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Improving Heart Surgery

A safe and easily available drug may be able to reduce brain injury during heart surgery, according to a ground-breaking New Zealand study.

Cardiac surgery has long been known as "good for the heart but bad for the brain", says Dr Simon Mitchell, director of Diving and Hyperbaric Medicine at the Royal New Zealand Naval Hospital in Auckland. Patients sometimes suffer brain injury, resulting in poorer cognitive function after surgery. One of the causes is exposure to air bubbles that enter the blood from the heart-lung machine used during the operation or from air left in the heart chambers after they are sewn up.

Mitchell counted the bubbles circulating in the patient's blood during 55 operations, where neither he nor the surgeons knew at the time which patients were receiving the drug lignocaine and which a placebo. The patients were given 11 neuropsychological tests and rated their own memory before, soon after and six months after the operation. Mitchell's team found that, on average, the 28 patients who received lignocaine reported improved memory and showed improved performance in six of the tests compared to the placebo group. Significantly more of the 27 patients who received the placebo showed markedly reduced performance on at least one of the tests.

"This finding of brain protection by a drug is of immense importance," he says. "Pharmacological protection in any area of brain injury is one of medicine's 'holy grails', but there are very few convincing demonstrations of brain protection in humans despite a huge degree of research attention."

The study was funded by the naval hospital, the English Freemasons of New Zealand and the Health Research Council of New Zealand. The investigating team included research psychologist Ora Pellet and Dr Des Gorman, Associate Professor of Medicine at the University of Auckland.

"This study has yet to be repeated overseas to confirm the result. If it is confirmed, it will definitely be a landmark study and may contribute to better outcomes for cardiac surgery patients around the world. It may also lead to the testing of lignocaine in other groups of patients with brain injury."

Lignocaine, a safe, cheap drug commonly used to stabilise heart rhythms, had not previously been tested in humans at risk of brain injury. Mitchell decided to do the trial as a result of US animal experiments which showed lignocaine protected against brain injury caused by bubbles in the blood. This led to lignocaine being used to treat divers with decompression illness, which also involves bubbles in the blood.

The team's next step is to investigate whether lignocaine has the same positive benefit when administered after rather than before the injury. If so, it may also be of use where brain injury cannot be anticipated, such as for head trauma victims.

One major discovery made in the course of the study led to the redesign of a market-leading blood reservoir, a component which acts as a header tank in heart-lung machines during cardiac surgery. Mitchell and Mr Tim Wilcox, chief of clinical perfusion at Green Lane Hospital, found that reservoirs designed with upwardly directed entry points produced a fountain which could introduce microscopic bubbles into blood. The reservoir manufacturing company flew Mitchell and Wilcox to Los Angeles to discuss these findings.

As a result, it is recommended that filters are used with existing bottom-entry reservoirs, and they are required to operate with a higher minimum volume of blood. The findings were also taken into account in the design of later model reservoirs.

The trial found that air left behind in the heart when it is closed at the end of the operation was the source of around 90% of the bubbles to which patients were exposed.

"It is very difficult to get all the air out of the heart chambers and pulmonary veins until the heart is actually beating and pumping blood again."

Green Lane surgeon Paget Milsom designed a circuit which would allow the heart to pump blood and bubbles back to the heart-lung machine for filtering, before the heart started pumping blood around the body.

This technique was used with 14 patients, some of whom were in the trial. They received a median of 101 bubbles compared with 1,650 left by conventional de-airing techniques.