Rapid Assessment of Platelet Function With a Modified Whole-Blood Aggregometer in Percutaneous Transluminal Coronary Angioplasty Patients Receiving Anti-GP IIb/IIIa Therapy
Background The glycoprotein (GP) IIb/IIIa receptor antagonist abciximab (c7E3 Fab, ReoPro) is approved for use in high-risk percutaneous transluminal coronary angioplasty (PTCA). At present, no “point of care” exists for measuring pharmacological GP IIb/IIIa blockade. To address this need, the Chrono-log Whole Blood Aggregometer, which measures platelet aggregation by electrical impedance, was adapted to test platelet function at the bedside.
Methods and Results GP IIb/IIIa receptor blockade, impedance (5 μg/mL collagen), and turbidimetric aggregation (5 and 20 μmol/L ADP) measurements were obtained on 14 PTCA patients who received the standard bolus plus a 12-hour infusion of abciximab. During abciximab administration, mean GP IIb/IIIa receptor blockade was >91%, and both impedance and turbidimetric aggregation were inhibited by ≥90%. At 12 hours after abciximab treatment, the mean inhibition of turbidimetric platelet aggregation to 5 and 20 μmol/L ADP was 65±20% and 49±14%, respectively, and inhibition of impedance aggregation was 69±12%. GP IIb/IIIa receptor blockade was 67±8%. At 36 hours after abciximab treatment (n=8), the mean inhibition of turbidimetric platelet aggregation to 5 and 20 μmol/L ADP was 44±21% and 30±14%, respectively, whereas impedance aggregation was inhibited by 60±14%. GP IIb/IIIa receptor blockade was 57±7%.
Conclusions During and at 12 hours after abciximab therapy, impedance and turbidimetric platelet aggregation to 5 μmol/L ADP were comparable and closely correlated with GP IIb/IIIa receptor blockade. However, at 36 hours after abciximab treatment, impedance platelet aggregation more closely paralleled GP IIb/IIIa receptor blockade and indicated a slower recovery of platelet function than turbidimetric aggregometry.
The antiplatelet agent abciximab (c7E3 Fab, ReoPro) reduces ischemic complications in patients undergoing high-risk PTCA1 and is approved for use in patients undergoing PTCA who are at high risk of abrupt closure. Abciximab is a human-murine chimeric Fab fragment of the monoclonal antibody 7E3 that was derived from immunization of a mouse with human platelets.2 The drug inhibits platelet function by engaging the GP IIb/IIIa receptor, thereby preventing the binding of adhesive ligands, such as fibrinogen and von Willebrand factor, to activated platelets. The use of abciximab, as well as the continuing development of other parenteral3 and oral4 GP IIb/IIIa antagonists, may necessitate rapid and accurate measurement of platelet function to optimize and monitor the effects of these therapies. In patients receiving abciximab5,6 and in animal models of coronary thrombosis,7–9 the ex vivo platelet aggregation response was virtually abolished when GP IIb/IIIa receptor blockade was maintained ≥80%. However, in some individuals, differences in peripheral and total body platelet count as well as in the number of GP IIb/IIIa receptors per platelet may influence optimal dosing of a GP IIb/IIIa antagonist. Consequently, it may be advantageous to monitor antiplatelet therapy to ensure that a therapeutic effect has been achieved initially and is sustained throughout the course of treatment. Also, under certain circumstances after discontinuation of therapy, it may be important to know if or when platelet function has been restored.
Currently available methods for assessing platelet function (eg, light transmittance aggregometry, bleeding time) were developed primarily to detect inherited and acquired platelet abnormalities and are not readily adaptable to a point-of-care setting. Bleeding time measurements are impractical in an intensive care or catheterization laboratory setting because of the need for prolonged monitoring periods under circumstances of profound GP IIb/IIIa blockade, and they are not highly reproducible.10 Major limitations of the current light transmittance (turbidimetric) platelet aggregation assays are the multicomponent equipment requirements, the relatively long time required to perform these analyses, and the need for technologists experienced in preparation of PRP and cell counting techniques. In contrast, electrical impedance aggregometry requires no cell separations and minimal preparation time (only 1:1 dilution of the blood with saline and a 5-minute incubation before the initiation of the assay) and is an approved clinical method for evaluating platelet function.11–13 The technique measures aggregation as an increase in the electrical impedance across two precious metal wires resulting from the accumulation of platelets in response to an agonist.14 Impedance aggregation can be completed within 15 minutes after a blood sample is obtained, and the method provides accurate results up to 3 hours after the sample has been attained.15 Comparisons of turbidimetric and impedance aggregation responses on blood samples from healthy donors show a good correlation between the techniques.15
The purpose of the present study was to evaluate electrical impedance aggregometry for measuring inhibition of platelet function produced by the pharmacological blockade of the platelet GP IIb/IIIa receptor in PTCA patients receiving standard abciximab therapy. Whole-blood aggregation was measured with a modified version of the Chrono-log Whole Blood Aggregometer that has been engineered with automated calibration and readout functions that minimize the amount of operator attention and time. The sensitivity of electrical impedance aggregation for measuring the effects of abciximab on platelet function was compared with the standard turbidimetric platelet aggregation technique. Estimates of platelet function determined by both aggregation techniques were compared to the extent of GP IIb/IIIa receptor blockade.
ADP and collagen were purchased from Chrono-log. Stabilized isotonic saline was from Baxter. The modified whole-blood platelet aggregometer was designed by Chrono-log Corp. All supplies needed for performing whole-blood aggregation (ie, cuvettes, stir bars) were purchased from Chrono-Log Corp. Microsequencing pipette tips (Seratips) were purchased from Marsh. Turbidimetric aggregation measurements were performed on a PAP-4C aggregometer (BioData Inc). The 11-amino-acid TRAP16 was synthesized at Centocor.
The study was conducted in two stages. The first stage was used to establish an appropriate agonist for use in the aggregation studies. The second stage was conducted to validate the use of electrical impedance aggregation in patients treated with abciximab who were undergoing PTCA. GP IIb/IIIa receptor blockade, turbidimetric, and electrical impedance aggregation measurements were performed on both study groups. Electrical impedance aggregation measurements were completed within 30 minutes of sample collection. Turbidimetric platelet aggregation and GP IIb/IIIa receptor blockade measurements were normally completed within 1 and 2 hours, respectively, after the blood samples were obtained. All platelet function measurements were performed in citrate anticoagulant because independent studies established that the binding and inhibitory profile of abciximab were not affected by the presence of chelating anticoagulant (unpublished observations).
Stage 1: Electrical Impedance Agonist Selection Study
The electrical impedance platelet aggregation responses to the agonists ADP (20 μmol/L) and TRAP (6 μmol/L) were obtained in a group of 8 subjects who were given abciximab. The electrical impedance aggregation response to collagen was also obtained in 2 of these patients for 72 hours after abciximab administration. All subjects received the 0.25-mg/kg bolus and the 12-hour infusion of abciximab. Three individuals received a 10-μg/min infusion, and 5 received a 0.125-μg · kg−1 · min−1 infusion regimen. All individuals received an oral dose of aspirin (325 mg) ≈12 hours before abciximab administration. The study was approved by the institutional review board of the National Medical Research Corp, and all patients gave written informed consent.
Stage 2: Electrical Impedance Aggregation Assessment Study
The trial was a single-center, open-label, nonrandomized study. Individuals participating in the trial were patients undergoing high-risk PTCA and received standard abciximab therapy (bolus dose of 0.25 mg/kg plus a 12-hour infusion of 10 μg/min). The study was approved by the institutional review board of Lancaster General Hospital, and all patients gave written informed consent.
Patients were pretreated with aspirin (325 mg) before the PTCA procedure. Within 15 minutes before balloon inflation, bolus increments of heparin were given to achieve an ACT of >300 seconds. After the procedure, heparin was continued at the discretion of the clinician; however, an infusion of 1000 IU/h was normally continued for 24 hours after the procedure and was adjusted to maintain the activated partial thromboplastin time within a range of 50 to 70 seconds. Within 1 to 2 hours after the procedure, the heparin infusion was temporarily discontinued to pull the arterial sheath, which was accomplished after the ACT was <180 seconds.
Stage 1: Agonist Selection Study
Blood samples were collected at five time points: predose and 2, 6, 12, and 24 hours after abciximab bolus (12 hours after infusion). Blood samples were drawn through the cap of the in-dwelling catheter through an 18-gauge needle into polypropylene syringes containing 1/100 volume of 40% trisodium citrate. Blood (5 mL) was drawn from the in-dwelling catheter and discarded before the test samples were obtained. After collection, the syringes were gently inverted two or three times to ensure complete mixing of the anticoagulant. After each blood draw, the catheter was flushed with 3 mL of normal saline solution and reflushed every 2 to 3 hours with saline until it was removed.
Stage 2: Electrical Impedance Aggregation Assessment Study
Blood samples were collected at five designated times: immediately before administration of abciximab but after the bolus dose of heparin was received, after abciximab bolus and 5 minutes before the first balloon inflation, 2 hours after abciximab bolus, 24 hours after abciximab bolus (12 hours after abciximab infusion), and 48 hours after abciximab bolus (36 hours after abciximab infusion). The predose sample was drawn 5 minutes before administration of the abciximab bolus, and the postabciximab, preballoon inflation blood samples were drawn within 10 to 15 minutes after the bolus dose of the drug was received. During times in which heparin dosing was being monitored (ie, the first three time points), blood samples were collected from the same access site as samples drawn for ACT measurements. Otherwise, blood was drawn by direct venipuncture from a peripheral vessel. Samples were collected into standard blue-top vacutainers (citrate anticoagulant). After collection, the blood tubes were gently inverted two or three times to ensure complete mixing of the anticoagulant.
Preparation of PRP and Platelet-Poor Plasma
PRP was prepared by centrifugation at 600g for 10 minutes in a Sorvall GLC4 tabletop centrifuge. Platelet-poor plasma was prepared by spinning the remaining blood at 1200g for 15 minutes. The platelet count of the PRP was determined by use of a Coulter counter ZM resistance particle counter. Platelet counts of the PRP were adjusted to 200 000±25 000 cells per μL with autologous platelet-poor plasma, except for samples (n=9) with platelet counts ≤175 000 cells per μL, in which no adjustments were made.
Platelet Aggregation Measurements
Light Transmittance Technique
Platelet aggregations of PRP samples were evaluated by the turbidimetric method.17 PRP samples (500 μL) were prewarmed to 37°C for 10 minutes before the aggregation studies were performed. For each PRP sample, the extent of light transmittance was monitored for 1 minute to ensure the absence of autoaggregation. Next, ADP was added to the cuvette (final concentration, 5 or 20 μmol/L), and aggregation response was monitored for 4 minutes. The extent of aggregation was defined as the maximal amount of light transmittance reached within 4 minutes after addition of the agonist. For each postabciximab administration sampling time and for each ADP concentration used, the percent of baseline aggregation was determined by the following calculation: maximum change in light transmission of test sample divided by the maximum change in light transmission of baseline sample. Finally, the product of the above calculation was multiplied by 100.
Electrical impedance aggregation measurements were performed on the modified whole-blood aggregometer, which was equipped with automated calibration and readout functions. The instrument has received a premarket 510(k) notification from the Food and Drug Administration Center for Devices and Radiologic Health, indicating that the modified Whole Blood Aggregometer is equivalent to the Chrono-log Corp Whole Blood Aggregometer (model 540).18 The technicians followed manufacturer instructions for proper cleaning and maintenance of the electrode. An aliquot of whole blood (0.5 mL) was diluted with an equivalent volume of isotonic saline and incubated for 5 minutes at 37°C. The impedance of each sample was monitored in sequential 1-minute intervals until a stable baseline was established (<5 mV drift per minute). After a stable baseline was established, the agonist was then added to the sample, aggregation was monitored for 6 minutes, and the final increase in ohms over this period was displayed as a numeric LED readout. In addition, a graphical printout (ie, chart tracing) of each electrical impedance aggregometry tracing was obtained. For each postabciximab administration sampling time, the percent of baseline aggregation was determined by the following calculation: maximum change in ohms of test sample divided by the maximum change in ohms of the baseline sample. Finally, the product of the above calculation was multiplied by 100.
GP IIb/IIIa Radioimmune Assay
A direct-binding radioimmune assay was used to measure the extent of abciximab-mediated GP IIb/IIIa receptor blockade.2 Briefly, PRP was incubated with 125I-labeled murine 7E3 IgG (18 μg/mL) for 30 minutes at room temperature. Replicate3 100-μL aliquots of the PRP incubation were then centrifuged through 200 μL of 30% sucrose to separate free from platelet-bound antibody. The number of molecules of antibody per platelet was calculated as the fraction of platelet-bound 125I-m7E3 IgG times the total antibody concentration (1.8 μg/mL) times the number of molecules per 1 μg (Avogadro’s number divided by the molecular weight of m7E3 IgG; 155 000). Finally, the previous product of the calculation was divided by the number of platelets in the sample.
Patient demographics and baseline platelet function data are expressed as means, medians, ranges, and ±1 SD of the mean. For continuous measurements, the primary descriptive statistics were means and ±1 SD of the mean. An ANOVA was applied to assess for differences between GP IIb/IIIa receptor blockade, turbidimetric, and electrical impedance aggregation measurements.
Stage 1: Electrical Impedance Agonist Selection Study
The electrical impedance aggregation responses to the agonists ADP, TRAP and collagen, the light transmittance aggregation responses to ADP and GP IIb/IIIa receptor blockade measurements were obtained on eight subjects who received a 0.25-mg/kg bolus and a 12-hour infusion of abciximab. The mean levels of GP IIb/IIIa receptor blockade, inhibition of the electrical impedance, and turbidimetric aggregation responses for the test population are summarized in Fig 1⇓. During abciximab treatment, mean GP IIb/IIIa receptor blockade was maintained above 80%, and the light transmittance platelet aggregation response to both 5 and 20 μmol/L ADP was ≥80% inhibited, except at 2 hours, when average percent inhibition with 20 μmol/L ADP was 65%. Also, throughout abciximab treatment, average level of inhibition of the impedance platelet aggregation responses to ADP and TRAP was 100%. At 12 hours after abciximab infusion, the average level of GP IIb/IIIa receptor blockade dropped to 65%, and the level of inhibition of turbidimetric aggregation to 5 and 20 μmol/L ADP was 65% and 56%, respectively. However, no recovery in the impedance aggregation response to either TRAP or ADP was detected.
In addition to ADP and TRAP, the impedance aggregation response to collagen was obtained on 2 patients (Fig 2⇓). In both individuals, GP IIb/IIIa receptor blockade was maintained >80%, and both turbidimetric aggregation responses to ADP and the impedance aggregation responses to all agonists were profoundly inhibited during abciximab administration. At 12 hours after abciximab treatment, when GP IIb/IIIa receptor blockade in both patients was 68%, comparable decreases were observed in the inhibition of the turbidimetric aggregation responses to 5 and 20 μmol/L ADP (72% and 70% inhibition, respectively) and the electrical impedance aggregation responses to 5 μg/mL collagen (72% inhibition). At this time, however, the impedance aggregation response to both TRAP and ADP were still completely inhibited. At 72 hours after treatment, inhibition of the impedance aggregation response to collagen decreased to 20%. Because collagen was the most sensitive agonist for detecting recovery of platelet function by impedance aggregometry, it was chosen for monitoring the impedance aggregation response in abciximab-treated PTCA patients in the second stage of the study.
Stage 2: Electrical Impedance Aggregation Assessment Study
Patient Demographics and Baseline Platelet Function Data
Eleven men and 3 women were enrolled in the study. The average age of the group was 66 years (range, 43 to 80 years). The average weight for the group was 85 kg (range, 66 to 137 kg). The baseline values for platelet aggregation, abciximab binding sites per platelet, and platelet count for the study population (n=16) are shown in the Table⇓. The baseline platelet counts of each patient were within the normal range of 150 000 to 450 000 cells per μL. The mean platelet count was 228 000 cells per μL and ranged from 150 000 to 337 000 cells per μL. The average number of abciximab receptors was 101 200±26 400 molecules per platelet, matching previously reported estimates.19 The predose platelet samples from each patient generated a vigorous turbidimetric aggregation response to both concentrations of ADP. The mean percent increases in light transmittance to 5 and 20 μmol/L ADP were 63±7.8% and 73±6.9%, respectively.
In contrast to the study group in stage 1, cardiac patients undergoing PTCA are routinely administered heparin and other medications before and during the intervention. Therapeutic levels of heparin used during PTCA have been shown both in vitro and ex vivo to enhance platelet reactivity.20,21 Because heparin and other concomitant medications that are normally administered to PTCA patients might alter platelet function, all baseline pharmacodynamic measurements were performed on blood that was drawn 5 minutes after a bolus administration of heparin. In addition, baseline electrical impedance aggregation responses to a number of collagen concentrations were measured to identify the minimal collagen concentration that would elicit the maximal aggregation response in PTCA patients. The results are shown in the Table⇑, in which the level of aggregation (measured as impedance) is correlated with the amount of collagen agonist. A 50% increase (18 versus 27 ohms) in the mean baseline platelet aggregation response was observed with 5 μg/mL compared with 2.5 μg/mL collagen. Doubling the concentration of collagen resulted in an additional, yet smaller, increase (13%; 31 verses 27 ohms) in the extent of the aggregation response. Because near-maximal electrical impedance aggregation was achieved with 5 μg/mL collagen, the electrical impedance aggregation results of all postabciximab treatment samples to 5 μg/mL of collagen are presented.
The turbidimetric (5 μmol/L ADP), and impedance (5 μg/mL collagen) aggregometry tracings of a representative patient are shown in Fig 3⇓. Before abciximab treatment, the platelets generate a rapid and vigorous aggregation response in both the turbidimetric and electrical impedance methods. In contrast, during abciximab therapy (preballoon inflation and 2 hours after bolus), both turbidimetric and impedance aggregation responses were almost or completely abolished, as illustrated by virtually no increase in either light transmittance or ohms during the aggregation monitoring period. Twelve hours after cessation of abciximab therapy (24 hours after abciximab bolus), partial recovery of platelet aggregation was detected with both aggregation methods, and platelet function continued to increase at 36 hours after cessation of therapy. However, it should be noted that neither aggregation response recovered to baseline at 36 hours after therapy (48 hours after bolus).
The effect of abciximab administration on GP IIb/IIIa receptor blockade, ex vivo electrical impedance, and turbidimetric platelet aggregation are illustrated in Fig 4⇓. Immediately before device activation (15 minutes after abciximab bolus), mean GP IIb/IIIa receptor blockade was 96±2.1% and ranged from 91% to 99%. The light transmittance platelet aggregation responses to 5 and 20 μmol/L ADP were inhibited 98±2.6% and 94±2.5%, respectively. Comparable levels of inhibition of platelet aggregation to collagen were also observed with impedance aggregation (96±4.5%). The differences between the amount of inhibition of impedance aggregation and the levels of inhibition of turbidimetric aggregation to both concentrations of ADP (P=.742 [5 μmol/L ADP] and 0.133 [20 μmol/L ADP], respectively), as well as the extent of GP IIb/IIIa receptor blockade (P=.971), were not statistically significant. These results illustrate that both the turbidimetric and impedance aggregation responses in PTCA patients treated with the standard dose of abciximab were well inhibited immediately before coronary intervention.
Comparable levels of inhibition of platelet function were also observed after 2 hours of abciximab administration. The mean level of GP IIb/IIIa receptor blockade was 92±3.1% and ranged from 86% to 96%. At this time, the turbidimetric platelet aggregation responses to 5 and 20 μmol/L ADP were inhibited by 98±2.3% and 90±3.7%, respectively. Likewise, the impedance platelet aggregation response to collagen was inhibited by 90±8.6%.
The recovery in platelet function was also assessed after cessation of abciximab treatment. At 12 hours after abciximab administration, the level of GP IIb/IIIa receptor blockade (67±8.3%) was comparable to the extent of inhibition of platelet aggregation observed with the impedance method (69±12.7%) and turbidimetrically with 5 μmol/L ADP (65±19.9%). The differences between the level of inhibition of impedance and turbidimetric aggregation responses to 5 μmol/L ADP (P=.918), as well as the level of GP IIb/IIIa receptor blockade (P=.983), were not statistically significant. At this time, however, the turbidimetric platelet aggregation response to 20 μmol/L ADP was less inhibited (49±14.2%) and was significantly different from the level of electrical impedance aggregation (P=.004).
By 36 hours after abciximab treatment (48 hours after abciximab bolus), the mean level of inhibition of impedance aggregation (60±14.7%) again coincided with the extent of GP IIb/IIIa receptor blockade (57±7.7%), and the differences between these measurements were not statistically significant (P=.983). However, a higher degree of recovery of platelet function was detected by the turbidimetric method with both concentrations of ADP. The turbidimetric platelet aggregation responses to 5 and 20 μmol/L ADP were inhibited by 44±21% (P=.193) and 30±15% (P=.004), respectively. Therefore, after cessation of abciximab therapy, the extent of GP IIb/IIIa receptor blockade correlated closely with the level of inhibition of platelet function as measured by electrical impedance. However, turbidimetric platelet aggregometry measurements recorded a greater degree of platelet functional recovery after abciximab treatment than either the recovery of GP IIb/IIIa receptor blockade or platelet aggregation as measured by electrical impedance.
The purpose of the present study was to assess the feasibility of using an electrical impedance aggregometer as a point-of-care instrument for monitoring platelet function in patients undergoing PTCA who received the potent antiplatelet agent abciximab. Impedance aggregation measurements were performed by use of standard instrumentation that was adapted for use at the bedside. The study was designed to compare electrical impedance platelet aggregation to both the radiometric GP IIb/IIIa receptor blockade assay and ADP-mediated turbidimetric platelet aggregation, which have been broadly used to monitor the pharmacodynamic effects of abciximab in both animal7–9 and clinical studies.5,6 On the basis of these previous studies, the therapeutic target level for coronary artery angioplasty was defined as ≥80% GP IIb/IIIa receptor blockade. The results of the present study show that in the patient population evaluated, electrical impedance and turbidimetric methods yielded comparable levels of inhibition of platelet aggregation during abciximab treatment. Profound inhibition of both light transmittance and electrical impedance aggregation was observed during abciximab treatment when GP IIb/IIIa receptor blockade was >90% inhibited. Inhibition of both electrical impedance and turbidimetric platelet aggregation was observed within 15 minutes of abciximab administration, thereby illustrating the rapid onset of the pharmacological effects of the drug. The level of inhibition of platelet function with both aggregation techniques was sustained at 2 hours after abciximab bolus. These observations suggest that electrical impedance aggregation can be used by clinicians to monitor the antiplatelet effects of abciximab to ensure that a therapeutic benefit has been attained initially and is sustained during the 12-hour course of treatment.
In the posttreatment phase, impedance and light transmittance aggregation both displayed a gradual recovery in platelet function. However, the level of turbidimetric aggregation recovered to a faster extent than the decline in GP IIb/IIIa blockade, and the rate of recovery was proportional to the strength of the agonist. In contrast, recovery of electrical impedance aggregation was slower during the postabciximab infusion phase and closely paralleled the extent of GP IIb/IIIa receptor blockade. Twelve hours after cessation of abciximab therapy, when GP IIb/IIIa receptor blockade was 67%, the extent inhibition of platelet function with electrical impedance (69%) and turbidimetric aggregation to 5 μmol/L ADP (65%) were equivalent. At 36 hours after therapy, inhibition of electrical impedance aggregation was again comparable to the level of GP IIb/IIIa receptor blockade, while lower levels of inhibition of turbidimetric platelet aggregation were detected with both 5 and 20 μmol/L concentrations of ADP. Because the degree of recovery of electrical impedance aggregation more closely matched the amount of GP IIb/IIIa receptor blockade, the electrical impedance aggregation response seemed more sensitive to the inhibitory effects of abciximab than turbidimetric platelet aggregation and may be a more precise physiological indicator of platelet activity. This increased sensitivity at intermediate GP IIb/IIIa receptor blockade is not unique to electrical impedance aggregation. The effects of abciximab administration on shear-induced formation of large platelet aggregate formation were still apparent 1 week after treatment, whereas light transmittance aggregation responses to high levels of ADP (20 μmol/L) are normally fully restored within 36 to 48 hours after treatment.22 Thus, although all methods of assessment indicate nearly complete inhibition of platelet function during abciximab treatment, the different methods of platelet function testing lead to different estimates for the extent and duration of inhibition after cessation of treatment. The increased sensitivity of electrical impedance aggregometry to the antiplatelet effects of abciximab was not the result of platelets receiving less agonist stimulation than in turbidimetric measurements. In the postabciximab treatment phase of the agonist selection (stage 1) study, the same concentration of ADP (20 μmol/L) reported a partial recovery of platelet function turbidimetrically, whereas no restoration of platelet aggregation was observed with impedance aggregometry (Fig 1⇑). The concentration of collagen used in the study was shown to elicit the maximal impedance aggregation response and release of dense granule constituents in platelets from normal human donors.13 In addition, 5 μg/mL of collagen demonstrated near-maximal impedance aggregation responses from the test population. Collagen, unlike ADP, also is classified as a strong platelet agonist in that it can stimulate granule release in the absence of platelet-platelet contact,23 and an in vitro dose titration study demonstrated that abciximab elicited comparable platelet aggregation inhibitory capacities by light transmission aggregation to both 5 μg/mL collagen and 5 μmol/L ADP (data not shown). Rather, it is more likely that technical differences between the two aggregation techniques are responsible for the different estimations of antiplatelet effects of abciximab during the posttreatment recovery phase. Turbidimetric platelet aggregation requires the separation of platelets from other blood cells that also may play an important role in modulating platelet behavior. Leukocytes release prostacyclin24 and platelet activation factor,25,26 and erythrocytes have receptors for prostacyclin27 and release ADP.28 In addition, the extensive sample preparation that is required for turbidimetric platelet aggregation may result in the degradation of short half-life mediators (ie, prostacyclin and thromboxane A2). Finally, the heterogeneous size and density of circulating platelets make it likely that subpopulations of platelets may be lost during the centrifugation step for preparing PRP.29 In contrast, impedance aggregometry requires no cell separation and minimal preparation time, and platelets are tested in a more physiological medium containing all blood cells that could modulate platelet behavior. Taken in their entirety, inherent differences of impedance and turbidimetric aggregation techniques could explain the dissimilar recovery rates of platelet function under conditions of intermediate GP IIb/IIIa blockade. Because electrical impedance aggregometry requires less sample manipulation than turbidimetric platelet aggregation, it may be more convenient for assessing the pharmacological effects of abciximab during treatment and may more accurately reflect the persistence of physiologically significant GP IIb/IIIa receptor blockade.
In conclusion, electrical impedance aggregometry is a rapid, simple, and accurate method for measuring the pharmacological effects of abciximab. The assay, therefore, may be useful for monitoring abciximab and other antiplatelet therapies. The increased sensitivity of electrical impedance aggregation at intermediate GP IIb/IIIa receptor blockade may be a more accurate reflection of platelet function in vivo than turbidimetric platelet aggregation. Electrical impedance aggregation may be a useful tool for the physician who wants to assess the degree of platelet function in an abciximab-treated patient requiring either emergency coronary bypass surgery or platelet transfusion.
Selected Abbreviations and Acronyms
|ACT||=||activated clotting time|
|PTCA||=||percutaneous transluminal coronary angioplasty|
|TRAP||=||thrombin receptor activation peptide|
We would like to thank Joann Tuzi, RN, Lou Ann Frey, RN, and Kay Nepper, RN, of the Mid Atlantic Heart Institute for their help in coordinating the study and Lakshmi Damaraju, PhD, for performing the statistical analyses.
- Received May 15, 1997.
- Revision received August 14, 1997.
- Accepted August 22, 1997.
- Copyright © 1997 by American Heart Association
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