Pressure reactivity index or PRx is tool for monitoring cerebral autoregulation in the

intensive care Intensive care medicine, also called critical care medicine, is a medical specialty that deals with seriously or critically ill patients who have, are at risk of, or are recovering from conditions that may be life-threatening. It includes pro ...

setting for patients with severe

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 ...

subarachnoid haemorrhage Subarachnoid hemorrhage (SAH) is bleeding Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural openi ...

, in order to guide therapy to protect the brain from dangerously high or low

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 ...

. PRx uses mathematical

algorithms In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing c ...

to calculate the

correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...

between

arterial blood pressure Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...

and

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 ...

. PRx assesses for correlations at low frequencies, below 0.5 Hz, and thus ignores individual

pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...

s while capturing the effects of respiratory-driven variation in arterial pressure as well as other longer-acting stimuli. Under normal conditions,

cerebral autoregulation Cerebral autoregulation is a process in mammals, which aims to maintain adequate and stable cerebral blood flow. While most systems of the body show some degree of autoregulation, the brain is very sensitive to over- and underperfusion. Cerebral au ...

ensures that cerebral blood flow is unchanged despite variations in blood pressure by regulating the cerebral vessels. For example, if the blood pressure increases, the cerebral vessels

vasoconstrict Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessel ...

to keep cerebral blood flow normal, whereas a decrease in blood pressure would lead to

vasodilation Vasodilation is the widening of blood vessels. It results from relaxation of smooth muscle cells within the vessel walls, in particular in the large veins, large arteries, and smaller arterioles. The process is the opposite of vasoconstriction, ...

of the cerebral vessels to increase blood flow. The cerebrovascular reactions to changes in blood pressure generates a corresponding effect on the intracranial pressure. When the blood pressure increases and the vessels vasoconstrict, the cerebral blood volume is reduced. According to the Monro-Kellie doctrine, less cerebral blood volume leads to a reduction in the intracranial pressure. If the blood pressure instead would decrease, the cerebral vessels would vasodilatate, with a resulting increase in cerebral blood volume.Donnelly J, Czosnyka M, Adams H, Robba C, Steiner LA, Cardim D. Pressure Reactivity-Based Optimal Cerebral Perfusion Pressure in a Traumatic Brain Injury Cohort. ''Acta Neurochir Suppl''. 2018; 126:209-212.

/ref>Czosnyka M, Czosnyka Z, Smielewski P. Pressure reactivity index: journey through the past 20 years. ''Acta Neurochir (Wien)''. 2017 Nov; 159(11):2063-2065.

/ref>Lang EW, Kasprowicz M, Smielewski P, Santos E, Pickard J, Czosnyka M. Short pressure reactivity index versus long pressure reactivity index in the management of traumatic brain injury. ''J Neurosurg''. 2015 Mar; 122(3):588-94.

/ref>

Definition

In the original article, it is stated that "Time-averaged values of ICP, ABP, CPP, (CPP = ICP - ABP), and the middle cerebral artery blood FV were calculated using waveform time integration (average values of 256 consecutive samples) for 5-second intervals. Linear (Pearson's) moving correlation coefficients between 40 past consecutive 5-second averages of ICP and ABP, designated as the PRx, were computed. Computations were repeated with a moving window every 5 seconds." Later research has shown that analysis of lower frequency data (minute-by-minute) can have similar results in autoregulation monitoring. In the 20 year follow up article, they state "we programmed our computers, running ICM (intensive care monitor) software, to calculate a moving correlation coefficient from 30 consecutive 10-s averages of ICP and ABP waveforms. We called this the PRx index (pressure reactivity index)". This is also the definition provided on the homepage promoting ICM+, a software that can calculate PRx. In 2022 a retrospective analysis identified five types of artifacts in terms of pressure reactivity index: "rectangular, fast impulse, isoline drift, saw tooth, and constant ICP value," and concluded that the effects of these artifacts on the PRx index are variable.

PRx and outcome prediction

A high PRx indicating disturbed pressure autoregulation predicts poor outcome in

PRx as a treatment target

PRx varies with the concurrent

cerebral perfusion pressure Cerebral perfusion pressure, or CPP, is the net pressure gradient causing cerebral blood flow to the brain (brain perfusion). It must be maintained within narrow limits because too little pressure could cause brain tissue to become ischemic (having ...

(CPP) in a U-shaped way. It has been suggested that the CPP with the lowest PRx is optimal (CPPopt) and CPP-values close to optimal have been associated with better outcome.Steiner, L.A., Czosnyka, M., Piechnik, S.K., Smielewski, P., Chatfield, D., Menon, D.K. and Pickard, J.D. (2002). Continuous monitoring of cerebrovascular pressure reactivity allows determination of optimal cerebral perfusion pressure in patients with traumatic brain injury. Critical care medicine 30, 733-738. CPP values above CPPopt are believed to cause hyperemia, i.e. to high cerebral blood flow that may cause cerebral edema and intracranial hypertension, whereas CPP values below CPPopt are believed to cause hypoperfusion and ischemia resulting in tissue damage.

References

{{CNS diseases of the nervous system Intensive care medicine Neurotrauma Neurophysiology

Definition

PRx and outcome prediction

PRx as a treatment target

See also

References