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.

Standards update on perfusion equipment and practice

Standards applying to the manufacture, testing and labeling of perfusion components and equipment, as well as those dealing with clinical use of extracorporeal circulation, have been promulgated by both standards-setting organizations and professional organizations. The rationale and purpose for device standards are discussed, and many organizations and the processes involved in developing standards are described. Perfusion checklists used during equipment set-up, use and at termination have a long track record of acceptance by clinicians. Evolving techniques have prompted revisions to the basic perfusion checklist, which should be considered a guide for development of institution-specific checklists. Current and future work by international standards-setting organizations is described.

Postbypass plasma surface tension and its role in bubble filtration

Plasma samples from patients undergoing open-heart surgery were obtained at the end of cardiopulmonary bypass for surface tension analysis. The samples were measured with a Du Nouy tensiometer for true static surface tension. The surface tension values from 40 patients were analysed and correlated with plasma free haemoglobin levels, cardiopulmonary bypass time and oxygenator type. The mean surface tension value for the plasma samples was 49·96 ± 2·98 dynes/cm. No statistical correlation was found between the surface tension and the three variables. Surface tension phenomena have important implications in the removal of gaseous microemboli by screen filtration of arterial blood as it is applied to the bubble point concept. Another aspect of surface tension involves the concept of capillarity which considers the forces tending to push a bubble through a blood vessel thus preventing the obstruction of blood flow. The results of this study provide actual values of plasma surface tension from patients undergoing cardiopulmonary bypass.

The evolution of perfusion education in America

Clinical cardiopulmonary perfusion has evolved significantly during its 50 years. The science, technology and educational processes related to cardiopulmonary bypass have undergone continual change and metamorphosis. Perfusionists were initially trained on the job or in the laboratory, but with the advent of myocardial revascularization and the explosion in cardiac surgery, the need for formal educational training programs became apparent. The American Society of Extracorporeal Technology began the arduous processes of credentialing and developing certification guidelines that have continued under the American Board of Cardiovascular Perfusion. Licen-sure has also been adopted in many states during the past 10 years where nearly 50% of all perfusionists are covered by some legislative act. One additonal challenge has been developing minimal standards in which perfusionists practice. The initial standards included the use of a checklist and have evolved to monitoring and recording the significant events and parameters of each clinical procedure. The education of perfusionists will continue to be a challenge related primarily to reduced fiscal resources, the applicant pool and the ever-changing demands for extracorporeal circulation services. According to demographic and census information, it is expected the overall number of cardiovascular interventions will increase dramatically towards the latter end of the current decade in the US. The question arises as to what role perfusionists will play, as treatment strategies continue to change. If the number of education programs and new graduates continue to decline, the total number of perfusionists will decline, especially as many senior perfusionists leave the field or retire. It may be difficult to predict the total number of cardiovascular interventions that will be required in the next several years. However, if the total number of cardiovascular interventions that require perfusion services increase as a function of the aging population and if the overall incidence of cardiovascular treatment does not change, it will be imperative that perfusion education ramp up and train more perfusionists to meet the supply and demand or a shortage of skilled personnel may develop.

Verification of a computational cardiovascular system model comparing the hemodynamics of a continuous flow to a synchronous valveless pulsatile flow left ventricular assist device

The purpose of this investigation is to use a computational model to compare a synchronized valveless pulsatile left ventricular assist device with continuous flow left ventricular assist devices at the same level of device flow, and to verify the model with in vivo porcine data. A dynamic system model of the human cardiovascular system was developed to simulate the support of a healthy or failing native heart from a continuous flow left ventricular assist device or a synchronous pulsatile valveless dual-piston positive displacement pump. These results were compared with measurements made during in vivo porcine experiments. Results from the simulation model and from the in vivo counterpart show that the pulsatile pump provides higher cardiac output, left ventricular unloading, cardiac pulsatility, and aortic valve flow as compared with the continuous flow model at the same level of support. The dynamic system model developed for this investigation can effectively simulate human cardiovascular support by a synchronous pulsatile or continuous flow ventricular assist device.

Perfusion safety: new initiatives and enduring principles

Perfusion safety has been studied and discussed extensively for decades. Many initiatives occurred through efforts of professional organizations to achieve recognition, establish accreditation and certification, promote consensus practice guidelines, and develop peer-reviewed journals as sources for dissemination of clinical information. Newer initiatives have their basis in other disciplines and include systems approach, Quality Assurance/Quality Improvement processes, error recognition, evidence-based methodologies, registries, equipment automation, simulation, and the Internet. Use of previously established resources such as written protocols, checklists, safety devices, and enhanced communication skills has persisted to the present in promoting perfusion safety and has reduced current complication rates to negligible levels.

Early techniques of extracorporeal circulation

While the techniques and equipment used 50 years ago may seem primitive by today's standards, they did permit cardiac surgery to rapidly develop. As early as 1951, Karlson would write: 'Recent advances in surgery have made operations upon the heart much more than surgical stunts, and have opened a hitherto relatively untouched field of surgical endeavor. A few years later, Mustard would defend his abysmal series of repairs of congenital cardiac defects by saying, 'Our techniques of perfusion and operation have proved feasible in three human cases, but the results are not adequate to operate freely on good-risk patients at the present time. It is hoped that improvements in techniques will soon make this possible'. By the end of the decade of the 1950s. Gross would express the following: In the support of human patients on pump oxygenators, the intricacies of technic are many, and the extent of problems is broad, but much clarity of thought is now evident. We can set forth certain statements considered to represent truths or valuable viewpoints, since in most cases they have derived from scores of laboratory experiences during which some 800 dogs have been used by us, or else they have been crystallized from bitter experience at the operating table, or shattering disappointments in postoperative failures. Fortunately, many of the conclusions have sprung from happy and rewarding results exhibited by patients who have weathered corrective surgery in a very satisfactory manner. In closing, after reviewing the early perfusion literature, it is evident that many techniques used in the 1930s, 1940s, and 1950s persist to the present. Some techniques fell out of favor, such as elective hyperkalemic arrest, only to be re-established decades later. Simplicity is the hallmark of the most enduring techniques, a thought expressed by Lillehei in 1955. Somewhat amazingly, full automation of the extracorporeal circuit existed on some of the very early machines, but perhaps because of the explosive growth of cardiac surgery and the need for disposable circuits that could be rapidly assembled, the safety aspects of heart-lung machines were neglected for many decades. Some would argue it still has not realized its potential for elimination of error in the conduct of cardiopulmonary bypass. However, cardiac surgery would not wait, and in 1956, Osborn would boldly state: 'Extracorporeal circulation for surgery of the heart has now come of age.

Percutaneous cardiopulmonary bypass for cardiac emergencies

Percutaneous cardiopulmonary support systems (PCPS) are compact, battery-powered, portable heart-lung machines that can be implemented rapidly in any area of the hospital using thin-walled cannulae inserted via the femoral vessels. PCPS provides temporary circulatory support by actively aspirating blood from the patient's venous system using a centrifugal pump and hollow fiber membrane oxygenator for gas exchange. A review of clinical reports has delineated several indications for emergent applications, with the most frequent being cardiac arrest (CA) or cardiogenic shock (CS). Survival is more likely in patients with CS (40%) compared to CA (21%). Implementation of PCPS after unwitnessed CA or cardiopulmonary resuscitation > 30 min yields a patient survival rate of < 10%. The likelihood of patient survival after emergent PCPS is most often related to the patient undergoing a definitive anatomic surgical repair such as coronary artery bypass or pulmonary embolectomy. If the need for circulatory support extends beyond 6 h, conversion to conventional long-term extracorporeal membrane oxygenation or a ventricular assist device is recommended.

Leukocyte filtration in lung transplantation

Controlled reperfusion of the transplanted lung has been used in nine consecutive patients to decrease manifestations of lung reperfusion injury. An extracorporeal circuit containing a roller pump, heat exchanger and leukodepleting filter is primed with substrate-enhanced reperfusion solution mixed with approximately 2000 ml of the patient's blood. This solution is slowly recirculated to remove leukocytes prior to reperfusion. When the pulmonary anastomoses are completed, the pulmonary artery is cannulated through the untied anastomosis using a catheter containing a pressure lumen for measurement of infusion pressure. An atrial clamp is left in place on the patient's native atrial cuff to decrease the risk of systemic air embolism during the brief period of reperfusion from the extracorporeal reservoir. During reperfusion, the water bath to the heat exchanger is kept at 35 degrees C and the flow rate for reperfusion solution is between 150 and 200 m/min, keeping the pulmonary artery pressure <14 mmHg. Eight of nine patients were ventilated on 40% inspired oxygen within a few hours of operation and 7/9 were extubated on or before postoperative day 1. Six of nine patients are long-term survivors.

Coronary sinus rupture with retrograde cardioplegia

Coronary sinus (CS) rupture occurring during retrograde cardioplegia (RCP) is a rare complication. Patients with left ventricular hypertrophy are at higher risk for injury to the CS. The patient was a 66-year-old female with hypertension, ischemic cardiomyopathy and dysrhythmias, who had evidence of an anterior wall myocardial infarction, congestive heart failure and angina. During coronary artery bypass surgery, antegrade cardioplegia was initially administered, but aortic insufficiency prevented adequate myocardial cooling. RCP was then administered and the heart cooled appropriately. After approximately 300 ml of blood cardioplegic solution had been given, the CS pressure suddenly dropped from 30 mmHg to zero. RCP administration was stopped, and the surgeon palpated a hematoma over the area of the CS, which later ruptured upon rotation of the heart. A primary repair could not be performed, so a pericardial patch was placed over the area of disruption, which appeared to provide adequate hemostasis. The patient was weaned from cardiopulmonary bypass (CPB), but began to bleed freely from the CS distal to the pericardial patch. The patient was placed back on CPB to allow further repair of the CS, but the tissues were thin and friable and the ventricle disassociated from the ventricular septum. The situation was deemed not salvageable and further attempts at repair were stopped. The perfusionist should monitor infusion pressures and the CS waveform during RCP delivery. Changes in the waveform may indicate cannula malposition, loss of balloon seal, or, more rarely, CS rupture; such changes should prompt immediate cessation of RCP delivery.

Development and current status of a new intracorporeal membrane oxygenator (IVOX)

The IVOX (intravascular oxygenator) is an intracorporeal, hollow-fibre membrane oxygenator and carbon dioxide (CO 2) removal device. The IVOX is surgically placed into the vena cava via a femoral or jugular venotomy. Oxygen (O 2) is pulled through the hollow fibres by a vacuum pump controlled by a flow meter. There is no extracorporeal circulation of blood. Gas exchange occurs as the patient's blood flows over several hundred hollow fibres. Inlet and outlet gas conduits exit a small skin incision for inflow of 0 2 and outflow of CO 2. Studies in sheep show that the IVOX can support approximately 30% of gas exchange requirements. The position of the IVOX in the vena cava does not affect haemodynamics or cause thromboembolic complications. It can remain in place for up to 22 days without affecting haematologic or blood chemistry parameters. The IVOX is currently undergoing clinical trials at selected medical centres in patients with acute respiratory failure.

Technique of controlled reperfusion of the transplanted lung in humans

BACKGROUND

Reperfusion injury remains a significant and sometimes fatal problem in clinical lung transplantation. Controlled reperfusion of the transplanted lung using white cell-filtered, nutrient-enriched blood has been shown recently to significantly ameliorate reperfusion damage in a porcine model. We modified this experimental technique and applied it to human lung transplantation.

METHODS

Approximately 1,500 mL of arterial blood was slowly collected in a cardiotomy reservoir during the lung implant, and mixed to make a 4:1 solution of blood:modified Buckberg perfusate. This solution was passed through a leukocyte filter and into the transplant pulmonary artery for 10 minutes, at a controlled rate (200 mL/min) and pressure (less than 20 mm Hg), immediately before removal of the vascular clamp.

RESULTS

Five patients underwent lung transplantation (1 bilateral, 4 single lung) using this technique. All patients were ventilated on a 40% fraction of inspired oxygen within a few hours and extubated on or before the first postoperative day.

CONCLUSIONS

Controlled reperfusion of the transplanted lung with white cell-filtered, nutrient-enriched blood has given excellent functional results in our small initial clinical series.

Laboratory testing of femoral venous cannulae: effect of size, position and negative pressure on flow

Femoral venous cannulae (17-28 French) were tested to compare flows obtained by their placement in a simulated inferior vena cava (IVC) or right atrium (RA) and by varying drainage pressures using gravity siphon drainage or a centrifugal pump in the venous line. The circuit consisted of conventional tubing and equipment including a segment of thin-walled latex tubing to simulate the IVC connected to a flexible reservoir to simulate the RA. The test fluid was a 40% glycerin solution. Flow was measured at height differentials of 30-60 cm (cannula-to-inlet of hard-shell venous reservoir) and with a -10 to -80 mmHg negative pressure created by the centrifugal pump. A roller pump returned the test fluid to a flexible bag to maintain a filling pressure of 0-1 mmHg. Flow increased modestly with an increasing height differential. When negative pressure was applied with the centrifugal pump, flow increased 10% and 18% (IVC and RA positions, respectively) compared to gravity siphon drainage conditions. There also was a tendency for flow to plateau or cease when the centrifugal pump was used at higher levels of negative pressure or when larger cannulae were used. We conclude: (1) position of smaller cannulae in the RA yield better flows than when the cannulae are larger and placed in the IVC; (2) smaller-sized cannulae are capable of achieving higher flows when the centrifugal pump is used; (3) cannulae must be properly positioned to achieve maximum flow; (4) the centrifugal pump will augment flow, but should be regulated to avoid extreme negative pressures; and (5) cannula design has no demonstrable effect on flow.

Surface tension effects of heparin coating on arterial line filters

An investigation was conducted to determine the effects that heparin-coated screen arterial line filters have on the surface tension of cardiopulmonary bypass (CPB) priming solution. Five brands of non-heparin coated arterial line filters (Bard H625, Bentley AF1040, Intersept 40mum, Pall EC3840 and Pall SP3840) and four brands of heparin-coated filters (Bard H640, Bentley AF1040C and AF1040D and Intersept 40mum) were tested in a closed-loop circuit containing two litres of Plasma-Lyte A and pumped at a rate of five litres per minute. Samples were collected at 0.5, 20, 60 and 120 minutes to determine the surface tension of the recirculated solution. The non-heparin coated arterial line filters showed no significant changes in surface tension, either between the individual groups or over time. The benzalkonium-heparin coated filters (Bard H640, Bentley AF1040C and Intersept 40mum) all showed significant decreases in surface tension when compared to zero circulation time or to the noncoated groups. The largest drop in surface tension occurred within the first five minutes of recirculation. The circuit with a Bentley AF 1040D (a new nonbenzalkonium process) coated filter showed no significant change in surface tension.

Devices and monitoring during neonatal ECMO: survey results

A survey of active ECMO centres regarding neonatal ECMO equipment and personnel was obtained by telephone interview in late summer 1989. Forty-seven of the centres in the USA listed in the Ann Arbor ELSO (Extracorporeal Life Support Organization) Registry at the time ( greater than 90%) were contacted and all participated. Nearly all use a roller pump, while less than 5% use a centrifugal pump. All programmes use a SciMed membrane oxygenator and 90% a SciMed heat exchanger. Heat exchanger water sources include the Gaymar T-pump (42%), Seabrook (25%) and Cincinnati Sub-Zero (23%) units. Eighty-seven per cent use a bladder box servo-regulated to the roller pump; these are most often custom-made (69%) but 13% of programmes use a commercially available (Seabrook) bladder box. Ten per cent use a pressure-regulated roller pump rather than a conventional (displacement) bladder box to detect decreases in venous return. Nearly 80% monitor circuit line pressures between the pump and patient. Seventeen per cent use an air bubble detector on the arterial side of the circuit. Only 10% use an arterial bubble trap and 6% an arterial line filter. Seventy-five per cent do not monitor gas line pressures into the membrane lung, but one-third do use a gas line pop-off valve to prevent elevated gas phase pressures. Seventy per cent reported use of continuous in-line measurement of mixed venous oxygen saturation; no programme reported any blood chemistries being monitored in line.(ABSTRACT TRUNCATED AT 250 WORDS)