Letter Regarding Article by Jin et al, “Is Obesity a Risk Factor for Mortality in Coronary Artery Bypass Surgery?”
To the Editor:
To date, there are more than 20 published reports, with that by Jin et al1 being the latest, that describe the relationship of body mass index/obesity with postoperative CABG mortality. Almost all studies report the fact that risk of mortality is high in underweight patients and decreases as an inverse function of BMI, until the threshold of class III obesity is reached, where the risk becomes high again. Gurm et al2 recently showed that a similar trend prevails in patients undergoing PTCA. This takes the perspective away from the surgical procedure and puts the focus on finding confounding factors related to obesity that might be at the root of this pattern.
A possible explanation of high risk in underweight patients is the small diameter of coronary arteries. This is a controversial issue, because variable results have been observed in several studies in South Asians.3 Another explanation comes from the results of several prospective studies on hypertension that have shown lean hypertensive subjects to be at a greater risk of cardiovascular death. Lean hypertensive subjects have a higher prevalence of isolated systolic hypertension, which confers a higher cardiovascular risk than isolated diastolic hypertension or systolic-diastolic hypertension, the latter 2 types being more prevalent in obese hypertensive individuals.
We therefore recommend further data analysis with stratification of hypertensive patients into those with isolated systolic hypertension and those with isolated diastolic hypertension or systolic-diastolic hypertension. Analysis of data after exclusion of patients with isolated systolic hypertension will make the results more meaningful and may help in better risk prediction.
Dr Ahmad and colleagues suggested doing further analyses with the stratification of hypertension patients by systolic hypertension, diastolic hypertension, or systolic-diastolic hypertension. Although we cannot do this because hypertension was collected as a yes-or-no variable, we did further analysis on the relationship of hypertension, body size, and mortality for isolated CABG surgery.
The prevalence of hypertension was increased with increasing body size: 36% of the underweight patients had hypertension, and the percentage went up to 50% for extremely obese patients. The raw mortality rates for underweight patients with or without hypertension were 15.6% (n=32) and 3.4% (n=58), respectively, whereas the mortality rates for non-underweight patients with or without hypertension were similar to each other, at 2.3% (n=6306) and 2.1% (n=9581), respectively. When hypertension was forced into our current logistic regression model, the OR was 1.076 (P=0.528), which indicated that hypertension was an insignificant additional risk factor for mortality. Also, we forced the variable “underweight” (body mass index <18.5 kg/m2) into the model, and its OR was1.821 (P=0.165). This means underweight is not a significant additional risk factor, which is in agreement with the results we found in our original study.1 Then, we added the interaction term of hypertension and underweight into the model and found that its OR was 3.6 (P=0.020). This is an interesting finding that should be further investigated: lean hypertensive patients may be more at risk when undergoing CABG surgery.
As Dr Ahmad and colleagues mentioned, some studies found that lean hypertensive subjects were at higher risk of cardiovascular death. One study found that the reason was due to deleterious lifestyles, particularly smoking and excessive alcohol intake.2 Further research may be needed to discover the reason that lean hypertensive individuals are more at risk for CABG surgery.
Jin R, Grunkemeier GL, Furnary AP, Handy JR Jr. Is obesity a risk factor for mortality in coronary artery bypass surgery? Circulation. 2005; 111: 3359–3365.
Stamler R, Ford CE, Stamler J. Why do lean hypertensives have higher mortality rates than other hypertensives? Findings of the Hypertension Detection and Follow-up Program. Hypertension. 1991; 17: 553–564.