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

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.

The effect of retrograde autologous priming volume on haemodilution and transfusion requirements during cardiac surgery.

OBJECTIVES: Many cardiac procedures using cardiopulmonary bypass (CPB) still require intraoperative transfusion. Retrograde autologous priming (RAP) has been introduced to decrease haemodilution and the blood transfusion rate. This study is designed to determine the influence or RAP on intraoperative haematocrit, transfusion and its clinical consequences. METHODS: The RAP effect was retrospectively studied in 753 patients during contemporary cardiac surgery, targeting a haematocrit of 25%. Multivariate linear regression analysis was performed to identify the independent factors influencing intraoperative haematocrit, transfusion rate and transfusion quantity. RESULTS: RAP was used in 498 patients and compared with 255 controls. RAP decreased the haemodilution level (nadir haematocrit 26.8 standard deviation [SD] 4.0% in RAP vs 25.8 SD 3.6% in controls; P = 0.001) and transfusion frequency (26.1 vs 33.3%, P = 0.04), despite smaller patients (body surface area [BSA] 1.86 SD 0.20 m(2) vs 1.91 SD 0.21 m(2) in RAP vs controls; P = 0.002) with lower preoperative haematocrit (38.9 SD 4.4% vs 40.5 SD 4.6%; P < 0.001). Optimal RAP volume was overall 475 ml (ROC area 0.55; 95% confidence interval [CI] 0.50-0.60; P = 0.04) and 375 ml in patients with BSA <1.7 m(2) (ROC area 0.63; 95% CI 0.54-0.73; P = 0.008) to decrease the transfusion incidence. Multivariate analysis revealed RAP volume as a significant determinant of nadir haematocrit (ß = 0.003, 95% CI 0.002-0.004, P < 0.001) and transfusion rate (odds ratio (OR) = 0.997, 95% CI 0.996-0.999, P < 0.001), independent of BSA, gender and preoperative haematocrit. CONCLUSIONS: Retrograde autologous priming is an effective adjunct to decrease the blood transfusion rate, coping with the CPB-related haemodilution and its adverse clinical effects. A RAP volume individualized to each patient offers most benefit as part of a multidisciplinary blood conservation approach.

Extracorporeal bubbles: a word of caution.

OBJECTIVES: Gaseous microemboli (GME) are a cause of neurocognitive deficit postcardiac surgery with cardiopulmonary bypass. However, the measurement of microbubbles during cardiopulmonary bypass is not easy, because blood is an opaque fluid and contains microparticles. Recently, two new microbubble counters, the Gampt BC200 and the emboli detection and classification (EDAC) quantifier, were developed for use during cardiac surgery. The accuracy of both devices was validated against industrial standards. METHODS: A clear blood analogue (30%/70% glycerol-water) was pumped, by means of a rollerpump out of a partially filled arterial line screen filter with a nominal pore size of 20 microm. Downstream the pump, all test sensors and an optical cuvette, were inserted in a vertically mounted 3/8 inch tubing. Measurements were taken at flows of 405, 1080, 3000 and 6000 ml/min. Backlight shadowgraphy and optical counting were used as industrial reference techniques for measuring size and counts. RESULTS: On average the EDAC quantifier underestimates average bubble diameter by 35+/-1%, 13+/-8%, 71+/-7% and 33+/-4% at 405, 1080, 3000 and 6000 ml/min, respectively. The Gampt BC200 has good sizing agreement at 1080 ml/min (+5+/-17%) and 3000 ml/min (+33+/-6%) but overestimates average diameter by 220+/-40% and 295+/-49% at 405 and 6000 ml/min, respectively. Both devices only partially count the number of microbubbles at higher flows. At 3 l/min the EDAC counts 38%, the Gampt 18% of total counts and at 6 l/min both the EDAC and Gampt only count 3% of total counts. CONCLUSIONS: Both the EDAC and Gampt can be used in a clinical setting for monitoring basal GME production. However, both devices have some major limitations when used for studying 'worst case' scenarios. One should take great caution when correlating measured data with neurocognitive outcome. Finally, results obtained by one device in a first study cannot be compared nor exchanged with results from the other device in a second study.