American Heart Journal
Volume 143, Issue 1 , Page E1, January 2002

Systemic platelet effects of contrast media: Implications for cardiologic research and clinical practice☆

London, United Kingdom

From the Departments of a Cardiology and b Haematology, Royal Brompton and Harefield National Health Service Trust, London, United Kingdom

Accepted 11 July 2001.

Article Outline

Abstract 

Background Angiographic contrast media cause platelet activation and decrease aggregability in vitro. We have previously shown in vitro a significant antiplatelet effect of contrast media at the concentrations obtained locally in the coronary artery during angioplasty. It is not known, however, whether a systemic effect is present. Method Thirty patients undergoing diagnostic coronary angiography were prospectively randomized to receive the nonionic medium iohexol, ionic low-molecular-weight medium ioxaglate, or ionic high-molecular-weight medium diatrizoate. Platelet aggregability was measured before and after the investigation with whole blood electrical impedance aggregometry (WBEA) with collagen agonist and the PFA-100 (Dade, Miami, Fla) platelet function analyzer with combined shear, collagen, and adenosine diphosphate as agonists. Results With WBEA, with iohexol no difference in impedance change was seen: (medians and ranges) before, 9.8 Ω (4.8-19.2 Ω) versus after, 9.6 Ω (2-19.2 Ω) (P not significant [NS]). With ioxaglate a significant fall was seen: before, 8.6 Ω (6.4-15.2 Ω) versus after, 6.6 Ω (0-12.4 Ω) (P = .004). With diatrizoate a significant and greater fall was seen: before, 10.8 Ω (6.4-17.6 Ω) versus after, 6.6 Ω (0-10.8 Ω) (P = .002). With PFA, no difference in closure time was seen with any medium: iohexol before, 99 seconds (79-142 seconds) versus after, 142 seconds (63-128 seconds) (P NS); ioxaglate before, 120 seconds (75-258 seconds) versus after, 95 seconds (74-258 seconds) (P NS); and diatrizoate before, 114.5 seconds (65-250 seconds) versus after, 100.5 seconds (72-300 seconds) (P NS). Conclusions Ionic but not nonionic contrast media have a systemic antiplatelet effect at diagnostic angiographic doses when measured with WBEA. Such an effect has not been shown before. This may explain the observed improved clinical outcome with ionic contrast media but also might confound platelet studies in coronary angioplasty. (Am Heart J 2002; 143:e1.)

 

Radio-opaque angiographic contrast media are widely used in coronary and peripheral angiography, balloon angioplasty, and stent implantation. Any effect of contrast media on platelet function may be important to clinical outcome but also may confound studies of platelet function in angioplasty.

There have been a number of reports regarding the hemostatic effects of different contrast media in vitro. Thrombin generation appears to be inhibited by ionic contrast media in nonanticoagulated whole blood.1 It has been demonstrated at this and other centers that contrast media cause platelet activation as measured by glycoprotein IIb/IIIa expression and degranulation measured by P-selectin expression, with nonionic media causing more activation than ionic media.2, 3, 4, 5, 6 Tests of platelet aggregation, however, yield different results: all media appear to inhibit aggregation and ionic media appear to cause greater inhibition than nonionic media.7, 8, 9 The situation is further complicated by the choice of anticoagulant used. Almost all studies have used citrate, but Heptinstall et al3 studied the effects of the non-calcium-chelating anticoagulant hirudin, which is a specific inhibitor of thrombin. With use of hirudin and whole blood these authors showed, in keeping with other studies, that the ionic medium ioxaglate inhibits aggregation but the nonionic agent iodixanol actually increases it.

The platelet activation and aggregation findings appear contradictory, and a number of explanations have been offered10 but do not fully explain the findings. Irrespective of this debate, the important question is which, if any, laboratory parameter is biologically relevant in patients.

Platelet function is affected by both red blood cells and leukocytes.11 There is also evidence that the mechanical trauma of centrifugation may activate platelets. Therefore there are reservations about methods requiring the preparation of platelet rich plasma.

It is known that high shear is present at the site of coronary thrombosis, which can induce platelet aggregation.12 Thus in vivo intra-arterial stimulus to platelet aggregation involves both mechanical shear and chemical agonists13 in a whole blood environment. It therefore seems appropriate to study the effect of contrast media under such conditions to attempt to recreate the in vivo situation.

We have previously reported our findings using the low shear method of whole blood impedance aggregometry and the high shear PFA-100 (Dade, Miami, Fla) device to study three commonly used contrast media, the nonionic medium iohexol, the ionic low-molecular-weight medium ioxaglate, and the ionic high-molecular-weight medium diatrizoate in vitro. We observed a profound antiplatelet effect in vitro at 10% concentration by volume with all 3 media, which is likely to be significant in the coronary artery at the time of a bolus injection. Diatrizoate was the most potent, iohexol the least potent, and ioxaglate of intermediate antiplatelet potency.14

In considering a platelet effect of contrast in the coronary artery there are 2 phases: the high intracoronary concentration achieved during the contrast injection itself and the much lower but relatively more sustained systemic concentration to which the coronary artery will also be exposed as the agent distributes in the circulating volume before rapid renal excretion. Finally, although the contrast media undergo rapid renal excretion, the platelet effect of the medium may not be immediately reversible and therefore not dependent on the maintenance of a given blood contrast medium concentration.

The clinical significance of these hemostatic differences is not certain, although higher rates of thrombus formation and less favorable outcomes after coronary angioplasty have been reported with use of nonionic contrast media than with ionic agents.15, 16 If such a difference exists it could be due to a local intracoronary effect or a less potent but more sustained systemic effect.

It is not known, however, whether such a systemic antiplatelet effect is caused by contrast media. Although Albanese et al2 found no evidence of platelet activation in peripheral blood after diagnostic cardiac catheterization, this was in a small group (n = 5) and there is no firm consensus. A systemic platelet effect might be relevant to clinical outcome and also may be a very important confounding variable for interventional platelet research.

We therefore studied the effect of the same 3 contrast media on platelet aggregation by the same method as previously in our in vivo study,14 namely, whole blood electrical aggregometry (WBEA) and the PFA-100 “platelet function analyzer.” In the current study, however, we analyzed blood ex vivo from patients with coronary artery disease receiving contrast media as part of diagnostic cardiac catheterization.

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Method 

Pilot studies 

In planning this research, we first estimated the systemic contrast concentration likely to occur during administration of 50 mL of medium during an investigation. Calculation with use of a single compartment distribution model and assuming a circulating volume of 5 L indicated that concentrations as high as 1% by volume could occur, although in practice the whole dose is not given as a single bolus. In vitro studies following up on our previous work14 reducing the contrast concentration from 10% indicated that we could detect an antiplatelet effect of diatrizoate at 1% by volume, and therefore it appeared possible that a systemic effect could occur and be detected with these methods. A small pilot study was performed with an unrandomized group of patients undergoing diagnostic coronary angiography with diatrizoate (n = 7). WBEA collagen 5 μg/mL was performed on femoral artery sheath blood before and after the procedure, in which approximately 55 mL of contrast was used. This study indicated a fall in impedance change after exposure to the medium: medians and ranges, before, 8.8 Ω (4.8-14 Ω) versus after, 2.8 Ω (2.4-13.2 Ω) (P = .03). PFA–adenosine diphosphate (ADP) analysis of the same sample failed to show any change: before, 89 seconds (76-254 seconds) versus after, 95 seconds (85-300 seconds) (P not significant [NS]). On the basis of these in vitro and vivo findings, a randomized in vivo study was performed.

Patients 

Thirty patients seen at this institution for routine coronary angiography for the investigation of stable angina were recruited and prospectively randomized to receive 1 of the 3 contrast media. All were receiving aspirin at a maintenance dose of 75 mg. Randomization was stratified so that 10 patients received each medium. Patient characteristics by randomization group are shown in Table I. Many patients were being treated with more than one drug.

Table I. Patient characteristics for each of the 3 media received and contrast volumes
DiatrizoateIoxaglateIohexol
No101010
Age (y) (median, range)59.4 (44-73)57.0 (30-71)62.5 (42-76)
Sex (male/female)6/49/17/2
β-Blocker547
Calcium antagonist243
Nitrate226
Statin348
Diuretic410
Angiotensin-converting enzyme inhibitor231
Amiodarone110
Contrast volume (mL)53 (49-58)55 (53-60)57 (50-65)

All patients received between 49 and 65 mL of contrast during the examination (Table I). No patient received heparin.

Sampling procedure 

Blood samples were taken from each patient through a short 7F femoral artery sheath inserted as part of the diagnostic procedure. At the start of the procedure and immediately after the last angiogram blood was drawn into a polyethylene syringe through a 21-gauge needle mounted and “aspirated” into Vacutainer blood-collecting tubes (Becton-Dickinson, Rutherford, NJ) containing citrate anticoagulant 0.105 mol/L.

Platelet function studies 

All samples were studies by WBEA and PFA. WBEA was performed according to the method of Cardinal and Flower17 with a 2-channel Chronolog 560 electric aggregometer (Chronolog, Haverton, Pa). A total of 0.5 mL of blood was mixed with 0.5 mL of 0.9% saline solution in polystyrene cuvettes; the stirrer bar speed was 800 revolutions/min and a matched pair of platinum impedance electrodes was used. The agonist of aggregation used was collagen 5 μg/mL final concentration (type 1 equine tendon collagen, Chronolog), which was added with a 5-μL glass bore micropipette.

PFA was performed by use of the 2-channel PFA-100 platelet function analyzer (Dade, Miami, Fla). The PFA-100 device was initially designed as a potential “in vitro equivalent of the bleeding time” and has potential in this regard.18 It is useful for detecting abnormalities of primary hemostasis19 but also has potential in investigating hemostasis in the context of cardiovascular risk and response to treatment, particularly in view of its small size and ease of use. It is becoming more widely used in cardiovascular research and, because it uses high shear rates in addition to chemical agonists and can detect the antiplatelet effects of contrast media,14 it was considered appropriate for this study.

The device uses citrate anticoagulated venous blood, which is aspirated through a steel tube series resistance and then through a 150-μm aperture in a nitrocellulose membrane achieving a shear rate of approximately 5000 to 6000S-1 to attempt to recreate physiologic shear rates. The membrane is loaded with 2 μg of type I collagen and either 50 μg of ADP or 10 μg epinephrine in disposable single-use cartridges. The test result is the time taken for the aperture to occlude completely with platelet thrombus. This device is commercially available, has been described in detail,20 and has undergone validation.21 All assays were performed in duplicate.

Reporting of results and statistical analysis 

WBEA results are reported as impedance changes in ohms at 6 minutes with a greater impedance change representing greater platelet aggregation. PFA-100 results are reported as “closure time,” which is the time in seconds taken for the experimental aperture to occlude. A shorter closure time therefore represents greater platelet aggregation. Statistical comparisons were paired analyses between the means of the duplicate runs at each time point. Normality of distribution could not be ensured; therefore nonparametric analysis was performed with the Wilcoxon paired test with GraphPad statistical software (GraphPad Software, Inc., San Diego, Calif).

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Results 

WBEA 

With iohexol, no difference in impedance change was seen after exposure to the medium: medians and ranges, before, 9.8 Ω (4.8-19.2 Ω) versus after, 9.6 Ω (2-19.2 Ω) (P NS) (Figure 1, A ).

With ioxaglate, a significant fall in impedance change was seen after exposure to the medium: medians and ranges, before, 8.6 Ω (6.4-15.2 Ω) versus after, 6.6 Ω (0-12.4 Ω) (P = .004) (Figure 1, B ).

With diatrizoate, a significant and greater fall in impedance change was seen after exposure to the medium: medians and ranges, before, 10.8 Ω (6.4-17.6 Ω) versus after, 6.6 Ω (0-10.8 Ω) (P = .002) (Figure 1, C ).

Thus a significant antiplatelet effect was seen with ioxaglate and a significant and greater effect was seen with diatrizoate, although the pre-exposure impedance values for patients exposed to diatrizoate were higher. The range of effects of the agents is in keeping with that of our in vitro study, with diatrizoate having the most potent effect, iohexol the least, and ioxaglate an intermediate effect.

PFA 

With iohexol, no difference in closure time was seen after exposure to the medium: medians and ranges, before, 99 seconds (79-142 seconds) versus after, 104 seconds (63-128 seconds) (P NS) (Figure 2, A ).

With ioxaglate, no difference in closure time was seen after exposure to the medium: medians and ranges, before, 120 seconds (75-258 seconds) versus after, 95 seconds (74-258 seconds) (P NS) (Figure 2, B ).

With diatrizoate, no difference in closure time was seen after exposure to the medium: medians and ranges, before, 115 seconds (65-250 seconds) versus after, 101 seconds (72-300 seconds) (P NS) (Figure 2, C ).

With use of PFA, therefore, no systemic platelet effect of the media was observed.

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Conclusions 

Angiographic contrast media have significant platelet effects whether measured by flow cytometric parameters of activation4 or by aggregometric methods.14 These studies have used high concentrations of media in vitro. A systemic effect in vivo has, however, not been reported previously.

In this study we have shown, by use of aggregometric methods, that a systemic antiplatelet effect is observed after the administration of ionic contrast media in the setting of diagnostic coronary angiography.

Study limitations 

We looked at patients undergoing diagnostic cardiac catheterization who received about 50 mL of contrast; patients undergoing percutaneous intracoronary interventions (PCI) often receive 4 to 5 times this volume. This, however, was a deliberate strategy to study the effects of the media in the absence of coronary intervention or other drugs such as heparin. A significant limitation of the study was the relatively small sample size and the potential risk factors associated with it. On the other hand, however, our results would be expected to underestimate the effect seen with larger doses of contrast in PCI. With use of such large volumes, in addition to the ionic agent, the nonionic agent iohexol might have induced a measurable systemic antiplatelet effect.

Preanalytical variables introduced by the sampling technique are always a concern with platelet studies, but these variables were standardized for all patients. The use of citrate anticoagulant is widespread in platelet research and it is used routinely by our group. It does, however, chelate calcium ions and may affect the results. Indeed, Heptinstall et al3 have reported different effects of contrast media in vivo with use of hirudin. Finally, the WBEA and PFA are limited, as are other platelet tests, particularly with regard to reproducibility. Nevertheless, they may be more relevant to the situation in the coronary artery than are other platelet tests because they use a combination of chemical agonists and shear, and they reflect aggregation rather than degranulation or glycoprotein expression.

Implications of findings for research 

There is considerable interest in platelet function in coronary artery disease and particularly with regard to PCI in the era of modern antiplatelet agents including ADP receptor antagonists and glycoprotein IIb/IIIa blockers. This study indicates that platelet studies need to consider the effects of contrast media even in peripheral sampling protocols. It is likely that at least some of the previously published pre-/post-PCI studies have been confounded by this effect. This underlines another of the major potential pitfalls in platelet research and the need for careful study design and measured conclusions.

Clinical significance of findings 

Although the study was not designed to address clinical end points, the findings are consistent with and provide a mechanism for the view raised by the study of Grines at al16 that ionic contrast media result in fewer ischemic complications of percutaneous transluminal coronary angioplasty than do nonionic agents. It is likely that the potential benefits of an ionic agent would be most marked in the setting of high-risk PCI.

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References 

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☆ Reprint requests: Dr Simon Davies, Department of Interventional Cardiology, Royal Brompton Hospital, Sydney Street, London SW3 6NP, United Kingdom. E-mail: s.davies@rbh.nthames.nhs.uk

PII: S0002-8703(02)36766-8

doi:10.1067/mhj.2002.119998

American Heart Journal
Volume 143, Issue 1 , Page E1, January 2002

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