Altered cerebral vasoregulation in hypertension and stroke

Early Mobilisation after Stroke: Review of the Literature

BACKGROUND

Mobilisation of acute stroke patients--in bed and out of bed as early as possible--is currently recommended to prevent general and neurological complications. However, mobilisation protocols are poorly defined and need to be standardised in order to evaluate their clinical benefits. The purpose of this study is to highlight recent advances in recommendations for mobilisation of acute stroke and early neurological rehabilitation as found in the recent literature.

METHODS

A Medline research was conducted from January 1950 to August 2005.

RESULTS

Data synthesis of the observational studies and meta-analyses suggests that for most patients with acute stroke, early mobilisation is not harmful and may well be beneficial. However, no controlled study exists comparing early (e.g. 1-3 days) versus late (e.g. 1-2 weeks) mobilisation. Based on the available data and on pathophysiological considerations, we have developed a partially individualised procedure with precise entry and exit points for early mobilisation according to clinical status.

CONCLUSIONS

Insufficient data are available to prove the beneficial effects of early mobilisation after stroke, although early neurological rehabilitation as part of routine stroke unit care seems to contribute to good long-term outcome. This article presents a standardised procedure for early mobilisation that may be tested in a randomised study between early mobilisation out of bed (3 days) compared to late mobilisation (6 days).

Cross-correlation of instantaneous phase increments in pressure-flow fluctuations: applications to cerebral autoregulation

We investigate the relationship between the blood flow velocities (BFV) in the middle cerebral arteries and beat-to-beat blood pressure (BP) recorded from a finger in healthy and post-stroke subjects during the quasisteady state after perturbation for four different physiologic conditions: supine rest, head-up tilt, hyperventilation, and CO2 rebreathing in upright position. To evaluate whether instantaneous BP changes in the steady state are coupled with instantaneous changes in the BFV, we compare dynamical patterns in the instantaneous phases of these signals, obtained from the Hilbert transform, as a function of time. We find that in post-stroke subjects the instantaneous phase increments of BP and BFV exhibit well-pronounced patterns that remain stable in time for all four physiologic conditions, while in healthy subjects these patterns are different, less pronounced, and more variable. We propose an approach based on the cross-correlation of the instantaneous phase increments to quantify the coupling between BP and BFV signals. We find that the maximum correlation strength is different for the two groups and for the different conditions. For healthy subjects the amplitude of the cross-correlation between the instantaneous phase increments of BP and BFV is small and attenuates within 3-5 heartbeats. In contrast, for post-stroke subjects, this amplitude is significantly larger and cross-correlations persist up to 20 heartbeats. Further, we show that the instantaneous phase increments of BP and BFV are cross-correlated even within a single heartbeat cycle. We compare the results of our approach with three complementary methods: direct BP-BFV cross-correlation, transfer function analysis, and phase synchronization analysis. Our findings provide insight into the mechanism of cerebral vascular control in healthy subjects, suggesting that this control mechanism may involve rapid adjustments (within a heartbeat) of the cerebral vessels, so that BFV remains steady in response to changes in peripheral BP.

Cerebral blood flow monitoring in clinical practice

The brain depends on a continuous flow of blood to provide it with oxygen and glucose needed to maintain normal function and structural integrity, thus cerebral blood flow is normally tightly regulated. A decrease in cerebral blood flow to ischemic levels may be tolerated for only minutes to hours, depending on the severity of the ischemia. If cerebral blood flow ceases completely, brain cell death occurs within minutes. A variety of conditions are encountered clinically, such as stroke or traumatic brain injury, where an actual or potential alteration in cerebral blood flow puts the brain at risk for ischemia and infarction. In this article, the physiology of cerebral blood flow will be presented as a basis for understanding cerebral blood flow regulation and the rationale for clinical interventions to optimize cerebral blood flow. Techniques currently available to assess cerebral blood flow and clinical situations in which cerebral blood flow is measured will be discussed. Clinical interventions will be presented briefly.

Dynamic Cerebral Autoregulation in Acute Lacunar and Middle Cerebral Artery Territory Ischemic Stroke

Background and Purpose— We addressed whether dynamic cerebral autoregulation (dCA) is affected in middle cerebral artery (MCA) territory (MCAS) and lacunar ischemic stroke (LS).

Methods— Blood pressure (MAP) and MCA velocity (V) were measured in 10 patients with large MCAS (National Institutes of Health Stroke score, 17±2; mean±SEM), in 10 with LS (score, 9±1), and in 10 reference subjects. dCA was evaluated in time (delay of the MCA V mean counter-regulation during changes in MAP) and frequency domains (cross-spectral MCA V mean -to-MAP phase lead).

Results— In reference subjects, latencies for MAP increments (5.3±0.5 seconds) and decrements (5.6±0.5 seconds) were comparable, and low frequency MCA V mean -to-MAP phase lead was 56±5 and 59±5° (left and right hemisphere). In MCAS, these latencies were 4.6±0.7 and 5.6±0.5 seconds in the nonischemic hemisphere and not detectable in the ischemic hemisphere. In the unaffected hemisphere, phase lead was 61±6° versus 26±6° on the ischemic side ( P <0.05). In LS, no latency and smaller phase lead bilaterally (32±6 and 33±5°) conformed to globally impaired dCA.

Conclusions— In large MCAS infarcts, dynamic cerebral autoregulation was impaired in the affected hemisphere. In LS, dynamic cerebral autoregulation was impaired bilaterally, a finding consistent with the hypothesis of bilateral small vessel disease in patients with lacunar infarcts.

Treatment of massive cerebral infarction

Stroke is the third leading cause of death in the United States, with a person dying every 3 minutes of a stroke. Massive ischemic stroke accounts for 10% to 20% of ischemic strokes, has traditionally been associated with a high mortality and morbidity, and requires a unique management strategy. Recent advances in management, fueled by an increased understanding of the pathophysiology, may help decrease mortality and improve outcomes. Rapid access to reperfusion therapies remains the most critical element of stroke care and the cornerstone of therapy. This article focuses on newer therapies, including osmotic therapy, hypothermia, maintained normothermia, strict glycemic control, induced hypertension, and hemicraniectomy, all of which show promise for reducing mortality and improving functional outcome. These interventions have become integrated into neurologic intensive care units around the world. They are complicated, require a high level of expertise, and carry a significant learning curve. In order for these new management techniques to be effective, an expedited, aggressive, meticulous, and potentially prolonged medical management approach is needed. To accomplish this there is a growing need for focused specialists in the areas of neurointensive care and stroke.

High-resolution ultrahigh-field MRI of stroke

BACKGROUND

Ultrahigh-field MRI at 8 T offers unprecedented resolution for imaging brain structures and microvasculature.

OBJECTIVE

The aim of this study is to apply high-resolution MRI for stroke imaging and to characterize findings at 1.5 and 8 T.

METHODS

Seventeen subjects with minor ischemic infarcts were studied using T2-weighted gradient echo (GE) and rapid acquisition with relaxation enhancement (RARE) images at 8 T with resolution up to 200 microm. In 10 subjects, T1- and T2-weighted fast spin echo (FSE) and fluid-attenuated inversion recovery (FLAIR) images were also acquired at 1.5-T MRI.

RESULTS

The 8-T images showed infarcts as sharply demarcated areas of high-signal intensity (n=21) and revealed more infarctions than 1.5-T images (n=14) (P<.003). The low-signal intensity areas that surrounded infarctions were suggestive of hemosiderin deposits. The 8-T characteristics of microvessels terminating within the infractions were distinct from normal vasculature. The 8-T images revealed an angioma at the site of a second stroke, not apparent on 1.5-T images.

CONCLUSIONS

Ultrahigh-field MRI at 8 T is feasible for stroke imaging. The 8-T MRI visualized infarcts and microvasculature with high resolution, revealing infarcts and vascular pathologies that were not apparent at 1.5 T.

Assessment of cerebral autoregulation from ectopic heartbeats

Transfer function analysis of spontaneous fluctuations in BP (blood pressure) and CBFV (cerebral blood flow velocity) has been widely used to study dynamic CA (cerebral autoregulation). The inverse Fourier transform and its integral, giving the impulse and step responses, have been used to gain perspective of the state of dynamic CA from the frequency and time domains respectively. The occurrence of ectopic heartbeats in the data has usually been treated as an artefact. Data containing multiple ectopic heartbeats were selected from a data set compiled for an acute stroke study which also included bilateral middle CBFV, concurrent surface ECG and non-invasive beat-to-beat BP recordings. Transfer function analysis and impulse and step responses were calculated from these data by (i) retaining ectopic heartbeats, (ii) after removal of ectopic heartbeats and replacement by linear interpolation, and (iii) using a narrow window of data surrounding selected ectopic heartbeats. Coherent averaging of the raw data of the selected ectopic heartbeats also allowed direct visualization of the relationship between BP changes and CBFV. The impulse and step responses were similar in shape whether or not ectopic heartbeats had been removed and showed characteristics of active dynamic CA. Removal of ectopic heartbeats from the CBFV and BP tracings, by linear interpolation or other methods, is not necessary to provide reliable estimates of dynamic autoregulation in subjects with ectopic heartbeat rates of up to eight per min. Additionally, impulse-like disturbances of BP induced by single-beat ectopic heartbeats provide enough information to characterize the autoregulatory response of the subject in agreement with more traditional methods of dynamic autoregulation assessment.

What do we (not) know about the management of blood pressure in acute stroke?

Although it is indisputable that high blood pressure should be treated to prevent a first or a recurrent stroke, the management of high blood pressure in the first hours and days after stroke remains controversial. There is no high-quality evidence from randomized controlled trials to guide treatment in the 80% of patients who have elevated blood pressure during the first days after stroke. Theoretically, there are pros and cons for manipulating blood pressure after onset of stroke. Most treatment guidelines suggest leaving blood pressure untouched based on pathophysiologic principles. Post-hoc analyses from randomized trials, however, suggest that elevated blood pressure is associated with recurrent stroke and higher mortality, even after adjustment for potential confounders. On the other hand, preliminary studies have suggested that voluntarily increasing blood pressure might be beneficial in a selected subgroup of patients. In this overview, we present a summary of recent studies on this topic.

Multimodal pressure-flow method to assess dynamics of cerebral autoregulation in stroke and hypertension

Background

This study evaluated the effects of stroke on regulation of cerebral blood flow in response to fluctuations in systemic blood pressure (BP). The autoregulatory dynamics are difficult to assess because of the nonstationarity and nonlinearity of the component signals.

Methods

We studied 15 normotensive, 20 hypertensive and 15 minor stroke subjects (48.0 ± 1.3 years). BP and blood flow velocities (BFV) from middle cerebral arteries (MCA) were measured during the Valsalva maneuver (VM) using transcranial Doppler ultrasound.

Results

A new technique, multimodal pressure-flow analysis (MMPF), was implemented to analyze these short, nonstationary signals. MMPF analysis decomposes complex BP and BFV signals into multiple empirical modes, representing their instantaneous frequency-amplitude modulation. The empirical mode corresponding to the VM BP profile was used to construct the continuous phase diagram and to identify the minimum and maximum values from the residual BP (BP_R) and BFV (BFV_R) signals. The BP-BFV phase shift was calculated as the difference between the phase corresponding to the BP_R and BFV_R minimum (maximum) values. BP-BFV phase shifts were significantly different between groups. In the normotensive group, the BFV_R minimum and maximum preceded the BP_R minimum and maximum, respectively, leading to large positive values of BP-BFV shifts.

Conclusion

In the stroke and hypertensive groups, the resulting BP-BFV phase shift was significantly smaller compared to the normotensive group. A standard autoregulation index did not differentiate the groups. The MMPF method enables evaluation of autoregulatory dynamics based on instantaneous BP-BFV phase analysis. Regulation of BP-BFV dynamics is altered with hypertension and after stroke, rendering blood flow dependent on blood pressure.

Cerebral pressure-flow relations in hypertensive elderly humans: transfer gain in different frequency domains

The dynamics of the cerebral vascular response to blood pressure changes in hypertensive humans is poorly understood. Because cerebral blood flow is dependent on adequate perfusion pressure, it is important to understand the effect of hypertension on the transfer of pressure to flow in the cerebrovascular system of elderly people. Therefore, we examined the effect of spontaneous and induced blood pressure changes on beat-to-beat and within-beat cerebral blood flow in three groups of elderly people: normotensive, controlled hypertensive, and uncontrolled hypertensive subjects. Cerebral blood flow velocity (transcranial Doppler), blood pressure (Finapres), heart rate, and end-tidal CO(2) were measured during the transition from a sit to stand position. Transfer function gains relating blood pressure to cerebral blood flow velocity were assessed during steady-state sitting and standing. Cerebral blood flow regulation was preserved in all three groups by using changes in cerebrovascular resistance, transfer function gains, and the autoregulatory index as indexes of cerebral autoregulation. Hypertensive subjects demonstrated better attenuation of cerebral blood flow fluctuations in response to blood pressure changes both within the beat (i.e., lower gain at the cardiac frequency) and in the low-frequency range (autoregulatory, 0.03-0.07 Hz). Despite a better pressure autoregulatory response, hypertensive subjects demonstrated reduced reactivity to CO(2). Thus otherwise healthy hypertensive elderly subjects, whether controlled or uncontrolled with antihypertensive medication, retain the ability to maintain cerebral blood flow in the face of acute changes in perfusion pressure. Pressure regulation of cerebral blood flow is unrelated to cerebrovascular reactivity to CO(2).

Assessing dynamic cerebral autoregulation after stroke using a novel technique of combining transcranial Doppler ultrasonography and rhythmic handgrip

OBJECTIVES

Dynamic cerebral autoregulation (CA) is impaired after stroke. Methods employed to assess this phenomenon usually involve deliberate alterations in blood pressure (BP) by physical means. We performed a pilot study to assess dynamic CA in acute stroke patients using a novel technique of combining transcranial Doppler (TCD) ultrasonography with rhythmic handgrip.

METHODS

Ten patients with ischaemic stroke in the middle cerebral artery (MCA) territory were studied. We performed continuous recordings of bilateral MCA velocities and used rhythmic handgrip to induce BP oscillations. Changes in autoregulation were indicated by changes in phase shift and gain of MCA velocity in relation to BP. Patients were examined at <7 days, 6 weeks, and 3 months after stroke.

RESULTS

There were no significant differences in phase shift or gain between the affected and unaffected cerebral hemispheres. Combining the results from both hemispheres, there was a trend of increasing phase shift (P=0.04) and decreasing gain (P=0.24) over the first three months after stroke, indicating improving CA. Rhythmic handgrip produced an average percentage change in BP (peak-to-trough) of 10% around the mean, and the frequency of the induced BP oscillations was very similar to that of the rhythmic handgrip.

CONCLUSIONS

Combining TCD with rhythmic handgrip appeared to be a useful technique for assessing dynamic CA and it deserves further studies. In this pilot study, there was some evidence that CA might improve up to 3 months after ischaemic stroke.

Anandamide content is increased and CB1 cannabinoid receptor blockade is protective during transient, focal cerebral ischemia

The role of endocannabinoid signaling in the response of the brain to injury is tantalizing but not clear. In this study, transient middle cerebral artery occlusion (MCAo) was used to produce ischemia/reperfusion injury. Brain content of N-arachidonoylethanolamine (AEA) and 2-arachidonoylglycerol were determined during MCAo. Whole brain AEA content was significantly increased after 30, 60 and 120 min MCAo compared with sham-operated brain. The increase in AEA was localized to the ischemic hemisphere after 30 min MCAo, but at 60 and 120 min, was also increased in the contralateral hemisphere. 2-Arachidonoylglycerol content was unaffected by MCAo. In a second set of studies, injury was assessed 24 h after 2 h MCAo. Rats administered a single dose (3 mg/kg) of the cannabinoid receptor type 1 (CB1) receptor antagonist SR141716 prior to MCAo exhibited a 50% reduction in infarct volume and a 40% improvement in neurological function compared with vehicle control. A second CB1 receptor antagonist, LY320135 (6 mg/kg), also significantly improved neurological function. The CB1 receptor agonist, WIN 55212-2 (0.1-1 mg/kg) did not affect either infarct volume or neurological score.