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FACP. Colegio de médicos de Tarragona Nº 4305520 / fgcapriles@gmail.com




Monday, May 30, 2016

The Cerebral Circulation and Sepsis-Associated Delirium

PulmCCM - May 28, 2016 - By JE
"The Journal of Intensive Care has newly published a series of sepsis-related organ dysfunction reviews. Additionally, a comprehensive yet concise overview of the cerebral circulation was just disseminated. This summary draws on both of these terrific primary resources as a point-of-departure for discussion of sepsis-associated delirium [SAD].
Cerebral blood flow [CBF] ultimately depends on 1. the arterial pressure head at the entrance to the cranial vault, 2. the cerebrovascular resistance and 3. the down-stream pressure typically estimated as the intra-cerebral pressure [ICP]. For pulmonary physiology enthusiasts, there is a glaring analogy here between alveolar pressure and intra-cerebral pressure. That is, waterfall physiology [or West Zone II in the lungs] tends to occur within the cranial vault as ICP supersedes jugular venous pressure. In other words, the pressure gradient is between the arterial input pressure and the pressure surrounding the veins [i.e. ICP]. This occurs because, normally, the central venous pressure [and therefore jugular venous pressure] approaches atmospheric pressure. Importantly, in states of very high central venous pressure [CVP] [e.g. acute cor pulmonale] and low ICP [e.g. hemicraniectomy, CSF over-drainage], the cerebral perfusion pressure may hold the CVP – rather than the ICP – as its pressure sink. In such a scenario, the transmural pressure of bridging veins will rise giving risk to their rupture..."
Figure 1: The normal cerebral autoregulation curve in orange. The x-axis is cerebral perfusion pressure [CPP] in mmHg, defined as the mean arterial pressure [MAP] less the intra-cerebral pressure [ICP]. Cerebral blood flow [CBF] is on the y-axis and is in litres/minute. The cartoon depicts the change in calibre of cerebral arterioles to defend CBF in response to changes in CPP. The pressure reactivity index [PRx] - in dashed lime green - is described in the text.