Continuous cerebral autoregulation monitoring by improved cross-correlation analysis: comparison with the cuff deflation test

Are We Ready for Clinical Therapy based on Cerebral Autoregulation? A Pro-con Debate

Cerebral autoregulation (CA) is a physiological mechanism that maintains constant cerebral blood flow regardless of changes in cerebral perfusion pressure and prevents brain damage caused by hypoperfusion or hyperperfusion. In recent decades, researchers have investigated the range of systemic blood pressures and clinical management strategies over which cerebral vasculature modifies intracranial hemodynamics to maintain cerebral perfusion. However, proposed clinical interventions to optimize autoregulation status have not demonstrated clear clinical benefit. As future trials are designed, it is crucial to comprehend the underlying cause of our inability to produce robust clinical evidence supporting the concept of CA-targeted management. This article examines the technological advances in monitoring techniques and the accuracy of continuous assessment of autoregulation techniques used in intraoperative and intensive care settings today. It also examines how increasing knowledge of CA from recent clinical trials contributes to a greater understanding of secondary brain injury in many disease processes, despite the fact that the lack of robust evidence influencing outcomes has prevented the translation of CA-guided algorithms into clinical practice.

Dynamic autoregulation of cerebral blood flow measured non-invasively with fast diffuse correlation spectroscopy

Cerebral autoregulation (CA) maintains cerebral blood flow (CBF) in the presence of systemic blood pressure changes. Brain injury can cause loss of CA and resulting dysregulation of CBF, and the degree of CA impairment is a functional indicator of cerebral tissue health. Here, we demonstrate a new approach to noninvasively estimate cerebral autoregulation in healthy adult volunteers. The approach employs pulsatile CBF measurements obtained using high-speed diffuse correlation spectroscopy (DCS). Rapid thigh-cuff deflation initiates a chain of responses that permits estimation of rates of dynamic autoregulation in the cerebral microvasculature. The regulation rate estimated with DCS in the microvasculature (median: 0.26 s-1, inter quartile range: 0.19 s-1) agrees well (R = 0.81, slope = 0.9) with regulation rates measured by transcranial Doppler ultrasound (TCD) in the proximal vasculature (median: 0.28 s-1, inter quartile range: 0.10 s-1). We also obtained an index of systemic autoregulation in concurrently measured scalp microvasculature. Systemic autoregulation begins later than cerebral autoregulation and exhibited a different rate (0.55 s-1, inter quartile range: 0.72 s-1). Our work demonstrates the potential of diffuse correlation spectroscopy for bedside monitoring of cerebral autoregulation in the microvasculature of patients with brain injury.

Pressure Autoregulation Measurement Techniques in Adult Traumatic Brain Injury, Part I: A Scoping Review of Intermittent/Semi-Intermittent Methods

The purpose of this study was to perform a systematic, scoping review of commonly described intermittent/semi-intermittent autoregulation measurement techniques in adult traumatic brain injury (TBI). Nine separate systematic reviews were conducted for each intermittent technique: computed tomographic perfusion (CTP)/Xenon-CT (Xe-CT), positron emission tomography (PET), magnetic resonance imaging (MRI), arteriovenous difference in oxygen (AVDO₂) technique, thigh cuff deflation technique (TCDT), transient hyperemic response test (THRT), orthostatic hypotension test (OHT), mean flow index (Mx), and transfer function autoregulation index (TF-ARI). MEDLINE^®, BIOSIS, EMBASE, Global Health, Scopus, Cochrane Library (inception to December 2016), and reference lists of relevant articles were searched. A two tier filter of references was conducted. The total number of articles utilizing each of the nine searched techniques for intermittent/semi-intermittent autoregulation techniques in adult TBI were: CTP/Xe-CT (10), PET (6), MRI (0), AVDO₂ (10), ARI-based TCDT (9), THRT (6), OHT (3), Mx (17), and TF-ARI (6). The premise behind all of the intermittent techniques is manipulation of systemic blood pressure/blood volume via either chemical (such as vasopressors) or mechanical (such as thigh cuffs or carotid compression) means. Exceptionally, Mx and TF-ARI are based on spontaneous fluctuations of cerebral perfusion pressure (CPP) or mean arterial pressure (MAP). The method for assessing the cerebral circulation during these manipulations varies, with both imaging-based techniques and TCD utilized. Despite the limited literature for intermittent/semi-intermittent techniques in adult TBI (minus Mx), it is important to acknowledge the availability of such tests. They have provided fundamental insight into human autoregulatory capacity, leading to the development of continuous and more commonly applied techniques in the intensive care unit (ICU). Numerous methods of intermittent/semi-intermittent pressure autoregulation assessment in adult TBI exist, including: CTP/Xe-CT, PET, AVDO₂ technique, TCDT-based ARI, THRT, OHT, Mx, and TF-ARI. MRI-based techniques in adult TBI are yet to be described, with the main focus of MRI techniques on metabolic-based cerebrovascular reactivity (CVR) and not pressure-based autoregulation.

Cerebral autoregulation in the microvasculature measured with near-infrared spectroscopy

Cerebral autoregulation (CA) is the mechanism that allows the brain to maintain a stable blood flow despite changes in blood pressure. Dynamic CA can be quantified based on continuous measurements of systemic mean arterial pressure (MAP) and global cerebral blood flow. Here, we show that dynamic CA can be quantified also from local measurements that are sensitive to the microvasculature. We used near-infrared spectroscopy (NIRS) to measure temporal changes in oxy- and deoxy-hemoglobin concentrations in the prefrontal cortex of 11 human subjects. A novel hemodynamic model translates those changes into changes of cerebral blood volume and blood flow. The interplay between them is described by transfer function analysis, specifically by a high-pass filter whose cutoff frequency describes the autoregulation efficiency. We have used pneumatic thigh cuffs to induce MAP perturbation by a fast release during rest and during hyperventilation, which is known to enhance autoregulation. Based on our model, we found that the autoregulation cutoff frequency increased during hyperventilation in comparison to normal breathing in 10 out of 11 subjects, indicating a greater autoregulation efficiency. We have shown that autoregulation can reliably be measured noninvasively in the microvasculature, opening up the possibility of localized CA monitoring with NIRS.

Influence of PEEP on cerebral blood flow and cerebrovascular autoregulation in patients with acute respiratory distress syndrome

BACKGROUND

High levels of positive end-expiratory pressure (PEEP), as part of the treatment in patients with acute respiratory distress syndrome (ARDS), may prevent alveolar collapse and maintain oxygenation. PEEP potentially reduces cerebral venous return, increases intracranial blood volume, and may, therefore, affect cerebral blood flow (CBF) and cerebrovascular autoregulation (AR). This study investigates the effect of PEEP on CBF and AR in patients with respiratory failure.

METHODS

CBF velocity was measured using transcranial doppler and correlated with the invasive arterial blood pressure curve to calculate the index of AR Mx (Mx>0.3 indicates impaired AR). Mx was measured at lower PEEP levels and after increasing PEEP. Only an increase of Mx of >0.2 was considered to be clinically relevant. Two 1-sided Wilcoxon tests.

RESULTS

Twenty mechanically ventilated patients with ARDS were included. Elevation of PEEP from 9.2±1 to 14.3±1 cm H2O did not influence CBF velocity but increased Mx from 0.317±0.35 to 0.414±0.32 (difference ≤0.2). Mx was >0.3 in 11/20 patients during baseline measurements, indicating impaired AR.

CONCLUSIONS

Surprisingly, AR was impaired in 55% of the patients with ARDS. This should be taken into account when managing cerebral perfusion pressure to avoid cerebral hyperperfusion or hypoperfusion. Increasing PEEP from 9.2 to 14.3 cm H2O had no further clinically relevant effect on AR, independent of preexisting AR impairment.

Pressure-derived versus pressure wave amplitude–derived indices of cerebrovascular pressure reactivity in relation to early clinical state and 12-month outcome following aneurysmal subarachnoid hemorrhage

Object

Indices of cerebrovascular pressure reactivity (CPR) represent surrogate markers of cerebral autoregulation. Given that intracranial pressure (ICP) wave amplitude–guided management, as compared with static ICP-guided management, improves outcome following aneurysmal subarachnoid hemorrhage (SAH), indices of CPR derived from pressure wave amplitudes should be further explored. This study was undertaken to investigate the value of CPR indices derived from static ICP–arterial blood pressure (ABP) values (pressure reactivity index [PRx]) versus ICP-ABP wave amplitudes (ICP-ABP wave amplitude correlation [IAAC]) in relation to the early clinical state and 12-month outcome in patients with aneurysmal SAH.

Methods

The authors conducted a single-center clinical trial enrolling patients with aneurysmal SAH. The CPR indices of PRx and IAAC of Week 1 after hemorrhage were related to the early clinical state (Glasgow Coma Scale [GCS] score) and 12-month outcome (modified Rankin Scale score).

Results

Ninety-four patients were included in the study. The IAAC, but not the PRx, increased with decreasing GCS score; that is, the higher the IAAC, the worse the clinical state. The PRx could differentiate between survivors and nonsurvivors only, whereas the IAAC clearly distinguished the groups “independent,” “dependent,” and “dead.” In patients with an average IAAC ≥ 0.2, mortality was approximately 3-fold higher than in those with an IAAC < 0.2.

Conclusions

The IAAC, which is based on single ICP-ABP wave identification, relates significantly to the early clinical state and 12-month outcome following aneurysmal SAH. Impaired cerebrovascular pressure regulation during the 1st week after a bleed relates to a worse outcome. Clinical trial registration no.: NCT00248690.

Correlation of clinical outcome with pressure-, oxygen-, and flow-related indices of cerebrovascular reactivity in patients following aneurysmal SAH

BACKGROUND

Impaired cerebrovascular reactivity (CR) has been reported to be associated with adverse outcome in patients with aneurysmal subarachnoid hemorrhage (aSAH). However, CR may be determined using different paradigms and it is unclear, which measurement method most suitable reflects the clinical course or is able to predict clinical deterioration.

METHODS

Twenty-one aSAH patients were included in this study. Following occlusion of the aneurysms with or without implantation of nicardipine prolonged release implants (NPRIs), mean arterial and mean intracranial pressure, regional tissue oxygenation, and regional cerebral blood flow was determined. Based on these data, pressure-, oxygen-, and flow-related autoregulatory indices (PRx, ORx, FRx) were simultaneously calculated and correlated with outcome parameters including the Glasgow outcome score (GOS) and the modified Rankin (mRankin) scale.

RESULTS

Eight patients showed newly developed cerebral infarcts. Low values of GOS and mRankin scale highly correlated with the incidence of cerebral infarcts (GOS, P = 0.001; mRankin, P = 0.003). However, indices of CR did not differ significantly between the infarction (I) and the noninfarction group (NI) (PRx, I, 0.058 +/- 0.096, NI, 0.097 +/- 0.203, P = 0.538; ORx, I, 0.162 +/- 0.316, NI, 0.094 +/- 0.176, P = 0.690; FRx, I, 0.395 +/- 0.200, NI, 0.265 +/- 0.177, P = 0.119). No correlation was found between indices and clinical outcome parameters (all not significant). However, ORx and FRx correlated well (P = 0.016).

CONCLUSIONS

Due to the low number of included subjects, the obtained results are preliminary. However, they indicate that either the present technique of index-determination is not sensitive enough or that there is no strong relation between the measured indices and clinical outcome. Future verification is required of continuous against already established non-continuous monitoring techniques of CR in order to relate both to clinical outcome.

Intraaortic Balloon Pump Counterpulsation and Cerebral Autoregulation: an observational study

Background

The use of Intra-aortic counterpulsation is a well established supportive therapy for patients in cardiac failure or after cardiac surgery. Blood pressure variations induced by counterpulsation are transmitted to the cerebral arteries, challenging cerebral autoregulatory mechanisms in order to maintain a stable cerebral blood flow. This study aims to assess the effects on cerebral autoregulation and variability of cerebral blood flow due to intra-aortic balloon pump and inflation ratio weaning.

Methods

Cerebral blood flow was measured using transcranial Doppler, in a convenience sample of twenty patients requiring balloon counterpulsation for refractory cardiogenic shock (N = 7) or a single inotrope to maintain mean arterial pressure following an elective placement of an intra-aortic balloon pump for cardiac surgery (N = 13). Simultaneous blood pressure at the aortic root was recorded via the intra-aortic balloon pump. Cerebral blood flow velocities were recorded for six minute intervals at a 1:1 balloon inflation-ratio (augmentation of all cardiac beats) and during progressive reductions of the inflation-ratio to 1:3 (augmentation of one every third cardiac beat). Real time comparisons of peak cerebral blood flow velocities with systolic blood pressure were performed using cross-correlation analysis. The primary endpoint was assessment of cerebral autoregulation using the time delay between the peak signals for cerebral blood flow velocity and systolic blood pressure, according to established criteria. The variability of cerebral blood flow was also assessed using non-linear statistics.

Results

During the 1:1 inflation-ratio, the mean time delay between aortic blood pressure and cerebral blood flow was -0.016 seconds (95% CI: -0.023,-0.011); during 1:3 inflation-ratio mean time delay was significantly longer at -0.010 seconds (95% CI: -0.016, -0.004, P < 0.0001). Finally, upon return to a 1:1 inflation-ratio, time delays recovered to those measured at baseline. During inflation-ratio reduction, cerebral blood flow irregularities reduced over time, whilst cerebral blood flow variability at end-diastole decreased in patients with cardiogenic shock.

Conclusions

Weaning counterpulsation from 1:1 to 1:3 inflation ratio leads to a progressive reduction in time delays between systolic blood pressure and peak cerebral blood flow velocities suggesting that although preserved, there is a significant delay in the establishment of cerebral autoregulatory mechanisms. In addition, cerebral blood flow irregularities (i.e. surrogate of flow adaptability) decrease and a loss of cerebral blood flow chaotic pattern occurs during the end-diastolic phase of each beat in patients with cardiogenic shock.