Evidence for in vivo Platelet Activation Following the Injection of Conventional Unfractionated Heparin

Thromboprophylaxis in Patients with COVID-19: Systematic Review of National and International Clinical Guidance Reports

BACKGROUND

Venous thromboembolism (VTE) is common among patients with severe coronavirus disease 2019 (COVID-19). Anticoagulation in hospitalized COVID-19 patients has been associated with survival benefits; however, the optimal thromboprophylaxis strategy has not yet been defined. <P> Objective: To identify published guidance reports by national and international societies regarding thromboprophylaxis strategies in COVID-19 patients in different settings (outpatients, hospitalized, post-discharge). <P> Methods: A systematic review of the literature (Pubmed/EMBASE) was conducted independently by two investigators. <P> Results: Among 1942 initially identified articles, 33 guidance documents were included: 20 published by national and 13 by international societies. These documents provide recommendations mainly for hospitalized (97% of reports) and post-discharge (75%) COVID-19 patients, and less so for outpatients (34%). Thrombotic and bleeding risk stratification prior to any treatment decision is the cornerstone of all suggested thromboprophylaxis strategies; 81% of the documents recommend thromboprophylaxis for all hospitalized patients with a prophylactic dosage of low molecular weight heparin irrespective of VTE risk. Intermediate or therapeutic dose intensity is recommended in high VTE risk patients by 56% and 28% of documents, respectively. Mechanical thromboprophylaxis is suggested in case of high bleeding risk or contraindication to pharmacological thromboprophylaxis (59% of documents). Extended pharmacological thromboprophylaxis is recommended for patients with high VTE risk after hospital discharge (63% of documents). For non-hospitalized outpatients, 28% of documents recommend pharmacological thromboprophylaxis for high VTE risk. <P> Conclusion: The current guidance identifies thromboprophylaxis in COVID-19 patients, especially during hospitalization, as of major importance for the prevention of VTE. Recommendations are derived from limited evidence from observational studies.

Homocysteine activates platelets in vitro

The mechanism of thrombogenicity in hyperhomocysteinemia remains controversial. The authors investigated the association between elevated plasma homocysteine levels, platelet function, and blood coagulation. Blood was collected from healthy subjects and patients with critical limb ischemia. Basal platelet counts and platelet aggregation as well as flow cytometry were performed to assess spontaneous- and agonist-induced platelet aggregation as well as P-selectin and Glycoprotein IIb/IIIa expression at different homocysteine concentrations. Thromboelastography was performed, and platelet shape change was assessed, using a channelyzer, by measuring median platelet volume. Lactate dehydrogenase was measured, to indirectly assess red blood cell membrane integrity, after homocysteine exposure. The study results suggest that platelet activation and hypercoagulability occur after exposure to homocysteine, especially in patients with critical limb ischemia. Homocysteine concentrations of approximately 50 micromol/L appear to be the level at which these changes occur in vitro, and this effect on platelets appears to be indirect.

A low molecular weight heparin, nadroparin (Fraxiparine), inhibits thrombin-induced platelet shape change and does not enhance spontaneous platelet aggregation

Therapeutic concentrations, added in vitro, of a low molecular weight heparin (LMWH), nadroparin (Fraxiparine), inhibit thrombin-induced platelet shape change (PSC), an early stage of human platelet activation. PSC was monitored by measuring the median platelet volume (MPV) using a high resolution channelyzer. The inhibitory action of nadroparin was specific to thrombin since this LMWH did not influence PSC induced by other agonists. In whole blood, unfractionated heparin (UH) but not nadroparin, significantly enhanced spontaneous platelet aggregation and increased MPV when compared with LMWH or saline controls. In conclusion, the LMWH, nadroparin, exerts less platelet activation than UH.