Cerebral Perfusion Pressure in TBI

OVERVIEW

• Cerebral Perfusion Pressure (CPP) = MAP – ICP or CVP (whichever is highest)
• Cerebral Blood Flow (CBF) = CPP/CVR             [CVR = cerebral vascular resistance]
• Brain Trauma Foundation (BTF) guidelines support a target CPP of 50-70 mmHg in patients with severe Traumatic Brain Injury

AUTOREGULATION

• Under normal circumstances, the brain is able to maintain a relatively constant CBF of approximately 50 mL per 100 g/min over a wide range of CPP (approximately 60 to 150 mm Hg).
• Autoregulation may be absent or altered in the injured brain
• See CBF vs SBP graph here (Fig 1)

ADVANTAGES OF CPP

• easily monitored
• can be monitored continuously
• nursing staff familiar
• endorsed by BTF (target CPP 50-70 mmHg)
• may prevent secondary injury from hypoperfusion (e.g. ischemia) or hyperperfusion (e.g. increased edema)
• if ICP is increasing and CPP compromised then medical management and decompression can be carried out prior to life threatening herniation of brain contents
• can be integrated with other monitoring (e.g. clinical and radiological)

LIMITATIONS OF CPP

• optimal CPP may be time/ patient/ pathology specifc
• only a surrogate for cerebral blood flow (CBF)
• cerebral vascular resistance is variable so changes in CBF may not detected by CPP
• does not allow for differential autoregulation between normal and injured brain
• therapy to maintain CPP can be harmful (e.g. lung injury, fluid overload, side effects of vasopressors)
• no Class I data to support use — indeed some evidence suggests that it makes no difference, and some that it may worsen outcomes
• poor correlation between CPP and indices of brain oxygenation
• to accurately use requires insertion of ICP monitor and associated complications (e.g. bleeding, subdural haematoma, infection)
• subject to measurement errors (e.g. ICP monitor, arterial line)
• no standardised calibration site for measurement of MAP when calculating CPP
  — for a person with 30 degrees elevation head and 30 cm distance between heart and the head, the difference in measured MAP/CPP levels will be 11 mmHg depending on if the blood pressure transducer is calibrated in the heart or head level
  — NASGBI and SBNS recommend zeroing the arterial transducer for calculation of cerebral perfusion pressure in the management of traumatic brain injury at the level of the tragus (see position statement here); which corresponds to the level of the foramen of Monro/ middle cranial fossa (as opposed to the level of the heart)

References and Links

• Brain Trauma Foundation Guidelines – Guidelines for the Management of Severe TBI

Journal articles

• Kirkman MA, Smith M. Intracranial pressure monitoring, cerebral perfusion pressure estimation, and ICP/CPP-guided therapy: a standard of care or optional extra after brain injury? Br J Anaesth. 2014 Jan;112(1):35-46. PMID: 24293327.
• Rao V, Klepstad P, Losvik OK, Solheim O. Confusion with cerebral perfusion pressure in a literature review of current guidelines and survey of clinical practise. Scand J Trauma Resusc Emerg Med. 2013 Nov 21;21(1):78. PMCID: 3843545
• Rosner MJ, Rosner SD, Johnson AH. Cerebral perfusion pressure: management protocol and clinical results. J Neurosurg. 1995 Dec;83(6):949-62. PubMed PMID: 7490638.
• Tameem A, Kroviddi H. Cerebral physiology. Contin Educ Anaesth Crit Care Pain 2013;13(4):113-118  [Free Full Text]
• White H, Venkatesh B. Cerebral perfusion pressure in neurotrauma: a review. Anesth Analg. 2008 Sep;107(3):979-88. PMID: 18713917. [Free Full Text]