Do clinically relevant circulating concentrations of radiographic contrast agents inhibit platelet-dependent arterial thrombosis?

Intra-arterial iodinated radiographic contrast material injection administration in a rat middle cerebral artery occlusion and reperfusion model: possible effects on intracerebral hemorrhage

BACKGROUND AND PURPOSE

Observations in human interventional stroke treatment led us to hypothesize that iodinated radiographic contrast material use may contribute to intracerebral hemorrhage. Effects of intra-arterial iodinated radiographic contrast material on hemorrhagic transformation after middle cerebral artery occlusion and reperfusion were studied in a placebo-controlled, blinded preclinical study in rats.

METHODS

Four groups of male Sprague-Dawley rats were studied: saline group (n=8), contrast group (n=12), heparin group (n=9), and contrast+heparin group (n=9). The middle cerebral artery was occluded for 5 hours using suture placement. Heparin was infused before suture removal and reperfusion. Saline and/or contrast were infused immediately during reperfusion. Incidence, location, and size of hemorrhage were determined by brain necropsy inspection at 24 hours.

RESULTS

There was a significant increase in incidence of cortical hemorrhage from control (37.5%), contrast (75.0%), heparin (77.8%) to contrast+heparin (100%; Cochran-Mantel-Haenszel correlation, P<0.01). Both pooled contrast groups (85.7%) and pooled heparin groups (88.9%) had higher rates of cortical intracerebral hemorrhage compared with the control group (P<0.05). Similar trends for increased cortical intracerebral hemorrhage were seen in the contrast-only (P=0.18) and heparin-only (P=0.18) groups. There was a trend for decreased infarct edema in rats receiving contrast versus those without (P=0.06).

CONCLUSIONS

Intraarterial iodinated radiographic contrast material may increase cortical intracerebral hemorrhage, similar to heparin. Iodinated radiographic contrast material effect may be additive to heparin effect on the incidence of cortical intracerebral hemorrhage.

In vivo veritas: Thrombosis mechanisms in animal models

Experimental models have enhanced our understanding of atherothrombosis pathophysiology and have played a major role in the search for adequate therapeutic interventions. Various animal models have been developed to simulate thrombosis and to study in vivo parameters related to hemodynamics and rheology that lead to thrombogenesis. Although no model completely mimics the human condition, much can be learned from existing models about specific biologic processes in disease causation and therapeutic intervention. In general, large animals such as pigs and monkeys have been better suited to study atherosclerosis and arterial and venous thrombosis than smaller species such as rats, rabbits, and dogs. On the other hand, mouse models of arterial and venous thrombosis have attracted increasing interest over the past two decades, owing to direct availability of a growing number of genetically modified mice, improved technical feasibility, standardization of new models of local thrombosis, and low maintenance costs. To simulate rupture of an atherosclerotic plaque, models of arterial thrombosis often involve vascular injury, which can be achieved by several means. There is no animal model that is sufficiently tall, that can mimic the ability of humans to walk upright, and that possesses the calf muscle pump that plays an important role in human venous hemodynamics. A number of spontaneous or genetically engineered animals with overexpression or deletion of various elements in the coagulation, platelet, and fibrinolysis pathways are now available. These animal models can replicate important aspects of thrombosis in humans, and provide a valuable resource in the development of novel concepts of disease mechanisms in human patients.

INTRACRANIAL SUBARACHNOID HEMORRHAGE FOLLOWING LUMBAR MYELOGRAPHY IN TWO DOGS

Intracranial subarachnoid hemorrhage is a rare but serious complication of lumbar puncture in humans. Possible sequelae include increased intracranial pressure, cerebral vasospasm, or mass effect, which can result in dysfunction or brain herniation. We describe two dogs that developed intracranial subarachnoid hemorrhage following lumbar myelography. In both dogs, myelography was performed by lumbar injection of iohexol (Omnipaque). Both the dogs underwent uneventful ventral decompressive surgery for disk herniation; however, the dogs failed to recover consciousness or spontaneous respiration following anesthesia. Neurologic assessment in both dogs postoperatively suggested loss of brain stem function, and the dogs were euthanized. There was diffuse subarachnoid hemorrhage and leptomeningeal hemorrhage throughout the entire length of the spinal cord, brain stem, and ventrum of brain. No evidence of infectious or inflammatory etiology was identified. The diagnosis for cause of brain death was acute subarachnoid hemorrhage. Our findings suggest that fatal subarachnoid hemorrhage is a potential complication of lumbar myelography in dogs. The cause of subarachnoid hemorrhage is not known, but may be due to traumatic lumbar tap or idiosyncratic response to contrast medium. Subsequent brain death may be a result of mass effect and increased intracranial pressure, cerebral vasospasm, or interaction between subarachnoid hemorrhage and contrast medium.

Differential effects of the iodinated contrast agents Ioxaglate, Iohexol and Iodixanol on thrombus formation and fibrinolysis

INTRODUCTION

Contrast media (CM) possess both pro-thrombotic and anticoagulant properties. Here we investigate the effect of three classes of CM; Iohexol, Iodixanol and Ioxaglate, on thrombus formation and fibrinolysis in vitro and evaluated the contribution of platelets to this process.

MATERIALS AND METHODS

Non-anticoagulated blood was mixed with CM or saline (50% or 20% (v/v)) for 1 min then citrated. Thrombi were produced in vitro under flow in a Chandler loop. Thrombus structure was visualized by immunohistochemistry using FITC-fibrin/ogen antibodies, and propidium iodide to identify nucleated blood cells. To measure fibrino/thrombolysis release of FITC-labeled fibrinogen, added prior to thrombus formation, was measured over 24 h. Platelet degranulation was analyzed by whole blood flow-cytometry.

RESULTS

No thrombi formed from blood incubated with Ioxaglate. Thrombi formed with Iohexol or Iodixanol weighed > OR = 10x more than saline controls (116+/-52 and 230+/-128 mg vs. 11+/-3 mg, respectively; p<0.0005), and were more resistant to thrombolysis as evidenced by the release of FITC over 24 h (19.1+/-8.9 and 31.9+/-17.2 U vs. 65.1+/-19.1 U, respectively; p<0.02). Thrombi formed with Iodixanol and Iohexol had larger, more diffuse platelet-rich head areas and tail regions composed of a more open fibrinogen/fibrin meshwork enclosing denser RBC rich areas compared to controls. Iodixanol and Ioxaglate did not increase platelet degranulation, but Iohexol caused a significant increase compared to the saline control (percentage of platelets expressing P-selectin 68.9+/-23.43% vs. 4.93+/-1.44%, respectively; p=0.001).

CONCLUSION

Thus thrombi formed with Iodixanol or Iohexol will be larger and more resistant to thrombolysis, whereas Ioxaglate may reduce the risk of thrombus formation.

In vivo comparative antithrombotic effects of ioxaglate and iohexol and interaction with the platelet antiaggregant clopidogrel

RATIONALE AND OBJECTIVES

Experiments were designed to (1) compare the effects of iodinated contrast media (CM) on a rat model of arterial thrombosis, (2) evaluate which element of the ioxaglate solution supports its antithrombotic activity, and (3) investigate the interaction of ionic and non-ionic CM with the antiplatelet agent clopidogrel.

MATERIALS AND METHODS

Carotid thrombosis was induced in rats by extravascular application of a filter paper soaked in FeCl3 (35% vol/wt), proximal to an ultrasonic flow probe. (1) The antithrombotic potential of low-osmolar ionic (ioxaglate Na/meglumine) or nonionic contrast media (iohexol and iodixanol) (all 1600 mg iodine/kg, IV) was assessed by measuring the time to occlusion (TTO) of the carotid artery and the thrombus weight (TW). (2) Isotonic saline and iso-osmolar (280 mOsm/kg) and hyperosmolar (560 mOsm/kg) solutions of meglumine hydrochloride, meglumine ioxaglate (560 mOsm/kg), sodium ioxaglate (600 mOsm/kg) and sodium and meglumine ioxaglate (commercial solution) were tested under similar conditions. (3) Interaction with clopidogrel was tested by injecting lower dose of CM (960 mg iodine/kg) 2 hours after clopidogrel (2 mg/kg per os).

RESULTS

(1) Ioxaglate prolonged TTO when compared with saline (30.0 +/- 1.1 minute vs. 19.6 +/- 2.4 minutes, P< 0.001), whereas iohexol had no effect (21.3 +/- 1.3 minutes). Ioxaglate's effect was associated with a reduction in TW with ioxaglate versus saline (2.6 +/- 0.4 mg and 4.7 +/- 0.7 mg, respectively, P< 0.05) whereas TW remained unchanged in the iohexol group (4.2 +/- 0.4 mg). The nonionic dimer iodixanol induced a direct vasoconstrictor effect on the carotid artery and was consequently excluded from the study. (2) Neither iso-osmolar nor hyperosmolar solutions of meglumine had any effect on TTO whereas both sodium and meglumine salts of ioxaglic acid prolonged TTO, suggesting that the antithrombotic effect of ioxaglate is mediated by the ioxaglic acid moiety alone as neither meglumine, osmolality or sodium played a significant role. (3) A synergistic effect on TTO was found when ioxaglate was associated with clopidogrel whereas no such effect was observed with iohexol.

CONCLUSIONS

These data show a greater in vivo antithrombotic potential for the ionic contrast medium ioxaglate than for the non-ionic contrast medium iohexol and, for the first time, a synergistic effect between a contrast medium and a platelet antiaggregant drug in vivo.