Identifying regions of compromised hemodynamics in symptomatic carotid occlusion by cerebrovascular reactivity and oxygen extraction fraction

Cerebral vasomotor reactivity predicts the development of acute stroke in patients with internal carotid artery stenosis

OBJECTIVE

To investigate the relationship between cerebral vasomotor reactivity (VMR) and acute stroke in patients with internal carotid artery stenosis.

METHODS

54 patients with internal carotid artery stenosis were enrolled. VMR was calculated by transcranial Doppler monitoring of the velocity of blood flow. 3-Dimensional dynamic contrast enhanced magnetic resonance angiography was used to detect stenosis, and diffusion weighted imaging was used to detect infarction.

RESULTS

VMR value was significantly lower in patients with carotid artery stenosis than in control group (T=3.112, P=0.002), and significantly lower in patients with aortic atherosclerotic stroke than in non-infarct group (T=10.930, P=0.000). However, VMR value was significantly higher in patients with new-onset small-artery occlusion stroke than in non-infarction group (T=-2.538, P=0.013). Scatter plots showed that aortic atherosclerotic stroke occurred mainly in patients with severe internal carotid artery stenosis, and VMR value in cerebral artery significantly decreased.

CONCLUSION

Decreased VMR value is an important prognostic factor for the occurrence of aortic atherosclerotic stroke, and can be used as a reference for preoperative hemodynamic evaluation in patients with internal carotid artery stenosis.

Noninvasive optical monitoring of critical closing pressure and arteriole compliance in human subjects

The critical closing pressure ( CrCP) of the cerebral circulation depends on both tissue intracranial pressure and vasomotor tone. CrCP defines the arterial blood pressure ( ABP) at which cerebral blood flow approaches zero, and their difference ( ABP -  CrCP) is an accurate estimate of cerebral perfusion pressure. Here we demonstrate a novel non-invasive technique for continuous monitoring of CrCP at the bedside. The methodology combines optical diffuse correlation spectroscopy (DCS) measurements of pulsatile cerebral blood flow in arterioles with concurrent ABP data during the cardiac cycle. Together, the two waveforms permit calculation of CrCP via the two-compartment Windkessel model for flow in the cerebral arterioles. Measurements of CrCP by optics (DCS) and transcranial Doppler ultrasound (TCD) were carried out in 18 healthy adults; they demonstrated good agreement (R = 0.66, slope = 1.14 ± 0.23) with means of 11.1 ± 5.0 and 13.0 ± 7.5 mmHg, respectively. Additionally, a potentially useful and rarely measured arteriole compliance parameter was derived from the phase difference between ABP and DCS arteriole blood flow waveforms. The measurements provide evidence that DCS signals originate predominantly from arteriole blood flow and are well suited for long-term continuous monitoring of CrCP and assessment of arteriole compliance in the clinic.

Cerebral watershed infarcts may be induced by hemodynamic changes in blood flow

OBJECTIVES

A watershed infarct is defined as an ischemic lesion at the border zones between territories of two major arteries. The pathogenesis of watershed infarcts, specifically whether they are caused by hemodynamic or embolic mechanisms, has long been debated. In this study, we aimed to examine whether watershed infarcts can be induced by altering the hemodynamic conditions in rats.

MATERIALS AND METHODS

In phase one, to determine the proper clamping duration for a reproducible infarct, 30 rats were equally divided into 5 subgroups and underwent bilateral common carotid artery (CCA) clamping for different durations (0.5, 1.0, 1.5, 2.0, and 3.0 hours). In phase two, to analyze the types of infarcts induced by bilateral CCA clamping, 40 rats were subjected to bilateral CCA clamping for 2 hours. As a control, 8 rats underwent all the operation procedures except bilateral CCA clamping. We performed 7.0T magnetic resonance imaging on the surviving rats on the second day to evaluate the extent of the infarcts. We further identified and examined the infarcts with brain slices stained using 2, 3, 5-triphenyltetrazolium chloride (TTC) on the third day.

RESULTS

After 2 hours of bilateral CCA clamping, cerebral infarction occurred in 42% of surviving rats (13/31). The majority of the ischemic lesions were located in watershed regions of the brain, demonstrated by both MRI and TTC staining.

CONCLUSION

Watershed infarcts were induced through changing hemodynamic conditions by bilateral CCA clamping in rats. This method may lead to the development of a reliable rodent model for watershed infarcts.

Results of Prospective Cohort Study on Symptomatic Cerebrovascular Occlusive Disease Showing Mild Hemodynamic Compromise [Japanese Extracranial-Intracranial Bypass Trial (JET)-2 Study]

The purpose of this study is to determine the true threshold of cerebral blood flow (CBF) and cerebrovascular reactivity (CVR) for subsequent ischemic stroke without extracranial-intracranial (EC-IC) bypass surgery in patients with hemodynamic ischemia due to symptomatic major cerebral arterial occlusive diseases. Patients were categorized based on rest CBF and CVR into four subgroups as follows: Group A, 80% < CBF < 90% and CVR < 10%; Group B, CBF < 80% and 10% < CVR < 20%; Group C, 80% < CBF < 90% and 10% < CVR < 20%; and Group D, CBF < 90% and 20% < CVR < 30%. Patients were followed up for 2 years under best medical treatment by the stroke neurologists. Primary and secondary end points were defined as all adverse events and ipsilateral stroke recurrence respectively. A total of 132 patients were enrolled. All adverse events were observed in 9 patients (3.5%/year) and ipsilateral stroke recurrence was observed only in 2 patients (0.8%/year). There was no significant difference among the four subgroups in terms of the rate of both primary and secondary end points. Compared with the medical arm of the Japanese EC-IC bypass trial (JET) study including patients with CBF < 80% and CVR < 10% as a historical control, the incidence of ipsilateral stroke recurrence was significantly lower in the present study. Patients with symptomatic major cerebral arterial occlusive diseases and mild hemodynamic compromise have a good prognosis under medical treatment. EC-IC bypass surgery is unlikely to benefit patients with CBF > 80% or CVR > 10%.

Approaches to brain stress testing: BOLD magnetic resonance imaging with computer-controlled delivery of carbon dioxide

Background

An impaired vascular response in the brain regionally may indicate reduced vascular reserve and vulnerability to ischemic injury. Changing the carbon dioxide (CO₂) tension in arterial blood is commonly used as a cerebral vasoactive stimulus to assess the cerebral vascular response, changing cerebral blood flow (CBF) by up to 5–11 percent/mmHg in normal adults. Here we describe two approaches to generating the CO₂ challenge using a computer-controlled gas blender to administer: i) a square wave change in CO₂ and, ii) a ramp stimulus, consisting of a continuously graded change in CO₂ over a range. Responses were assessed regionally by blood oxygen level dependent (BOLD) magnetic resonance imaging (MRI).

Methodology/Principal Findings

We studied 8 patients with known cerebrovascular disease (carotid stenosis or occlusion) and 2 healthy subjects. The square wave stimulus was used to study the dynamics of the vascular response, while the ramp stimulus assessed the steady-state response to CO₂. Cerebrovascular reactivity (CVR) maps were registered by color coding and overlaid on the anatomical scans generated with 3 Tesla MRI to assess the corresponding BOLD signal change/mmHg change in CO₂, voxel-by-voxel. Using a fractal temporal approach, detrended fluctuation analysis (DFA) maps of the processed raw BOLD signal per voxel over the same CO₂ range were generated. Regions of BOLD signal decrease with increased CO₂ (coded blue) were seen in all of these high-risk patients, indicating regions of impaired CVR. All patients also demonstrated regions of altered signal structure on DFA maps (Hurst exponents less than 0.5; coded blue) indicative of anti-persistent noise. While ‘blue’ CVR maps remained essentially stable over the time of analysis, ‘blue’ DFA maps improved.

Conclusions/Significance

This combined dual stimulus and dual analysis approach may be complementary in identifying vulnerable brain regions and thus constitute a regional as well as global brain stress test.