Changes in platelet function with inflammation in patients undergoing vascular surgery.

The role of platelets in ischaemic events is well established. Aspirin represents the default antiplatelet and blocks the metabolism of arachidonic acid (AA) at the cyclo-oxygenase enzyme (COX). AA is commonly used as a test of response to aspirin, but recent data raise uncertainty about the validity of this approach. Specifically, in some patients AA-induced clotting is not suppressed, but the level of COX-dependent AA metabolite, thromboxane B2 (TXB2) is negligible. Furthermore, AA-induced whole blood clotting varies dynamically in individuals, who are aspirin responsive according to TXB2 levels. The aim of this study was to assess the level of AA-, ADP- and thrombin-mediated platelet reactivity in patients on aspirin before, during, and after major vascular surgery, which represents a model of on/off vascular inflammation. Firstly, we hypothesized, that in association with this inflammatory episode AA-, ADP- and thrombin-induced clotting would change in a dynamic manner. Secondly, that AA-induced clotting will be modified despite complete suppression of platelet TXB2 production by aspirin throughout the periprocedural period, possibly via a lipoxygenase-mediated mechanism. Fourty patients underwent major vascular surgery (open abdominal aortic aneurysm operation, infrainguinal bypass for subcritical limb ischaemia or peripheral aneurysm repair with bypass). They were all on 75 mg of aspirin prior to and throughout the perioperative period and received 5000 units of unfractionated heparin intraoperatively. AA-, ADP-, and thrombin-induced clotting, AA metabolites (TXB2 and 12-Hyroxyeicosatetraenoic acid (12-HETE)) and inflammatory markers (CRP, IL-6, TNF-α and CD40) were measured pre-procedure and at 2, 24, 48 hours, 3 to 5 days and 3 months after surgery. AA-, ADP- and thrombin-induced platelet reactivity was assessed using thrombelastography. TXB2, 12-HETE, IL-6, TNF-α, CD40 were determined using the sequential competitive binding Enzyme-Linked ImmunoAssay technique and CRP was determined using an immune-turbidimetric test on human serum. There was a transient rise in inflammatory markers in the early perioperative period (CRP at 24, 48 hours and 3 to 5 days p < 0.001 and IL-6 at 2, 24, 48 hours and 3 to 5 days p < 0.001 as compared to baseline). Patients had negligible levels of TXB2 throughout, confirming a consistent therapeutic response to aspirin. There was a transient rise in thrombin-mediated clotting (MAThrombin at 48 hours p = 0.001 and 3 to 5 days p < 0.001) and a fall in AA- and ADP-induced clotting in the early post op period (both MAAA and MAADP p = 0.001 at 2 hours). At 3 months, the level of AA- and ADP-induced clotting was significantly higher than at baseline (p = 0.008 for MAAA and p = 0.002 for MAADP), hence demonstrating a rebound effect. These data demonstrate a novel dynamic variation in platelet aggregation with acute vascular inflammation, including AA-induced whole blood clotting which is apparently COX-1 independent.

Detection of individual responses to clopidogrel: Validation of a novel, rapid analysis using thrombelastography 6s.

INTRODUCTION: There is potential value in testing individual response to P2Y12 inhibitors to predict ischemic and bleeding risk in patients undergoing percutaneous coronary intervention. The aims of this study were: (1) to validate the ability of a novel point of care (POC) assay, thrombelastography (TEG) 6s, to detect changes in adenosine diphosphate (ADP)-induced whole blood clotting in volunteers and patients given clopidogrel using TEG 5000 as a reference and (2) to compare a novel, rapid parameter, area under the curve at 15 minutes (AUC15), with the traditional maximum clot amplitude (MA) in TEG 6s. METHODS: A total of 25 participants were included in whom ADP-induced clotting was measured at 4 time points: (1) 12 healthy volunteers given 600 mg of clopidogrel; (2) 12 patients with ACS given 600 mg of clopidogrel; (3) 1 healthy volunteer given 600 mg of clopidogrel on 5 separate occasions. All samples were tested using conventional TEG 5000 and the new POC TEG 6S, and a new parameter called AUC15 was compared with MA in TEG 6s. RESULTS: (1) TEG 5000 and TEG 6s both detected changes in ADP-induced platelet activation. Bland-Altman analysis demonstrated a good level of agreement between them. (2) For TEG 6S, correlation between MA and the novel AUC15 was strong for both thrombin and ADP channels (R2  = 0.867, R = .936, P < .001), and the AUC15 result was available on average 13.3 minutes earlier. CONCLUSIONS: Thrombelastography 6s is a rapid, easy to use and accurate test of ADP-induced clotting using TEG 5000 as a reference. A novel parameter, AUC15, is a viable, time-saving option for this test and has potential value in personalized P2Y12 inhibitor therapy.

Is arachidonic acid stimulation really a test for the response to aspirin? Time to think again?

INTRODUCTION: Platelets play a key role in pathogenesis of atherothrombosis. Activated platelets initiate thrombus formation. Antiplatelet therapy (APT) modifies these properties. APT involves aspirin. The existence of 'aspirin resistance' is reported in many populations with cardiovascular disease. The prevalence of this phenomenon is highly variable, affecting more than 50% of patient subgroups in some papers. Areas covered: This review describes the prevalence of 'aspirin resistance', analyses why there is so much apparent variation and addresses whether the commonly used tests of aspirin response are in fact accurately assessing its functional performance. The clinical consequences if arachidonic acid(AA)-mediated assays do not accurately assess the functional performance of aspirin could be important. Expert commentary: Two important issues arise, firstly, that it can no longer be considered robust to use AA-induced platelet activation as a true diagnostic test of functional response to aspirin. It is clear that the output from PFT using AA as an agonist are not even a surrogate for the pharmacological activity of aspirin. Secondly, current data raise important and clinically relevant questions about, how AA stimulation induces clotting in individuals in whom aspirin is effective at its COX-1 target. The evidence indicates at least one recruitable, COX-1-independent pathway that is associated with vascular inflammation.

Stent Thrombosis Patients with Hyporesponsiveness to Clopidogrel, Prasugrel, and Ticagrelor: A Case Series Using Short Thromboelastography.

Patients after percutaneous coronary intervention (PCI) with stent implantation and functional hyporesponsiveness to P2Y12 inhibitors are at higher risk of ischaemic events, particularly stent thrombosis (ST). It is currently not routine practice to assess the functional response to these agents. However, concern over functional hyporesponsiveness to clopidogrel has led to widespread uptake of prasugrel and ticagrelor as the default P2Y12 inhibitor after stent implantation in patients with acute coronary syndrome. Here we report, for the first time, 3 cases in which patients who have had ST exhibit hyporesponsiveness to clopidogrel, prasugrel, and ticagrelor.