Nimodipine has no effect on the cerebral circulation in conscious pigs, despite an increase in cardiac output

Cardiac Output and Cerebral Blood Flow: A Systematic Review of Cardio-Cerebral Coupling

Control of cerebral blood flow (CBF) is crucial to the management of neurocritically ill patients. Small studies which have examined the role of cardiac output (CO) as a determinant of CBF have inconsistently demonstrated evidence of cardio-cerebral coupling. Putative physiological mechanisms underpinning such coupling include changes in arterial blood pressure pulsatility, which would produce vasodilation through increased oscillatory wall-shear-stress and baroreceptor mediated reflex sympatholysis, and changes in venous backpressure which may improve cerebral perfusion pressure. We sought to summarize and contextualize the literature on the relationship between CO and CBF and discuss the implications of cardio-cerebral coupling for neurocritical care. A systematic review of the literature yielded 41 studies; all were of low-quality and at high-risk of bias. Results were heterogenous, with evidence for both corroboration and confutation of a relationship between CO and CBF in both normal and abnormal cerebrovascular states. Common limitations of studies were lack of instantaneous CBF measures with reliance on transcranial Doppler-derived blood flow velocity as a surrogate, inability to control for fluctuations in established determinants of CBF (eg, PaCO2), and direct effects on CBF by the interventions used to alter CO. Currently, the literature is insufficiently robust to confirm an independent relationship between CO and CBF. Hypothetically, the presence of cardio-cerebral coupling would have important implications for clinical practice. Manipulation of CBF could occur without the risks associated with extremes of arterial pressure, potentially improving therapy for those with cerebral ischemia of various etiologies. However, current literature is insufficiently robust to confirm an independent relationship between CO and CBF, and further studies with improved methodology are required before therapeutic interventions can be based on cardio-cerebral coupling.

Cerebral microvascular effects of nimodipine in combination with soman

Nimodipine, a calcium antagonist, has been shown to increase the detoxification of soman. In this study the cerebral microcirculatory effects of nimodipine and the acetylcholinesterase inhibitor soman was studied. Anaesthetised rats were administered nimodipine, 10 mg kg(-1) or vehicle intra-peritoneally, and 1h later exposed to 45 μg kg(-1) soman intravenously. The regional blood flows were measured using the microsphere method. Nimodipine and soman markedly increased the cerebral blood flow (CBF) and reduced the vascular resistance. Total CBF increased by 146% after nimodipine and by 105% after soman administration. Combined administration of nimodipine and soman caused additional but not fully additive effects on CBF and vascular resistance, indicating possible different mechanisms of the two agents. A part of the nimodipine induced increased detoxification after AChE-inhibition may be associated with this cerebral vasodilation.

Cerebral oxygenation and haemodynamic effects induced by nimodipine in healthy subjects

The cerebrovascular effects of nimodipine are still poorly understood even in the healthy condition; in particular, its effects on tissue oxygenation have never been investigated. The aim of the present study was to investigate changes in cerebral oxygenation and blood volume upon oral administration of nimodipine (90 mg) in the healthy condition. In eight subjects, changes in cerebral tissue oxygenation and blood volume were determined simultaneously with changes in blood velocity of the middle cerebral artery (VMCA) by using, respectively, near infrared spectroscopy (NIRS) and transcranial Doppler ultrasonography (TCD). The subjects also underwent noninvasive assessment of arterial blood pressure (ABP) and end-tidal CO2. TCD and NIRS CO2 reactivity indices were al-so extracted. Nimodipine significantly reduced ABP (11±13%) and increased heart rate, as well as NIRS oxygenation(6.0±4.8%) and blood volume indices (9.4±10.1%), while V(MCA) was not significantly decreased (2.0±3.5%). Nimodipine slightly but significantly reduced the V(MCA) response to changes in pCO2 whereas the CO2 reactivity of NIRS parameters was improved. The observed changes in cerebral tissue oxygenation and blood volume indicate nimodipine-induced cerebrovascular dilation and increased perfusion, while the effect on V(MCA)possibly results from dilation of the insonated artery. The present results cast doubt on the putative nimodipine-induced impairment of CO2 reactivity.

Nimodipine affects the microcirculation and modulates the vascular effects of acetylcholinesterase inhibition

The present investigation was undertaken in order to study whether microvascular effects of the calcium antagonist nimodipine induces changes that can explain an increased detoxification of the highly toxic cholinesterase inhibitor soman. Anaesthetised, tracheotomised and artificially ventilated rats were treated intra-peritoneally (ip) with nimodipine, 10 mg kg(-1) or vehicle followed one hour later by the exposure to 45 microg kg(-1) soman (iv). Nimodipine per se induced a vasodilation in the intestine, myocardium and other muscles. In the abdominal skin soman elicited a significant vasoconstriction that was turned into an increased blood flow after nimodipine pre-treatment. A slight vasoconstriction in diaphragm of soman intoxicated rats was turned into a significant vasodilation by nimodipine pre-treatment. In the intestinal parts no effect of soman was detected. However, in nimodipine pretreated animals soman induced a significant vasoconstriction. The capacity of soman detoxifying processes, i.e. enzymatic hydrolysis and covalent binding to different esterases, is unequally distributed throughout the body. Together with the knowledge of the detoxifying processes of cholinesterase inhibition the results support our theory, that nimodipine alters the peripheral blood flow in a beneficial way resulting in improved detoxification ability.

The effect of the calcium antagonist nimodipine on the detoxification of soman in anaesthetized rabbits

The effect of nimodipine, a vasoactive calcium antagonist, on the disappearance of soman from blood was studied in anaesthetized rabbits intoxicated with soman (10.8 micrograms kg-1 i.v.). Blood samples from the left heart ventricle and femoral artery were used to investigate soman detoxification. The concentrations of the soman isomers C+P- and C-P- in blood samples were determined by gas chromatography coupled with high-resolution mass spectrometry. During the sampling, 15-300 s after soman injection, the soman concentration in control animals decreased from 50 to 0.029 ng mL-1; in animals pre-treated with nimodipine (10 mg kg-1) it decreased from 15 to 0.033 ng mL-1. In animals pre-treated with nimodipine the soman concentration was significantly reduced during the first minute of sampling. No differences were detected between soman concentrations in samples from the heart and femoral artery. Acetylcholinesterase inhibition was also used as an indicator of soman activity; there was no difference between the activity of this enzyme in different peripheral organs of control and nimodipine-treated animals. Nimodipine reduces the initial concentration of soman in the blood, which might be of significance in the treatment of soman intoxication.

Calcium channels in cerebral arteries

Electrophysiological evidence shows the existence of voltage-operated Ca2+ channels of the L- and, in some cases, T- and B-, type in the smooth muscle cells of major cerebral arteries and arterioles. Current intensity through L-type Ca2+ channels is higher in cerebral than in peripheral arteries, which points to a greater dependence on extracellular Ca2+ of contractile responses in cerebral arteries. The increase in cytosolic Ca2+ concentration is the key event leading both to maintenance of basal cerebrovascular tone and to contraction of cerebral arteries in response to depolarization and agonist-receptor interaction. Such an increase results from increased transmembrane influx of Ca2+ through L-type Ca2+ channels, as well as from the release of Ca2+ from intracellular Ca2+ stores. Ca2+ entry modulators (dihydropyridines, phenylalkylamines, benzothiazepines, and diphenylpiperazines) bind to allosterically coupled sites in the Ca2+ channel, thus inhibiting (Ca2+ entry blockers) or stimulating (Ca2+ entry activators) Ca2+ influx and, therefore, contractile responses of the cerebral arteries. In vivo, Ca2+ entry blockers increase pial vascular caliber and cerebral blood flow by their direct action on the cerebroarterial wall. However, such an action also takes place on several peripheral vascular beds, which leads to hypotension. Therefore, the brain cannot be considered a "privileged" organ when the vasodilatatory action of Ca2+ entry blockers is considered. Since increased cytosolic Ca2+ concentration (and, therefore, activation of Ca2+ channels) plays a crucial role in the pathogenesis of ischemic brain damage (e.g., acute stroke and subarachnoid hemorrhage), Ca2+ entry blockers could be useful cytoprotective drugs. However, with the exception of nimodipine in the management of subarachnoid hemorrhage, clinical trials have yielded results that are not so promising as one could expect from those obtained in experimental research.

The protective effect of nimodipine, a calcium antagonist, and its influence on soman clearance in the anaesthetized rabbit

The effect of pretreatment with a vasoactive compound, nimodipine, on soman intoxication in peripheral organs of rabbits was studied by measuring changes in the cholinesterase and acetylcholinesterase activity and by measuring clearance of soman in blood using gas chromatography/high resolution mass spectrometry. In animals receiving soman only, initial blood concentrations were approximately 100 ng mL-1 and were still detectable after 5 min. The clearance rate of soman in blood markedly increased following nimodipine pretreatment such that soman was below the detection limit (0.002-0.003 ng mL-1) in all samples. Soman injection caused a significant inhibition of the acetylcholinesterase activity in serum, and in brain. In rabbits pretreated with nimodipine, no significant inhibition of acetylcholinesterase activity occurred after soman injection. In view of the effects of nimodipine on soman clearance and on the acetylcholinesterase and cholinesterase inhibition during soman intoxication, we suggest that nimodipine has profound circulatory effects, which during soman intoxication, increase the vascular perfusion through the body and thereby increase the detoxifying capacity.

Cardiovascular profile of 5 novel nitrate-esters: a comparative study with nitroglycerin in pigs with and without left ventricular dysfunction

1. Four cumulative 10 min intravenous infusions of 0.05, 0.2, 0.5 and 2.0 mg min-1 were used to compare the cardiovascular profile of 5 novel nitrate-esters dissolved in Intralipid 10% to that of nitroglycerin (GTN) in conscious pigs. 2. Infusion of Intralipid 10% alone had no effect on any of the systemic haemodynamic parameters. GTN infusions decreased mean arterial blood pressure dose-dependently from 94 +/- 2 mmHg to 79 +/- 3 mmHg (P less than 0.05) and raised cardiac output from 2.74 +/- 0.09 l min-1 to 3.40 +/- 0.18 l min-1 (P less than 0.05) due to an increase in heart rate (by up to 43 +/- 3%), as stroke volume decreased slightly. Systemic vascular resistance decreased (by 32 +/- 3%) and left ventricular end-diastolic pressure fell from 5.2 +/- 0.4 mmHg to 2.2 +/- 0.5 mmHg (both P less than 0.05). 3. The novel compounds CEDO 8811, CEDO 8834 and CEDO 8901 increased cardiac output only at the highest dose (7%, 8% and 9%, respectively). There was no change in mean arterial blood pressure as the increase in cardiac output was counterbalanced by arterial vasodilatation. All three compounds reduced left ventricular end-diastolic pressure slightly. 4. CEDO 8816 was a more potent arterial and venodilator than the aforementioned CEDO compounds, as the decreases in systemic vascular resistance and left ventricular end-diastolic pressure were already significant at lower doses. The fall in stroke volume was fully compensated by the increase in heart rate and as a result cardiac output increased by 11 +/- 3% (P less than 0.05) at the highest dose. 5. CEDO 8956 was the most potent vasodilator of the novel compounds and exhibited a cardiovascular profile similar to that of GTN. Left ventricular end-diastolic pressure decreased significantly during infusion of 0.2mgmin-'. Mean arterial blood pressure decreased by 11 +/- 2% (P < 0.05) in spite of an increase in cardiac output by up to 20 +/- 2% (P < 0.05), due to a decrease (by 27 +/- 1%, P <0.05) in systemic vascular resistance. The increases in heart rate (20 +/- 5%, P < 0.05) and LVdP/dtmax (38 +/- 4%, P < 0.05) were, however, considerably less after CEDO 8956 than after GTN. 6. The potential of CEDO 8956 in the treatment of chronic left ventricular dysfunction was evaluated during administration to conscious pigs (21-23 kg), in which the left circumflex coronary artery was ligated 4 weeks earlier. In these animals, baseline values for cardiac output and LVdP/dtx were lower and those of systemic vascular resistance and left ventricular end-diastolic pressure were higher than in the first group of experiments. 7. Both GTN and CEDO 8956 in doses of 0.05 to 2.0 mg inm increased cardiac output dosedependently (by up to 34% and 19%, respectively). The decrease in systemic vascular resistance was larger with GTN (35%) than with CEDO 8956 (17%), which resulted in a 13% decrease in mean arterial pressure during infusion of GTN, whereas there was no change in mean arterial pressure during infusion of CEDO 8956. Both compounds increased LVdP/dt,,,,X (by 48% and 30%, respectively) and lowered left ventricular end-diastolic pressure to normal levels. 8. At a dose of 1.0Omg min- 1, both GTN and CEDO 8956 increased left ventricular blood flow parallel to the increase in myocardial oxygen demand. At this dose, GTN also caused vasodilatation in the vascular beds of the brain, kidneys and adrenals. With CEDO 8956 no significant changes were achieved. 9. We conclude that the cardiovascular profile of CEDO 8956 in both normal animals and in animals with chronic left ventricular dysfunction warrants further study on its usefulness in the treatment of a number of cardiovascular disorders.