Vascular autoregulation:
- A myogenic mechanism where wall tension directly results in vascular smooth muscle constriction.
- An increase in MAP → vasoconstriction within 5 to 15 seconds → decrease CBV + ICP
- A decrease in MAP → vasodilation → increase in ICP.
Autoregulation disruption
- Can occur on the days following injury
- Magnitude and risk of disruption depending on the severity of injury.
- Increase in MAP → Vasodilation → increase ICP
MAP challenge to check if auto regulation is intact
- General
- Performed under stable conditions to prevent confounding (e.g., no other active changes in care should be made during the challenge, including adjustments in sedation, analgesia, EVD drainage or other physiological parameters).
- Technique:
- Record baseline monitor parameters at the beginning of the challenge (e.g., ICP, MAP and CPP).
- Initiate or titrate a vasopressor to increase the MAP by 10 mm Hg for up to 20 min.
- Observe the interaction between the MAP, ICP, CPP and PbtO2 during the challenge.
- Record monitor parameters at the end of the challenge.
- Evaluate the observed responses and recorded values for evidence of sPAR status. Disrupted sPAR will present as a sustained increase in ICP with MAP elevation.
- Adjust the target MAP back to baseline (disrupted sPAR) or to the chosen new, elevated target (intact sPAR).
Calculation of PRx
- Monitoring and correlation of spontaneous slow waves in BP and ICP allows calculation of a pressure reactivity index (PRx) which can be used as a continuous bedside assessment of CA.
Pressure reactivity index (PRx):
- A method in quantifying vascular reactivity
- Cerebral vasculature reactivity
- Ranges between – 1 and +1.
- A negative correlation indicates good vasoreactivity (ICP declines as a result of arterial vasoconstriction when MAP increases) and autoregulatory protection,
- A positive correlation indicates the loss of autoregulation and worse prognosis.
- Clinical evidence
- PRx < 0.3 correlates with good outcome on dichotomized GOS.
- PRx is frequently compromised on the first day after TBI
- Loss of autoregulation in the first 48 hours has strong indication for additional secondary injury including cerebral hypoxia and further ischaemic insults.
- Slow wave fluctuations in MAP and ICP lasting 30 seconds to several minutes are common in ventilated patients after head injury.
- Clinical use
- Since preservation of autoregulation is a crucial protective mechanism against transient MAP derangements.
- ‘CPP optimal’ (CPPopt)
- The CPP threshold at which PRx is minimized to maximize autoregulatory protection
- PRx changes dynamically with CPP
- Continuous PRx monitoring in TBI patients determines a real- time CPPopt dependent on the severity of injury
- Deviation from the CPP less than CPPopt appears to increase risk of mortality
- CPP more than CPPopt appears to discriminate between good and poor outcome on dichotomized GOS
- It is yet to be determined whether targeting CPPopt is a valid therapeutic target
- Ways to find CPPopt
- Cambridge way using their formula PRx:
- Edinburgh way by changing the pressure
- CPPopt is a CPP that allows the PbtO2 to be >20 mmHg
- Give inotrope to increase MAP → check PbtO2 rises to above 20 mmHg
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