Regular readers will remember my interest in the influence of specific vasopressors on cerebral oxygenation. You can have a look at my postdoc work here and posts related to that topic here, here, here, here and here.
We have just published a new study regarding that issue, this time in diabetics undergoing cardiac surgery. A potential reduction in brain oxygenation following administration of norepinephrine or phenylephrine could elevate the risk of cerebral ischemia in these patients, a clinical population already at risk for the development of vascular cognitive impairments, transient ischemic attack and stroke. Still, we did not know to date the influence of norepinephrine and phenylephrine on cerebral oxygenation measured by near-infrared spectroscopy during cardiopulmonary bypass in diabetics.
Accordingly, we decided to quantify changes in cerebral oxygenation during administration of norepinephrine and phenylephrine during cardiopulmonary bypass in diabetics and non-diabetics (our goal was not to compare the two vasopressors).
To do so, norepinephrine (6 diabetics; 8 non-diabetics) or phenylephrine (8 diabetics; 9 non-diabetics) was administered intravenously to maintain mean arterial pressure above 60 mmHg during cardiopulmonary bypass.
Mean arterial pressure, venous temperature, arterial oxygenation, and cerebral oxygenation were recorded before anesthesia induction (baseline) and continuously during cardiopulmonary bypass.
Interestingly, cerebral oxygenation was lowered to a greater extent in diabetics vs. non-diabetics with the administration of norepinephrine (-14±13% vs. 3±12%; p<0.05), and tended to be lowered to a greater extent in diabetics vs. non-diabetics with the administration of phenylephrine [-12±8% vs. -6±7%; p=0.1] during cardiopulmonary bypass.
Basically, the results of our study suggest that administration of norepinephrine to restore mean arterial pressure during cardiopulmonary bypass is associated with a reduction in cerebral oxygenation (measured by near-infrared spectroscopy) in diabetics but not in non-diabetics. Administration of phenylephrine is associated with a trend towards a greater reduction in cerebral oxygenation in diabetics compared to non-diabetics.
Although these findings are really interesting, we have highlighted some issues in our discussion:
1) Further studies are necessary in order to support our findings while strictly controlling for potential confounders;
2) Further studies are necessary in order to investigate which mechanism is responsible for the reduction in cerebral oxygenation during cardiopulmonary bypass with the utilization of norepinephrine and phenylephrine in these patients;
3) The small number of subjects precludes the generalization of these findings to the whole population of diabetics undergoing cardiac surgery;
4) 60 mmHg is not necessarily the lower limit of cerebral autoregulation for all patients undergoing cardiac surgery. Accordingly, maintaining mean arterial pressure at 60 mmHg represents a potential limitation of this study, since brain blood flow could be (or not) autoregulated at this mean arterial pressure in these patients;
5) The lowering in cerebral oxygenation, measured by near-infrared spectrocopy, following administration of norepinephrine, could partly be explained by changes in skin blood flow.
Brassard P, Pelletier C, Martin M, Gagné N, Poirier P, Ainslie PN, Caouette M, Bussières JS. Influence of norepinephrine and phenylephrine on frontal lobe oxygenation during cardiopulmonary bypass in patients with diabetes. J Cardiothorac Vasc Anesth. 2013 Dec 17. pii: S1053-0770(13)00513-2. doi: 10.1053/j.jvca.2013.09.006. [Epub ahead of print]