Cerebral perfusion pressure as a therapeutic target

Question 1 from the second paper of 2009 and Question 24 from the second paper of 2006 asked for a detailed discussion about the utility of using CPP as a therapeutic target. Unfortunately, the college model answer was very brief; it probably reflects some sort of minimal level of competence. An ideal synopsis of this issue is offered by LITFL. No resource improves on the brevity or clarity of their short note on this concept. If brevity and clarity are not your thing, then you should probably read this 2013 review article from the BJA, by Kirman and Smith, and the canonical "Guidelines for the management of severe traumatic brain injury" from the Brain Trauma Foundation.

In brief:

Relationship of blood pressure, intracranial pressure, cerebral perfusion and blood flow

Cerebral perfusion pressure is the driving pressure gradient which produces flow in the cerebral circulation against the resistance of cerebral vessels. Thus, it is the difference in mean cereberal arterial pressure and the mean cereral venous pressure. Because we can hardly measure the latter and we only guess at the former, a useful approximation is to subtract intracranial pressure (ICP) from the mean systemic arterial pressure (MAP). Thus, CPP = (MAP - ICP).

However, it is not a measure of cerebral blood flow. Flow is a very different property; cerebral blood flow is a function of both the pressure gradient and the resistance. Cerebral vascular resistance might change randomly and regionally, all without any change in systemic MAP. Not only that, but "flow" alone does not determine cereberal oxygenation - there are even more factors involved in this, such as the oxygen carrying capacity of red cells, the viscosity of the blood, etc etc...

Rationale for cerebral perfusion pressure monitoring

Why are we even interested in cerebral perfusion pressure?

• ICP on its own is probably somewhat useless, given that it relates very indirectly to cerebral blood flow.
• When cerebral autoregulation is impaired, the CPP correlates linearly with cerebral blood flow.
• Thus, one can estimate that if one's cerebrovascular autoregulation is regionally impaired, with a CPP around 60mmHg one should have optimal blood flow both to the regions with impaired autoregulation and to the regions where it is preserved.
• Thus, CPP rather than ICP should be the main parameter to guide therapy.
• Targeting a CPP also forces one to actively manage hypotension, which has known negative associations in traumatic brain injury.

Evidence behind this rationale:

• A CPP under 50mmHg seems to be associated with increased brain lactate in the boundary of injured cortex.
• A CPP under 60mmHg seems to be associated with poor outcome in TBI.
• A CPP greater than 60mmHg seems to have little influence on outcome in TBI.
• A CPP over 95mmHg seems to be associated with a poor outcome in TBI .

Advantages of cerebral perfusion pressure monitoring

There are some advantages of using CPP as a treatment target in traumatic brain injury. The brain-injured patient is unable to autoregulate their cerebral blood flow, and thus they rely on you to make sure that their cerebral arterial pressure remains reasonably high. Its easy enough to monitor it continuously if you have both an arterial line and an ICP monitor.

Arguments from convenience and safety

• Cheap and easy to monitor, if the patient already has an arterial line and an ICP monitor.
• Automatic algorithms perform the calculations for you.
• It has been around for some decades, and the staff are familiar with it.
• It can be monitored continuously.
• Continuous monitoring allows one to observe severe fluctuations, and react to them
• CPP monitoring does not negate clinical and radiological assessment of cerebral perfusion- rather, it augments them.

Arguments from authority

• Endorsed by the Brain Trauma Foundation (target CPP 50-70 mmHg).
  • In their model answer to Question 1 from the second paper of 2009 the College makes a statement regarding there being no Class I evidence to suport the use of CPP-guided blood pressure management in traumatic brain injury. True, but the entire set of BTF guidelines only has one single piece of Class I evidence, which is "don't you ever use steroids". Thus, one cannot single out the CPP guidelines as somehow "baseless" on the grounds that no investigator has thus far dared to randomise a group of brain-injured patients to an experimental group in whom the CPP is totally unmanaged.
• Endorsed by the Neuroanaesthesia Society of Great Britain and Ireland (NASGBI) and the Society of British Neurological Surgeons (SBNS) - see their position statement (apparently CPP monitoring "remains the standard of care", though the Societies acknowledge that "there has been recent controversy". This statement references the 2014 Kirkman and Smith article.

Arguments from pragmatic bedside concerns

• If you can calculate the CPP, that means you are monitoring the ICP and the MAP.
• Ergo, if you ignore the CPP, that means you think that either the MAP alone or the ICP alone are more important as therapeutic endpoints (contrary to internationally accepted guidelines). Or you would prefer to rely on something even more wacky and experimental. Sure, its an option. But what would the coroner say?
• Thus, you should target the CPP while there is nothing better to target.

Arguments from evidence

• A trial of CPP vs ICP-targeted management (Kumar et al, 2014) found a 20% difference in 90-day mortality in favour of the CPP-guided group (18.2% vs 38.2%);  however it must be mentioned that the study population were 110 children with meningitis, rather than TBI patients.

Disadvantages of cerebral perfusion pressure monitoring

Counter-arguments from theoretical physiology

• CPP is only a surrogate for cerebral blood flow (CBF).
• As cereberal vascular resistance varies (eg. with varying levels of vasoactive agents and sedatives, and with changes in respiration) the cerebral blood flow will also vary- but the measured CPP could remain the same. You simply wouldn't know.
• With heterogeneous brain pathology, individual variations in cerebral vascular health, and hourly variations in the level of sedation, the "optimal" CPP for an ideal CBF will likely be very different in any given patient, at any given moment. Targeting a CPP tends to ignore the fact that no brain is uniformly injured, and that there are regions which are still able to autoregulate their bloodflow.
• Brain oxygenation is really what we are interested in, but CPP does not correlate very well with the PbO₂ (although when grouped with ICP it sort-of correlates with good perfusion, provided it is over 60mmHg)

Counter-aguments from the uselessness of guidelines

• The recommendations for the use of CPP monitoring are by no means firm. The BTF only has Level II recommedations to make regarding keeping the CPP under 70, and Level III recommendations regarding keeping it above 50.
• The site of transducer calibration should be at the level of the Foramen of Monro, as per NASGBI/SBNS guidelines - but according to a 2013 survey, everybody is ignoring this guideline and referencing the right atrium routinely.
  • This is worrying, because for patients with 50° head elevation, there is an 18mmHg CPP difference - which means that with a heart-referenced CPP of 60mmHg, the "true" CPP is around 42mmHg.
  • This is even more worrying, because the two papers (Changaris and Clifton) referenced by the Brain Trauma Foundation in support of their over-60mmHg CPP target both zeroed their transducers at the level of the heart. In fact, all the studies used by the BTF either used wildly different pressure reference levels, or simply failed to report their methodology. What was the true pressure at the tragus? Who knows.

Counter-arguments from pragmatic bedside concerns

• To maintain a CPP of 60mmHg at the Foramen of Monro with an ICP of 20mmHg, one must maintain a heart-reference-level MAP of 98mmHg, which would probably correspond to an SBP of around 150-170mmHg. This is not a benign blood pressure. In one's desperate bid to protect the brain, one may inadvertantly flood the patient with fluid and cause pulmonary oedema, or exacerbate existing neurogenic pulmonary oedema.
• Thus, therapy to maintain CPP can be harmful.
• Specifically, the monitoring of CPP requires the monitoring of ICP, which requires a hole to be drilled in your head, with all the inconvenience thereof.
• Even with an ICP monitor and an arterial line, you are making measurements which are subject to error, and then you are amplifying that error by calculating a derived parameter from them.

Counter-arguments from hard evidence

• There is no Class I data to support the use of CPP monitoring in TBI.
• Specifically, there is no well-designed trial comparing CPP to another validated therapeutic goal in the setting of adult TBI.
• There is evidence that mindless pursuit of the CPP can be harmful: ARDS tends to develop.