Investigations into the mechanisms of coronary vasodilation by contrast media in dogs

Hepatic blood perfusion estimated by dynamic contrast-enhanced computed tomography in pigs: limitations of the slope method

OBJECTIVE

The aim of this study was to determine whether dynamic contrast-enhanced computed tomography (DCE-CT) and the slope method can provide absolute measures of hepatic blood perfusion from the hepatic artery (HA) and portal vein (PV) at experimentally varied blood flow rates.

MATERIALS AND METHODS

Ten anesthetized 40-kg pigs underwent DCE-CT of the liver during periods of normocapnia (normal flow), hypocapnia (decreased flow), and hypercapnia (increased flow), which were induced by adjusting the ventilation. Reference blood flows in the HA and PV were measured continuously by surgically placed ultrasound transit-time flowmeters. For each capnic condition, the DCE-CT-estimated absolute hepatic blood perfusion from the HA and PV were calculated using the slope method and compared with flowmeter-based absolute measurements of hepatic perfusions and relative errors were analyzed.

RESULTS

The relative errors (mean ± SEM) of the DCE-CT based perfusion estimates were -21% ± 23% for HA and 81% ± 31% for PV during normocapnia, 9% ± 23% for HA and 92% ± 42% for PV during hypocapnia, and 64% ± 28% for HA and -2% ± 20% for PV during hypercapnia. The mean relative errors for HA were not significantly different from 0 during hypocapnia and normocapnia, and the DCE-CT slope method could detect relative changes in HA perfusion between scans. Infusion of contrast agent led to significantly increased hepatic blood perfusion, which biased the PV perfusion estimates.

CONCLUSIONS

Using the DCE-CT slope method, HA perfusion estimates were accurate at low and normal flow rates, whereas PV perfusion estimates were inaccurate and imprecise. At high flow rate, both HA perfusion estimates were significantly biased.

Myocardial perfusion defect in patients with coronary artery disease demonstrated by 64-multidetector computed tomography at rest

BACKGROUND

The first-pass imaging of 64-multidetector computed tomography (MDCT) using pharmacological stress has been used to assess myocardial perfusion. However, detection of myocardial ischemia at rest using MDCT has yet to be elucidated. We studied the incidence of myocardial perfusion defect (MPD) by 64-MDCT at rest and the effect of coronary revascularization therapy on MPD in patients with coronary artery disease.

HYPOTHESIS

MPD by 64-MDCT at rest indicates myocardial ischemia.

METHODS

We studied 76 patients with coronary artery disease who underwent 64-MDCT before and after revascularization therapy and 55 patients who did not undergo revascularization therapy. According to percent diameter stenosis, we defined group A, B, C, and D to have stenosis between 70% and 90%, 50% and 69%, 30% and 49%, and 10% and 29%, respectively. We evaluated regional myocardial contrast enhancement by long and short axis planes. MPD was defined as hypoenhancement area of some extent with CT value <50 HU during diastole.

RESULTS

MPD was found in 60.0% and 32.4% of group A and B patients, respectively (P = 0.0176). The incidence was 4.8% and 0% in group C and D patients, respectively (P<0.0001 compared with group A and B). All patients in group A and B and 2 patients with MPD in group C underwent coronary revascularization therapy. MPD disappeared after revascularization therapy in all but 3 group A patients. No patients showed new MPD after revascularization therapy.

CONCLUSIONS

Our results demonstrate that a significant percentage of patients with significant coronary artery stenosis show MPD by 64-MDCT at rest, and these MPDs may represent myocardial ischemia.

Effects of iodinated contrast media on common carotid and brachial artery blood flow and wall shear stress

The aim of our study was to evaluate the effect of the intravenous contrast media iomeprol on wall shear stress, blood flow and vascular parameters in the common carotid and brachial artery. Thirty outpatients undergoing thoracic or abdominal spiral CT scans were studied. The internal diameter and flow velocity of the common carotid and brachial artery were evaluated by ultrasound, and blood viscosity was measured before and after low osmolality iomeprol (Iomeron 350) injection. The wall shear stress, blood flow and pulsatility index were calculated. To test the differences between groups, the Wilcoxon rank test and Mann Whitney U test were applied. Blood viscosity decreased slightly, but significantly after contrast media (4.6+/-0.7 vs. 4.5+/-0.7 mPa.s, P = 0.02). Contrarily, blood flow and wall shear stress did not change in the common carotid artery, but significantly decreased in the brachial artery (0.9+/-0.4 vs. 0.6+/-0.3 ml/s, P < 0.0001, and 41.5+/-13.9 vs. 35.3+/-11.0 dynes/cm2, P < 0.002, respectively), whereas the pulsatility index significantly increased in the brachial artery (5.0+/-3.3 vs. 7.5+/-5.3, P < 0.001). Iomeprol injection causes blood flow and wall shear stress reduction of the brachial artery; the rise in the pulsatility index suggests an increase in peripheral vascular resistance. Further investigation is needed to evaluate whether these modifications can be clinically relevant.

Contrast media effect on cerebral blood flow regulation after performance of cerebral or coronary angiography

OBJECTIVE

The immediate hemodynamic effects of contrast agents are well documented. Less is known about their longer-lasting effects. We investigated the later effect of iopromide on the neurovascular coupling mechanism after the performance of cerebral or coronary angiography. Neurovascular coupling is a fine-tuned and reliable mechanism adapting cerebral blood flow to cortical activity. When performing a visual stimulation and measuring the resultant flow velocity change in the posterior cerebral artery transcranial Doppler has been used to determine vascular integrity.

METHODS

The visually evoked blood flow velocity response in the posterior cerebral artery was measured with transcranial Doppler before, 1-2 h after and the day after angiography. The overshoot of the flow velocity response and the stable flow velocity level at the end of the stimulation phase were used to compare the different conditions.

RESULTS

A significantly diminished overshoot was found 1-2 h after angiography only in patients undergoing cerebral angiography. The stable blood flow velocity levels at the end of the stimulation phase remained unchanged in all cases.

CONCLUSIONS

Our finding demonstrates effects of contrast media on dynamic blood flow regulation of the cerebral vasculature 1-2 h after application of direct contrast agent suggesting a possible concentration effect of the contrast agent. However, since the relative blood flow velocity increase under stable blood flow conditions remained constant, this dynamic alteration does not cause a reduction in cerebral blood flow.

The impact of coronary artery disease on the coronary vasomotor response to nonionic contrast media

BACKGROUND

Coronary artery disease (CAD) alters the vasomotor response to a variety of pharmacological agents. We tested the hypothesis that CAD also has an impact on the coronary vasomotor response to radiologic contrast media.

METHODS AND RESULTS

We performed quantitative coronary angiography in 42 patients without angiographic evidence of CAD and 38 patients with CAD in the left coronary artery. Angiographically smooth coronary segments (n=235) were analyzed for changes on luminal diameters and coronary venous oxygen saturation in response to 3 media: the nonionic dimer iodixanol, the nonionic monomer iopromide, and the ionic agent ioxaglate. In subjects without CAD, we assessed the effects of intracoronary administration of the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine and of the cyclooxygenase inhibitor indomethacin on such changes. Iodixanol induced coronary vasodilation in subjects without CAD (8.8+/-8.6%, P<0.001). Patients with CAD exhibited no significant diameter changes in segments >/=20 mm apart from a stenosis (4.7+/-9.4%, P=NS) and significant constriction in segments <20 mm from a stenosis (-3.8+/-4.6%, P<0. 05). Similar results were obtained with iopromide, but no changes were found with ioxaglate. All contrast media induced transient (<35 seconds) increases in coronary venous oxygen saturation in all subjects. Indomethacin, but not N(G)-monomethyl-L-arginine, blunted the vasodilating effect of iodixanol and iopromide (by 80% and 76%, respectively; P<0.001).

CONCLUSIONS

Nonionic contrast media induce a vasodilatory response in normal vessels not by a mechanism involving increased flow or endothelial nitric oxide synthesis, but rather by depending on preserved vascular cyclooxygenase activity. CAD changes normal epicardial vasodilatory response into vasoconstriction.

Effect of contrast media on coronary vascular resistance: contrast-induced coronary vasodilation

STUDY OBJECTIVES

To determine if the vasodilatory response to the intracoronary injection of ionic and nonionic contrast media in intact pigs is dependent on nitric oxide (NO). The mechanisms responsible for inducing the increase in coronary blood flow in response to the intracoronary injection of contrast media during angiography are still not entirely understood. There is evidence to suggest that the response could be partially mediated by NO.

PARTICIPANTS

We studied 14 anesthetized, open-chested pigs receiving ventilation.

MEASUREMENTS AND RESULTS

Changes in coronary blood flow and coronary vascular resistance were measured in response to the coronary artery injection of saline solution (0.5 mol/L, isosmolar with plasma) and three different contrast agents: meglumine sodium ioxaglate (Hexabrix; Mallinckrodt Medical; Point-Claire, Quebec, Canada), a low osmolar ionic contrast agent; iohexol (Omnipaque 300; Sanofi Winthrop; Markham, Ontario, Canada), a nonionic contrast agent; and diatrizoate meglumine 66%, diatrizoate sodium 10% (MD-76; Mallinckrodt Medical), an ionic contrast agent. Measurements were made during three experimental conditions: the coronary artery infusion of (1) saline solution, control; (2) L-nitro-arginine (LNNA; 10(-3) mol/L and 10(-2) mol/L), a competitive inhibitor of NO synthase; and (3)L-arginine 10(-1) mol/L, a substrate for NO synthase. The infusion of LNNA produced an increase in baseline coronary vascular resistance (p < 0.001), but it did not attenuate the vasodilatory response to the infusion of the contrast agents. Both the high and low osmolar ionic and nonionic contrast media caused a decrease in baseline coronary vascular resistance. For all three conditions, MD-76, which has the highest osmolality, produced the greatest decrease in coronary vascular resistance.

CONCLUSION

The vasodilatory response of the coronary vasculature to contrast agents is directly related to osmolality and is not mediated by NO.