Question 1

College Answer

Definiton : CPP = MAP-ICP

• Advantages:
• Easily monitored
• Can be monitored continuously
• Nursing staff familiar
• BTF guidelines endorse use of CPP at 60

Limitations:

• Used as a surrogate for CBF.CVR is variable and therefore changes in CBF not detected by CPP
• Does not allow for differential autoregulation in the normal and injured brain.
• Therapy to maintain CPP can result in lung injury
• No Class I data to support use
• Poor correlation between CPP and indices of brain oxygenation

Discussion

The general topic of cerebral bloodflow autoregulation is discussed elsewhere.

More specific information regarding using CPP as a therapeutic target in traumatic brain injury is explored in the Required Reading section.

In brief:

Advantages and Disadvantages of CPP-Guided Therapy

Category	Advantages	Limitations
Physiology	Better relationship to cerebral blood flow than the ICP or MAP alone CPP under 50mmHg is associated with increased brain lactate	Only a surrogate for cerebral blood flow As cereberal vascular resistance varies the measured CPP will remain the same. The "optimal" CPP for an ideal CBF will differ in heterogenous brain apthology, as well as at different times and in different patients CPP does not correlate very well with brain tissue oxygenation.
Pragmatism	Cheap and easy to monitor The staff are familiar with it. It can be monitored continuously. On can react to changes more easily. Augments clinical and radiological assessments of TBI Probably better than MAP or ICP alone	It is unclear where to zero the transducer. Even when guidelines for this are available, they are not being followed.
Support	Endorsed by the Brain Trauma Foundation (target CPP 50-70 mmHg). Endorsed by the Neuroanaesthesia Society of Great Britain and Ireland (NASGBI) and the Society of British Neurological Surgeons (SBNS)	The BTF only has Level II and Level III recommendations regarding CPP. All the studies used by the BTF used very different pressure reference levels to measure the CPP, or failed to report their methodology.
Evidence	Low CPP and high CPP are both associated with poor outcomes. A trial of CPP vs ICP-targeted management found improved mortality (18.2% vs 38.2%) with CPP as the goal	There is no Class I evidence in support of this practice. There is evidence that CPP-guided therapy can be harmful: ARDS tends to develop. Mortality benefit mainly seen in one trial and is otherwise from retrospective cohort studies

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).

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.

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...

In short, as has been demonstrated experimentally, CPP does not correlate well with brain tissue oxygenation.

Furthermore, 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. 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.

Lastly, 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.

Oh's Intensive Care manual

Chapter 52   (pp. 580)  Cerebral  protection by Victoria  Heaviside  and  Michelle  Hayes

 

Harper, A. MURRAY. "Autoregulation of cerebral blood flow: influence of the arterial blood pressure on the blood flow through the cerebral cortex." Journal of neurology, neurosurgery, and psychiatry 29.5 (1966): 398.

 

Phillips, Stephen J., and Jack P. Whisnant. "Hypertension and the brain."Archives of internal medicine 152.5 (1992): 938-945.

 

Paulson, O. B., S. Strandgaard, and L. Edvinsson. "Cerebral autoregulation." Cerebrovascular and brain metabolism reviews 2.2 (1989): 161-192.

 

Busija, David W., and Donald D. Heistad. Factors involved in the physiological regulation of the cerebral circulation. Springer Berlin Heidelberg, 1984.

 

Eriksson, Evert A., et al. "Cerebral perfusion pressure and intracranial pressure are not surrogates for brain tissue oxygenation in traumatic brain injury."Clinical Neurophysiology 123.6 (2012): 1255-1260.