Bubbles and bypass: an update

Bubbles in the bloodstream are not a normal condition -yet they remain a fact of cardiopulmonary bypass (CPB), having been extensively studied and documented since its inception some 50 years ago. While detectable levels of gaseous microemboli (GME) have decreased significantly in recent years and gross air embolism has been nearly eliminated due to increased awareness of etiologies and technological advances, methods of use of current perfusion systems continue to elicit concerns over how best to totally eliminate GME during open-heart procedures. A few studies have correlated adverse neurocognitive manifestations associated with excessive quantities of GME.

Newer techniques currently in vogue, such as vacuum-assisted venous drainage, low-prime perfusion circuits, and carbon dioxide flooding of the operative field, have, in some instances, exacerbated the problem of gas embolism or engendered secondary complications in the safe conduct of CPB. Doppler monitoring (circuit or transcranial) primarily remains a research tool to detect GME emanating from the circuit or passing into the patients’ cerebral vasculature. Newer developments not yet widely available, such as multiple-frequency harmonics, may finally provide a tool to distinguish particulate microemboli from GME and further delineate the clinical significance of GME.

Bubble tracking through computational fluid dynamics in arterial line filters for cardiopulmonary bypass

Gaseous embolism is still a concern in cardiopulmonary bypass, and the use of arterial line filters (ALFs) is widespread because of their recognized role in increasing safety. Currently, the methods used for the optimization/evaluation of ALF designs are based on a trial-and-error approach. In this work, we propose a method to objectively assess the air-handling capabilities of ALFs, using computational fluid dynamics (CFD) simulations to track the trajectory of large numbers of bubbles traversing the device under examination. We applied the CFD method to ALF prototypes, whose design featured the classical purge/screen configuration, to establish the relative roles of the bubble-trap and bubble-barrier deairing effects. Simulations were run at the maximum rated blood flows. Clusters of hundred bubbles in the micro- to macroembolic size range (10-1,000 microm diameter) were tracked. The results quantified the relative amount of bubbles whose fate is either to reach the purge line or to be intercepted by the filter screen. For microbubbles, the analysis detailed how the screen barrier is exploited, mapping the percent distribution of the intercepted bubbles along the screen surface. We conclude that the method proposed increases knowledge and awareness in designing an optimal exploitation of the filtration mechanisms involved in ALFs.

Dynamic bubble trap can replace an arterial filter during cardiopulmonary bypass surgery

Objective: The arterial filter (AF) and the dynamic bubble trap (DBT) reduce the number of air microbubbles passing through these devices. The aim of the study was to confirm that the DBT diminishes microbubbles in the arterial line similar to, or better than, the AF, and can replace it.

Methods: In a clinical study, we evaluated 60 patients undergoing cardiopulmonary bypass surgery, divided into two groups (30 patients each). In the first group, we used an open extracorporeal system, and in the second group, a closed system. For 15 patients in each group, the AF was incorporated, the other 15 patients received the DBT. The microbubbles were counted before and after the AF or DBT, using two-channel-ultrasonic Doppler devices.

Results: The exposure of patients to small bubbles (<45μm) is significantly higher in the AF than in the DBT group. The DBT reduces large bubbles (<45μm) better than the AF, with a rate exceeding 16%.

Conclusion: The use of the DBT instead of the AF yields higher air micro-bubble removal efficacy, allowing replacement of the AF, assuming the AF is used for air removal purpose only.

Investigation on the ability of an ultrasound bubble detector to deliver size measurements of gaseous bubbles in fluid lines by using a glass bead model

Detectors based on ultrasonic principles are today's state of the art devices to detect gaseous bubbles that may be present in extracorporeal circuits (ECC) for various reasons. Referring to theoretical considerations and other studies, it also seems possible to use this technology to measure the size of detected bubbles, thus offering the chance to evaluate their potential hazardous effect if introduced into a patient's circulation. Based on these considerations, a commercially available ultrasound bubble detector has been developed by Hatteland Instrumentering, Norway, to deliver bubble size measurements by means of supplementary software. This device consists of an ultrasound sensor that can be clamped onto the ECC tubing, and the necessary electronic equipment to amplify and rectify the received signals. It is supplemented by software that processes these signals and presents them as specific data. On the basis of our knowledge and experience with bubble detection by ultrasound technology, we believe it is particularly difficult to meet all the requirements for size measurements, especially if these are to be achieved by using a mathematical procedure rather than exact devices. Therefore, we tried to evaluate the quality of the offered bubble detector in measuring bubble sizes. After establishing a standardized test stand, including a roller pump and a temperature sensor, we performed several sets of experiments using the manufacturers software and a program specifically designed at our department for this purpose. The first set revealed that the manufacturer's recommended calibration material did not meet essential requirements as established by other authors. Having solved that problem, we could actually demonstrate that the ultrasonic field, as generated by the bubble detector, has been correctly calculated by the manufacturer. Simply, it is a field having the strongest reflecting region in the center, subsequently losing strength toward the ECC tubing's edge. The following set of experiments revealed that the supplementary software not only does not compensate for the ultrasonic field's inhomogeneity, but, furthermore, delivers results that are inappropriate to the applied calibration material. In the last set of experiments, we were able to demonstrate that the signals as recorded by the bubble detector heavily depend upon the circulating fluid's temperature, a fact that the manufacturer does not address. Therefore, it seems impossible to resolve all these sensor related problems by ever-increasing mathematical intervention. We believe it is more appropriate to develop a new kind of ultrasound device, free of these shortcomings. This seems to be particularly useful, because the problem of determining the size of gaseous bubbles in ECC is not yet solved.

Can an oxygenator design potentially contribute to air embolism in cardiopulmonary bypass? A novel method for the determination of the air removal capabilities of neonatal membrane oxygenators

At present, air handling of a membrane oxygenator is generally studied by using an ultrasonic sound bubble counter. However, this is not a quantitative method and it does not give any information on where air was entrapped in the oxygenator and if it eventually was removed through the membrane for gas exchange. The study presented here gives a novel technique for the determination of the air-handling characteristics of a membrane oxygenator. The study aimed at defining not only the amount of air released by the oxygenator, but also the amount of air trapped within the oxygenator and/or removed through the gas exchange membrane. Two neonatal membrane oxygenators without the use of an arterial filter were investigated: the Polystan Microsafe and the Dideco Lilliput. Although the air trap function of both oxygenators when challenged with a bolus of air was similar, the Microsafe obtained this effect mainly by capturing the air in the heat exchanger compartment while the Lilliput did remove a large amount of air through the membrane. In conclusion, the difference in trap function was most striking during continuous infusion of air. Immediate contact with a microporous membrane, avoidance of high velocities within the oxygenator, pressure drop, transit time and construction of the fibre mat all contribute to the air-handling characteristics of a membrane oxygenator.

Bubble generation and venous air filtration by hard-shell venous reservoirs: a comparative study

We have previously shown significant bubble formation in Medtronic Maxima hard-shell venous reservoirs (HSVRs). In the present study, we not only investigated the mechanism of this bubble formation, but also the extent of bubble clearance by membrane oxygenators and arterial line filters. In addition, we also compared the performance of five HSVRs with respect to bubble formation and venous air filtration. Salvaged clinical CPB circuits containing different HSVRs were studied by downstream Doppler monitoring under fixed flow-decreasing volume, fixed volume-increasing flow, and entrained venous air conditions. Bubbles formed in the Medtronic Maxima top entry HSVR at volumes below 800 ml and flows above 3.5 l min-1, and were incompletely removed by a membrane oxygenator and arterial line filter. Decreased bubbling was seen when the reservoir atmosphere was flushed with CO2, suggesting that these bubbles formed in a fountain at the venous inflow. The Medtronic Maxima Forte HSVR formed significantly fewer bubbles at low volumes, and filtered venous air effectively. Negligible bubble formation occurred in the Sorin, Terumo, or Baxter reservoirs. The minimum recommended operating volume for the Medtronic Maxima top entry reservoir should be reset at 600 ml and this device should always be used with an arterial filter. Bubble formation is substantially reduced in the new Medtronic Maxima Forte HSVR and this device is a good filter for venous air.

Cardiopulmonary bypass for adult patients: a survey of equipment and techniques

OBJECTIVES

The techniques and equipment used for cardiopulmonary bypass for adult cardiac surgery vary among institutions and may change over time. This study sought to document the changing patterns of practice.

DESIGN

Voluntary survey of meeting participants.

SETTING

13th Annual San Diego Cardiothoracic Surgery Symposium (February 1993).

PARTICIPANTS

There were 331 responses from perfusionists (79.5%), cardiac surgeons (11.2%), and anesthesiologists (6.3%). The majority of these participants were from institutions where more than 1,000 cardiac operations were performed annually.

MEASUREMENTS AND MAIN RESULTS

It was found that 91.5% of the respondents used membrane oxygenators currently, compared with 5% in 1982 (as reported in a previous survey). Over 80% of the institutions surveyed used some type of perioperative cell-salvaging technique. Arterial line filters were used by 92% of the respondents compared with 64% in 1982. Also, 80% of the respondents were aware of the availability of leukocyte-depleting filters.

CONCLUSIONS

The probable reasons for the increased utilization of membrane oxygenators and arterial line filters include less damage to the formed elements of blood, fewer gaseous microemboli, and better control of carbon dioxide elimination and oxygenation. The authors anticipate that future surveys will document increased use of leukocyte-depleting filters because of the literature implicating neutrophils as mediators of tissue destruction in various disease processes, including myocardial reperfusion injury.

Cerebral perfusion during canine hypothermic cardiopulmonary bypass: effect of arterial carbon dioxide tension

Cerebral blood flow (radioactive microspheres), intracranial pressure (subdural bolt), and retinal histopathology were examined in 20 dogs undergoing 150 minutes of hypothermic (28 degrees C) cardiopulmonary bypass to compare alpha-stat (arterial carbon dioxide tension, 40 +/- 1 mm Hg; n = 10) and pH-stat (arterial carbon dioxide tension, 61 +/- 1 mm Hg; n = 10) techniques of arterial carbon dioxide tension management. Pump flow (80 mL.kg-1.min-1), mean aortic pressure (78 +/- 2 mm Hg), and hemoglobin level (87 +/- 3 g/L [8.7 +/- 0.3 g/dL]) were maintained constant. During bypass, intracranial pressure progressively increased in the alpha-stat group from 6.0 +/- 1.0 to 13.9 +/- 1.8 mm Hg (p less than 0.05) and in the pH-stat group from 7.7 +/- 1.1 to 14.7 +/- 1.4 mm Hg (p less than 0.05), although there was no evidence of loss of intracranial compliance or intracranial edema formation as assessed by brain water content. With cooling, cerebral blood flow decreased by 56% to 62% in the alpha-stat group (p less than 0.05) and by 48% to 56% in the pH-stat group (p less than 0.05). However, 30 minutes after rewarming to 37 degrees C, cerebral blood flow in both groups failed to increase and remained significantly depressed compared with baseline values. Both groups showed similar amounts of ischemic retinal damage, with degeneration of bipolar cells found in the inner nuclear layer in 67% of animals. We conclude that, independent of the arterial carbon dioxide tension management technique, (1) cerebral perfusion decreased comparably during prolonged hypothermic bypass, (2) intracranial pressure increases progressively, (3) ischemic damage to retinal cells occurs despite maintenance of aortic pressure and flow, and (4) a significant reduction in cerebral perfusion persists after rewarming.