Australasian Anaesthesia 2011 - Australian and New Zealand ...

Cardiac output monitoring in non-cardiac surgery: how and why? 179Cardiac output monitoring in non-cardiac surgery: how and why?A/PROF. PHILIP PEYTON MD PHD FANZCADepartment of Anaesthesia, Austin Hospital, Melbourne, VICPhilip Peyton is a Staff Anaesthetist with an interest in research, with a focus on haemodynamic monitoring andlung gas exchange physiology. His MD was in the field of pulmonary blood flow measurement, and his PhD ininhalational agent pharmacokinetics.Conflict of Interest Statement: Philip Peyton and Austin Health are applicants in a US Patent Application. No.12/743224 “System and method for monitoring cardiac output”.BACKGROUNDThe number of technologies for monitoring of cardiac output in anaesthesia and critical care has grown substantiallyover the last 15-20 years. The aim has been to provide less invasive and hazardous alternatives to the pulmonaryartery catheter to optimise patient haemodynamics and fluid management. In recent years this has spurredinvestigation into improvements in patient outcomes. Research continues into the best combinations of fluidvolumes, use of crystalloid versus colloid solutions, and inotrope and vasopressor use to maintain physiologicalhomeostasis in the face of the blood and fluid losses, acid-base and thermoregulatory disturbance and cardiovascularinstability that can accompany anaesthesia for major surgery.The older techniques for measurement of cardiac output included methods that were not adapted to real-timeuse in the perioperative setting. Indicator dilution methods using injection of indicators such as indocyanine greenwere “single shot” techniques, as were methods based on uptake of inert gas, like nitrous oxide, by the lung.Thermodilution has been useful tool on the perioperative setting, but requires invasive cannulation with a right heartcatheter, as does the oxygen Fick method. The routine use of thermodilution and the pulmonary artery catheter isoften confined to cardiac surgery and high risk patients undergoing major surgery, such as liver transplantation.The newer less invasive techniques for cardiac output measurement fall broadly into the categories of pulsecontour based methods, Doppler devices, thoracic bioimpedance and partial CO 2 rebreathing. In combination witharterial and central venous blood pressure measurement, the data from these devices allow determination of cardiacoutput as well as systemic vascular resistance (SVR) or left ventricular afterload among other derived variables,which can then guide inotrope, vasopressor and volume therapy (Figure 1). In addition, many devices provideindices of ventricular preload to guide volume replacement therapy. However, while invasive measurement of bloodpressure is now common in clinical practice, use of minimally invasive techniques for stroke volume or cardiacoutput measurement remains far from routine. The limited use of these technologies is largely due to the cost ofthese devices and their disposable components, and concerns about their accuracy and precision.The principles behind these techniques, their reliability and their potential to influence patient management willbe reviewed.Figure 1. The components of haemodynamic assessment, and their modifiersPerfusion pressure(Preload)Fluids/VolumeCardiacoutput(Afterload)VascularResistanceInotropes &ChronotropyVasopressors