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CABG mortality by sex and age

BMI and CABG mortality
Which CABG procedure
A reader asked the question about what were the death rates after coronary artery bypass grafting (CABG) and whether there were any patient criteria that predicted a higher likelihood of dying. Good questions these, and there is some quite interesting evidence to look at.

In the United States the National Cardiovascular Network collects information on patient characteristics, post CABG complications and (short-term) in-hospital mortality, using trained personnel from a number of hospitals. Between October 1993 and December 1999 there were complete data on just over 51,000 CABG patients; 30% were women. Mortality and major post surgery complications were examined by age and sex, together with interactions with pre-existing patient characteristics, like comorbid conditions.

Crude mortality rates by age and sex are shown in Figure 1. Women had higher death rates than men, an effect apparent until the ninth decade of life. Women, and younger women in particular, were more likely to have other conditions like diabetes, heart failure, stroke and hypertension. Making allowances for these reduced the difference between men and women, but did not eliminate it.

Figure 1: In-hospital CABG mortality

Women also tended to suffer more postoperative complications than men (Table 1), except for bleeding requiring reoperation. Again, the difference between women and men was most marked at younger age.

Table 1: Post CABG complications

Percentage with complication

Complication Men
Bleeding requiring reoperation 3.0 2.9
Postoperative MI 1.3 1.7
Neurological complication 3.8 5.3
Renal failure 4.0 5.0

The cause of the higher mortality and complication rates in younger women is not known. While younger women are more ill, this does not account for all the difference, and an unknown risk factor has been postulated.

BMI and CABG mortality

In the United States a randomised trial of CABG and percutaneous transluminal coronary angioplasty (PCTA) has been going on, and patients declining randomisation have been entered into a concurrent observational registry. Patients have been followed up for five years. Short and long-term outcomes have been analysed according to initial BMI for all CABG patients [2].

Examination of the 1,503 patients undergoing CABG according to BMI showed no effect of BMI on in-hospital events. Three and five-year (Figure 2) mortality after CABG was very significantly affected by BMI. The reason for the link between BMI and mortality is likely to be an increased presence of multiple coronary risk factors in obese patients.

Figure 2: Five year mortality and BMI

Which CABG procedure

Most people undergoing CABG need three bypass grafts, with a single internal mammary artery graft and two vein grafts. The evidence for the use of arterial graft is good, but what about the evidence for using two internal mammary grafts instead of one? A thoughtful meta-analysis indicates that two is likely to be better than one [3].

The review sought studies comparing single and bilateral internal mammary grafts that had at least 100 patients in each group and with at least four years of follow up. Study quality was assessed on a variety of criteria. Particularly interesting was the quality assessment of non-randomised studies (Table 2) that included comparability at baseline.

Table 2: Quality assessment of non-randomised studies

Cohort selection was assessed on the answers to three questions

Were details of criteria for assignment of patients to treatment provided?

One star awarded for relevant details.

How representative was the exposed cohort?

One star if representative of typical patient undergoing CABG, no star if groups of patients were selected or selection of group was not described.

How was the non-exposed cohort selected?

One star if drawn from the same community as the exposed cohort; no star if drawn from a different source, or selection of group not described.

Cohort comparability was assessed on the basis of study design or analysis of cohort differences

No differences between the two groups, or differences controlled for, in particular with reference to age, sex, ventricular function or diabetic status (two stars). One star was assigned if one of these four characteristics was not reported, even if there were no other differences between the groups, and the other characteristics had been controlled for. No star was assigned if the two groups differed.

Outcome was assessed by two criteria

Assessment of outcome

One star for information ascertained by record linkage or interview, no star if this information was not reported or ascertained in some other way.

Adequacy of cohort follow-up

One star if no patient or fewer than 20% of patients were lost to follow-up; no star if more than 20% of patients were lost to follow-up, or if the researchers did not provide relevant information.

There were 10 reports included, and seven had information that could be combined. None of the reports was a randomised trial, but most scored highly in the quality assessment exercise. The bulk of the patients (81%) and deaths (87%) were in two large studies.

The seven studies had 15,962 patients, and there were 679/4693 (14%) deaths at longest follow up with bilateral interior mammary artery grafts, and 2482/11269 (22%) for single mammary artery grafts. This was statistically significant, with a hazard ratio of 0.80 (0.70 to 0.94) (Figure 3). If one were to calculate an NNT for bilateral versus single internal mammary artery grafts to prevent one long-term death it would be 13 (11-16). Redo surgery rates were also lower with bilateral grafts (8% versus 40% in the largest trial).

Figure 3: Single and bilateral IMA grafts


There is a fascinating wealth of evidence of different types available in the field of surgery, including many systematic reviews and meta-analyses of randomised trials, together with large comprehensive registries, and much thoughtful stuff besides. For CABG even a cursory examination of evidence tells us much about likely short- and long-term survival. But we have also to remember that in 1958 randomised trials told us that internal mammary artery ligation for angina was ineffective. So randomisation is not to be overlooked! Caveat lector.


  1. V Vaccarino et al. Sex differences in hospital mortality after coronary artery bypass surgery. Evidence for higher mortality in young women. Circulation 2002 105: 1176-1181.
  2. HS Gurm et al. The impact of body mass index on short- and long-term outcomes in patients undergoing coronary revascularization. Journal of the American Collage of Cardiology 2002 39: 834-840.
  3. DP Taggart et al. Effect of arterial revascularisation on survival: a systematic review of studies comparing bilateral and single internal mammary arteries. Lancet 2001 358: 870-875.
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