Optical verification and in-vitro characterization of two commercially available acoustic bubble counters for cardiopulmonary bypass systems

Abstract

Introduction: Gaseous microemboli (GME) introduced during cardiac surgery are considered as a potential source of morbidity, which has driven the development of the first bubble counters. Two new generation bubble counters, introduced in the early 2000s, claim correct sizing and counting of GME. This in-vitro study aims to validate the accuracy of two bubble counters using monodisperse bubbles in a highly controlled setting at low GME concentrations.

Methods: Monodisperse GME with a radius of 43 µm were produced in a microfluidic chip. Directly after their formation, they were injected one-by-one into the BCC200 and the EDAC sensors. GME size and count, measured with the bubble counters, were optically verified using high-speed imaging.

Results: During best-case scenarios or low GME concentrations of GME with a size of 43 µm in radius in an in-vitro setup, the BCC200 overestimates GME size by a factor of 2 to 3 while the EDAC underestimates the average GME size by at least a factor of two. The BCC200 overestimates the GME concentration by approximately 20% while the EDAC overestimates the concentration by nearly one order of magnitude. Nevertheless, the calculated total GME volume is only over-predicted by a factor 2 since the EDAC underestimates the actual GME size. For the BCC200, the total GME volume was over-predicted by 25 times due to the over-estimation of GME size.

Conclusions: The measured errors in the absolute sizing/counting of GME do not imply that all results obtained using the bubble counters are insignificant or invalid. A relative change in bubble size or bubble concentration can accurately be measured. However, care must be taken in the interpretation of the results and their absolute values. Moreover, the devices cannot be used interchangeably when reporting GME activity. Nevertheless, both devices can be used to study the relative air removal characteristics of CPB components or for the quantitative monitoring of GME production during CPB interventions.

Keywords: GME; bubble counter; cardiopulmonary bypass; emboli; microbubble; validation.

Conflict of interest statement

Declaration of Conflicting Interests: The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.

(A) Schematic representation of the pulsed-Doppler technique. A piezo element transmits ultrasound pulses under a fixed angle θ with respect to the blood flow direction. In between the transmitted pulses, the scattered pressure waves originating from a bubble passing through the acoustic field are recorded by the same piezo element. (B) The amplitudes of the recorded scattered waves first increase then decrease when a bubble passes through the acoustic field. A single bubble passing through the acoustic field produces a primary echo with amplitude A1. It may also produce secondary echoes of lower amplitude A2 due to reflections of the primary echo from the walls of the bubble counter probe.

Figure 2.

(A) Schematic representation of setup 1. Bubbles produced in the microfluidic device were directly injected into a 4 L/min flow of water through a 3/8 inch CPB tube and, subsequently, detected by the bubble counter sensor. (B) Optical window at the bubble injection location for optical verification using a high-speed camera. (C) Micrograph of the microfluidic bubble production device. (D) Every single injected bubble was optically verified.

Figure 3.

(A) Bubbles were guided by a capillary into an acoustically transparent silicone tube that guided the bubbles through the detectors at fixed locations. (B) Cross-section of a 0.95-cm diameter CPB tube mounted in the BCC200 sensor. (C) Cross-section of the CPB tube mounted in the EDAC sensor. Measurements were performed with the silicone tube at the locations marked by the red dots.

Figure 4.

(A) The injected normalized bubble size distribution (black) and the normalized size distribution measured with the BCC200 (blue) measured with setup 1. The total acoustic number count was 3316 and the total optical number count was 2811. (B) Repeated measurement with the EDAC. The total acoustic number count was 24,248 and the total optical number count was 2743.

Figure 5.

(A) Optically and acoustically measured bubble size distributions with the BCC200 system at position [0,0] in Figure 3B. (B) Bubble counts as a function of time measured with the BCC200 system. (C) Acoustically measured size distribution (gray areas) as a function of the horizontal (C) and vertical (D) bubble position in the BCC200 system. Blue horizontal lines represent the mode of the size distribution. The standard deviation is represented by the vertical black dashes. Optically measured bubble radius is represented by the black dots. Figures E and F show the ratio of the acoustic to the optical bubble count for horizontal and vertical bubble positions, respectively.

Figure 6.

(A) Optically and acoustically measured size distributions with the EDAC system at position [0,0.5] in Figure 3C. (B) Bubble counts as a function of time measured with the EDAC system. Acoustically measured size distribution (gray areas) as a function of the horizontal (C) and vertical (D) bubble position in the EDAC system. The black horizontal lines represent the mode of the size distribution of which the standard deviation on both sides is shown by the vertical black error bars. The optically measured bubble radius is represented by the black dots. Figures E and F show the acoustic to optical bubble count ratio for horizontal and vertical bubble positions, respectively.