Differences in arterial and venous thromboelastography parameters: Potential roles of shear stress and oxygen content

Objective: To investigate potential mechanisms for the differences in thromboelastography variables observed between arterial blood samples and venous blood samples. Design: Prospective cohort study. Setting: University hospital. Participants: Patients undergoing cardiac surgery (n = 33). Interventions: After the withdrawal of 10 mL of discarded blood (>3 deadspace volumes), 3 blood samples were withdrawn simultaneously from the central venous port of the pulmonary artery catheter (CVP), the radial arterial catheter (ART), and the side port of the 9F sheath introducer (SI). Measurements and Main Results: Thromboelastography was done simultaneously on each sample. All thromboelastography analyses were performed with 1% celite and heparinase according to the manufacturer's guidelines. A total of 80 ART, SI, and CVP comparisons were obtained. Mean hematocrit values were not different between sampling sites (27 ± 4 v27 ± 4 v27 ± 3). Thromboelastography R time values (mean ± SD) were CVP, 8 ± 3; ART, 10 ± 3; and SI, 13 ± 5 (p = 0.004). Thromboelastography maximal amplitude (MA) values (mean ± SD) were CVP, 60.4 ± 11.7; ART, 56.2 ± 11.4; and SI, 50.5 ± 13.2 (p = 0.008). Calculated maximal shear stresses were CVP, 48 dyne/cm₂; ART, 36 dyne/cm₂; and SI, 0.3 dyne/cm₂. Blood samples obtained from the CVP (highest shear stress) resulted in faster (shorter R) and stronger (larger MA) coagulation compared with the arterial site (intermediate shear stress) and sheath introducer (lowest shear stress). Conclusion: These data show that differences exist in thromboelastography values between arterial and venous blood samples and, more importantly, show that the differences observed are not related to differences in oxygen content. These differences seem to be related to differences in catheter lumen diameter and, presumably, shear forces.