title: Platelet activation after presyncope by lower body negative pressure in humans.
abstract: Central hypovolemia elevates hemostatic activity which is essential for preventing exsanguination after trauma, but platelet activation to central hypovolemia has not been described. We hypothesized that central hypovolemia induced by lower body negative pressure (LBNP) activates platelets. Eight healthy subjects were exposed to progressive central hypovolemia by LBNP until presyncope. At baseline and 5 min after presyncope, hemostatic activity of venous blood was evaluated by flow cytometry, thrombelastography, and plasma markers of coagulation and fibrinolysis. Cell counts were also determined. Flow cytometry revealed that LBNP increased mean fluorescence intensity of PAC-1 by 1959±455 units (P<0.001) and percent of fluorescence-positive platelets by 27±18%-points (P = 0.013). Thrombelastography demonstrated that coagulation was accelerated (R-time decreased by 0.8±0.4 min (P = 0.001)) and that clot lysis increased (LY60 by 6.0±5.8%-points (P = 0.034)). Plasma coagulation factor VIII and von Willebrand factor ristocetin cofactor activity increased (P = 0.011 and P = 0.024, respectively), demonstrating increased coagulation activity, while von Willebrand factor antigen was unchanged. Plasma protein C activity and tissue-type plasminogen activator increased (P = 0.007 and P = 0.017, respectively), and D-dimer increased by 0.03±0.02 mg l(-1) (P = 0.031), demonstrating increased fibrinolytic activity. Plasma prothrombin time and activated partial thromboplastin time were unchanged. Platelet count increased by 15±13% (P = 0.014) and red blood cells by 9±4% (P = 0.002). In humans, LBNP-induced presyncope activates platelets, as evidenced by increased exposure of active glycoprotein IIb/IIIa, accelerates coagulation. LBNP activates fibrinolysis, similar to hemorrhage, but does not alter coagulation screening tests, such as prothrombin time and activated partial thromboplastin time. LBNP results in increased platelet counts, but also in hemoconcentration.
authors.affiliation: University of Texas Health Science Center at San Antonio, San Antonio, Texas, United States of America.
authors.forename: Morten
authors.initials: M
authors.lastname: Zaar
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Chriselda G
authors.initials: CG
authors.lastname: Fedyk
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Heather F
authors.initials: HF
authors.lastname: Pidcoke
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Scherer
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Kathy L
authors.initials: KL
authors.lastname: Ryan
authors.affiliation: University of North Texas Health Science Center Fort Worth, Fort Worth, Texas, United States of America.
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Rickards
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Carmen
authors.initials: C
authors.lastname: Hinojosa-Laborde
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Victor A
authors.initials: VA
authors.lastname: Convertino
authors.affiliation: United States Army Institute of Surgical Research, Fort Sam Houston, Texas, United States of America.
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Cap
mesh_terms: D000328:Adult; D015415:Biomarkers / Q000097:blood; D001777:Blood Coagulation; D005260:Female; D005434:Flow Cytometry; D006728:Hormones / Q000097:blood; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D015539:Platelet Activation; D012083:Renin / Q000097:blood; D013575:Syncope / Q000097:blood*; D013916:Thrombelastography
chemical_list: D015415:Biomarkers; D006728:Hormones; D012083:Renin