Comparison of transcranial Doppler flow velocity and cerebral blood flow during focal ischemia in rabbits

The Role of Ultrasound as a Diagnostic and Therapeutic Tool in Experimental Animal Models of Stroke: A Review

Ultrasound is a noninvasive technique that provides real-time imaging with excellent resolution, and several studies demonstrated the potential of ultrasound in acute ischemic stroke monitoring. However, only a few studies were performed using animal models, of which many showed ultrasound to be a safe and effective tool also in therapeutic applications. The full potential of ultrasound application in experimental stroke is yet to be explored to further determine the limitations of this technique and to ensure the accuracy of translational research. This review covers the current status of ultrasound applied to monitoring and treatment in experimental animal models of stroke and examines the safety, limitations, and future perspectives.

Evaluation of transcranial Doppler flow velocity changes in intracerebral hemorrhage rats using ultrasonography

This study investigated the blood flow velocity changes in seven major arteries in rat brain before and after intracerebral hemorrhage (ICH) using high frequency transcranial Doppler (TCD) ultrasonography (13-4 MHz). Eighteen adult Sprague Dawley rats received either the collagenase-injection surgery (ICH, n=12) or the saline-injection surgery (control, n=6) after baseline TCD flow velocity values were recorded. The TCD flow velocity changes were measured at 0.5h after the surgery and daily for the following 8 days in seven major cerebral arteries, including bilateral internal carotid arteries, bilateral middle cerebral arteries (MCAs), bilateral posterior cerebral arteries (PCAs), and basilar artery. The results showed a significant decrease of TCD flow velocity in the right MCA at 0.5 h and 24 h after the collagenase-injection surgery, and in the right PCA at 0.5 h in the ICH group rats. The TCD flow velocities in these two arteries gradually increased and then returned to the baseline values in the following days. The control group rats did not show significant changes in TCD flow velocity in all monitored arteries. This study demonstrates the feasibility and reliability of monitoring TCD flow velocity in cerebral arteries using ultrasonography technique in a rat ICH model. The results of this study extend our knowledge in the cerebrovascular changes during intracranial hemorrhage and suggest a possibility of clinical application of TCD ultrasonography in studying the dynamic cerebral circulation after strokes. Moreover, this method could be extensively applied in further studies using potential neuroprotective treatments that affect the cerebral dynamics in the intracerebral hemorrhage.

Evaluation of cerebral blood flow changes in focal cerebral ischemia rats by using transcranial Doppler ultrasonography

Ischemic stroke is typically characterized by the disruption of cerebral blood flow. This study aimed to consecutively evaluate the cerebral blood flow changes in a focal ischemia rat model during the occlusion-reperfusion procedure and along the recovery stage after stroke. In 12 Sprague Dawley (SD) rats, a middle cerebral artery occlusion/reperfusion (MCAo/r) surgery was conducted, which combines a permanent occlusion of the right common carotid artery (CCA), external carotid artery (ECA) and a transient occlusion of the right internal carotid artery (ICA) and middle cerebral artery (MCA) with a monofilament introduced from the proximal ICA towards the distal right ICA then removed after 90 min. Blood flow velocity (BFV) from the concerned arteries were measured using ultrasonography (13-4 MHz) at the basal stage before the surgery, after the reperfusion stage and during the post-stroke status. At reperfusion stage and after, BFV increased significantly in the left ICA and in the basilar artery (BA) (starting from post-24 h, p < 0.05 vs. basal). Moreover, BFV were reversed in the distal right ICA and reflow was recorded in the right MCA. Time-average maximum BFV in the right MCA at reperfusion and post-stroke 24-96 h was decreased significantly (p < 0.05 vs. basal). The reversed flow in the right ICA was enabled by the settlement of the collateral supply through the circle of Willis which consisted in higher BFV in the opposite ICA and in the BA still 24 h, although the proximal right ICA remain occluded. Ultrasound measurement of BFV helps to provide information on the redistribution of the blood flow supply after the onset of stroke.

Continuous neuromonitoring using transcranial Doppler reflects blood flow during carbon dioxide challenge in primates with global cerebral ischemia

OBJECTIVE

At present, there is no consensus on the optimal monitoring method for cerebral blood flow (CBF) in neurointensive care patients. The aim of the present study was to investigate whether continuous transcranial Doppler (TCD) monitoring with modulation of partial pressure of CO2 reflects CBF changes. This hypothesis was tested in 2 pathological settings in which cerebral ischemia can be imminent: after an episode of cerebral ischemia and during vasospasm after subarachnoid hemorrhage.

METHODS

Sixteen cynomolgus monkeys were divided into 3 groups: 1) chemoregulation in control animals to assess the physiological range of CBF regulation, 2) chemoregulation during vasospasm after subarachnoid hemorrhage, and 3) chemoregulation after transient cerebral ischemia. We surgically placed a thermal CBF probe over the cortex perfused by the right middle cerebral artery. Corresponding TCD values were acquired simultaneously while partial pressure of CO2 was changed within a range of 25 to 65 mm Hg (chemoregulation). A correlation coefficient of CBF with TCD values of greater than r equals 0.8 was considered clinically relevant.

RESULTS

CBF and CBF velocity correlated strongly after cerebral ischemia (r = 0.83, P < 0.001). Correlations were poor in chemoregulation controls (r = 0.2) and in the vasospasm group (r = 0.55).

CONCLUSION

The present study provides experimental support that, in clearly defined conditions, continuous TCD monitoring combined with chemoregulation testing may provide an estimate of CBF in the early postischemic period.

Ultrasonic assessment of cerebral blood flow changes during ischemia-reperfusion in 7-day-old rats

A model of ischemic brain injury in 7-day-old rat pups has been developed to study perinatal ischemia. It combines permanent occlusion of the distal left middle cerebral artery (LMCA) and transient occlusion of homolateral common carotid artery (LCCA). At removal of the clip on LCCA, reflow allowed brain reperfusion through cortical anastomoses. In 10 rat pups, we measured blood flow velocities (BFV) in main cerebral arteries with 12-MHz ultrasound imaging. At basal states, peak systolic BFV in proximal LMCA was 16.0 +/- 3.0 cm.s(-1). Occlusion of LMCA did not yield significant modifications. Occlusion of LCCA involved only a decrease in BFV to 9.5 +/- 2.6 cm.s(-1) (p < 0.001). Indeed, LMCA was then supply by the right internal carotid and the vertebral arteries through the circle of Willis. In three rat pups, release of occlusion of LCCA was followed by restoration of BFV in the left internal carotid artery and in LMCA, in seven pups, by a reversed flow in the LICA and lower BFV in LMCA (11.9 +/- 2.3, p < 0.05). BFV returned to basal values from h5 to h48 in all animals. In addition, ultrasound imaging is a useful, reproducible, non invasive, easy-to-repeat, method to assess and monitor arterial cerebral blood flow supply in small animals. It helps to characterize changes occurring during cerebral ischemia and reperfusion, particularly the depth of the hypoperfusion, as well as the variability of reflow. In preclinical studies, this method could help to identify what can be assigned to a neuroprotective treatment and what depends on changes in cerebral blood flow supply.

Detection and monitoring of cerebral hemodynamic disturbances with transcranial color-coded duplex sonography in patients after head injury

Reduced cerebral blood flow after severe head injury results in an increased risk of ischemic brain damage. Blood flow should therefore be monitored with a simple, reliable method. Transcranial color-coded Doppler sonography (TCCS) is an accepted tool for the diagnosis of cerebral vasospasm; however, its usefulness in evaluating patients with head injury has not been proven. Cerebral blood-flow velocity in the middle, anterior, and posterior cerebral arteries was measured with a 2.5 MHz probe (Aplio SSA 770A, Toshiba, Japan) in 36 subjects with moderate or severe head injury. Serial measurements of resistance index (RI), peak-systolic, end-diastolic, and mean velocity in the middle cerebral arteries were performed 2-24 h after head trauma and in the subsequent days during hospitalization. Immediately after head trauma, increased RI values, and unusually decreased blood-flow velocity (mainly in MCA) were observed. Microcirculation disturbances were suspected because the end-diastolic velocity had substantially diminished. Changes in blood-flow parameters correlated with the clinical state, and in most cases, a poor prognosis. In some patients, blood-flow velocity increased above the normal reference limit and this implied poor prognosis. Transcranial color-coded Doppler sonography is a reliable, repeatable, and accessible tool that provides information about cerebral blood-flow disturbances and may hold diagnostic and prognostic importance.

Improvements in the intraluminal thread technique to induce focal cerebral ischaemia in rabbits

Although the intraluminal thread technique has been used to induce focal cerebral ischaemia in rabbits, its success rate is not high. We, therefore, attempted to improve the stability and reproducibility of this method by using thread tips of appropriate diameter as determined from the anatomical characteristics of the carotid and cerebral arteries of New Zealand white rabbits. Following intraarterial injection of casting material, we tested threads of four different tip diameters to determine the optimal thread tip that could occlude the middle cerebral artery (MCA). 2,3,5-Triphenyltertrazolium chloride (TTC) staining showed that, consistent with the intraluminal diameter of the MCA from the arterial casts ( 0.50 +/- 0.06 mm), thread with 0.51-0.55 mm tip diameter was optimal for the occlusion of the MCA. Ability to induce focal cerebral ischaemia was also dependent on variations in the anatomy of the internal carotid artery (ICA), especially the origin of the occipital artery. Our results suggest that use of appropriately sized thread and accurate manipulation of its tip significantly improves the stability and reproducibility of this model.

Short-term effect of cigarette smoking on CO(2)-induced vasomotor reactivity in man: a study with near-infrared spectroscopy and tanscranial Doppler sonography

Cigarette smoking is a major risk factor for stroke, and quitting reduces the stroke risk within a few years. The aim of our study was to clarify whether CO(2)-induced vasomotor reactivity (VMR) is impaired in smokers after smoking a cigarette as a possible factor of an increased stroke risk. We compared VMR of 23 healthy smokers assessed at baseline, immediately, and 30 min after smoking a cigarette (1.2 mg nicotine) with values from nonsmoking, age-matched controls (n=24), obtained at identical time intervals. Cerebral blood flow velocities (CBFV) of both middle cerebral arteries (transcranial Doppler sonography), changes in concentration of cerebral oxygenated, deoxygenated, and total hemoglobin (HbO(2), Hb, and HbT, near-infrared spectroscopy), mean arterial blood pressure (MAP), and skin blood flow were recorded during normo- and hypercapnia. VMR was calculated as percentage change in CBFV and as micromolar change in concentration of HbO(2), Hb, and HbT per 1% increase in endtidal CO(2). CBFV in smokers was increased at baseline (left, p<0.05; right, p=0.05), immediately (p<0.01), and 30 min after smoking (p<0.05) as compared with nonsmokers. MAP rose immediately after smoking (p<0.01) and declined after 30 min. VMR in smokers at baseline did not differ from controls, decreased immediately after smoking (p<0.05), and normalized after 30 min (p>0.05). Increased baseline CBFV in smokers after smoking might be due to arteriolar dilation, increased MAP, and possibly constriction of basal cerebral arteries. Impaired VMR for about 30 min after smoking reflects endothelial dysfunction. This might contribute to the enhanced stroke risk in smokers.

Preserved CO(2) reactivity and increase in middle cerebral arterial blood flow velocity during laparoscopic surgery in children

In adult patients, the creation of pneumoperitoneum (PP) by means of carbon dioxide (CO(2)) insufflation leads to an increase in cerebral blood flow velocity (CBFV), which is thought to be caused by hypercapnia. We evaluated whether PP leads to an increase of CBFV in children, and whether this increase is directly related to PP. The effects of PP on middle cerebral artery blood flow velocity were investigated in 12 children (mean age 3 yr, range 15-63 mo) undergoing laparoscopic herniorrhaphy under general anesthesia with sevoflurane and nitrous oxide/oxygen. CBFV was measured by using transcranial Doppler ultrasonography. During CO(2) insufflation, the end-tidal CO(2) concentration was kept constant by adjustment of ventilation by increasing minute volume. The CBFV increased significantly at an intraabdominal pressure of 12 mm Hg compared with baseline from 68 +/- 11 cm/s to 81 +/- 12 cm/s (P < 0.05). CO(2) reactivity remained in the normal range (4.0% +/- 1.9%/mm Hg) during PP. We conclude that the induction of PP leads to an increase in middle cerebral artery blood flow velocity in young children independent from hypercapnia, whereas CO(2) reactivity remains normal.

IMPLICATIONS

Laparoscopic surgery is performed frequently in pediatric patients. Cerebral blood flow velocities increase during insufflation of the intraperitoneal cavity for minimally invasive surgery in children. The vasoreactivity as part of the cerebral autoregulation remains unaffected.