The effects of nifedipine and felodipine on cerebral blood flow during anoxic episodes

Role of nitric oxide-containing factors in the ventilatory and cardiovascular responses elicited by hypoxic challenge in isoflurane-anesthetized rats

Exposure to hypoxia elicits changes in mean arterial blood pressure (MAP), heart rate, and frequency of breathing (fR). The objective of this study was to determine the role of nitric oxide (NO) in the cardiovascular and ventilatory responses elicited by brief exposures to hypoxia in isoflurane-anesthetized rats. The rats were instrumented to record MAP, heart rate, and fR and then exposed to 90 s episodes of hypoxia (10% O2, 90% N2) before and after injection of vehicle, the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), or the inactive enantiomer D-NAME (both at 50 μmol/kg iv). Each episode of hypoxia elicited a decrease in MAP, bidirectional changes in heart rate (initial increase and then a decrease), and an increase in fR. These responses were similar before and after injection of vehicle or D-NAME. In contrast, the hypoxia-induced decreases in MAP were attenuated after administration of L-NAME. The initial increases in heart rate during hypoxia were amplified whereas the subsequent decreases in heart rate were attenuated in L-NAME-treated rats. Finally, the hypoxia-induced increases in fR were virtually identical before and after administration of L-NAME. These findings suggest that NO factors play a vital role in the expression of the cardiovascular but not the ventilatory responses elicited by brief episodes of hypoxia in isoflurane-anesthetized rats. Based on existing evidence that NO factors play a vital role in carotid body and central responses to hypoxia in conscious rats, our findings raise the novel possibility that isoflurane blunts this NO-dependent signaling.

The effects of graded hypercapnia on the activation flow coupling response due to forepaw stimulation in alpha-chloralose anesthetized rats

Activation flow coupling (AFC), changes in cerebral blood flow (CBF) due to changes in neural activity with functional stimulation, provides the physiological basis of many neuroimaging techniques. Hypercapnia leads to an increase in CBF while neural activity remains unaffected. Laser Doppler (LD) flowmetry was used to measure CBF changes (LD(CBF)) in the somatosensory cortex due to periodic electrical forepaw stimulation (4 s in duration) before and during graded hypercapnia (3% CO(2), 5% CO(2) and 10% CO(2)). With increasing CO(2) concentrations, the baseline LD(CBF) progressively increased. The peak height (PH) of the LD(CBF) response, expressed as a percent change from the observed baseline for each hypercapnic state, significantly decreased (P<0.05) with increasing CO(2) concentrations. However, the absolute magnitude of the LD(CBF) change was independent of CO(2) concentration. The temporal dynamics of the LD(CBF) response during hypercapnia were significantly prolonged compared to baseline conditions (P<0.05).

Effect of oral felodipine on ocular circulation

PURPOSE

To evaluate the effects of felodipine, a calcium channel blocker, on ocular circulation.

METHODS

In a double blind, randomized, crossover design, 10 volunteers received placebo or felodipine on 2 separate days. Bidirectional laser Doppler velocimetry (BLDV), laser Doppler flowmetry (LDF), and monochromatic fundus photography (MFP) were employed to assess retinal, choroidal, and optic nerve rim circulatory parameters. Measurements were obtained at baseline, 1.5, and 3 hours after dosing. Blood pressure, intraocular pressure (IOP), and heart rate were monitored, and perfusion pressure (PP) was calculated.

RESULTS

In comparison to placebo, significant average percentage decreases in diastolic blood pressure (BPd; p = 0.001), mean blood pressure (BPm; p = 0.003), and perfusion pressure (PP; p = 0.003) were observed 1.5 hours following felodipine. No significant differences were observed following placebo. Retinal, choroidal and optic nerve rim circulatory parameters showed no significant changes after placebo or felodipine. A significant positive correlation between BPd and optic nerve velocity (ONve1) was observed 3 hours following felodipine (R = +0.654; p = 0.040). In contrast, no significant correlation between BPd and ONve1 was noted (R = +0.198; p = 0.583) following placebo.

CONCLUSIONS

No significant change in retinal, optic nerve rim, or foveolar choroidal blood flow were observed, a factor that may be important in the treatment of systemic hypertension in patients with glaucoma.

A comparison of cerebral blood flow during basal, hypotensive, hypoxic and hypercapnic conditions between normal and streptozotocin diabetic rats

This study was undertaken to determine the differences (if any) in cerebral blood flow (CBF) between streptozotocin (STZ) diabetic and normal rats. CBF was studied in connection with episodes of hypoxia, hypercapnia and hypotension as compared to the basal condition. Overall basal CBF rates in streptozotocin diabetic rats were significantly higher than in normal animals. However, initial basal flow rates prior to the first challenge were insignificantly higher in the STZ diabetic group. The higher CBF rate in STZ diabetics was also seen during the peak flows of the hypoxic and hypercapnic challenges. Furthermore, although overall CBF decreased for both the normal and STZ diabetic groups during hypotension, higher CBFs were observed in the STZ diabetic group during this challenge. The percent increase in CBF above control resulting from hypoxia or hypercapnia and the changes in CBF resulting from hypotension were not significantly different in the STZ diabetic and normal groups. The results indicate that the STZ diabetic rat regulates CBF in the same manner as the normal rat in response to hypoxia, hypercapnia and hypertension. The STZ diabetic rat executes these CBF responses at a slightly higher CBF rate. In view of the finding that the regulation of CBF is unaltered in the STZ diabetic animal, it is hypothesized that the associated hyperglycemia may be the causative agent for the cerebral ischemic susceptibility associated with long-term diabetes mellitus rather than a failure of CBF regulation.

Acute effects of antihypertensive agents on cerebral blood flow in hypertensive rats

The acute effects of various antihypertensive agents on cerebral blood flow and mean arterial pressure (MAP) were studied in anesthetized (amobarbital 100 mg/kg) spontaneously hypertensive rats. Cerebral blood flow in the cortex and thalamus was measured by the hydrogen clearance method before and during a 60-min i.v. infusion of calcium antagonist (nifedipine), angiotensin converting enzyme inhibitor (captopril) or beta-blocker (propranolol). Nifedipine, 30 or 150 micrograms/kg per h, decreased dose dependently the MAP by 20 or 31%, and concomitantly increased cortical blood flow by 28 or 74%, and thalamic blood flow by 51 or 64%, respectively. Captopril, 10 or 100 mg/kg per h, decreased MAP by 7 or 14%, but changed cerebral blood flow minimally. In contrast, propranolol, 1.0 or 5.0 mg/kg per h, decreased MAP by 13 or 11%, with a concomitant reduction of cortical and thalamic blood flow by 20 or 15 and 33 or 37%, respectively. It is concluded that the changes in cerebral blood flow in response to hypotension are varied by antihypertensive drugs depending on the direct or indirect effect of the drugs (dilatation or constriction) on cerebral vessels. Nifedipine seems to dilate while propranolol constricts cerebral vessels.

Anaesthesia for closed embolisation of cerebral arteriovenous malformations

Cerebral arteriovenous malformation embolisation is a therapeutic, neuroradiological procedure involving injection of bucrylate glue into the nidus of the AV malformation to obliterate the abnormal vascular network. These procedures may involve significant risks, are often long and thereby necessitate the need for some form of sedation and for adequate monitoring of the cerebral, cardiovascular and respiratory systems. The anaesthetic management of a series of twenty patients undergoing embolisation of a cerebral arteriovenous malformation is outlined, seven general and nineteen neurolept anaesthetics being administered. Neurolept anaesthesia is the preferred technique as neurological assessment during the procedure is possible and complications may be diagnosed immediately. Systemic arterial hypotension may facilitate the embolisation process and various agents, including glyceryl trinitrate and sodium nitroprusside, have been employed for this purpose.