blood ties

An engineering team rethinks the key link in cardiac surgery, the heart-lung machine.

The heart is an amazing organ. in reality, it's a double pump, with one side pushing blood to the lungs and the other side pushing the oxygen-rich blood returning from the lungs out to the rest of the body.

Because of this complexity, when something goes wrong with the heart, fixing it is a complicated task. Stop the heart to make a repair and the lungs stop, too. In order for the surgeon to operate on the heart, an outside system must provide the pumping action to get blood through the body, and it must assume the function of the lungs, removing carbon dioxide from the blood and letting oxygen in.

During open-heart surgery, such as a bypass procedure, a machine takes over the function of both heart and lungs. This heart-lung machine allows the surgeon to carefully stop the heart, while the vital organs continue to receive blood and oxygen. The surgeon can then set about the delicate task of repairing the damaged heart.

The Synergy system uses a centrifugal pump to move a patient's blood.

Heart-lung machines, in use in one form or another since the 1950s, have saved countless lives. But they can also introduce complications of their own. And some of those complications can be life-threatening.

That's why a European engineering enterprise has developed a machine designed to get around some of the most serious side effects of the heart-lung machine.

The conventional heart-lung machine pumps blood through a single, disposable circuit. The circuit consists of a large bucket, or reservoir, used to hold extra blood volume; a peristaltic or roller pump that pushes blood through the body; a heat exchanger to regulate the patient's temperature; an oxygenator used to oxygenate the patient's blood and remove carbon dioxide waste, and a filter to help remove particulates, or emboli, from blood before sending it back to the patient.

Blood is drained by gravity from the patient, then pumped from the large reservoir through the heat exchanger, oxygenator, and filter, then back into the patient's body. All the components are attached to one another using various lengths of PVC tubing that are also attached to the patient. The nondisposable hardware portion of the heart lung machine is called the pump console and directly manages this circuit.

The pump console regulates pump flow, monitors the patient's vital parameters like blood pressure, temperature, and blood flow, and provides real-time information on key physiological parameters like blood oxygen saturation and levels of blood clotting.

Open-and-Shut Cases

This technology has improved significantly in terms of performance and reliability over the past 25 years, permitting surgeons to make a formerly risky procedure like open-heart surgery one of the most common procedures for the treatment of heart problems. In 2000, 686,000 open-heart procedures were performed in the United States alone, according to the American Heart Association.

While mortality rates have improved greatly over the years, morbidity and procedure-related postsurgery sickness are still possible complications. The circuit must be filled, or "primed," with a physiological saline solution before the procedure in order to remove the air from the circuit and to avoid removing too much blood from the patient. Priming can cause postoperative complications like low red blood cell counts due to over-dilution from the saline solution, particularly in smaller patients, who do not have much blood volume in the first place.

Another problem is agitating the blood by removing it from the body and running it through a plastic circuit.

Blood reacts to being in contact with strange surfaces, and blood components like clotting factors and platelets can become so agitated that the blood itself can present challenging problems for the patient after surgery.

This condition is called systemic inflammatory response and can significantly lengthen a stay in the intensive care unit. That translates into higher costs for the hospital, up to $1,000 a day in the ICU, a particularly important consideration in today's environment of cost-based managed health care.

In an effort to mitigate some of the issues relating to on-pump procedures, minimally invasive cardiac surgery, or MICS, doesn't stop the heart. Instead, surgeons work on it while it is still beating.

While recent advances in this field have been promising, the clinical results of recent multiyear studies, such as one published in The Annals of Thoracic Surgery in 2001 by David A. Bull and his colleagues at the University of Utah Health Sciences Center, indicate that no significant advantages using this approach are making themselves obvious. The minimally invasive approach can also be difficult technically for the surgeon, who must stitch a new blood vessel onto the surface of the heart while it is still beating.

A traditional heart-lung system uses multiple machines to pump blood through a single, disposable circuit.

"While the MICS approach will definitely be the long-term future of this market," said Matteo Glauber, chief cardiac surgeon of the Ospedale Pasquinucci in Massa, Italy, "we must do something now to address patient morbidity using the heart-lung machine, while the industry develops easier and more effective techniques for beating-heart surgery."

The engineering staff at Dideco SpA in Mirandola, Italy, responsible for improving the company's heart-lung bypass system, took this as a challenge. Dideco and its sister divisions, Cobe Cardiovascular in Arvada, Colo., and Stšckert Instruments of Munich, Germany, worked together to develop a system based on a heart-lung machine, but with a different approach they believe will lead to fewer post-op complications.

"We really had our work cut out for us," said Dideco engineering director Ivo Panzani. "First, to decide precisely how to improve what we have been doing successfully for 20 years, but also how to convince our management to accept such a high-risk project. There were no guarantees of its success, and in fact there still are no guarantees it will be successful. But, we are convinced this approach is the future of our on-pump business."

Dideco's system, called Synergy, functions as a heart-lung machine, but handles the draining of a patient's blood differently from the standard device. Typically, the perfusionist, who manages the heart-lung machine, uses the reservoir to drain the patient's blood. Synergy eliminates this reservoir completely and effectively uses the patient as the reservoir.

The Synergy device consists of the same type of components as in a heart-lung machine, but places them in a single integrated device. The components are a pump, a heat exchanger, a blood oxygenator, and a filter. In a traditional heart-lung circuit, each of these parts is separate, but in the Synergy system they are combined into one assembly.

Drawn From the Heart

The Synergy system is based on a centrifugal pump that can be used to actively suck the blood directly from the patient instead of draining blood by gravity. Synergy is small—30x20x20 cm—and so is its control console, with a footprint of 1.3 square meters, compared with 3 square meters for the traditional console.

As a result, the device can be placed closer to the patient. Therefore, less tubing runs to and from the patient. Priming volume also drops, from 1,500 or more cubic centimeters for a standard heart-lung system down to 900 cm³.

According to Roel de Vroege, a professor of clinical perfusion at the Free University Hospital at Amsterdam in the Netherlands, "Since the volume is so much smaller, you can use some of the patient's blood to push the saline prime out of the circuit and reduce the prime even more. We see almost no dilution of the patient's blood using this technique, which is possible also with traditional circuits, but the effect is nowhere as dramatic."

Rene Huybregts, a cardiac surgeon at the Free University Hospital and one of those responsible for the first clinical evaluations of Synergy in Europe, sees the reduction of blood dilution as critical to improving the recovery of the patient. "Lower prime means that we should not need to give transfusions in the intensive care unit, and everyone knows in this field that lower transfusion rates are better for patients in the long run."

Additionally, the Synergy system is totally coated with a phosphorylcholine biocompatible coating intended to mimic the endothelium, the inside surface of the body's blood vessels. The coating fools the blood into thinking it is still inside the body, even though it is circulating through plastic tubing. Since the blood still thinks it is in the body, the level of activation of the blood clotting mechanisms and systemic inflammatory response mechanisms in the blood are greatly reduced.

Additionally, a cell-washing machine called an autotransfusion machine, or cell saver, is used with this system to suction any blood released into the chest during surgery. Recent studies have shown that the spilled blood coming from the chest has been highly "activated." The use of the autotransfusion machine means that the highly activated blood is cleaned and effectively deactivated before being returned to the patient.

Autotransfusion is currently not used routinely with a traditional system, and a cell saver is used perhaps by only 30 percent of surgical centers in the United States and Europe. Typically, blood is filtered and debubbled in the reservoir of a traditional system. Because the Synergy system doesn't have a reservoir, it must use an autotransfusion system.

The Synergy system uses a centrifugal pump from Cobe Cardio-
vascular to handle the pumping duties.

"The Synergy concept doesn't work without this pump," said Dideco's Panzani. "We typically use a peristaltic roller pump in traditional heart-lung machines, but such a system in our Synergy approach is a little dangerous because the excess suction could damage the heart."

According to Panzani, the centrifugal pump reduces risk that can arise when a surgeon moves the heart around to work on it. The movement sometimes causes the stopped heart to collapse around the cannula. When the blood flow is interrupted, a centrifugal pump will stop trying to pull fluid. But a peristaltic roller pump, which is used to push blood in a conventional heart-lung machine, is a positive displacement pump. If it were used to draw blood and the heart were to collapse around the cannula, the pump would not stop drawing and could damage heart tissue.

Dideco faced other important design considerations as well. Typically with an oxygenator design, the focus is on using program simulations like computational fluid dynamics to detect and eliminate areas of recirculation and stagnation within the device. Such areas allow clots to form. If a clot ends up in the patient, it can create an embolism that causes a stroke.

The Synergy system uses an oxygenator to manage blood gases and a centrifugal pump to handle blood movement, and places both close to the patient.

In the case of Synergy, however, the initial problem was that the small size of the device caused excess shear, to which some components of the blood are extremely sensitive. Red blood cells exposed to excess shear will tear open, releasing free hemoglobin into the bloodstream that can potentially cause kidney malfunction. Platelets under excess shear become extremely agitated, heightening the systemic inflammatory response. Even with a biocompatible coating, excess shear can have the same effect postoperatively on the patient as an uncoated circuit.

Dideco used CFD simulation to define and eliminate areas of excess shear, particularly in the centrifugal pump and in areas where there were sudden velocity changes within the device. The results were validated by laboratory testing, focusing on levels of blood damage in bovine blood. All of these results were submitted to the U.S. Food and Drug Administration and to the auditing authorities in Europe prior to clinical use. The system was tested on animals at the University Veterinary Clinic in Pisa, Italy, and at the St. Gatien Clinic in Tours, France.

Mike van Driel, Dideco product manager and one of the engineers who helped develop the concept, said, "The bovine testing was critical because using a device that actively drains the patient directly is a big paradigm shift for our market, and poses some significant training issues. We were able to have a firm grasp on how to deal with these issues prior to initiation of human clinical evaluations here in Europe."

The Synergy system is currently in clinical use in 11 centers in Europe and at one in the United States, after having received premarket FDA approval in April last year. More than 300 units have been used to date in the European Union and the United States. The clinical use of the system in Europe and the U.S. will continue to be expanded as the techniques for managing the system are improved.

There are still some challenges to overcome in managing the system, including accidental introduction of air at the surgical site and the removal of such air from the system. Air occasionally enters the circuit due to excess negative pressures or small leaks at the cannulation site of the heart, and these air emboli must be removed.

In a traditional system, they are removed by the reservoir, while the reservoirless Synergy device uses a large-capacity bubble trap before the pump and an arterial filter after the pump to remove entrained air. Air sensors on the tubing lines also alert the operator ahead of time, and can be used in conjunction with a remote clamping system that will prevent air from being passed to the patient.

Increased automation in the mini heart-lung machine is in the pipeline, as is a second-generation device that is two-thirds the size of the current system.

So while the doctors wait for clinically significant strides in the next generation of minimally invasive cardiac surgery, Synergy represents another potentially helpful tool for on-pump cardiac procedures.

This article was prepared by staff writers in collaboration with outside contributors.

Return to Index