Factors controlling the intrasplenic transit of platelets.

The kinetics of 111In platelets have been investigated in human subjects using a gamma camera and computer system. Time activity curves were recorded over the spleen, chest and liver, following intravenous bolus injection of autologous radio-labelled platelets. Splenic blood flow expressed as the percentage of total blood volume entering the spleen in unit time, and intrasplenic platelet transit time were calculated from the time activity curves. Spleen size was recorded from the camera images. Splenic blood flow increased with increasing spleen size (r = 0.56, P less than 0.001). Platelet transit time showed an inverse relationship with a semi-quantitative estimate of splenic perfusion (i.e. splenic blood flow per unit volume spleen; r = 0.64, P less than 0.001) but was not related to spleen size. At high rates of estimated splenic perfusion transit time appeared to reach a minimum value of about 7 min, whereas at low rates of estimated perfusion, transit time rose sharply to reach levels greater than 20 min. The effect of polycythaemia, taken to indicate high intrasplenic haematocrit on platelet transit time, was also investigated. Patients with secondary polycythaemia had elevated transit times (19 min SEM 1.6, n = 10) compared with primary polycythaemia (9.7 min SEM 0.8, n = 5) and normals (9.6 min SEM 0.5, n = 5). It was concluded that splenic perfusion was important in determining the duration of platelet transit.