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Assessment of neurocognitive impairment after off-pump and on-pump techniques for coronary artery bypass graft surgery: prospective randomised controlled trial

BMJ 2002; 325 doi: https://doi.org/10.1136/bmj.325.7375.1268 (Published 30 November 2002) Cite this as: BMJ 2002;325:1268

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Re: alpha stat vs pH stat during cardiopulmonary bypass

The primary determinant of organ dysfunction and failure in patients
having cardiac surgery or indeed any form of elective surgery would seem
to be the inadequacy of mitochondrial oxidative phosphorylation as the
gastric tonometry data from Guy's and elsewhere suggest. The issue,
therefore, is whether the use of alpha stat measurements gives a better
measure of the adequacy of oxidative phosphorylation than the use of pH
stat measurements.

At any given temperature the intramitochondrial pH is higher than
blood pH which is higher than the cytosolic pH (1). More importantly the
pH gradient between these different compartments is maintained over a wide
range of temperatures, the pH in all compartments (alpha stat) being
linearly related to the temperature.

ATP synthesis is in terms of Mitchell's hypothesis dependent upon the
protonmotive energy charge or the magnitude of the proton gradient between
the mitochondrial matrix and the cytosol. One question is, therefore, does
the magnitiude of the proton gradient also remain constant as the
temperature changes. The issue is more complicated for the energy required
to drive active transport processes increases and the need for energy
released by ATP hydrolysis decreases as the temperature falls.

Tonometrics Inc looked at the effect of body temperature on the
measurements of pH made with tonometers some fifteen years ago and was
unable to establish that the temperature of a water bath into which a
tonometer was inserted in vitro made any difference to the manner in which
the tonometric pH was derived. I am not aware of any investigator who has
compared the predictive ability of alpha stat measuremetns with pH stat
measurements.

Mr Whitaker observes in his electronic response to this article that
Mr Taggartt at Oxford uses alpha stat measurments and implied that he
thought they might be of better predictive value for outcome than pH stat
measurements. If so are the data available for public scrutiny?

1. Hochachka PA, Somero GW. Biochemical adaptation. Oxford University
Press, New York, NY, 2002.

Competing interests:  
None declared

Competing interests: No competing interests

18 January 2003
Richard G Fiddian-Green
None
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