In Vitro Comparison of ECC Blood Flow Measurement Techniques

-----------------Four extracorporeal blood flow measurement devices were compared for accuracy: the digital display (Stockert-Shiley roller pump), the Sarns doppler flow probe, the Transonic ultrasonic flow probe and the Biomedicus electromagnetic flow probe. The effects of changing temperature and hematocrit at various flows were compared. Eighty-four blood flow measurements were recorded for hematocrits from 17.5% to 35.0%, temperatures from 25 to 37°C and flows from 3.0 to 5.5 LPM. A summary of the results follows: ACTQ(r2) HCT TEMP INTER MEAN STD. DEV %DIFF %DIFF Biomedicus .98 .0001 NS .0053 1.0 2.3 Sarns .96 .0203 NS NS 1.6 3.2 Shiley RPM .99 NS NS NS -2.3 1.5 Transonic .99 .0001 NS NS 4.3 1.4 ACTQ(r2) = correlation with actual flow, HCT = hematocrit effect on difference (p value), TEMP= temperature effect on difference (p value), INTER= 2way ANOVA interaction, MEAN/STD DEV %DIFF =average and one standard deviation of the percent difference between actual and reported flow, NS = non-significant. All flow measurement systems correlated with actual flow(p<.OOI). The Biomedicus, Sarns and Transonic errors were affected by hematocrit, while none were affected by temperature. (p<.05) Introduction -----------------Accurate knowledge of the blood flow rate during extracorporeal circulation (ECC) is an important aid in judging the adequacy of perfusion and in performing calculations such as oxygen delivery, oxygen consumption, systemic vascular resistance and cardiac index. The purpose of this study was to compare four flow measuring devices and determine the Address correspondence to: Thomas M. Akers ECT Department, CHRP, Medical University of South Carolina 171 Ashley Avenue, Charleston, South Carolina 29425 17 accuracy of each. The actual output was measured via a graduated container and compared to a roller pump digital display, an ultrasonic flow probe, a doppler flow probe and an electromagnetic flow probe. The effects of changing hematocrits and temperature on the accuracy of the flow measuring devices were also quantitated since fluctuations in these variables may result in reported flow rates that are significantly different from actual flow rates (1, 2, 4, 5). Concerning the roller pump, an analogy can be made comparing its output and the left heart output (2): LV Cardiac Output=Stroke Volume x Heart Rate (Eq. 1) or ECC Pump Flow Output =Pump Stroke Volume x RPM (Eq. 2) The roller pump stroke volume is determined by (3): Roller Pump Stroke Vol=Raceway Length x Pi x Radius2 (Eq. 3). For the roller pump, perfusionists find it most convenient to calibrate flow rate in liters per minute to a specific number of roller head RPM's; thereby enabling the flow reading from the digital display (4). The digital display has a calibration factor set and confirmed prior to each case/procedure by the perfusionist. Pump Blood Flow= Calibration Factor x RPM's (Eq.4) The calibration factor is assumed to remain constant with normal ECC technique variables. However, the roller pump flow readout may not always be an accurate estimate of flow rate, particularly if the occlusion is not set properly (2, 3). An ECC device which accurately measures blood flow rate independent of the variables which affect the roller pump output is desirable. Three such devices are available: the Transonic ultrasonic flow probe, the Sams doppler flow probe and the Biomedicus electromagnetic flow probe. The Transonic flow probe consists of a probe body housing an upstream and a downstream ultrasonic transducer on one side and a fixed acoustic reflector positioned midway between the two transducers on the opposite side. The circuitry within the flowmeter operates the flow probe through two cycles. First, an electrical excitation causes the downstream transducer to emit a plan wave of ultrasound. This wave passes through the tubing, bounces off the acoustic reflector, passes through the tubing again, and is received at the upstream transducer. The upstream transducer converts the received acoustic vibrations into electrical signals. The flowmeter analyzes the received signal and records an accurate measure of the time it took for the wave of ultrasound to pass from one transducer to the other. Next, the transmit-receive sequence of the upstream cycle is repeated, but with the transmitting and receiving functions of the transducers interchanged. Thus, the liquid flow is now transversed by the ultrasonic wave in the opposite direction. Again, the flowmeter derives and records from this transmit-receive sequence an accurate measure of the transit time. The transit time measured by the flow probe is affected by motion in the ultrasoundconducting medium. On the upstream cycle, the sound wave travels against one vector component of the flow on each half of its reflective pathway, which increases the total transit time by a specific amount. In the downstream cycle, the sound wave travels with a vector component of the flow on each half of its reflective pathway, which decreases the total transit time by the same amount. Flowmeter circuitry then subtracts the downstream transit time from the upstream transit time, resulting in a difference signal proportional to the volume flow of the moving liquid. The transit time shift difference signal is subsequently scaled to correspond with the predetermined calibration factor of the flow probe and displayed as the absolute volume rate of flow through the flow probe in ml/min or 1/min (a). The Sarns (b) doppler flow probe utilizes a transmitting transducer that emits an ultrasonic signal which reflects off red blood cells to a receiving transducer. Blood velocity is detected by way of the frequency shift of the reflected ultrasound. The blood velocity is multiplied by the internal cross-sectional area of the tubing to yield flow rate in 1/min (7). The Biomedicus electromagnetic flow probe contains an externally excited electromagnet which creates a magnetic field across the probe in a direction perpendicular to the blood flow. As ion-bearing blood flows through the lumen of the tubing it generates a voltage directly proportional in magnitude to the velocity of the flowing blood. The voltage creates a current which flows through the detecting electrodes to an amplifier where the voltage is dropped across a large resistor. From the voltage difference velocity of the blood is determined, and flow rate may be calculated by multiplying velocity by the internal cross-sectional of the probe lumen (5). Hematocrit and temperature are two ECC parameters that are routinely changing during cardiopulmonary bypass. Variations in either parameter have been known to alter roller pump output by altering blood viscosity and tubing flexibility (2, 6). a. Transonic TlOlD Manual and Supplement, Transonic Systems Inc., Ithaca, NY b. Sarns Delphin Centrifugal System, Operators Manual, Sarns 3M Health Care, Ann Arbor, MI 18 Variations in hematocrit have been shown to affect measurements by the doppler flow measuring devices, the electromagnetic flow measuring systems and the Transonic flow probes (5, 8). It has also been reported that the electromagnetic flow probe shows an increase in error at higher temperatures (9). Therefore, flow measurements were compared and analyzed while varying the hematrocrit and temperature over a variety of flows. This study was designed to observe each t1ow measurement system under several conditions similar to cardiopulmonary bypass. Due to the impracticality of simulating all bypass conditions, the design was limited to variations of blood flow, temperature and hematocrit. Conditions were used based on an "average" adult patient (Wt. = 70 kg, Ht. = 178 em, BSA = 1.87 m), with calculated flows based on cardiac indices of 1.6, 2.0, 2.4 and 2.8 l/min/m• Four temperatures (37, 32, 28 and 25oC) and four hematocrits (35, 29, 25 and 17.5%) characteristically seen on bypass were chosen. Methods and Materials The in vitro extracorporeal circuit in Diagram 1 was created utilizing a Stockert-Shiley roller pump (c), a Sams membrane oxygenator (d), a Baxter-Bentley custom periodic tubing pack (e), a CR Bard H4700 filtered cardiotomy (f) an Atrium closed drainage system (g) (ACDS), a Sarns doppler flow probe (b), a Bio-Medicus electromagnetic flow probe (h), a Transonic ultrasonic flow probe (i), and a Hemotherm heater/cooler U).