The cardiovascular (CV) system is responsible for the adequate supply of oxygen to the cells of the body, and effective CV function is essential for health and survival. Accurate quantitative assessment of CV performance is critical for diagnosis, intervention and appropriate management of circulation. While multiple technologies exist for monitoring the circulation many are invasive with associated risks, or based on monitoring of blood pressure or other circulatory surrogates and are modestly effective. Further, clinical assessment of haemodynamic variables has been proven to be unreliable, even in the hands of experienced clinicians. It is concerning that for such a vital measure of health, the clinical measurement and monitoring of haemodynamics is so inadequate and the application of protocols derived from such monitoring is so ineffective. The development of an accurate, noninvasive, safe and accessible measurement of central haemodynamics may result in the widespread implementation of physiologically rational and effective diagnostic and management protocols. There is thus a clinical need for improved methods of CV monitoring.
Doppler ultrasound is a proven and accurate method for quantitation of blood flow and forms a routine component of diagnostic echocardiography (echo). However the Doppler echo method is rarely applied to the monitoring of haemodynamics as the cardiac output (CO) measurement is complex to perform, time consuming and characterised by technical limitations. The Ultrasonic Cardiac Output Monitor (USCOM) was conceived to simplify and improve the Doppler echo method for specific application to CV haemodynamic monitoring.
This thesis describes the concepts and ultrasound principles leading to the development of the USCOM device, and the subsequent development and testing of the device to evaluate its applications in clinical practice. This objective of the process was to determine if the USCOM is a safe, accurate and accessible haemodynamic monitor that can improve medical care and advance the specialist ultrasound science of Doppler haemodynamics.
The first chapter of the thesis establishes the clinical need for an improved CV monitor by reviewing current evidence and practice. The pulmonary artery catheter (PAC), the current gold standard for measurement of CO and pulmonary artery pressures, was found not to have been appropriately validated nor proven in clinical practice and therefore not an adequate circulatory monitor. This chapter concludes that the PAC provides neither an accurate measure of CO nor a sensitive method for detecting haemodynamic changes associated with disease or therapy.
The second chapter involved an analysis of the current Doppler echo method for measurement of CO, and established that noninvasive continuous wave (CW) Doppler ultrasound has a sensitivity of 2% in controlled circulatory models. CW Doppler is thus an ideal alternative CV haemodynamic monitoring modality. It was also established that while the Doppler echo method is of proven accuracy, its use in clinical practice is limited by fundamental weaknesses. From these strengths and weaknesses in the Doppler echo method, improvements were proposed as features that are the foundation of the USCOM device.
The third chapter described the development of the USCOM device. The technology and algorithms were developed from ultrasound theory and population based haemodynamics. Software and engineering concepts were applied to shift Doppler from a once daily, complex, time consuming diagnostic measurements to a more simple, widely applicable technology that provides real time, longitudinal haemodynamic monitoring.
Chapter four described the process of validating the USCOM outputs. Validation involved comparison with a variety of different reference methods including PAC and non-PAC technologies. The device was proven to be both accurate and sensitive against true gold standards, including electromagnetic flow probes, and was found to be generate measures that were interchangeable with other clinical technologies currently in practice. The device was found to have an accuracy of 1mL/kg/min in neonates and a sensitivity to haemodynamic change of 5% across a range of outputs from 0.12L/min to 18.7L/min. The device was further demonstrated to be clinically useful and to improve clinical practice across a number of applications including sepsis, fluid management, heart failure and hypertension. As a noninvasive technology the age range across which the technology is clinically applicable included neonates, children, adults and the elderly. Like any monitoring technology, there are limitations with the principles, application and adoption of USCOM. These limitations were discussed and strategies to minimise their consequence proposed.
Chapter five hypothesises how a simple to use, noninvasive Doppler CV monitor may be further improved and applied to specific clinical applications to further advance the study and practice of haemodynamics. The potential for all age personalised haemodynamic diagnostic and management protocols and the development of new multimodal devices was discussed.
In conclusion this thesis reports that the USCOM device addresses a clinical need, is soundly based in ultrasound theory, provides an improved CV monitoring method, and is well validated against gold standards, and improves clinical care. Like all medical technologies an understanding of the principles, operation and limitations allows for a more effective application of the device. Finally a number of potential future developments of Doppler CV monitoring were outlined. The device was demonstrated to be a safe and cost effective technology with clinically important target applications including sepsis, heart failure and hypertension and objective guidance of fluid, inotropes and vaso-active therapies. There is an established need for an improved CV monitor, and the noninvasive USCOM is a safe, accurate and accessible haemodynamic monitor, and a viable alternative to current methods. USCOM has the potential to improve circulatory assessment and patient care across multiple clinical applications.