. Circulation 1998; 97: 1689–94.[Abstract/Free Full Text]

11 Herlitz J, Brandrup G, Emanuelsson H, et al. Determinants of time to discharge following coronary artery bypass grafting. Eur J Cardiothorac Surg 1997; 11: 533–8.[Abstract]

12 Sabourin CB, Funk M. Readmission of patients after coronary artery bypass graft surgery. Heart Lung 1999; 28: 243–50.[Medline]

13 Hollenbeak CS, Murphy DM, Koenig S, Woodward RS, Dunagan WC, Fraser VJ. The clinical and economic impact of deep chest surgical site infections following coronary artery bypass graft surgery. Chest 2000; 118: 397–402.[Abstract/Free Full Text]

14 Pelosi P, Croci M, Ravagnan I, Vicardi P, Gattinoni L. Total respiratory system, lung, and chest wall mechanics in sedated-paralyzed postoperative morbidly obese patients. Chest 1996; 109: 144–51.[Abstract/Free Full Text]

15 Brandt M, Harder K, Walluscheck KP, et al. Severe obesity does not adversely affect perioperative mortality and morbidity in coronary artery bypass surgery. Eur J Cardiothorac Surg 2001; 19: 662–6.[Abstract/Free Full Text]

16 National Institutes of Health. Definition of obesity. In: Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. Bethesda, Maryland: Department of Health and Human Services, National Institutes of Health, National Heart, Lung, and Blood Institute, 1998. Available from URL; http://www.nhlbi.nih.gov/guidelines/obesity/ob_gdlns.htm.

17 Office of Nutrition Policy and Promotion. Canadian Guidelines for Body Weight Classification in Adults. Health Canada Publications Centre, Ottawa, Canada 2003. Available from URL; http://www.hc-sc.gc.ca/fn-an/nutrition/weights-poids/guide-ld-adult/index_e.html.

18 Eichenberger AS, Proietti S, Wicky S, et al. Morbid obesity and postoperative pulmonary atelectasis: an under-estimated problem. Anesth Analg 2002; 95: 1788–92.[Abstract/Free Full Text]

19 Wong DT, Cheng DC, Kustra R, et al. Risk factors of delayed extubation, prolonged length of stay in the intensive care unit, and mortality in patients undergoing coronary artery bypass graft with fast-track cardiac anesthesia. A new cardiac risk score. Anesthesiology 1999; 91: 936–44.[Medline]

20 De Pergola G, Pannacciulli N. Coagulation and fibrinolysis abnormalities in obesity. J Endocrinol Invest 2002; 25: 899–904.[Medline]

21 Bowles LK, Cooper JA, Howarth DJ, Miller GJ, MacCallum PK. Associations of haemostatic variables with body mass index: a community-based study. Blood Coagul Fibrinolysis 2003; 14: 569–73.[Medline]





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