Cerebral perfusion pressure

Cerebral perfusion pressure (CPP) is the pressure gradient between the mean arterial pressure (MAP) and the intracranial pressure (ICP) ¹. Cerebral perfusion pressure is expressed by the following equation:

• Cerebral perfusion pressure = mean arterial pressure (MAP) – intracranial pressure (ICP)
• Normal cerebral perfusion pressure is 55 mm Hg to 60 mm Hg
  • An increase in intracranial pressure (ICP) can decrease the cerebral perfusion pressure (CPP)
  • A decrease in intracranial pressure (ICP) may improve cerebral perfusion pressure (CPP)
  • Remember, lowering mean arterial pressure (MAP) in a hypotensive patient may lower cerebral perfusion pressure (CPP).
  • A minimum cerebral perfusion pressure (CPP) should be maintained to avoid cerebral insult. It is age-dependent and is as follows:
    • Infants – 50 mm Hg,
    • Children – 60 mm Hg,
    • Adults – 70 mm Hg.
  • Cerebral flow flow is quite sensitive to oxygen and carbon dioxide.
  • Hypoxia causes vasodilation and therefore increases cerebral blood flow and may worsen intracranial pressure (ICP).
  • Hypercarbia also results in vasodilation and can alter intracranial pressure (ICP) via effects on cerebrospinal fluid (CSF) pH and increases cerebral blood flow.

Mean arterial pressure (MAP) can be determined from ²:

• Mean arterial pressure (MAP) = (cardiac output [CO] x systemic vascular resistance [SVR]) + central venous pressure [CVP]
  • Mean arterial pressure (MAP):
    • Maintain = 80 mm Hg
    • 60 mm Hg = cerebral vessels maximally dilated
    • < 60 mm Hg = cerebral ischemia
    • > 150mmHg = increased intracranial pressure (ICP)

Intracranial pressure (ICP) is the pressure inside the skull and thus in the brain tissue and cerebrospinal fluid (CSF). Intracranial pressure (ICP) is measured in millimeters of mercury (mmHg) and, at rest, is normally 7–15 mmHg for a supine adult ³. The body has various mechanisms by which it keeps the intracranial pressure (ICP) stable, with CSF pressures varying by about 1 mmHg in normal adults through shifts in production and absorption of CSF. Changes in intracranial pressure (ICP) are attributed to volume changes in one or more of the constituents contained in the cranium. CSF pressure has been shown to be influenced by abrupt changes in intrathoracic pressure during coughing (intra-abdominal pressure), valsalva maneuver, and communication with the vasculature (venous and arterial systems).

• An increase in intracranial pressure (ICP) can decrease the cerebral perfusion pressure. Intracranial pressure (ICP) is dependent on the volume of the following compartments:
  • Brain parenchyma (< 1300 mL)
  • Cerebrospinal fluid (100 – 150 mL)
  • Intravascular blood (100 – 150 mL)
  • Cushing reflex (hypertension, bradycardia, and respiratory irregularity) due to an increase in intracranial pressure (ICP)
  • Normal intracranial pressure (ICP) is age dependent (adult younger than ten years old, child 3-7 years old, infant 1.5-6 years old)
  • > 20 mm Hg= increased morbidity and mortality and should be treated. It is perhaps more important to maintain an adequate cerebral perfusion pressure.

In most organs, perfusion pressure is calculated as arterial pressure minus venous pressure. However, in the upright position, the venous pressure in the head is essentially zero, due to the affect of gravity along with the fact that the brain is above the heart. Thus, in the brain for most cases, the exiting pressure is the intracranial pressure (ICP) ⁴. The gradient between mean arterial pressure (MAP) and intracranial pressure (ICP), therefore, determines the magnitude of the force which drives the flow of blood to the brain. Increases in intracranial pressure or decreases in systemic pressure can result in impairment of the distribution of blood and oxygen to the brain. Conversely, cerebral perfusion pressure (CPP) can be maintained by either raising mean arterial pressure (MAP) or lowering intracranial pressure (ICP).

Cerebral vessels can, however, respond to fluctuations in cerebral perfusion pressure with changes in vascular resistance. This cerebrovascular autoregulation is capable of maintaining a adequate cerebral blood flow over a range of systemic blood pressure from approximately 60 to 150 mm Hg (see Figure 1 below).

Hypotension beyond this autoregulatory range due to septic shock, for instance, can lower cerebral perfusion pressure and put the brain at risk for ischemia and infarction. Head trauma, CNS infection, and metabolic disturbances can impair normal cerebrovascular autoregulation and thereby threaten cerebral perfusion.

Cerebral perfusion pressure monitoring is useful in the setting of closed head trauma, a situation in which the occurrence of cerebral ischemia due to elevated ICP can significantly influence outcome. Tissue which has pre-existing ischemia is especially vulnerable to damage in the setting of low cerebral perfusion pressure. Studies indicate that cerebral perfusion pressure should ideally be maintained at a minimum of 70 mm Hg following closed head injury. In the intensive care setting, cerebral perfusion pressure monitoring relies measuring both arterial pressure and intracranial pressure

Figure 1. Cerebral perfusion pressure

[Source ⁵ ] References

1. Mason P. Medical Neurobiology. Oxford University Press. (2011) ISBN:0195339975
2. Mean Arterial Pressure. https://www.cvphysiology.com/Blood%20Pressure/BP006
3. Monitoring the injured brain: ICP and CBF. Steiner, L.A. et al. British Journal of Anaesthesia, Volume 97, Issue 1, 26 – 38. https://doi.org/10.1093/bja/ael110
4. Cerebral perfusion pressure. http://casemed.case.edu/clerkships/neurology/NeurLrngObjectives/CPP.htm
5. Regenhardt, Robert & Das, Alvin & Stapleton, Christopher & Chandra, Ronil & Rabinov, James & Patel, Aman & Hirsch, Joshua & Leslie-Mazwi, Thabele. (2017). Blood Pressure and Penumbral Sustenance in Stroke from Large Vessel Occlusion. Frontiers in Neurology. 8. 10.3389/fneur.2017.00317.