Prevention of brain injury in cardiac surgery

Abstract

Background: Stroke and neurocognitive deficits may follow heart surgery and have been linked to peri-operative cerebral embolism. Lignocaine exhibits cerebral protection in animal models of cerebral arterial gas embolism. This study began as randomised trial of lignocaine in brain protection in left heart valve surgery patients. Carotid Doppler emboli counting, developed to control for emboli exposure in the trial groups, revealed that most emboli occurred at the termination of cardiopulmonary bypass (CPB), and that "deairing" techniques used to remove air from the heart were not effective. Doppler monitoring also suggested that emboli were generated by the hard shell venous reservoir (HSVR) component of the CPB circuit, and that contrary to popular perception, air entrained into the CPB venous return line did pass through the circuit back to the patient. Methods: Salvaged CPB circuits were used in vitro to investigate emboli generation by Medtronic Maxima HSVRS, and the passage of entrained venous line air through the CPB circuit. The efficacy of a novel left heart deairing technique was audited clinically using the Doppler device. Finally, a randomised double blind trial of lignocaine in cerebral protection during cardiac surgery was conducted. Sixty five patients underwent pre-operative neuropsychological (NP) testing and were randomised to receive lignocaine in a standard antiarrhythmic dose, or a placebo, in a double blinded infusion over 48 hours beginning at surgery. The NP tests were repeated at 10 days, 10 weeks and 6 months post-operatively. Results: The Medtronic Maxima HSVRs were found to generate bubbles when operated at blood volumes well above the manufacturer's recommended minimum. These bubbles, and air entrained to the CPB venous return line, were found to readily transit the CPB circuit. Patients deaired using the novel technique were exposed to more than 10-fold less emboli after removal of the aortic clamp and withdrawal of CPB. Lignocaine treated patients exhibited a significantly reduced incidence of NP deficits at 10 days and 10 weeks postoperatively, and reported better memory at 10 weeks and 6 months postoperatively. Conclusions: The Medtronic Maxima HSVRs should not be operated at blood volumes lower than 600 - 700 ml. Attempts should always be made to eliminate air entrainment to the CPB venous line, especially where vacuum assisted drainage is used. The novel de-airing technique is markedly superior to conventional methods. Lignocaine is a potentially useful cerebro-protective agent during cardiac surgery.