Jugular Bulb Saturation (1995)

In Response: We would like to thank Dr. Inglis for his interest in our article regarding the intraoperative use of jugular venous bulb catheters during neurosurgical procedures [1]. We are well aware of the controversy regarding the venous drainage of the brain, but the predominant view at the time of writing is that the difference in venous oxygen saturation between the two sides in the majority of instances is small [2], and that two thirds of the blood drained is from the ipsilateral hemisphere [3]. Thus, we adopted the approach of preferentially cannulating the jugular bulb ipsilateral to the surgical lesion, and presumably draining areas at the highest risk of cerebral ischemia. The recent reports of Chieregato et al. [4] and Stocchetti et al. [5] on bilateral jugular bulb venous monitoring and the differences between them certainly added more interesting discussion to this issue. One could hardly argue with the points that Dr. Inglis made in his letter, most of which we addressed in our original manuscript, although he did not make any specific recommendation. It is clear that a difference in oxygen saturation can exist between the two jugular bulbs, and this difference is likely accentuated in the presence of a severe unilateral injury. What is at issue, however, is which side is more representative of the global cerebral oxygen saturation. As illustrated by Robertson et al. [6] in their comments on the report by Stocchetti et al. [5] (not mentioned by Dr. Inglis), blood from the dominant jugular vein should be more representative of mixed cerebral venous blood. For example, consider the situation in which 75% of blood flow is going to the right jugular vein with an oxygen saturation of 70%, and 25% is going to the left side with an oxygen saturation of 50%. The true mixed cerebral venous saturation would be (70% times 0.75) + (50% times 0.25) = 65%. This value more closely approximates the saturation from the dominant side. Since Stocchetti et al. did not measure flow and were therefore unable to actually determine the dominant side, it is not surprising that they were equally unable to determine the most appropriate side for monitoring. In the absence of an intracranial pressure monitor and thus the ability to test the dominance of the jugular vein using the method described by Andrews et al. [7], we suggest it is appropriate to cannulate the jugular bulb ipsilateral to the operative hemisphere, which can be considered to be at a higher risk of ischemia due to surgical retraction and manipulation. In this context, we would like to emphasize the point that we made in the original manuscript; jugular bulb oxygen saturation monitoring, by its very nature, has high specificity but low sensitivity. Thus a normal saturation may conceal areas of potential ischemia, but a low saturation is indicative of the presence of regions with low flow. This is valid irrespective of the site chosen for jugular bulb oxygen saturation monitoring. Finally, we agree with Dr. Inglis that, with proper caution, monitoring jugular bulb oxygen saturation provides valuable physiologic information. Furthermore, similar to any other physiologic variable, jugular bulb oxygen saturation should always be interpreted within the context of the complete clinical picture. Basil F. Matta, MB, FRCA Department of Anaesthesia, Addenbrooke's Hospital, Cambridge CB2 2QQ, England Teresa S. Mayberg, MD Arthur M. Lam, MD, FRCPC Department of Anesthesiology, Harborview Medical Center, University of Washington, Seattle, WA 98104