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LICOX

Karim Brohi karim at trauma.org
Tue Oct 11 01:48:02 BST 2005


Guy,

You don't have to apologise :-)

I know the theory - but I'm not sure how much the pO2 of the interstitium contributes to these measurements.  Animal models show
PbrO2 correlating well with cerebral blood flow and PaO2 and possibly the cerebral metabolic rate for oxygen (though I have yet to
see convincing data on this) but not with much else.  Of course in vitro you can get the electrode to equilibrate with interstitial
fluid - but I doubt this has much effect in vivo.

Also I'm sure you're not suggesting that the probe is able to measure O2 tension at different distances between the blood vessel
endothelium and the neuron, so I'm not sure how the diffusion distance will come into play.   Neuronal O2 utilization will affect
the PvO2 which will pake up part (how much?) of the PbrO2 reading.
I can't find any work by Maas (or anyone else) that shows that brain tissue oximetry readings are reflecting interstitial O2 and not
some venous/arterial mixture.

Karim

-----Original Message-----
From: trauma-list-bounces at trauma.org [mailto:trauma-list-bounces at trauma.org] On Behalf Of Guy Jackson
Sent: 10 October 2005 12:08
To: Trauma & Critical Care mailing list
Subject: Re: LICOX


Karim,

Sorry, but it is even more complex than this. What you have argued is true at the capillary level, but the Licox samples the
interstitium. Thus local oedema can cause increase in what has been termed 'diffusion distance', thus reducing the levels of oxygen
the cells are exposed to; or the cells can become more active and reduce local levels of oxygen by increased usage (with the failure
of coupling of supply and demand that can occur). This may be one of the effects in spreading depression. All the factors you have
mentioned (SaO2, Hb, etc) are also relevant, and should be corrected first. However, once you have done so you must conceder the
concept of 'diffusion distance', in which case providing more dissolved oxygen (i.e.
over-pressure) might be of help. You can certainly raise interstitial levels of oxygen this way (measured either by Licox or
Neurotrend), as evidenced by the work of Andreas Maas (and others). The question that desperately needs to be answered is: Can you
improve outcome this way? The only interventional study I know of used a rise in CPP as it's intervention, and used ten time too few
patients when you do the power analysis. It is no surprise that it reached a p value of 0.1, but I am unsure what this means in this
setting.

Personally, I fully agree that Licox is a sensitive marker that something is wrong, but I think it can tell us a little more than
this (compared to say CBF monitoring). The trouble comes when you try and decipher the meaning from the data in front of you, given
the multiplicity of things that could be wrong. There are a lot of band wagons being jumped on out there, and things are only going
to get worse with new modalities (microdialysis, etc) coming along. What is needed is a system to organise the care of these often
complex patients. Here's the one I teach:

1. Sort out the ICU basics

2. Only when 1 is sorted, sort out the goal directed therapy (OK they can go together).

3. Sort out the extras.

4. Start again.

Cheers,

Guy

----- Original Message ----- 
From: "Karim Brohi" <karim at trauma.org>
To: "'Trauma & Critical Care mailing list'" <trauma-list at trauma.org>
Sent: Saturday, October 08, 2005 5:42 AM
Subject: RE: LICOX


Keith,

The LICOX brain tissue oxygen electrode is a very interesting and potentially very useful device, but as yet our understanding of it
is incomplete and it is very difficult to come up with protocols incorporating it into clinical use (it's hard enough to get units
to follow even baseline brain injury management guidelines).  Mis-use of the probe is rife and one potential misunderstanding of
what the probe is actually measuring is potentially very harmful (I'll elaborate). Clearly ICP and CPP is not the full story for
brain injury management, just as maintaining blood pressure at the expense of flow in shock has been shown to be detrimental at
certain times and in certain pathophysiological conditions.  What is clear is that not all brain injuries are the same, and that the
same brain injury responds differently at different times in its management.

The short answer to your question is that patients should be managed along standard traumatic brain injury guidelines.  When a
change in the PbrO2 (partial pressure of oxygen in the brain) is noted on the LICOX, this should prompt a search for the cause:
recheck ABG, ICP & CPP, ventriculostomy catheter, consider repeat CT etc. DO NOT simply turn up the FiO2.

What does the PbrO2 measure?  Well it is not measuring the neuronal PO2.

Here's my take on what PbrO2 is reflecting:  It IS measuring some ratio of arteriolar, capillary and venous O2.  So although there
is some measure of local neuronal oxygen utilization from the venous measurement, much of the measurement is derived from the PaO2.
ie.  PbrO2 = x.PaO2 + y.PvO2 (of that local area of the brain).  Thus while the LICOX is showing you some degree of "tissue
oxygenation" from the venous measurement it is heavily weighted towards any change in the PaO2.

But it's also not that simple, as data shows that PbrO2 is affected by flow (ie. cardiac output) as well as the PaO2.  In essence
then, the PbrO2 is a surrogate measure for oxygen delivery to the brain (or the area of brain tissue it is sitting in).  BUT, and
here is the important part, it is only measuring part of the story, and if you don't understand that, then if you base your
treatment only on the PbrO2 you will make severe errors.

Here's the O2 delivery equation you know:
O2 delivery = Cardiac Output * Oxygen concentration of blood
= CO* [O2 bound to haemoglobin + O2 dissolved in the blood] (using CO really to mean arterial flow in the brain) = CO *
[1.34*Hb*SaO2 + PaO2*0.0003] or = CO*[1.34*Hb*Sao2] + CO*[PaO2*0.0003]

and for the brain:
= CBF*[1.34*Hb*SaO2] + CBF*[PaO2*0.0003]

The PbrO2 is only measuring the right hand side of the equation - CBF*[PaO2*0.0003].  For brain injured patients who are being
appropriately managed by guidelines their Hb is adequate and o2sats should be in the 96-100% range - ie on the flat part of the Hb
dissociation curve. Increasing the PaO2 will have no effect on the SaO2.

Thus the left hand side of the equation is not reflected at all in the PbrO2 - and yet the right hand side contributes nothing to
brain oxygenation (because of the 0.0003).

So...  you're patient with a LICOX probe in develops a drop in the LICOX reading.  MANY people treat this by turning up the FiO2.
The LICOX reading will improve, because PaO2 will rise.  BUT brain oxygenation will be unchanged because CBF*1.34*Hb*SaO2 will be
unchanged.  The REASON the LICOX reading fell is because CBF, Hb or SaO2 fell - and most usually that will be because cerebral blood
flow has reduced.  Increasing the FiO2 will have had NO effect except to increase the LICOX reading.

So currently I believe the LICOX has a place, and that is as a sensitive indicator that something is going wrong (a canary if you
like) with cerebral blood flow and oxygen delivery - and therefore a change in the PbrO2 should prompt a search for this.  The LICOX
reading is not a number to be treated in exclusion.

Personally I also think it is difficult to evaluate the PbrO2 without a jugular bulb catheter in place measuring global O2
utilization (despite the problems with these catheters).  Of course the fundamental problem with all this is that can monitor the
hell out of the brain but we still only have about 5 possible therapeutic interventions.

Hope this is of value.  Be interested in other views on what PbrO2 measures or how people are using these probes. Are there many
units out there who are routinely putting LICOX probes in their brain injured patients?

Karim







-----Original Message-----
From: trauma-list-bounces at trauma.org [mailto:trauma-list-bounces at trauma.org]
On Behalf Of Lamb, Keith D.
Sent: 06 October 2005 03:04
To: 'Trauma & Critical Care mailing list'
Subject: LICOX




Anyone using LICOX to monitor brain tissue oxygenation in traumatic brain injury. If so, what are you experiences, your standard
parameters, and any data that you have to support its use?

Keith

Keith D. Lamb RCP, RRT
Team Leader/Charge Therapist
Newark, Delaware

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