Background

Cardiac output (CO) is defined as the volume of blood pumped by the heart in one minute - it is equal to the product of the heart rate and the stroke volume. The CO value provides a vital tool for the diagnosis, treatment, and prognosis both of critically ill patients and also of those recovering from surgical procedures.

Historically, CO measurements were derived from catheters inserted into the pulmonary artery - this technique is becoming less popular due to its invasive nature. CO measurements are now being derived from arterial pressure waveforms obtained from catheters inserted into the radial and/or femoral arteries. The changing fluid pressure within the catheter is measured with a pressure sensor - an example waveform is shown below:

CO instruments

CO instruments apply an algorithm to the pressure sensor output to estimate the CO value. These algorithms are developed by the instrument manufacturers and kept commercially confidential. They are likely to contain a number of parameters (e.g. ventricular contractility, arterial impedance and compliance, peripheral vascular resistances) that relate to a patient's physical state and may require an independent calibration technique (e.g. lithium dilution and transpulmonary thermodilution) to convert the pressure waveform to a cardiac output.

Requirements

Clinicians need confidence that the displayed CO values are correct. One potential problem is that the algorithms tend to be derived from clinical data obtained from relatively well patients, such as those recovering from surgical procedures. The behaviour of a critically ill patient's cardiovascular system may differ significantly, resulting in the predicted CO values being incorrect and leading to less than optimal diagnosis and treatment.

Project description

The aim of this project is to enable the results from different CO instruments to be compared by analysis of the same input waveforms. This requires the design and manufacture of a pressure waveform generator (PWG), an instrument capable of repeatably generating a range of representative pressure waveforms - a library of digitally-stored waveforms (obtained from critically ill patients in their care) has been methodically built up by Guy's & St Thomas' NHS Foundation Trust (GSTT) and will provide the input data for the PWG. GSTT will also lead the testing of the CO instrumentation with the PWG.

PWG design

The principle of operation of the PWG is shown in the following diagram:

The pressure is generated in the push pot by the shaker moving the piston within the pot, its position being monitored by the laser interferometer system.

A PID controller is also used to optimise the generated pressure waveform with respect to the required waveform. The following graph shows how well this system is presently working, the error (green) between the required (white) and measured (red) displacements rarely exceeding a few tens of bits in an amplitude of 5 000 bits:

This initial set-up is simply to prove the concept of the design - as the requirement is for the pressure waveforms to be generated in fluid-filled catheters, in the final version the push pot will be mounted vertically and half filled with liquid. This final system may also use a capacitance displacement sensor in place of the laser interferometer, and a reference pressure transducer may also be incorporated for comparative measurements.