An in vitro method to quantitate gaseous microemboli production of bubble oxygenators. 1982.

____________ _ An in vitro method to collect and quantify volumes of gaseous microemboli generated by a bubble oxygenator has been developed. Gaseous microemboli generated by a bubble oxygenator are captured and collected in a vortex separation test cell. The cells are pressurized and the resultant pressure changes are converted to volume gas readings using a standard calibration curve. A doppler type ultrasonic detector and this new technique were compared in order to determine correlation between the two techniques. Results of this study indicate that the ultrasonic detector "counts" do not quantitatively relate to volume of air captured in the test cells. The placement of the ultrasonic detector within the circuit can alter its readings, e.g., proximity to turbulent areas caused by connectors or positive/negative pressure excursions due to the blood pump. Significant differences were noted when fresh, bovine blood with physiologically balanced gases were substituted for lactated Ringer's solution and 100% oxygen due to differences in bubble stability and saturation levels. Based on these findings, the use of vortex collection cells can provide a more accurate profile of gaseous microemboli production by various cardiopulmonary devices. A circuit and procedure are described for this purpose. Address all inquiries to: L. M. Sakauye, Shiley, Inc., 17600 Gillette Avenue, Irvine, CA 92714 Presented at the 20th AmSECT International Conference, Hollywood, Florida, April 25-27, 1982. Volume 14, Number 5, 1982 Introduction _____________ _ The production of gaseous microemboli by cardiopulmonary devices • and possible deleterious effects A are well known. Several devices and procedures have been introduced to measure gaseous microemboli generated by an extracorporeal circuit5·6; however, a serious drawback has been the inability of the user to calibrate the instrument or procedure. Protocols describing techniques to produce calibration bubbles require equipment not generally available to most laboratories and rely on either doppler-type detectors or particle counters to verify size • Latex, glass or silicon beads are often used to calibrate these devices but accurate simulation of gaseous microemboli is not possible because the former are rigid, non-pliant and cannot combine to form larger bubbles. The procedure described in this paper incorporates a standard, reproducible calibration step using air which permits quantitative volume measurements of gaseous microemboli. Gaseous microemboli volumes are measured by taking advantage of Boyle's Law which states that as a fixed volume (V) of an ideal gas is subjected to an increase or decrease in pressure (P), it follows the equation: PV = k (where k is constant dependent on temperature) [1] As force is exerted on a bubble by a volume of incompressible liquid in a closed system, the pressure of the gas inside the bubble increases as the volume of gas decreases. Using this principle, pressure changes within rigid chambers can be correlated to gas volume changes. The Journal of Extra-Corporeal Technology 445 FIGURE 1: Vortex Separation Cell. Composed of clear polycarbonate blocks and joined in-tandem with stainless steel plug valves. The test cell (Figure 1) was designed to maximize the vortex flow. This acceleration traps the bubbles entering the cell. Data accumulated during development of the cells indicate that these cells can effectively capture and retain bubbles. At specific times, the cell is isolated from the circuit and pressure readings are taken by pressurizing the cell with a known volume of fluid. A pressure transducer attached to the cell is used to measure these pressure changes which are recorded on a strip chart recorder. The majority of previous studies conducted to evaluate gaseous microemboli production by bubble oxygenators have utilized doppler type ultrasonic detectors. The theory and mechanisms of these devices have been extensively documented8. Saline or lactated Ringer's solution and occasionally water appear to be the test media of choice. In nearly all cases, the ventilating gas has been 100% oxygen. While preliminary studies were conducted with lactated Ringer's solution and 100% oxygen, the bulk of the data presented 446 The Journal of Extra-Corporeal Technology FIGURE 2: Gaseous Microemboli Test Circuit. Volume 14, Number 5, 1982 FIGURE 3: Technique Laboratories' TM-8 Bubble Activity Monitor. Settings on "Bubble Size Microns" dial appears to represent instrument sensitivity, not actual bubble size. here was obtained using fresh, heparinized bovine blood and a modified room air gas mixture composed of 21% oxygen, 74% nitrogen, and 5% carbon dioxide. This gas mixture was selected so that "normal" arterial P02 values could be maintained. Significant differences in the bubble activity and volume of air collected were found when the test media was altered. ~ethods __________________________ _ Figure 2 illustrates the test circuit used in this study. Gravity flow of blood into the oxygenator was regulated by maintaining a constant arterial reservoir level and measured by monitoring the RPM of a calibrated blood pump. The TM-8 Microbubble Activity Monitora (Figure 3) was used to measure bubble activity at various locations on the test circuit. Blood flow into the vortex separation cells was monitored using an in-line electromagnetic flowmeterb. Pressure transducersc were attached on each of the vortex separation test cells and pressure change deflection curves were recorded on a multi-channel strip chart recorderct. Blood bypassed from the test cell was de-bubbled by the cardiotomy reservoire and returned to the blood reservoir. This de-bubbling a Technique Laboratories, Hants, England b Model 501, Carolina Medical Electronics, King, North Carolina 27021 'Model P50, Gould Statham Instruments, Inc., Hato Rey, Puerto Rico 00919 • Model 6720, Wantanabe Instruments Corp., Tokyo, Japan 'CARDF Cardiotomy Reservoir, Shiley, Inc., Irvine, California 92714 Volume 14, Number 5, 1982 was necessary to produce the zero counts as found in venous return lines of ex-vivo circuits. The test circuit was primed with either lactated Ringer's solution or blood adjusted to a 35% hct. The vortex separation test cells were isolated from the circuit and residual bubbles were evacuated. Additional fluid was added to completely fill the cell. Full scale and baseline deflections ("zero" air volume) were obtained by carefully injecting and withdrawing 50 p,l of fluid into the cell. After attaining a consistent ''zero'' the calibration curve was generated. The calibration curve was obtained by carefully introducing measured volumes of air into the primed, bubble free test cell. Using a gas-tight Hamilton syringe, 1 p,l aliquots of air were introduced into the test cell. After each injection, 50 p,l of fluid were introduced and removed to compress and decompress the air. The subsequent change in the cell's pressure was recorded on the strip chart recorder. After 10 p,l of air had been injected in this manner, 90 p,l of air were injected in 10 p,l aliquots followed by 900 p,l of air in 100 p,l aliquots. This produced a calibrated standard curve ranging from 1 p,l to 1000 p,l (Figure 4). The length of the deflection could be correlated to the volume of air trapped in the cell. The cell was then re-primed to remove the air, re-zeroed and isolated from the circuit in preparation for the test. The oxygenators were tested for one hour each at gas to blood flow ratio (V:Q) = 0.5:1, 1:1 and 2:1, where blood flow (Q) = 4 liters/minute. Temperature was maintained at 37°C ± 1°C using the The Journal of Extra-Corporeal Technology 447