Abstract presented at the annual meeting of the American Society of Anesthesiology,

San Francisco, California, October 12-17, 2007.

Introduction

The elderly represents one of the fastest growing segments of the American population. Current estimates are that persons ≥ 65 years of age form about 15% of the American society and this proportion is projected to increase to 26% by 2025 (₁). Another interesting epidemiologic parallel is the increasing prevalence of obesity in the general population particularly in the elderly (₂). Recent estimates suggest that 60% of adult Americans are overweight and ₃0.9% are obese (₃). The close link between high body mass index (BMI) and cardiovascular complications is well known. About 65% of patients with history of myocardial infarction have a body mass index (BMI) ≥ 25kg/m². Obesity has been described by the American Heart Association (AHA) and American College of Cardiology (ACC) as a major modifiable cardiovascular risk factor (₄). It is almost axiomatic among anesthesiologists and surgeons that the obese patient is at increased peri-operative risk. However there are very little data to support this widely-held notion. The role of obesity in the cardiac surgical patient is even more confusing; some studies have shown that obesity may not be a significant risk factor after cardiac surgery (₅,₆) while others showed that only post-operative sternal wound infections are commoner in obese compared to their lean counterparts (₇). Some of the reasons why obese patients may have a higher incidence of post-operative complications after cardiac surgery include, greater myocardial workload, inadequate myocardial protection of a hypertrophied heart, an imbalance in myocardial oxygen demand and supply, and decreased respiratory muscle reserve (₆).

All these studies examined the impact of obesity on early or late post-cardiac surgery outcome but none have looked at the role played by obesity during cardiac surgery i.e. intra-operative complications. Since intra-operative complications could contribute to early and late post-operative morbidity and mortality, it is essential to explore the factors that may contribute to the occurrence of adverse intra-operative events. This study examined the contribution of BMI to anesthetic and intra-operative complications in a cohort of elderly patients undergoing cardiac surgery under total or partial cardio-pulmonary artery bypass at the University of Michigan hospital.

Methods

Following Institutional Review Board approval, we carried out a retrospective review of our electronic peri-operative clinical information system (Centricity®; General Electric Healthcare, Waukesha, WI). A structured, electronic preoperative history and physical is completed for every patient by the attending anesthesiologist or senior resident assigned to care for the patient. The following demographic, anthropometric and clinical data were collected on all cardiac surgical patients aged ≥65yr managed between January 2003 and December 2006: age, gender, ethnic group, American Society of Anesthesiologist (ASA) status, height, weight and body mass index (BMI). Body Mass Index (BMI) was calculated as weight in kilograms divided by the square of the height in meters (BMI = kg/m²). Patients were classified as underweight (BMI ≤19.9kg/m²), normal weight (BMI = 20- 24.9kg/m²), overweight (BMI = 25–29.9kg/m²), obese (BMI = 30 - 35kg/m²) and morbidly obesity (BMI ≥ 35kg/m²). The proportion of obese and morbidly obese patients between the male and female patients was compared. The presence of co-morbid conditions such as hypertension, diabetes, obstructive sleep apnea, COPD, history of smoking, chronic renal insufficiency, hyper-lipidemia, and congestive cardiac failure were noted.

In addition, the type of cardiac surgery: coronary artery bypass graft (CABG), valve repair/replacement, aortic surgery or any of these combinations was recorded. Patients undergoing cardiac transplantation and ventricular assist device implantation were excluded from the analysis.

Anesthesia care in our facility is 100% supervised by attending cardiac anesthesiologists. The method and choice of induction of anesthesia is at the discretion of the attending staff. Typically the first attempt at mask ventilation and laryngoscopy are performed by a senior anesthesiology resident, or a cardiac anesthesia fellow. In all cases mask ventilation was accomplished with a disposable, clear plastic mask (King Systems Corporation Noblesville, IN), while laryngoscopy was performed with a fiberoptic laryngoscope handle and blade (Heine Inc.,Dover, NH).

Complications and medical co-morbidities were described under several broad headings. Airway complications included difficult mask ventilation, (measured using a 4-point scale previously described by Han et al (₈), difficult laryngoscopy (defined by Cormack and Lehane grade >2), use of gum elastic bougie, significant desaturation (defined recorded pre-bypass SPO₂ value ≤ 90%) and dental injury. Hypertension was defined as history of or use of anti-hypertensives. Diabetes was defined based on the use of oral hypoglycemic agents or insulin. Heart failure was defined by history of current therapy for heart failure or ≥3 New York Heart Association symptoms. Renal insufficiency was defined by history of chronic dialysis or serum creatinine >2.0mg/dl. Obstructive sleep apnea (OSA) was defined by patient report or presence of such diagnosis in the patient's medical records. Use of blood products on and off cardio-pulmonary bypass (CPB), need for and number inotropes following CPB were compared between obese and normal weight patients. The choice and number of inotropes is typically jointly determined by the cardiac surgeon and the attending anesthesiologist in charge of the case. Total duration of surgery, anesthesia, aortic cross clamp time as well as total CPB times was calculated.

Statistical Analysis

Results

A total of 697 elderly patients had cardiac surgery with cardio-pulmonary bypass during the study period. One patient was excluded because of missing BMI data. There were 420 (60.3%) males and 277 (39.7%) females. The mean age of the population was 73.4 ±5.7 years. There were 564 (80.9%) white American, 78 (11.2%) black American, 16 (2.3%) Hispanic American and 39(5.6%) others (including Asian, Pacific Islander, and Native American). Coronary artery bypass graft (CABG) was the commonest type of surgery (48%) while CABG with mitral or aortic valve repair was done in 22%, valve surgery alone in 18%, aortic surgery was 8% and others 4%.

The overall prevalence of overweight and obesity was 69.9 % (overweight = 37.6% and obese = 32.3%). There was no significant difference in the prevalence of overweight and obese between the ethnic groups. BMI showed a slight negative correlation with age (r = -0.165, p < 0.001). Females were significantly older and more likely to be underweight compared to males (Table 1).

Figure 1

Table 1: Demographic and anthropometric statistics of an elderly cardiac surgical population.

There was no significant difference in the distribution of obesity and morbid obesity between male and female elderly patients Table (1). Many of the well-known medical co-morbidities were significantly more prevalent in the obese than in normal weight patients (Table 2).

Figure 2

Table 2: Pre-operative co-morbidities between obese and non-obese patients.

Mask ventilation was easily accomplished in most patients although 102 (14.2%) patients were classified as difficult mask ventilation (Table 3).

Figure 3

Table 3: Details of Mask Ventilation

At the same time the incidence of difficult mask ventilation was higher in the obese than in normal weight patients (Table 4).

Figure 4

Table 4: Frequency of intra-operative complications between obese and non-obese cardiac surgical patients.

The overall incidence of difficult laryngoscopy and need for bougie were 11.7% and 7.1% respectively. There was no significant difference in the incidence of difficult laryngoscopy between obese and normal weight patient. No patient required fiberoptic intubation. Although there was no significant difference in the mean lowest pre-bypass SPO₂ between obese and normal weight patients (90.3±7.5 vs. 88.8±9.1); obese patients were more likely to have significant pre-bypass desaturation than their lean peers (Table 4). Additionally, obese patients had longer bypass time and required slightly larger mean cardioplegia volume than normal weight patients. Comparisons between normal and obese patients with respect to other intra-operative complications are detailed in Table 4.

Discussion

We have shown that the prevalence of overweight and obesity is high in this cohort of elderly cardiac surgical patients and that certain intra-operative complications are associated with high BMI. Contrary to general consensus, obesity was not associated with a higher incidence of difficult airway in the elderly and all patients in this series could be intubated without the need for awake or asleep fiberoptic intubation. It is however noteworthy that there is a high incidence of difficult mask ventilation in the obese elderly cardiac surgical patient. We observed a significantly higher incidence of difficult mask ventilation than figures published by previous authors (₉,₁₀). However, these authors did not specifically study the elderly population. It is possible that the combination of obesity and edentulous state could severely limit the adequacy of mask ventilation in the elderly. Mask ventilation is particularly important in the cardiac surgical patient because the standard “cardiac induction” involves the use of a non-depolarizing muscle relaxant (typically pancuronium) and a period of mask ventilation prior to laryngoscopy and intubation. Difficult or unstable mask ventilation prior to securing the airway could contribute to hypoxia and this may explain the higher incidence of significant pre-bypass desaturation in the obese patients compared to their healthy weight peers that we found in the present study.

The incidence of difficult laryngoscopy in our patients is similar to the observation by Ezri et al in patients presenting for coronary artery bypass surgery (₁₁). Our findings and those of others (₁₂) also confirm that the incidence of difficult laryngoscopy is higher in the cardiac surgical patients than the estimated incidence of 1.5% to 15% in the general surgical population. This may be a reflection of the higher prevalence of obesity in cardiac patients and the fact that cardiac patients are usually older. It is noteworthy that while three of the obese patients in the series by Ezri et al (₁₁) required fiberoptic intubation, none of our patients required this technique to secure their airway. There are no reports of difficult laryngoscopy in the elderly cardiac surgical population so our report is a necessary first step to define the scope of the problem in this group and to identify modifiable risk factors. The association of obesity with difficult laryngoscopy remains controversial, with some studies finding no association (₁₃) and others finding some (₁₄,₁₅). Difficult laryngoscopy was not associated with high BMI in our elderly patient population. It is possible that the distribution of fat in the obese elderly patient population is different from the obese adult population. For example, fat pads in the face and neck region are known to be associated with difficult laryngoscopy and tracheal intubation (₁₆).

Elderly obese patients in our study were more likely to require multiple vasopressors after cardiopulmonary bypass. This may be due to the higher prevalence of cardiovascular disease in obese vs. non-obese patients. It has been postulated that because obese patients have greater myocardial workload, greater myocardial hypertrophy, and possibly inadequate myocardial protection during CPB, they are more susceptible to post-operative complications (₁₇). These conditions could also predispose the obese patient to requiring multiple vasopressors post-CPB.

Study Limitations

Correspondence to

O.O. NAFIU Department of Anesthesiology University of Michigan Room UH 1H247 Ann Arbor, Michigan 48109-0048. Phone: 734-936-4280. Fax: 734-936-9091. e-mail: onafiu@med.umich.edu