Transcranial Doppler Pulsatility Index Research Articles

Accuracy of Physical Examination and Imaging Findings for the Diagnosis of Elevated Intracranial Pressure.

LR+: 2.0 for pupillary dilation LR+: 2.20 for compression or absence of the basal cisterns on CT LR+: 1.92 for midline shift > 10 mm on CT LR−: 0.84 for pupillary dilation LR−: 0.23 for compression or absence of the basal cisterns on CT LR−: 0.89 for midline shift > 10 mm on CT Elevated intracranial pressure (ICP) is a common but severe complication of several medical and traumatic conditions.1, 2 Prolonged increases in ICP are associated with poor patient outcomes in a variety of conditions, including traumatic brain injury (TBI), spontaneous subarachnoid or intracerebral hemorrhage, space-occupying lesion, meningitis, cerebral infarct, and cerebral edema from hepatic encephalopathy.3-5 Invasive ICP monitoring is the reference standard for cases suspected of increased ICP but is not universally available and has several potential complications, including intracranial infection and hemorrhage.5, 6 Consequently, clinicians must often rely on noninvasive methods for assessing increased ICP. The systematic review and meta-analysis discussed here included all retrospective, prospective observational, and randomized controlled trials of adult patients (age ≥ 16 years) in the emergency department or intensive care unit that assessed physical examination findings, brain computed tomography (CT), ocular nerve sheath diameter on ultrasound, or transcranial Doppler indices.7 The reference standard consisted of either an ICP reading> 20 mm Hg on invasive ICP monitoring or craniectomy with an operative diagnosis of elevated ICP. The primary outcome was the diagnostic accuracy of the aforementioned tests for diagnosing increased ICP. The systematic review identified 40 studies (n = 5,123 patients) who met the inclusion criteria. Twenty-four studies were prospective cohort studies, 15 were retrospective, and one was a randomized controlled trial. Twenty studies included patients with TBI, three subarachnoid hemorrhage, two intracerebral hemorrhage, two hepatic failure, one ischemic stroke, and 12 mixed populations of primary brain injury. Only three physical examination findings (pupillary dilatation, motor posturing, and altered mental status) had adequate relevant studies to allow for a meta-analysis. Table 1 demonstrates the findings associated with elevated ICP. Measuring ocular nerve sheath diameter with ultrasound appeared to have a high discriminatory power with the pooled area under the receiver operating characteristic (AUROC) curve of 0.94 (95% confidence interval [CI] = 0.91 to 0.96). The sensitivity and specificity of the test were dependent on the cutoff used for increased optic nerve sheath in each study, precluding the possibility of meta-analysis to calculate pooled sensitivity and specificity. Authors calculated AUROC values for transcranial doppler pulsatility index (TCD-PI) to detect ICP ≥ 20 mm Hg based on three studies, finding a pooled AUROC value of 0.85 (95% CI = 0.78 to 0.91) with use of combined TCD arterial blood pressure methods. This study has several important limitations. Most of the included studies were relatively small, with only 13 studies enrolling more than 100 people. Additionally, the prevalence varied significantly between studies which may lead to spectrum bias, and evidence quality for findings suggestive of elevated ICP was predominantly low or moderate, with only midline shift > 10 mm on CT associated with high evidence quality. Nearly one-third of the studies were retrospective in nature. There was also significant heterogeneity with regard to the etiology and severity of the associated injuries, with no definition of the search strategy pertaining to the specific etiology of elevated ICP. Clinical signs were evaluated independently, which is not typical of clinical practice, and it is unclear how the diagnostic accuracy would change when using combinations of findings. Moreover, the inclusion of studies with either invasive monitoring or intraoperative diagnosis may lead to misclassification of the target condition in the latter case. Advances in imaging quality may limit the external validity of older studies with regard to current imaging. Furthermore, while 10 studies (1,035 patients) evaluated ocular nerve sheath diameter, the variations in ocular nerve sheath diameter thresholds precluded the ability to perform meta-analyses on this group. Finally, there were limited data on transcranial Doppler with differences in both the parameters assessed between studies. Based on the existing evidence, most findings are insufficiently sensitive or specific for the diagnosis of increased ICP. Therefore, physical examination findings, CT, ocular nerve sheath diameter, and transcranial Doppler do not appear to be reliable in identifying or excluding increased ICP.

Diagnosis of elevated intracranial pressure in critically ill adults: systematic review and meta-analysis

ObjectivesTo summarise and compare the accuracy of physical examination, computed tomography (CT), sonography of the optic nerve sheath diameter (ONSD), and transcranial Doppler pulsatility index (TCD-PI) for the diagnosis of...

Is The Combination of Glasgow Coma Scale and Transcranial Doppler Pulsatility Index Improving The Prediction of Outcome in Traumatic Brain Injury Patients?

Background: Glasgow coma scale (GCS) is a familiar scoring system with a standard statistical association with neurological outcome, it has many limitations that minimize its ability in prediction of Traumatic brain injury (TBI) patients' outcome. Transcranial Doppler (TCD) is a noninvasive aid in this field that can improve outcome prediction. Objective: This study aims to explore the effect of combination of GCS and Pulsatility Index (PI) in the prediction of outcome of TBI patients. Patients and Method: This study was performed in Zagazig University Hospital (ZUH). 103 traumatic brain injured patients were engaged in the investigations with hospital days of 1, 2, 3, and 7 or until patient discharge. The study was performed by TCD along with GCS recordings. Prognosis was also assessed by the Glasgow outcome scale (GOS). Results: The sensitivity and negative predictive value of PI was more than GCS in TBI patients (GCS 3-15), as they were (67.5% versus 50%) and (81.1% versus 74.3%), respectively. The combination of both PI and GCS increased the sensitivity and the negative predictive value up to 70% and 83.3%, respectively. PI had higher sensitivity, positive and negative predictive values than GCS in the identification of secondary neurologic deterioration (SND) in mild and moderate TBI patients (73.3% versus 40.0%, 61.1% versus 50.0%, and 92.0% versus 83.9%, respectively), while the combined value of both PI and GCS increased both sensitivity and positive predictive value up to 76.7% and 66.7%, respectively. Conclusion: The combination of GCS and PI would improve the prediction of outcome.

Is the Combination of Glasgow Coma Scale and Transcranial Doppler Pulsatility Index Improve the Prediction of Outcome in Traumatic brain injury Patients?

Is the Combination of Glasgow Coma Scale and Transcranial Doppler Pulsatility Index Improve the Prediction of Outcome in Traumatic brain injury Patients?

Ultrasound Measures of Brain Pulsatility Correlate with Subcortical Brain Volumes in Healthy Young Adults

Increasing evidence suggests that brain pulsatility is involved in the pathophysiology of various neurological and psychiatric disorders. However, it remains unclear whether high brain pulsatility is damaging to or protective of the brain in normal conditions, and this could depend on the age of the individual and the methods used to measure brain pulsatility. The goal of our study was to investigate associations between subcortical volumes and brain pulsatility as assessed with ultrasound in healthy young adults using both a conventional method (transcranial Doppler pulsatility index [TCD-PI]) and the innovative method of tissue pulsatility imaging (TPI), which allows a high level of detection of small brain movements (micrometers). Twenty-five females aged 18–55 with no history of significant medical disorder underwent magnetic resonance imaging and ultrasound assessment. The volumes of six subcortical regions known to be particularly sensitive to change in cerebral blood flow were measured and compared with brain pulsatility as assessed with TCD-PI and TPI. TCD-PI and TPI measures positively correlated with all subcortical regions, with the caudate nucleus having the strongest association. Linear regressions found that TCD-PI and TPI measures of brain pulsatility explained 16% to 67% of the variance of the subcortical volumes. Our results suggest that a greater pulsatility as assessed with ultrasound in healthy young adults may constitute a protective factor for brain structure. Ultrasound measures of brain pulsatility may be appropriate to provide costless, non-invasive, portable and highly sensitive markers of cerebral blood flow pulsatility related to brain structure.

Relationship Between Brain Pulsatility and Cerebral Perfusion Pressure: Replicated Validation Using Different Drivers of CPP Change

BackgroundDetermination of relationships between transcranial Doppler (TCD)-based spectral pulsatility index (sPI) and pulse amplitude (AMP) of intracranial pressure (ICP) in 2 groups of severe traumatic brain injury (TBI) patients (a) displaying plateau waves and (b) with unstable mean arterial pressure (MAP).MethodsWe retrospectively reviewed patients with severe TBI and continuous TCD monitoring displaying either plateau waves or unstable MAP from 1992 to 1998. We utilized linear and nonlinear regression techniques to describe both cohorts: cerebral perfusion pressure (CPP) versus AMP, CPP versus sPI, mean ICP versus ICP AMP, mean ICP versus sPI, and AMP versus sPI.ResultsNonlinear regression techniques were employed to analyze the relationships with CPP. In plateau wave and unstable MAP patients, CPP versus sPI displayed an inverse nonlinear relationship (R2 = 0.820 vs. R2 = 0.610, respectively), with the CPP versus sPI relationship best modeled by the following function in both cases: PI = a + (b/CPP). Similarly, in both groups, CPP versus AMP displayed an inverse nonlinear relationship (R2 = 0.610 vs. R2 = 0.360, respectively). Positive linear correlations were displayed in both the plateau wave and unstable MAP cohorts between: ICP versus AMP, ICP versus sPI, AMP versus sPI.ConclusionsThere is an inverse relationship through nonlinear regression between CPP versus AMP and CPP versus sPI display. This provides evidence to support a previously-proposed model of TCD pulsatility index. ICP shows a positive linear correlation with AMP and sPI, which is also established between AMP and sPI.

Non-invasive Monitoring of Intracranial Pressure Using Transcranial Doppler Ultrasonography: Is It Possible?

Although intracranial pressure (ICP) is essential to guide management of patients suffering from acute brain diseases, this signal is often neglected outside the neurocritical care environment. This is mainly attributed to the intrinsic risks of the available invasive techniques, which have prevented ICP monitoring in many conditions affecting the intracranial homeostasis, from mild traumatic brain injury to liver encephalopathy. In such scenario, methods for non-invasive monitoring of ICP (nICP) could improve clinical management of these conditions. A review of the literature was performed on PUBMED using the search keywords ‘Transcranial Doppler non-invasive intracranial pressure.’ Transcranial Doppler (TCD) is a technique primarily aimed at assessing the cerebrovascular dynamics through the cerebral blood flow velocity (FV). Its applicability for nICP assessment emerged from observation that some TCD-derived parameters change during increase of ICP, such as the shape of FV pulse waveform or pulsatility index. Methods were grouped as: based on TCD pulsatility index; aimed at non-invasive estimation of cerebral perfusion pressure and model-based methods. Published studies present with different accuracies, with prediction abilities (AUCs) for detection of ICP ≥20 mmHg ranging from 0.62 to 0.92. This discrepancy could result from inconsistent assessment measures and application in different conditions, from traumatic brain injury to hydrocephalus and stroke. Most of the reports stress a potential advantage of TCD as it provides the possibility to monitor changes of ICP in time. Overall accuracy for TCD-based methods ranges around ±12 mmHg, with a great potential of tracing dynamical changes of ICP in time, particularly those of vasogenic nature.

Abstract TP190: Bone Mineral Density is Inversely Related to Cerebral Pulsatility Index in Patients With Acute Ischemic Stroke

Introduction: Atherosclerosis and osteoporosis are two major public health problems. These are both multifactorial and degenerative diseases that accompany aging. Co-morbidities are common, with a high prevalence of cardiovascular risk factors including hypertension, smoking, and sedentary lifestyle. Several studies have suggested a relationship between atherosclerosis and osteoporosis, with a significant correlation between coronary artery calcium and bone mineral density (BMD). The pulsatility index (PI) is designed to measure vascular resistance and characterizes the shape of the spectral waveform. The PI was increasing with age but alterations in PI are the reflection of multifactorial, pleiotropic events occurring in the cardio and cerebral vascular system. Atherosclerosis of the large proximal arteries and the intracranial vessels are important factors for increased PI. Hypothesis: We hypothesized that the transcranial Doppler (TCD) PI would reflect the atherosclerotic component of intracranial vessels which in turn is related to systemic atherosclerosis. In this context, we investigated the relationship between TCD PI and BMD in acute ischemic stroke patients. Methods: Patients having their first ischemic stroke within 7 days were screened for the study. Doppler signals from the main stem of the middle cerebral artery (MCA) were obtained transtemporally with a traditional 2-MHz transducer at depths of 56, 58, and 60 mm. BMD was measured at total hip and lumbar spine in the anterior-posterior projection, using dual-energy x-ray absorptiometry. Results: A total of 146 patients were enrolled in the study and the mean age was 68.3±10.34 years. Multiple linear regression analysis reveals that age (p<0.001) and BMD T-score (p=0.028) were significantly associated with increased TCD PI. Old age and decreased T-score were significantly related to increased TCD PI. Age adjusted scatterplots showed that BMD T-score was inversely related to TCD PI in acute ischemic stroke patients (r=-0.202, p=0.028). Conclusions: Our findings indicate that decreased BMD may be associated with atherosclerosis in the cerebral circulation.

Serum Uric Acid Is Associated with Cerebral White Matter Hyperintensities in Patients with Acute Lacunar Infarction

The Effect of Cilostazol in Acute Lacunar Infarction Based on Pulsatility Index of Transcranial Doppler (ECLIPse) study showed a significant decrease in transcranial Doppler pulsatility index (PI) with cilostazol treatment after 90 days of acute lacunar infarction. The aim of this analysis was to perform a subgroup analysis of the ECLIPse study to explore the relationship between serum uric acid (UA) and the volume of white matter hyperintensities (WMH) in patients with acute lacunar infarction. The ECLIPse was a multicenter, randomized, double-blind, placebo-controlled study conducted in Korea. For this subgroup analysis, WMH volume was measured for those subjects for whom FLAIR or T2-weighted images were available using semiautomated computerized software. Of the 203 patients in 8 hospitals in the ECLIPse study, 130 in 6 hospitals were entered for this subgroup analysis. The mean age was 64.7 ± 9.95 years, and 20.8% were women. The mean WMH volume was 11.57 cm(3) (.13 to 68.45, median 4.86) and mean serum UA was 5.2 mg/dL (1.5 to 8.9). Multiple linear regression analysis revealed that age (P < .001) and serum UA (P = .013) were significantly associated with WMH volume. Age-adjusted scatterplots showed that serum UA level was positively related to WMH volume in patients with acute lacunar infarction (r = 0.275, P = .003). This study showed that serum UA was associated with cerebral WMH in patients with acute lacunar infarction.

Frontal electroencephalogram variables are associated with the outcome and stage of hepatic encephalopathy in acute liver failure.

Acute liver failure (ALF) and hepatic encephalopathy (HE) can lead to an elevated intracranial pressure (ICP) and death within days. The impaired liver function increases the risks of invasive ICP monitoring, whereas noninvasive methods remain inadequate. The purpose of our study was to explore reliable noninvasive methods of neuromonitoring for patients with ALF in the intensive care unit (ICU) setting; more specifically, we wanted to track changes in HE and predict the outcomes of ALF patients treated with albumin dialysis. The study included 20 patients with severe ALF at admission who had been referred to the ICU of the liver transplantation (LT) center for albumin dialysis treatment and evaluation for transplantation. Data were collected from all study patients in the form of continuous frontal electroencephalography (EEG) recordings and transcranial Doppler (TCD) measurements of cerebral blood flow. Among the studied EEG variables, the 50% spectral edge frequency decreased and the delta power increased as the HE stage increased. Both variables were predictive of the stage of HE [prediction probability (PK) of 50% spectral edge frequency = 0.23, standard error (SE) = 0.03; PK of delta power = 0.76, SE = 0.03]. The total wavelet subband entropy, a novel variable that we used for tracking abnormal EEG activity, predicted the outcome of ALF patients treated with albumin dialysis (PK = 0.88, SE = 0.09). With a threshold value of 1.6, the TCD pulsatility index had an odds ratio of 1.1 (95% confidence interval = 0.1-9.3) for a poor outcome (LT or death). In conclusion, EEG variables are useful for the monitoring of HE and can be used to predict outcomes of ALF. TCD measurements do not predict patient outcomes.

Central aortic pressure and pulsatility index in acute ischemic stroke.

We investigated the relationship between transcranial Doppler (TCD) pulsatility index (PI) and central aortic pressure by measurement of the aortic augmentation index (AIx). We enrolled 148 consecutive patients with a diagnosis of acute ischemic stroke. Patients were eligible for the study if they experienced their first ischemic stroke within the preceding 7 days and were 45 years of age or older. At Day 7 (±2) after stroke onset, TCD studies were performed and AIx was measured by applanation tonometry on the same days. The mean age was 66.3 (47-90) years and 37.8% were women. The mean middle cerebral artery (MCA) PI was significantly related with age (r =.361), hypertension (r = .184), peripheral systolic blood pressure (SBP; r = .211), peripheral pulse pressure (PP; r = .396), aortic SBP (r = .184), aortic DBP (r = -.181), and aortic PP (r = .371). The basilar artery (BA) PI was significantly related with age (r = .311), peripheral DBP (r = -.267), peripheral PP (r = .358), aortic DBP (r = -.266), and aortic PP (r = .347). TCD PI was significantly related with central aortic pressure, especially PP. The PI in the MCA and BA is closely associated with the pulsatile component of BP in the systemic circulation.

The use of transcranial doppler pulsatility index to guide intracranial pressure monitoring in intoxicated traumatic brain injury patients

Introduction: Management of intracranial pressure (ICP) represents a cornerstone in the care of traumatic brain injury (TBI) patients. On occasions, intoxicated TBI represent a clinical challenge since their initial imaging is not significantly correlating with elevated ICP yet their neurological exam concerns toward that possibility. Materials and Methods: We present an initial series of intoxicated TBI patients in which the trans-cranial Doppler (TCD) were conducted to noninvasively judge the ICP and correlate those with the clinical findings. Results: A total of four patients is reported in this series. About 50% of the patients had evidence of elevated ICP based on TCD parameters confirmed by ICP monitors. Conclusion: From our series, TCD screening examinations were helpful in expediting an objective guided decision for intracranial pressure monitoring.

Informativity of pulsatility index and cerebral autoregulation in hydrocephalus

A high correlation between transcranial Doppler pulsatility index (PI) increase and intracranial hypertension has been recently demonstrated in most neurosurgical patients. But in patients with hydrocephalus PI is sometimes controversial. This may be due to a different degree of cerebral autoregulation (CA) under condition of cerebral perfusion pressure decrease. To compare the results of PI and CA assessment in patients with hydrocephalus. Twenty-six patients (aged 16–52; male – 9, female – 17) with various types of hydrocephalus were studied. We monitored blood flow velocity in middle cerebral arteries with Multi Dop X and systemic blood pressure with Finapres-2300. CA was assessed with cuff test (evaluation of ARI) and cross-spectral analysis of spontaneous oscillations of cerebral and systemic hemodynamics within the range of Mayer's waves (evaluation of phase shift in radians – PS). Depending on presence of intracranial hypertension (ICH), all patients have been divided in two groups. Mean values of PI did not differ significantly in both groups. At the same time ARI and PS were considerably ( p < 0.01) higher in patients without signs of ICH. In group of patients with ICH postoperative clinical improvement was accompanied with considerable ( p < 0.05) increase of PS. In group of patients without ICH we did not observe any positive changes in neurological state postoperatively. Preoperative CA assessment being more informative than PI evaluation can increase transcranial Doppler valuability in noninvasive diagnostics of cerebral spinal fluid dynamics and may be helpful in clarifying indications for operation in patients with hydrocephalus.

Transcranial Doppler pulsatility index is a poor predictor of hydrocephalus in patients with aneurysmal subarachnoid haemorrhage

Hydrocephalus is a common complication of aneurysmal subarachnoid haemorrhage (aSAH). The increase in intracranial pressure is associated with increased mortality and morbidity. Early recognition and intervention in these patients is essential in order to achieve favourable outcome. In the literature the value of noninvasive measurement of transcranial Doppler (TCD)-derived pulsatility index (PI) in predicting increased intracranial pressure remains questionable. The aim of this study was to examine the value of PI in predicting hydrocephalus in patients with aSAH.

Transcranial Doppler Pulsatility Index: What it is and What it Isn’t

Transcranial Doppler (TCD) pulsatility index (PI) has traditionally been interpreted as a descriptor of distal cerebrovascular resistance (CVR). We sought to evaluate the relationship between PI and CVR in situations, where CVR increases (mild hypocapnia) and decreases (plateau waves of intracranial pressure-ICP). Recordings from patients with head-injury undergoing monitoring of arterial blood pressure (ABP), ICP, cerebral perfusion pressure (CPP), and TCD assessed cerebral blood flow velocities (FV) were analyzed. The Gosling pulsatility index (PI) was compared between baseline and ICP plateau waves (n=20 patients) or short term (30-60min) hypocapnia (n=31). In addition, a modeling study was conducted with the "spectral" PI (calculated using fundamental harmonic of FV) resulting in a theoretical formula expressing the dependence of PI on balance of cerebrovascular impedances. PI increased significantly (p<0.001) while CVR decreased (p<0.001) during plateau waves. During hypocapnia PI and CVR increased (p<0.001). The modeling formula explained more than 65% of the variability of Gosling PI and 90% of the variability of the "spectral" PI (R=0.81 and R=0.95, respectively). TCD pulsatility index can be easily and quickly assessed but is usually misinterpreted as a descriptor of CVR. The mathematical model presents a complex relationship between PI and multiple haemodynamic variables.

Transcranial Doppler Pulsatility Index for Initial Management of Brain-Injured Patients

To the Editor: We read with interest the article by Behrens et al,1 concerning the lack of accuracy of transcranial Doppler (TCD) to predict intracranial pressure (ICP). The authors measured the pulsatility index (PI) in 8 patients with idiopathic normal pressure hydrocephalus (INPH) and communicating CSF systems. The ICP-PI relationship was then explored through the lumbar infusion technique to get a range of ICP values between 0 and 50 mm Hg in each individual. The authors found a large interindividual variation in the slopes of ICP-PI correlations that should preclude any predictability of the ICP as measured by PI. Although original in its approach, this study has several limitations. First, it is probably premature to draw firm conclusions from data collected in 8 patients. The message could have been more appropriate if circumvented to patients with INPH only. A good estimation of ICP was recently shown with PI measurements in 45 ventilated head-injured patients with actually measured ICP below 40 mm Hg.2 That study confirmed previous data with an estimation of ICP using PI in 81 patients with various intracranial disorders.3 Second, there is large evidence that TCD measurements must be interpreted according to potential confounding factors, such as Paco2, blood hematocrit, arterial blood pressure, and heart rate. This point was extensively addressed in animals subjected to manipulation of cerebrovascular resistance, ie, CO2 challenge, arterial hypotension, and intracranial hypertension.4 The simulation model used in the present study took arterial blood pressure only in spontaneously breathing patients, which should represent an important limitation to the interpretation of data. Third, in patients with severe head injury, abnormal PI measurements on admission prompted interventions to restore cerebral perfusion pressure.5 We found PI as one TCD measurements to distinguish between patients at risk for secondary neurological deterioration after minor-to-moderate brain trauma.6 With no evidence of hypocapnia, elevated PI values can thus be viewed as the result of reduced compliance throughout the entire brain, including increased rigidity of the cerebral arteries and elevated ICP. It is obvious that TCD provides only a crude estimation of brain hemodynamics and ICP and it cannot be substituted for the actually measured ICP. However, its permanent accessibility in the emergency setting should promote the use of TCD to improve the initial management of brain-injured patients. Unfortunately, the present study does not provide enough evidence to abandon the use of TCD. Pierre Bouzat Gilles Francony Bertrand Fauvage Jean-François Payen Grenoble, France

Transcranial Doppler pulsatility index is not a reliable indicator of intracranial pressure in children with severe traumatic brain injury

Background The TCD-derived PI has been associated with ICP in adult studies but has not been well investigated in children. We examined the relationship between PI and ICP and CPP in children with severe TBI. Methods Data were prospectively collected from consecutive TCD studies in children with severe TBI undergoing ICP monitoring. Ipsilateral ICP and CPP values were examined with Spearman correlation coefficient (mean values and raw observations), with a GEE, and as binary values (1 and 20 mm Hg, respectively). Results Thirty-four children underwent 275 TCD studies. There was a weak relationship between mean values of ICP and PI ( P = .04, r = 0.36), but not when raw observations ( P = .54) or GEE ( P = .23) were used. Pulsatility index was 0.76 when ICP was lower than 20 mm Hg and 0.86 when ICP was 20 mm Hg or higher. When PI was 1 or higher, ICP was lower than 20 mm Hg in 62.5% (25 of 40 studies), and when ICP was 20 mm Hg or higher, PI was lower than 1 in 75% (46 of 61 studies). The sensitivity and specificity of a PI threshold of 1 for examining the ICP threshold of 20 mm Hg were 25% and 88%, respectively. The relationship between CPP and PI was stronger ( P = .001, r = −0.41), but there were too few observations below 50 mm Hg to examine PI at this threshold. Conclusion The absolute value of the PI is not a reliable noninvasive indicator of ICP in children with severe TBI. Further study is required to examine the relationship between PI and a CPP threshold of 50 mm Hg.

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

The methods for continuous assessment of cerebral autoregulation using correlation, phase shift, or transmission (either in time- or frequency-domain) were introduced a decade ago. They express dynamic relationships between slow waves of transcranial Doppler (TCD), blood flow velocity (FV) and cerebral perfusion pressure (CPP), or arterial pressure (ABP). We review a methodology and clinical application of indices useful for monitoring cerebral autoregulation and pressure-reactivity in various scenarios of neuro-critical care. 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.

Transcranial Doppler Identification of Changing Autoregulatory Thresholds after Autoregulatory Impairment

OBJECTIVE Transcranial Doppler (TCD) flow velocity (FV) assessment may provide a useful index of autoregulatory impairment after severe head injury. It may define a therapeutic end point against which cerebral perfusion pressure (CPP) can be titrated. This study examines the relationship between cerebral blood flow (CBF) and TCD FV indices in a laboratory model before and after autoregulatory impairment. METHODS CPP, CBF, and middle cerebral artery TCD FV were measured continuously in nine anesthetized and ventilated sheep. CPP was decreased by hemorrhagic hypotension. The process was repeated after impairment of autoregulation by cisternal infusion, which maintained CPP at 0 mm Hg for 15 minutes. Points of significant change (i.e., breakpoints) from baseline values for each of the measured flow parameters were identified by using a ratio of variance technique. RESULTS Before any significant change in CBF or systolic TCD, diastolic TCD FV decreased (mean breakpoint, 69 mm Hg; range, 56–78 mm Hg) as CPP was reduced. This divergence of diastolic and systolic TCD FV, which occurred before autoregulatory failure, was associated with an increasing TCD pulsatility index (mean breakpoint, 63 mm Hg; range, 53–70 mm Hg). At diastolic TCD FV congruent with 10 cm/s, systolic TCD FV (mean breakpoint, 48 mm Hg; range, 46–53 mm Hg) and CBF (mean breakpoint, 49 mm Hg; range, 47–51 mm Hg) decreased rapidly, indicating autoregulatory failure. After autoregulatory impairment, the breakpoints for all four indices shifted to higher CPP values (mean, 16 mm Hg). CONCLUSION TCD FV assessment identified two CPP thresholds of autoregulatory loss. Before autoregulatory failure, an earlier phase of autoregulatory disturbance may be detected by divergent systolic and diastolic TCD FVs. It is important to note that this phase may be detected before CBF decreases. These TCD FV breakpoints depend on the state of autoregulatory impairment and may provide potential targets for CPP-directed therapy.

Transcranial Doppler Identification of Changing Autoregulatory Thresholds after Autoregulatory Impairment

Transcranial Doppler (TCD) flow velocity (FV) assessment may provide a useful index of autoregulatory impairment after severe head injury. It may define a therapeutic end point against which cerebral perfusion pressure (CPP) can be titrated. This study examines the relationship between cerebral blood flow (CBF) and TCD FV indices in a laboratory model before and after autoregulatory impairment. CPP, CBF, and middle cerebral artery TCD FV were measured continuously in nine anesthetized and ventilated sheep. CPP was decreased by hemorrhagic hypotension. The process was repeated after impairment of autoregulation by cisternal infusion, which maintained CPP at 0 mm Hg for 15 minutes. Points of significant change (i.e., breakpoints) from baseline values for each of the measured flow parameters were identified by using a ratio of variance technique. Before any significant change in CBF or systolic TCD, diastolic TCD FV decreased (mean breakpoint, 69 mm Hg; range, 56-78 mm Hg) as CPP was reduced. This divergence of diastolic and systolic TCD FV, which occurred before autoregulatory failure, was associated with an increasing TCD pulsatility index (mean breakpoint, 63 mm Hg; range, 53-70 mm Hg). At diastolic TCD FV congruent with 10 cm/s, systolic TCD FV (mean breakpoint, 48 mm Hg; range, 46-53 mm Hg) and CBF (mean breakpoint, 49 mm Hg; range, 47-51 mm Hg) decreased rapidly, indicating autoregulatory failure. After autoregulatory impairment, the breakpoints for all four indices shifted to higher CPP values (mean, 16 mm Hg). TCD FV assessment identified two CPP thresholds of autoregulatory loss. Before autoregulatory failure, an earlier phase of autoregulatory disturbance may be detected by divergent systolic and diastolic TCD FVs. It is important to note that this phase may be detected before CBF decreases. These TCD FV breakpoints depend on the state of autoregulatory impairment and may provide potential targets for CPP-directed therapy.