Cerebral perfusion pressure, or CPP, is the net
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
gradient causing
cerebral blood flow
Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arterie ...
to the brain (brain
perfusion
Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is deliver ...
). It must be maintained within narrow limits because too little pressure could cause brain tissue to become
ischemic
Ischemia or ischaemia is a restriction in blood supply to any tissue, muscle group, or organ of the body, causing a shortage of oxygen that is needed for cellular metabolism (to keep tissue alive). Ischemia is generally caused by problems wi ...
(having inadequate blood flow), and too much could raise
intracranial pressure
Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury (mmHg) and at rest, is normally 7–15 Millimeter of mercury, mmHg ...
(ICP).
Definitions
The
cranium
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...
encloses a fixed-volume space that holds three components:
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
,
cerebrospinal fluid
Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.
CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the bra ...
(CSF), and very soft tissue (the
brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
). While both the blood and CSF have poor compression capacity, the brain is easily compressible.
Every increase of ICP can cause a change in tissue perfusion and an increase in stroke events.
From resistance
CPP can be defined as the pressure gradient causing
cerebral blood flow
Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arterie ...
(CBF) such that
:
where:
:CVR is cerebrovascular resistance
By intracranial pressure
An alternative definition of CPP is:
:
where:
:MAP is
mean arterial pressure
In medicine, the mean arterial pressure (MAP) is an average blood pressure in an individual during a single cardiac cycle. MAP is altered by cardiac output and systemic vascular resistance.
Testing
Mean arterial pressure can be measured dire ...
:ICP is
intracranial pressure
Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury (mmHg) and at rest, is normally 7–15 Millimeter of mercury, mmHg ...
:JVP is
jugular venous pressure
The jugular venous pressure (JVP, sometimes referred to as ''jugular venous pulse'') is the indirectly observed pressure over the venous system via visualization of the internal jugular vein. It can be useful in the differentiation of different for ...
This definition may be more appropriate if considering the circulatory system in the brain as a
Starling resistor, where an external pressure (in this case, the intracranial pressure) causes decreased blood flow through the vessels. In this sense, more specifically, the cerebral perfusion pressure can be defined as either:
:
(if ICP is higher than JVP)
or
:
(if JVP is higher than ICP).
Physiologically, increased
intracranial pressure
Intracranial pressure (ICP) is the pressure exerted by fluids such as cerebrospinal fluid (CSF) inside the skull and on the brain tissue. ICP is measured in millimeters of mercury (mmHg) and at rest, is normally 7–15 Millimeter of mercury, mmHg ...
(ICP) causes decreased blood perfusion of
brain cell
Brain cells make up the functional tissue of the brain. The rest of the brain tissue is structural or connective called the stroma which includes blood vessels. The two main types of cells in the brain are neurons, also known as nerve cells, an ...
s by mainly two mechanisms:
*Increased ICP constitutes an increased
interstitial hydrostatic pressure
The Starling equation describes the net Fluid dynamics, flow of fluid across a semipermeable membrane. It is named after Ernest Starling. It describes the balance between capillary pressure, interstitial pressure, and osmotic pressure. The classi ...
that, in turn, causes a decreased
driving force for capillary filtration from intracerebral blood vessels.
*Increased ICP compresses cerebral arteries, causing increased cerebrovascular resistance (CVR).
FLOW
Ranging from 20ml 100g-1 min-1 in white matter to 70ml 100g-1 min-1 in grey matter.
Autoregulation
Under normal circumstances a MAP between 60 and 160 mmHg and ICP about 10 mmHg (CPP of 50-150 mmHg) sufficient blood flow can be maintained with autoregulation.
Although the classic 'autoregulation curve' suggests that CBF is fully stable between these blood pressure values (known also as the limits of autoregulation), in practice spontaneous fluctuations can occur.
Outside of the limits of autoregulation, raising MAP raises CBF and raising ICP lowers it (this is one reason that increasing ICP in
traumatic brain injury
A traumatic brain injury (TBI), also known as an intracranial injury, is an injury to the brain caused by an external force. TBI can be classified based on severity (ranging from mild traumatic brain injury TBI/concussionto severe traumatic b ...
is potentially deadly). In trauma some recommend CPP not go below 70 mmHg.
Recommendations in children is at least 60 mmHg.
Within the autoregulatory range, as CPP falls there is, within seconds, vasodilatation of the cerebral resistance vessels, a fall in cerebrovascular resistance and a rise in cerebral-blood volume (CBV), and therefore CBF will return to baseline value within seconds (see as ref. Aaslid, Lindegaard, Sorteberg, and Nornes 1989: http://stroke.ahajournals.org/cgi/reprint/20/1/45.pdf). These adaptations to rapid changes in blood pressure (in contrast with changes that occur over periods of hours or days) are known as dynamic cerebral autoregulation.
Footnotes
References
*
* {{Cite journal, last=Walters , first=FJM , year=1998 , url=http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm , title=Intracranial Pressure and Cerebral Blood Flow , journal=
Physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
, issue=8, Article 4 , accessdate=2011-02-10 , url-status=dead , archiveurl=https://web.archive.org/web/20110514050024/http://www.nda.ox.ac.uk/wfsa/html/u08/u08_013.htm , archivedate=2011-05-14
Medical terminology
Neurology
Neurotrauma
Physiology