Modeling indicator dispersion in extracorporeal blood lines

The measurement of indicators such as saline diluted by blood flow provides important information on transport characteristics during extracorporeal blood treatments. When saline is injected and measured using the extracorporeal system, the effects caused by dispersion within the extracorporeal system have to be taken into consideration in order to adequately identify intracorporeal transport characteristics. It was the aim of this study to quantify the extracorporeal contribution and to obtain a transport function for specified sections of the extracorporeal system. The dispersion of saline following an impulsive input was measured in arterial and venous segments of customary extracorporeal blood lines with different distribution volumes (V = 23-87 mL) using a range of different blood flows (Qb = 200-450 ml/min). The dispersion was analyzed using a modified Gamma distribution function characterized by three parameters n, k, and tau, where n is real, positive, and n > or = 1, where k = Qb/Vt*n2, and where tau refers to the indicator appearance time at the sampling site. The parameters n, k, and tau were identified by fitting the model function to experimental data. The value of n was 2.3+/-0.5 and largely independent of the type of line segment, Qb, or Vt tau showed a strong dependence on Vt and Qb which was described by tau = Vt/Qb*(n-1)/n. Thus, with a given n, and when Vt and Qb are known, the transport function for saline in important sections of the extracorporeal circulation can be determined for specific experimental conditions. With this information indicator dilution curves measured in extracorporeal blood lines can be corrected for extracorporeal effects.