To the Editor:
I read with interest the report of Davis et al. 1and the accompanying editorial,2which describe and discuss the observation that the outcome of endovascular therapy for acute stroke is much worse when accompanied by general anesthesia compared with local anesthesia with sedation. The systolic blood pressures were higher in the sedated patients, and it was suggested that adequate blood pressure control could ameliorate the outcomes observed in the general anesthesia group.
Although I agree that adequate blood pressure control is always to be recommended, I wish to propose that there was another important difference between the two groups that was not addressed in these two reports and could well be significant in contributing to the outcome of the two groups: the partial pressure of arterial carbon dioxide (PaCO2).
The PaCO2of the sedated patients would have been greater than normal because of (hopefully mild) respiratory depression, whereas the PaCO2would have been lower than normal in the patients during general anesthesia because patients are traditionally hyperventilated, especially in neurosurgical cases. In response to PaCO2, there would be cerebral vasodilation in the sedated, hypercarbic group with spontaneous ventilation and cerebral vasoconstriction in the anesthetized, hypocarbic with controlled ventilation.
It has been shown that hyperventilation and hypocapnia in head-injured patients result in poor clinical outcome.3Similarly, it is quite plausible that hyperventilation is detrimental to patients with acute stroke. In fact, the report by Davis et al. could be interpreted as showing that hypercarbia might have a salutatory effect on the outcome for these patients.
It is unlikely that arterial blood gases were measured often enough in this retrospective study for meaningful comparisons between the groups. However, prospective studies could be designed to compare the effect of different levels of ventilation on the outcome of acute stroke patients requiring general anesthesia for endovascular therapy.
Until the results of such a study become available, I suggest that the difference in outcome between the two groups of patients (local anesthesia with sedation vs . general anesthesia) could, at least partially, be explained by the difference in PaCO2between the two groups, and therefore should have been discussed in the article and editorial.