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| Guest Commentary |
by Andreas Liebold, MD, PhD
Universtat Rostock, Rostock, Germany |
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Mini-Perfusion Systems Around the Corner?
Evolution of mini-CPB systems
Following the introduction of the first cardiopulmonary bypass (CPB) machines in the mid 1950s there have been various improvements in extracorporeal perfusion technology. The membrane oxygenator replaced the bubble oxygenator. Rotary blood pumps have been evaluated against traditional roller pumps. Attempts have been made to progressively reduce the priming volume. Arterial line filtration, leukocyte depletion, and surface coating are further examples of notable improvements that resulted in measurable clinical progress. However, despite all of these historical improvements, the adverse effects of CPB (eg, inflammation, coagulation disturbance, hemolysis, and hemodilution) remained important problems since they carry relevant clinical risks for the patients (neurological impairment, organ dysfunction). Strategies to overcome CPB-linked adverse effects included neuroprotection, hypothermia, and anti-inflammatory drugs. But they have never been translated into innovative clinical perfusion concepts.
Mini-CPB systems are an attempt to cover the needs of an up-to-date perfusion system by eliminating or at least minimizing the most hazardous CPB drawbacks (eg, hemodilution, blood-air-contact, blood-foreign surface-contact). Their basic idea is to ensure adequate perfusion by a closed, extremely minimized extracorporeal circuit. The only “active” components are a rotary blood pump and a high performance membrane oxygenator (either in the form of two single components or as one integrated “pump-oxygenator”). By avoidance of a venous reservoir and by means of suction blood separation, blood-air-contact (which is probably the main cause of blood activation) can completely be eliminated. Hemodilution as well as mechanical blood trauma are minimized. Modern concepts of perfusion also include an utmost of safety and automatism. This is in part accomplished by driving consoles, that, although smart in design, utilize a variety of parameters in order to ease the work of the perfusionist.
Current role of mini-CPB systems
Today the vast majority of procedures is carried out with a standard open reservoir CPB. Off-pump techniques for coronary revascularization have not gained wide acceptance among surgeons, as the currently available information fails to show their clinical superiority. Moreover, the first critical reports remind of caution regarding the mid- and long-term results of off-pump surgery. Obviously, not only the tendency to incomplete revascularization but also technical difficulties in constructing coronary anastomoses on the beating heart affect the surgeons decision to perform CABG rather with CPB. Given that CPB will further be needed in the majority of cases, its refinement with the aim of improved biocompatibility is of general interest. This is reflected by the growing number of scientific abstracts addressing the topic of minimized CPB systems. Likewise, the number of manufacturers of extracorporeal technology providing mini-CPB systems is increasing.
Clinical experience, safety
It is somewhat surprising that only few authors reported their experiences with mini-CPB systems, although ten thousands of systems already have been purchased (referring to the industry). Feasibility studies, including our own experience in >700 patients revealed that mini-CPB systems can serve as a full substitute to conventional CPB in coronary surgery, either as a temporary support on the beating heart or as a full support on the arrested heart. Early results regarding perioperative morbidity are very promising. Using mini-CPB as a semi-closed system with aortic or pulmonary artery suction permits the extension of mini-CPB to aortic valve replacement. Positioning a vent catheter in the pulmonary artery limits the aspiration of air and the vent line can be connected to the CPB circuit.
The most frequently asked questions regard safety, especially air entrapment into the closed circulation. Some mini-CPB systems feature air evacuation devices, some do not. In all instances of active venous return (negative pressure) air can enter the circuit along the venous line. Causes include surgical failure like atrial purse string insufficiency, atrial wall injury, or line disconnection. Small amounts of air are detected and eliminated by the membrane oxygenator. Large amounts of air (line disconnection) cause a stop of the blood pump. The system then needs to be re-primed using the aortic pressure. No such incidences have been reported so far. Nevertheless, the companies are working on further improvements including an intelligent air removal making the mini-CPB systems suitable also for open heart (valve) procedures.
Value of mini-CPB systems
The theoretical advantages of mini-CPB over standard CPB systems should advocate their general use. The clinical benefits of mini-CPB are obvious (less transfusion requirements, less organ complications). However, they must be confirmed by a large prospective-randomized study comparing mini-CPB to standard open CPB. For surgeons, anaesthesiologists, and perfusionsist, there is a learning curve for this technique. Especially the anaesthesiologist is forced to keep a close watch on the vascular tone (liberal use of vasoactive substances) to obtain optimal performance of the pump. Since the perfusion is “volume-constant” (no reservoir) the technique requires a new understanding of hemodynamics during perfusion and a higher level of communication between the cardiac surgical team members.
In conclusion, the use of a mini-CPB system provides a level of excellence (hemodynamic support, safety) similar to traditional CPB but offers a variety of advantages. Technical improvement (air evacuation) and scientific proof are warranted.
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