The Application of the Ultrasound Cardiac Output Monitor (USCOM) to Anaesthesia Practice

Monitoring cardiac output (CO) and associated haemodynamic variables, and their use to facilitate circulatory optimization, can play a very important role in both anaesthesia and critical care medicine. The pulmonary artery catheter (PAC) is regarded as the clinical gold standard in CO measurement. However, in recent years its safety and benefits have been questioned and this has resulted in a significant decline in its use. Following this decline, several less invasive methods of CO measurement have been introduced into clinical practice and they include thoracic electrical impedance and arterial pulse contour analysis.;For the first part of my thesis work that looked at the repeatability of the USCOM. I developed a method of performing a series of six USCOM scans under stable haemodynamic conditions that measured the coefficient of variation (CV) of several key USCOM variables: Stroke volume index (SVI), CO, Peak velocity (Vpk) and Flow time corrected (FTc). I applied the method to 180 patients of all ages (1 to over 80- years) following induction of anaesthesia for routine surgery. I evaluated the quality of each USCOM scan using a 12-point Cattermole score (CS). I also developed a 10-point insonation (IS) score that evaluated the ability to insonate aortic blood flow using the USCOM. My main findings from this study were that the repeatability of USCOM scan readings during anaesthesia had a CV of < 5% in most anaesthetized patients and that the repeatability was clinically acceptable. However, increasing age and a low ejection volume (i.e. SVI) made insonation more difficult and reduced the quality of the USCOM scan. I concluded that under such circumstances the reading from the USCOM could not always be trusted and that a scoring system of scan quality should be used.;For my second part of my thesis work that looked at the trending ability of the USCOM, I compared USCOM CO readings to those from a more established method, oesophageal Doppler, perioperatively. I recruited twenty patients having major surgery in whom CO was expected to change over a wide range of values intraoperatively. I collected paired readings from both devices at regular intervals throughout surgery. Although it was not my primary intention to assess the accuracy or precision of these two methods, I performed a repeated measures Bland-Altman analysis. I assessed trending by plotting the individual patient scatter plots and performing linear regression and correlation. Collectively, I performed a concordance analysis on all twenty sets of data.;For the third part of my thesis work that looked at the utility of the USCOM during anaesthesia, I (a) collected a number of case reports where using the USCOM perioperatively had proved useful and (b) performed a clinical study to evaluate a new monitor that used bioreactance to measure CO, where USCOM and the CardioQ were used as dual reference methods. During anaesthesia I found that (i) the USCOM helped to identify several patients with undiagnosed aortic valve stenosis, (ii) identified high CO due to arterial-venous shunting as a cause of pulmonary hypertension and heart failure, and (iii) helped in the management of patients with severe cardiomyopathy and heart failure. I also developed a methodology that enabled me to evaluate the performance of other CO technologies during anaesthesia using the USCOM in combination with the oesophageal Doppler.;In conclusion, USCOM required some degree of skill and understanding to be used effectively during anaesthesia and to obtain reliable data. In particular, caution is needed when interpreting USCOM scan data form the elderly patient. Although its accuracy appears to be no better that any other non-Doppler CO method, it does provide reliable trend data that shows the change in CO. Thus, it can be used to facilitate haemodynamic management in difficult to manage anaesthetised patients with cardiovascular impairment, as well as simply monitoring CO changes during anaesthesia. It can also be used as a research tool to evaluate other CO monitoring modalities. Thus, the USCOM is a useful monitor in anaesthetic practice. (Abstract shortened by UMI.).