Happy New Year to all!
So trying to catch up on some reading, here is an interesting paper I came across. Young et al did a retrospective study on post-arrest BP, in an attempt to answer the very pertinent and important question as to whether or not a higher MAP may confer better neurological recovery, which is a very sensical hypothesis. After all, a brain with potential swelling, both of tissue and endothelium, may “need” a higher BP. Some societies have advocated for a higher MAP than is usually targeted (i.e. 65) and in studies this has been anywhere from 60 to 100. In their particular institution (Vanderbilt) the protocol aimed for 80-90.
Here it is:
So what did they find?
Basically, they were unable to demonstrate that a higher MAP – in this case defined as achieving 80 mmhm – improved anything, with a follow up to 3 months. There was also no increased mortality related to the use of vasopressors.
So, why might this be? Well, I think there are a couple of important principles to review, especially for the novices reading this.
1. Pressure does not equal flow. The relationship between pressure and flow is a complex one and depends on the interaction between the pump (CO) and the circuit resistance (SVR). Pressure rises when resistance is increased, output is increased, or both. If resistance is increased without increasing output – or by a disproportionate increase in resistance vs output, flow decreases. The effect of vasopressors such as norepinephrine is complex, with both vasoconstriction and increased cardiac output (both via beta stimulation and via increased venous return), and depends on volume status, alpha sensitivity and the recruitable cardiac reserve.
So…? This means that on the surface, a BP number tells you little about flow. The same MAP may represent a highly vasoconstrictor, low-flow state, or a normal flow state. Obviously, a certain minimum pressure is required, to drive the flow from artery down thru the capillaries, but what that number is is unclear. So when looking at any study using simply MAP without another assessment of flow, one cannot draw a conclusion that improving hemodynamics may not help the situation. How does one assess this – in all likelihood an integrated approach using ultrasound (volume status, cardiac function), tissue saturation (cerebral/somatic oximetry) and possibly other technologies, including simple physical exam looking at skin mottling. This type of information could categorize patients into flow categories and make results much more interesting and applicable.
Note that this isn’t really criticism on the authors – it would be impossible to do this on a retrospective study, but simply food for thought for further studies to come.
2. The N=1 principle: remember that we are never treating hundreds of patients at once, and we do not have to decide what is best for most (which is what an RCT generally answers) but what is best for the one patient we are treating. Hence, looking at any one patient and saying that the target BP should be 65 vs 80 based on this study is incorrect. What we should be saying is that aiming for a higher MAP may not be necessary if we feel that the patient is well perfused at 65. How each of us figures that out will depend on individual skills and available technologies, but to simply aim for 65 without further thought and assessment is relinquishing your MD in favour of the printed word, essentially what any paper protocol could do.
In the next post I’ll discuss the use of tissue oximetry and how it can be used as part of a strategy to optimize vasopressor use and MAP targets.
Thanks and love to hear your opinions!
Oh, and don’t forget to register for CCUS 2015 at http://www.ccusinstitute.org, and for more info at http://wp.me/p1avUV-bh. In those couple of days, Paul Marik, Scott Weingart (@emcrit), Josh Farkas (@pulmcrit), and a bunch of other totally amazing speakers will be talking about this stuff, and more!