Effects of preexisting bypass graft on rCBF and SSEP's following acute stroke in dogs.

Regional cerebral blood flow (rCBF) was measured with radiolabeled microspheres in a canine model of superficial temporal artery-middle cerebral artery (STA-MCA) bypass and acute ischemia. Ischemic zone flows in seven dogs with the bypass first closed and then open showed no significant contribution of bypass flow in the intact vascular system. Following acute proximal occlusion, rCBF was preserved by bypass flow. A significant flow decrease ensued when the bypass was then clipped, confirming the adequacy of the lesion and the protective effect of the bypass. Reopening the bypass after 15 minutes of ischemia restored 76% of the previous flow. This was a significant increase from the global ischemia values, and was not statistically different from preocclusive values. Preocclusion somatosensory evoked potentials (SSEP's) in these animals showed a consistent biphasic wave at 8 to 10 msec after stimulation. This wave, with some decrease in amplitude, was preserved by bypass flow following creation of the arterial lesion. Bypass clipping abolished these ipsilateral SSEP's. Variable return of SSEP's occurred following reopening of the graft, but the recordings never reached preischemic amplitudes. This experimental study shows that, in this model, a prophylactic bypass subjected to immediate demand (with no time for "maturation") can adequately augment cortical rCBF and is superior to delayed revascularization. The data lend theoretical support to placement of a prophylactic STA-MCA bypass prior to elective carotid artery sacrifice or in surgery where the risk of acute vascular injury is high.

Sequential measurement of somatosensory evoked potentials in a canine model of hemispheric ischaemia.

We have previously shown that occlusion of 6 canine intracranial vessels produces profound hemispheric ischaemia and abolishes the ipsilateral SSEP. In the present study, this work was extended to ascertain the differential contributions of 6 intracranial vessels to SSEP function. Five dogs underwent craniotomy, brain retractions, and ipsilateral arterial microdissection, and were prepared for SSEP recordings. Stimulation-recording sequences were performed firstly prior to craniotomy, secondly following craniotomy and microdissection and thirdly following sequential occlusion of each of 6 vessels: (a) anterior cerebral (A2), (b) ethmoidal, (c) ophthalmic, (d) middle cerebral (MCA), (e) posterior communicating (PCoA), and (f) posterior cerebral artery. Individual SSEP amplitudes and latencies were measured and averaged and values for each wave were compared statistically to baseline values and to the immediately preceding value. Two thalamocortical potential components were identified: an early wave (6-10 ms) and a late wave (10-15 ms). No significant amplitude declines between waves occurred with A2, ethmoidal, and ophthalmic occlusions. Following MCA occlusion, amplitude decreased to 49.1% and 34.5% of baseline in the early and late waves respectively; a significant decrease (p less than 0.05) both from baseline and from post-ophthalmic occlusion. Further occlusions (PCoA and posterior cerebral) produced progressive declines in amplitude to a nadir of 12.5% (early) and 2.1% (late) of baseline. Although latency did increase slightly from baseline with each occlusion (5-7% total), no consistent and significant latency changes were identified.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hypercapnia on cerebral blood flow following prophylactic and delayed experimental superficial temporal artery-middle cerebral artery bypass.

Cerebral blood flow was studied in dogs to ascertain whether preexisting superficial temporal artery-middle cerebral artery bypass could preserve hypercapnic reactivity following acute ischemia and whether postischemic-delayed revascularization would restore hypercapnic reactivity. In six dogs flow was preserved and some degree of hypercapnic response remained following proximal occlusion with a patent bypass. During complete ischemia (bypass occluded) there was no hypercapnic reactivity in the ischemic zone. Significant flow was restored to the ischemic area following bypass reopening, but a cerebral blood flow decrease was seen with subsequent hypercapnia. In the opposite (control) hemisphere hypercapnia always produced significant cerebral blood flow increases. These data support the superiority of prophylactic over delayed superficial temporal artery-middle cerebral artery bypass in appropriate clinical situations.

Cerebral autoregulation following prophylactic and delayed experimental STA-MCA bypass.

Cerebral autoregulation is impaired in ischaemic regions. We hypothesized that pre-existing STA-MCA bypass would be superior to delayed revascularization in maintaining ipsilateral rCBF and preserving cerebral autoregulation following experimental stroke. Two series of dogs were tested to evaluate this hypothesis, but which was disproved for the chosen experimental conditions. In the first, eight dogs underwent craniotomy, STA-MCA bypass, and radiolabeled microsphere rCBF determinations. Blood pressure was manipulated with intravenous adenosine and levarterenol. Ischaemic zone rCBF was measured at MAP 60 mm Hg (97.2 ml.min-1.100 g-1) and MAP 140 mm Hg (113.6) (p = NS), in the intact arterial system with the patent bypass in place. An hemispheric ipsilateral ischaemic lesion was then created, and three further microsphere rCBF determinations were made at MAP 60 mm Hg (41.7 ml.min-1.100 g-1), MAP 100 mm Hg (52.6) and MAP 140 mm Hg (58.3). There were no significant differences between these measurements (ANOVA p = NS). In a second series of five animals the bypass was placed and the stroke lesion created first. Ischaemic zone rCBF was then measured at MAP 60 mm Hg (35 ml.min-1 . 100 g-1) and MAP 140 mm Hg (44 ml.min-1 . 100g-1) (p = NS), with the patent bypass in place. The bypass was then clamped for 15 minutes and profound ischaemia confirmed (5 ml.min-1 . 100 g-1, p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation of experimental rCBF determinations in goats with flow measurements from a Doppler-modified carotid artery shunt.

A carotid artery shunt system has been developed that continuously monitors blood flow rates by embedding a Doppler crystal in the shunt wall. The crystal ranges through a "liquid lens" that enables it to be placed without violation of the shunt lumen. Because the crystal is at a fixed angle (45 degrees) to the axis of blood flow and the diameter of the lumen remains constant, a linear relationship exists between flow rates and the Doppler velocity signal. This shunt system was previously tested in vitro using a pulsatile pump and was found to be accurate to within 4.7% of the actual flow rate. In the present study, animal (goat) experiments were performed consisting of simultaneous carotid shunt flow and bilateral rCBF measurements by the radiolabeled microsphere technique to determine in vivo the accuracy of this Doppler modified shunt and to ascertain the ability of shunt flow to increase in the face of acute contralateral carotid occlusion. Data from five animals show that in vivo shunt flow can be recorded to within 13% of control rCBF and that shunt flow increases nearly 50% under conditions of distal demand (contralateral carotid occlusion). This device may prove useful in laboratory studies of carotid shunt dynamics and in clinical practice to quickly detect correctable shunt flow abnormalities.