Monitoring of Cerebrovascular Autoregulation: Facts, Myths, and Missing Links

Abstract

Poor autoregulation and loss of pressure-reactivity are independent predictors of fatal outcome following head injury. Autoregulation is impaired by too low or too high CPP when compared to autoregulation with normal CPP (usually between 60 and 85 mmHg; and these limits are highly individual). Hemispheric asymmetry of the bi-laterally assessed autoregulation has been associated with asymmetry of CT scan findings: autoregulation was found to be worse ipsilateral to contusion or lateralized edema causing midline shift. The pressure-reactivity (PRx index) correlated with a state of low CBF and CMRO2 revealed using PET studies. The PRx is easier to monitor over prolonged periods of time than the TCD-based indices as it does not require fixation of external probes. Continuous monitoring with the PRx can be used to direct CPP-oriented therapy by determining the optimal CPP for pressure-reactivity. Autoregulation indices are able to reflect transient changes of autoregulation, as seen during plateau waves of ICP. However, minute-to-minute assessment of autoregulation has a poor signal-to-noise ratio. Averaging across time (30 min) or by combining with other relevant parameters improves the accuracy. MYTHS: It is debatable whether the TCD-based indices in head injured patients can be calculated using ABP instead of CPP. Thresholds for functional and disturbed autoregulation dramatically depends on arterial tension of CO2--therefore, comparison between patients cannot be performed without comparing their PaCO2. The TCD pulsatility index cannot accurately detect the lower limit of autoregulation. MISSING LINKS: We still do not know whether autoregulation-oriented therapy can be understood as a consensus between CPP-directed protocols and the Lund-concept. What are the links between endothelial function and autoregulation indices? Can autoregulation after head injury be improved with statins or EPO, as in subarachnoid hemorrhage? In conclusion, monitoring cerebral autoregulation can be used in a variety of clinical scenarios and may be helpful in delineating optimal therapeutic strategies.

Predicting the Limits of Cerebral Autoregulation During Cardiopulmonary Bypass

“…The mean velocity index cut-off indicating the LLA is not clearly known but it is likely to be between 0.25 and 0.5. 14,21,24 For the purpose of this study we defined the LLA as the MAP where mean velocity index incrementally increased from < 0.4 to ≥ 0.4. When mean velocity index was ≥ 0.4 at all MAP during CPB, the autoregulation threshold was defined as that MAP where mean velocity index had the lowest value.…”

“…14,21,24,39 We acknowledge that the use of a mean velocity index of ≥ 0.4 for defining the LLA is somewhat arbitrary. This is in part due to the nature of autoregulation whereby vasoreactivity responsible for mediating this response continues until extremely low blood pressure.…”

“…11 Real-time autoregulation monitoring can be accomplished by the continuous calculation of the correlation between transcranial Doppler-measured cerebral blood flow velocity of the middle cerebral artery and cerebral perfusion pressure (termed mean velocity index). [12][13][14] Our group has demonstrated in laboratory experiments and in patients undergoing cardiac surgery that near-infrared spectroscopy (NIRS) signals provide an acceptable surrogate for monitoring changes in cerebral blood flow for autoregulation monitoring. [15][16][17] The latter approach might provide a method for individualizing MAP during CPB without the need for specialized equipment and without the limitations of transcranial Doppler monitoring (e.g., need for trans-temporal insonating window, interference from electric cautery, movement artifact, etc).…”

Neurosonology of intracranial pressure increase

“…Çap ölçümü optik diskin 3-mm arkasında yapılmalıdır (41). yöntemlerle karbondioksit parsiyel basıncının arttırılmasına TCD yanıtı ("Karbondioksit vazomotor reaktivite testi") ile de belirlenebilir (52).…”

Positive end-expiratory pressure oscillation facilitates brain vascular reactivity monitoring

“…Current strategies for PRx monitoring require prolonged recordings of multiple PRx measurements averaged across time (for event detection) or across CPP (for CPP optimization) (8). Such averaging is necessary to reduce noise in PRx due to inherent, incoherent, physiologic variability of ABP and ICP slow waves (10).…”