Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Cardiac performance is a function of the fluid flowing into the heart. The determinants of flow include, the preload, the performance of the heart (contractility and relaxation), and the afterload or systemic vascular resistance (SVR). However it is hypothesised that these parameters, while useful, do not completely describe cardiac function.
Importantly, the blood pressure (BP) is a product of the cardiac output (CO) and the vascular function, measured as SVR. A more complete representation of cardiovascular function will include combined measures of both flow and pressure.
The concept of Cardiac Power Output (CPO) and Stroke Work (SW) have been advanced as possible improvements to the basic assessment methods that can be derived from stroke volume (SV) and CO values, and BP.
An improved understanding of cardiovascular function can be determined by combining flow volume and pressure measures.
However, arterial pressure varies throughout the vascular network and so the point of measurement of the blood pressure is important when determining the usefulness and application of the measurement.
Currently Doppler is an accurate measure of blood flow but other accurate measures of flow could be utilised. While BP can be measured at any point in the cardiovascular network. The measurement methods vary from manometric methods located within the heart or vessels, acoustic sensors, plethysmography and tonometry. The science of each of these measurement methods is well established but their combination to create more informative measures of cardiovascular function which can be monitored has not been established.
Cardiac Power Output (CPO) in Watts is defined as effectively a product of mean Blood Pressure (BP) and Cardiac Output (CO).
Stroke Work (SW) is defined as the amount of energy in Joules that is given to the blood in a single heart stroke. With Cardiac Work (CW) further defined as the product of SW and the patient's heart rate.
CO and SV, which form the basis for calculating measures of CPO and SW, are typically measured using instantaneous single measures from echocardiography or invasive catheter derived measures. Previously, the most common method for calculating CPO and SW was Thermodilution, involving an invasive procedure based on the use of a pulmonary artery catheter (PAC) also known as the Swan-Ganz thermodilution catheter and provides a measure of right heart blood pressures. Using the PAC thermodilution CO can be measured, from which the CPO and SW can be derived.
These measures are typically made at one time and may take up to an hour to measure using invasive catheter methods. During this period the values of the measured parameters may change along with the patients condition. As CPO and SW vary beat to beat, reflecting changes in physiology and pathophysiology, a method of measuring SV and CO in real time for calculating CPO and SW will substantially improves these previously described methods.
Further the inclusion of more accurate and reliable measures of SV and CO may allow improved calculation of SW and CPO and may better describe cardiac and cardiovascular function.
Hence there is a need in the art for providing real-time, preferably non-invasive, measurement of the right and left ventricular blood flow of a patient and to provide a cardiac inotropy measure.