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Ann Thorac Surg 2004;78:1513-1514
© 2004 The Society of Thoracic Surgeons
Guy's Hospital, St. Thomas St, London SE1 9RT, UK
donaldc{at}doctors.org.uk
To the Editor:
I was interested to read the randomized study by Lund and colleagues [1] in which they reported decreased intraoperative cerebral microemboli in patients having off-pump coronary artery bypass grafting compared with those having an on-pump operation. The findings are important and confirm the results of other nonrandomized, studies. However, arterial line filters are known to reduce microemboli during cardiopulmonary bypass [2]. Some arterial line filters are better than others at limiting microemboli [3]. Therefore, it would be very helpful to know whether arterial line filters were used in the on-pump group and, if so, what type. On the basis of the number of microemboli reported, I expect that filters were used, but this should have been noted. The method of pH control during hypothermia is also important in determining neuropsychological outcome and, possibly, microemboli delivery to the brain [4]. When comparing two methods such as on-pump and off-pump operations and measuring postoperative neuropsychological change, albeit as a secondary outcome, one should demonstrate that each method is being made as safe in neuroprotective terms as possible. It cannot be assumed that alpha-stat strategy and arterial lines are universally used.
When relating the number of microemboli to neuropsychological outcome, Lund and colleagues were rightly circumspect. The Doppler consensus criterion for detecting microemboli is based on an arbitrary size of gaseous or particulate emboli, and the pathological effect of both types remains speculative. The use of methods for distinguishing between gaseous and solid microemboli has not yet been reported in clinical practice. As Lund and associates pointed out, both inflammation and altered cerebral blood flow may also potentially affect neuropsychological outcome.
In regard to cerebral blood flow, transcranial Doppler is a useful method to simultaneously record both middle cerebral artery blood velocity and microembolic events. It is disappointing that Lund and colleagues did not take the opportunity to measure or report blood velocity in the two study groups. There may have been profound differences in velocity between the on-pump and off-pump groups. In addition, it would have been helpful to know how the authors controlled systolic blood pressure or whether they measured cardiac output and central venous pressure during the off-pump procedures. These hemodynamic variables can influence cerebral blood flow. In the absence of such information, it remains possible that an adverse effect of off-pump surgical procedures on cerebral blood flow counteracts the beneficial reduction in microemboli. This is one potential explanation for the absence of a difference in neuropsychological outcome between the two groups. Another hypothesis is that it is the reduced cerebral blood flow in off-pump operations that causes fewer microemboli to be detected in the cerebral circulation rather than the technique of off-pump surgery producing less microemboli.
In terms of the analysis of neuropsychological outcome, I was surprised that although Lund and co-workers wrote that "a standardized total sum score assessing cognitive function across the selected 12 measures was computed," this was not reported in their results. As has previously been discussed [5], the alternative deficit scores used are less sensitive and can result in higher numbers of false-positive findings. This may also help to explain the similarly high incidence of decline in both the on-pump group and the off-pump group at 3 months postoperatively.
References
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