Introduction

Coronary artery bypass grafting (CABG) is performed on at least half a million U.S. citizens annually, providing relief from angina and other symptoms of coronary artery disease. Improvements in anesthesia and surgical techniques have meant that older people can now undergo this operation. It's been known for some time that this type of surgery is accompanied by a temporary decline in mental performance (or cognitive function) in a number of patients. Depending on how this is measured, rates between 35% and 80% of patients have been quoted. The decline measured on leaving hospital generally improves over the next 6 months in most people. Now physicians at Duke University have examined mental performance in bypass surgery patients five years after their operations. At first sight, the results are somewhat disturbing.

Method

261 patients who had bypass surgery were studied. A battery of tests of their mental function were done before surgery, at the time of hospital discharge, and then 6 weeks, 6 months, and 5 years later. Four mental ability areas were tested: verbal understanding and memory, orientation in space, powers of concentration and attention, and visual memory. To get an overall figure, each person's 4 scores were added together to provide a "composite cognitive index".

A calculated fixed difference from the average score (called a standard deviation) was used to define a decrease in mental performance. A drop of one standard deviation amounted to a decrease in performance of about 20% in the particular area being tested.

Only 172 of the 261 bypass patients provided full test data at all five time points. This was because of the physicians couldn't reach some of them, while others were too ill or had died. The average age of the subjects was 61, and most of them (72%) were men.

Results

On discharge from hospital, a reduction in mental performance, as defined above, was found in 53% of the participants. By 6 weeks, the situation had improved - only 36% had reduced mental performance, and by 6 months this number had dropped to 24%. However, the 5-year results showed that as many as 42% of the subjects had a decline in mental functioning.

The participants were then analyzed as two different groups - those who showed a reduced mental performance at discharge, and those who did not. In the first group, the 5-year scores were found to be similar to the discharge scores, in spite of the apparent improvements at 6 weeks and 6 months. On the other hand, in those patients without any sign of reduction in mental performance at discharge, their scores at 5 years were about equal to their pre-operative scores.

Further analyses were done to see what could possibly predict a less-than-satisfactory outcome. The Duke physicians found that older people, those with a lower level of education, those with higher mental performance scores before surgery, as well as the those with a decline in performance at discharge, were all at greater risk of decreased mental function 5 years later. (There was a suggestion that poor heart function (decreased left ventricular ejection fraction) was more common in those who later had cognitive decline, but this was not a significant finding.)

Conclusions

What do these results mean? First, it seems likely that the reduced, or 'better', rates at 6 weeks and 6 months were because the subjects learnt how to answer the test questions better.

Second, we must realize that there was no control group in this survey. In other words, we don't know how many 61-year-olds would have a 20% decline in mental performance over 5 years even if they didn't undergo bypass surgery.

Short-term reduction in mental performance has been reported before in elderly people having non-heart surgery, and this can persist in up to 10% of them for at least two years. In other words, the findings in this study are not exclusive to heart bypass surgery.

The authors of the study aren't sure what's the cause of this apparent side-effect of bypass surgery. Is it because of reduced oxygen reaching the brain during surgery, because of the techniques used, or is it due to small fragments of thrombus breaking off during surgery and blocking small arteries in the brain? Perhaps it's a combination of both? Studies are under way to see if the it's possible to modify or avoid one or other of these possible causes.

We should not be panicked into rejecting coronary artery bypass surgery, which is clearly a most effective way of relieving severe coronary artery disease. A 20% reduction in mental functioning over 5 years in a proportion of patients may not, in fact, impact enormously on their quality-of life, compared to the crippling angina or heart failure they might otherwise suffer.

For people who do experience a significant decline in mental function after surgery, there are steps that can be taken to help restore normalcy, or at least prevent further deterioration - modifications in lifestyle (no smoking, less drinking, more exercise), diet, and medications.

Further studies must be done to determine the full extent of the problem, and to test improved anesthetic and surgical techniques that may help reduce this side-effect of an otherwise valuable procedure.