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Cerebral oxygen desaturations during single lung ventilation
- Thomas M Hemmerling (29 December 2008)
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Significance in cerebral oxygen desaturation
- Mark E Edsell (22 December 2008)
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Thomas M Hemmerling McGill University
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Editor – We thank Dr. Edsell for his interest in our study. We agree with Dr. Edsell that various conditions can influence the distribution of cerebral oxygen in venous and arterial vessels; we have recently demonstrated,(1) that lateral patient positioning can have an influence on the right and left cerebral oxygen saturation. Despite these differences – with the upper hemisphere generally having higher saturations than the lower hemisphere – being significant, they are so small that they cannot explain differences of the magnitude described in our article.(2) In addition, decreases of cerebral oxygen saturation of more than 15% from baseline cannot be explained by simple positioning of the patient and occur only during single lung ventilation (SLV). There was no correlation between the maximum cerebral oxygen saturation decrease and the maximum peripheral saturation decrease during SLV; we believe therefore that other factors might be more important, increase of intrathoracic pressure or impairment of venous flow during thoracic surgery with SLV, as Dr. Edsell pointed out, are some of the theories we currently investigate. Whatever pathophysiologic hypothesis will be proven, the very significant drops in cerebral oxygen saturation during thoracic surgery with SLV, of the same or even bigger magnitude as in cardiac surgery,(3) are certainly astonishing and potentially harmful to patients. Future studies will have to focus on the impact of cerebral desaturations on postoperative morbidity after thoracic surgery. References 1 Hemmerling TM, Kazan R, Bracco D. Inter-hemispheric cerebral oxygen saturation differences during thoracic surgery in lateral head positioning. Br J Anaesth. 2009; 102: 141-2. 2 Hemmerling TM, Bluteau MC, Kazan R, Bracco D. Significant decrease of cerebral oxygen saturation during single-lung ventilation measured using absolute oximetry. Br J Anaesth. 2008; 101: 870-5. 3 Murkin JM, Adams SJ, Novick RJ, et al. Monitoring brain oxygen saturation during coronary bypass surgery: a randomized, prospective study. Anesth Analg. 2007; 104:51-8. Conflict of Interest:None declared |
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Mark E Edsell
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Editor - Hemmerling and colleagues[1] attribute the reduction in regional cerebral oxygen saturation (rSO2) seen in their study to the pathophysiological changes caused by single lung ventilation but other causes should also be discussed. As the article describes, cerebral oximetry is calculated from the absorption of Near Infra-Red (NIR) light by haemoglobin in the entire microvascular tissue bed and thus reflects the oxygenation in all tissues and vessels in the locality of the probe. However, it does not take into account changes that may occur in the relative proportions in the venous and arterial blood in those tissues. Therefore, any factor that could potentially alter these proportions will lead to an apparent change in rSO2.[2] Causes of venous hypertension such as raised intrathoracic pressure, coughing, venous occlusion or patient position may lead to cerebral venous engorgement and an apparent drop in rSO2 without any real change in the cerebral oxygenation or extraction. Hypoxia too has been shown to change the arterio-venous blood proportions leading to changes in rSO2.[3] Whilst it remains likely that the primary cause for the reduction in rSO2 in this study is the hypoxia caused by SLV, other factors that could have contributed to this should also have been considered. Cerebral oximetry remains a useful tool for monitoring changes in cerebral oxygenation but, like all monitors, correctly interpreting these changes is paramount. Mark Edsell London, UK markedsell@docotrs.org.uk References 1. Hemmerling TM, Bluteau MC, Kazan R and Bracco D. Significant decrease of cerebral oxygen saturation during single-lung ventilation measured using absolute oximetry. Br. J. Anaesth. 2008 101: 870-87. 2. Pattinson KT, Imray CH, Wright AD. What does cerebral oximetry measure? Br J Anaesth 2005; 94(6): 863-4 3. Wolf CB, Imray CHE. Partitioning of arterial and venous volumes in the brain under hypoxic conditions. Adv Exp Med Biol 2003; 540: 19-23 Conflict of Interest:None declared |
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