Usage of organ protective vapors on the CPB in elderly patients

Abstract

Background: The fastest growing segment in today's society in the industrial nations is the segment of elderly with 65 years and older. Anaesthesia and surgery are known to contribute to the incidence of a syndrome described as Postoperative Neurocognitive Decline (POCD). Due to decreased neuronal plasticity in elderly patients, the Cardiopulmonary Bypass (CPB) has higher impact on cognitive function. POCD is described mainly as a decrease in memory and concentration span. Monk et al. describe the influence of accumulative deep anaesthesia time to result in a significant increase in one year mortality in elderly patients in otherwise uneventful surgical procedures. The incidence of POCD is described in cardiac patients incorporating the use of the CPB in up to 75% of the cases. Anaesthetic vapors such as Sevo and Isoflurane have been shown to present neuro and organ protective effects. Modern oxygenators are using different types of hollow fibre membranes that are specifically designed for the exchange of CO2 and O2. Due to different physico-chemical properties of the different membranes in different oxygenators for CPB the question arises, whether or not, different membranes are allowing for permeation of such vapors through the membrane. Published evidence about neuro and organ protective effects of vapors suggest an improved neuro cognitive outcome of elderly patients due to the use of anaesthetic vapors in the CPB. Aim of this presented study is to compare the permeability of porous and dense membranes for the delivery of anaesthetic vapors during CPB in cardiac procedures. Material and Methods: A realistic artificial CPB circuit is used consisting of a venous reservoir, an oxygenator with a vaporizer and a roller pump filled with 1.3l human blood (hematocrit of 30). Blood, gas and reservoir pressures as well as the blood temperature are continuously monitored. Continuous gas sampling is performed at the exhaust of the oxygenator, the reservoir and after the vaporizer. The blood is adjusted to a temperature of 30°C during de-nitrogenation for 30 minutes or until equilibration by flushing the reservoir and the oxygenator with 6l/min of 5% CO2 in Oxygen. The reservoir is closed and the gas delivery to the reservoir stopped. The flow to the oxygenator is adjusted to 1l/min of 5% CO2 in Oxygen and Sevo or Isoflurane respectively are added with 2,0%. The reservoir concentration of each vapor is recorded over time. After a stabile concentration in the reservoir is reached, the gas stream is switched to vapor free room air at 1l/min and the reservoir concentration of the vapor is recorded over time. Results: The Quadrox D, Maquet (dense polymethylpentene membrane) as compared to the Synthesis, Sorin (micro-porous polypropylene membrane) shows relative limited performance in delivery of vapors (Sevo < Isoflurane). Summary: As more and more elderly patients are treated in cardiac surgery using the CPB, anaesthetic vapors such as Sevo and Isoflurane are desirable as neuro protective and organ protective substances on the CPB in this segment of the population. Modern oxygenators are using different types of hollow fibre membranes and the question arises, if those different membranes allow larger molecules like Sevo and Isoflurane to effectively travel through the membrane to be delivered to the patient blood during CPB. We tested physico-chemical properties of different membranes. Oxygenators containing a plasma-tight as compared to oxygenators containing a porous membrane showed very limited performance in delivery of vapors through the membrane suggesting a limited use for clinically relevant and cost-effective delivery of vapors during CPB to achieve relevant and cost-effective neuro and organ protection. F. Wilfart et al., Delivery of Vapors on Cardiopulmonary Bypass Using Different Oxygenator Membranes, Proceedings of the Eighth IASTED International Conf. on Biomedical Engineering, Innsbruck, Austria, Feb. 16-18, 2011