Effect of saline injection mixing on accuracy of conductance lumen sizing of peripheral vessels

Abstract

Transient displacement of blood in vessel lumen with saline injection is necessary in the conductance method for measurement of arterial cross-sectional area (CSA). The displacement of blood is dictated by the interactions between arterial flow hemodynamics and saline injection dynamics. The objective of the present study is to understand how the accuracy of conductance measurements is affected by the saline injection. Computational simulations were performed to assess the error in predictions of arterial CSA using conductance measurements over a range of peripheral artery diameters (i.e., 4, 7, and 10 mm) with an introducing catheter (6 Fr.) for various blood flow and saline injection rates. The simulation results were validated using the conductance measurements of the phantoms with known diameters (i.e., 7 and 10 mm). The results demonstrated that a minimum ratio of saline injection rate to blood flow rate of 3 is needed to fully displace the blood and result in accurate measurement of CSA for the peripheral artery sizes considered. Furthermore, the error was shown to be minimized as the detection electrodes are positioned between the distal to the mixing zone induced by saline injection and far downstream (4-8 cm from the injection catheter tip). The present study shows that even for the large peripheral arteries (7-10 mm) where mixing can occur, an appropriate injection rate and detection position can produce accurate measurement of lumen size.

Figure 1. Schematic of the computational domain used for simulations.

(A) 2-D axisymmetric view along the longitudinal direction and dimensions of vessel-injection catheter-conductance catheter configuration. (B) 3-D computational domain illustrating in vitro conductance catheter measurements. D_L, D_IC, and D_CC respectively denote the vessel lumen diameter, injection catheter diameter, and conductance catheter diameter. In the present simulations, the injection and conductance catheter diameter were assumed to be 2 mm (corresponding to 6 Fr. guide-catheter) and 0.9 mm, respectively (corresponding to 0.035” guide-wire). L_E, L_D, and L_DT respectively denote the excitation electrode distance (set as 4 mm), detection electrode distance (set as 1 mm), and distance of detection electrode from the injection catheter tip. Note that the excitation electrode spacing was set as 20 mm for the larger vessel sizes considered (i.e., 7 and 10 mm).

Figure 2. Temporal evolution of (A) saline fraction field, (B) average saline fraction over the cross-section at detection electrodes center, and (C) corresponding voltage detection for the 7 mm diameter vessel (i.e., D_L = 7 mm) at flow rate ratio of 3 (i.e.,  = 3).

Red and blue colors denote pure blood and saline, respectively. Saline was assumed to be injected at t = 0 s. The voltage detection position (i.e., center of detection electrodes) is 20 mm from the injection catheter tip. Four black dashed lines in (A) denote the positions of excitation (outer pair) and detection (inner pair) electrodes.

Figure 3. Streamlines and blood fraction contours (left panels) and pressure contours (right panels) at the flow rate ratio of (A) 2, (B) 3, and (C) 5.

The vessel lumen diameter D_L is 7 mm.

Figure 4. Percent error in predicted diameter for three different ratios of saline injection rate to blood flow rate (i.e.,  = 2, 3, and 5) along the various axial positions of conductance catheter relative to injection catheter for a variety of lumen diameters (A) D_L = 4 mm, (B) D_L = 7 mm, and (C) D_L = 10 mm with a commonly used injection catheter size (i.e., D_IC = 6 Fr.).

Circle, rectangle, and triangle symbols denote the ratio of 2, 3, and 5, respectively. The detection electrodes distance relative to injection tip indicates the distance of detection electrodes center from the distal end of injection catheter.

Figure 5. Effect of combinations of saline injection and blood flow rate on conductance measurement at a given flow rate ratio.

(A) Streamlines and blood fraction contours in the 7 mm diameter vessel at the flow rate ratio of 3 for three different combinations of saline injection and blood flow rates (i.e., Q_S = 3 ml/s and Q_B = 1 ml/s, Q_S = 6 ml/s and Q_B = 2 ml/s, and Q_S = 9 ml/s and Q_B = 3 ml/s). (B) Percent error in diameter corresponding to the three different flow rate conditions. Circle, rectangle, and triangle symbols denote the flow condition of Q_S = 3 ml/s and Q_B = 1 ml/s, Q_S = 6 ml/s and Q_B = 2 ml/s, and Q_S = 9 ml/s and Q_B = 3 ml/s, respectively.

Figure 6. Comparison of the percent error in diameter calculated by simulations and conductance measurements of phantoms at four different ratios of injection rate to baseline flow rate (i.e.,  = 2, 2.5, 3.3, and 5) for two different sizes (A) D_L = 7 mm and (B) D_L = 10 mm.

Lines and symbols represent simulations and measurements, respectively. Circle and solid, rectangle and dashed, triangle and dashed-dot, and gradient and dashed dot-dot denote the flow rate ratio of 2, 2.5, 3.3, and 5, respectively. Half normal saline was used as a baseline flow medium. Each conductance measurement was made in three (n = 3) phantoms for each diameter. The largest error was found to be 4 and 3.2% for 7 and 10 mm diameter phantom, respectively.