Flow Measurement at the Pump Head of Centrifugal Pumps: Comparison of Ultrasonic Transit Time and Ultrasonic Doppler Systems

Abstract

Determination of blood flow is essential for monitoring rotary blood pumps. However, accurate measurement directly adjacent to the pump housing is difficult because of the highly irregular flow profiles near the fast spinning rotor. Therefore, a specially adapted flow probe based on the ultrasound transit time (USTT) principle was designed to evaluate the flow in centrifugal blood pumps. The probe can be directly mounted at the housing and creates 2 crossed measuring ultrasound beams. The mean value, Qm, of the 2 output signals corresponds to the blood flow and the difference, Qd, correlates to the vorticity of the flow profile in the pump outflow tract. In vitro measurements obtained an accuracy for mean flow values of better than +/-0.6 L/min in extreme working points and for vorticity values even as high as Qd = 3.5 L/min. Because of vorticity, however, the output signal contained considerable noise, and that required the application of a 10 Hz filter. Positioning of the ultrasound (US) beams parallel to the axial direction of the pump was superior to radial positioning. Additional measurement of the flow profile demonstrated that a large vorticity occurred (up to Qd equal to 3.5 L/min), and this vorticity was highly dependent upon the afterload of the pump. In vivo experiments demonstrated the reliability of the method. We concluded that USTT flow measurement can determine blood flow immediately adjacent to the pump housing with sufficient accuracy, and these measurements are superior to those from US-Doppler systems (which cannot handle the vorticity accurately enough) and electromagnetic devices (which lack zero stability).

Advanced Suction Detection for an Axial Flow Pump

“…However, none of these systems are currently commercially available for application in the clinical field, and the calculation of pressure difference between inflow and outflow from pump parameters (15,16) does not allow separation between changes in preload, afterload and friction losses. Therefore, the use of flow data and the indirect evaluation derived from flow patterns and current data were proposed (9,10,(16)(17)(18)(19).…”

Development of a Suction Detection System for Axial Blood Pumps

“…Yet, such retraining or recalibration would then also be necessary for both human experts and other imaginable automatic systems. Such redesign could also be done if the flow probe were to be eliminated; however, in clinical practice so far, the accuracy of these probes (16) has proven to be highly advantageous compared to indirect flow estimation.…”

“…Direct data from the low‐level filling pressures are very difficult to obtain with acceptable long‐term accuracy (12), and the calculation of the pressure difference between inflow and outflow from pump parameters (13,14) does not allow the distinction between changes in aortic and arterial pressures. Therefore, the use of flow data and the indirect evaluation derived from flow patterns and current data were proposed (9,10,13,15–17). Most of these approaches were based on the detection of rapid falling flanks, as they occur in the case of massive suction with nearly complete occlusion of the inflow cannulae, or in the increase of higher harmonics within the power spectra of these signals, which are caused mainly by these steep flanks in the time domain (10).…”

New method of cardiac output measurement using ultrasound velocity dilution in rats

“…One advantage of TTU flowmetry for vascular blood flow measurements is that vessel diameter does not need to be measured by the operator (30). Detailed descriptions of TTU have been provided elsewhere (3,5,23,31).…”