Background Cardiac function monitors are well known and, typically, produce an output waveform representative of the patient's cardiac output, which waveform is presented visually on a display unit.

One of the problems with prior art forms of apparatus is that they have not included any facility whereby changes in output over time can be readily compared ; or changes in output due to particular treatments can be readily determined.

It is therefore an object of this invention to provide a method of, and or apparatus for monitoring cardiac function which will go at least some way in addressing the above identified needs, or which will at least provide a useful choice.

Summary of the Invention Accordingly, in a first aspect the invention provides a cardiac function monitor of a type which displays a waveform representative of cardiac output, said rr. or'itor being characterised in that it provides for tne storage of one or more waveform components for subsequent display alongside at least

one waveform subsequently captured.

The monitor may provide for those waveform components to be stored to be selected manually. Alternatively, or in addition, said monitor may include means for automatically isolating and storing a waveform component.

If automated, the monitor is configured to ensure the corresponding part of each waveform is stored and displayed.

To this end, the monitor first identifies a characteristic point on the waveform then selects and stores the same relative part of the waveform extending from said characteristic point.

Typically this will be in both directions from said characteristic point.

The monitor is preferably configured to identify the end of systole of a complex and then store the complex around said end of systole.

In a second aspect the invention provides a method of comparing cardiac output waveforms which includes the use of the monitor set forth above.

In a third aspect the invention provides a method of determining over time the effect of treatment, or absence thereof, on cardiac function, said method including generating a visual waveform display of cardiac function; isolating and storing in memory a section of the waveform; after treatment, or the elapse of a period of time without treatment, displaying the stored section of waveform alongside a waveform subsequently generated for comparison.

Many variations in the way the present invention can be performed will present themselves to those skilled in the art.

The description which follows is intended as an illustration only of one means of performing the invention and the lack of description of variants should not be regarded as limiting.

Wherever possible, a description of a specific element should be deemed to include equivalents thereof whether in existence now or in the future. The scope of the invention should be limited by the appended claims alone.

Brief Description of the Drawings One form of the invention will now be described with reference to the accompanying drawings in which: Figure 1: shows a monitor display unit according to the invention; and Figure 2: shows a process flow diagram illustrating part of the invention described herein; Figure 3: shows a cardiac output waveform from which one or more complexes can be taken according to the invention; and Figure 4: shows a single complex isolated from the waveform shown in Figure 3.

Detailed Description The present invention provides apparatus for and/or a method of comparing cardiac function-particularly cardiac output,

over time and, thereby, allow clinicians to readily observe a decline in cardiac function or, alternatively, observe changes in such function due to treatment provided.

Referring to Figure 1, according to the invention a cardiac function monitor 5 is provided which includes a visual display 6 on which cardiac output waveforms 7 are displayed.

The monitor 5 is preferably of the Doppler ultrasound type which monitors cardiac function in real time. Accordingly the waveform 7 is a dynamic waveform which typically scrolls across the display 6 from right to left but may go from left to right as a refresh scroll.

By way of explanation, the waveform is composed of a number of complexes, each complex effectively being a heartbeat. In Figure 4 a single complete complex is shown along with part of the preceding complex. Within each complex there is a peak which represents systole-the period in which the heart muscle contracts to force blood from the heart. Having regard to the nature of the subject apparatus, the start and end of systole are significant events and, accordingly monitors of this nature typically allocate the start and end of systole with arrows such as are shown in Figures 3 and 4. The"A"arrows indicate the start of systole while the"B"arrows mark the end of systole. These arrows are placed by the processing software of the monitor and do not form part of the present invention.

A significant drawback with prior art forms of cardiac function monitor was that they provided no means to enable users such as clinicians, to determine the effect of treatment on cardiac function, or to simply and readily observe changes in function over extended periods of time. The present invention addresses this by providing for complexes to be isolated and stored in

memory. The isolated complexes can then be recalled at some later time and displayed on a section of the monitor 6 alongside the waveform 7 then being captured and displayed.

According to the particular embodiment of the invention described herein, the exercise of isolating and storing a complex may be undertaken manually, or may be undertaken under the control of the processor embodied in the monitor 5.

For manual operation, when the clinician wishes to store a complex for future reference, he or she depresses capture knob 8 on the monitor. This"freezes"part of the waveform, extant at the time of depression of the knob 8, in a margin 10 shown as the shaded portion at the right hand side of display unit 6. The monitor 5 preferably further includes a function where a discrete number of prior complexes, say up to 5 seconds of the waveform previous to freeze, are held in a memory so that the clinician can scroll back through those and isolate a particular complex for storage and later recall. This process requires the clinician to accurately position the complex of interest within the margin section 10 or under an indicator section showing the area to be stored.

When recalled, the complex is displayed in static form in the margin 10 whilst the real time waveform scrolls across the remainder of unit 6.

Obviously the process of freezing the waveform and then scrolling back to find a particular complex of interest involves the clinician in a level of concentrated activity which may be better employed in critical care situations. Accordingly the present invention preferably provides for complexes to be isolated and stored under the control of the processor embodied in monitor 5 so that, when recalled, complete complexes are displayed in a consistent manner in the margin 10.

To use the automated facility the clinician depresses knob 11 although it will be appreciated that the same knob as knob 8 could be depressed if enabled in a different manner or placed in an alternative setting.

When knob 11 is depressed, the monitor then selects and stores in memory the last complete complex.

In order to isolate the last full complex the processor preferably follows the routine shown in Figure 2. More specifically, the processor looks for the most recent"B"arrow in the preceding 10 seconds. If there is no such B arrow then the processor generates an error message. Assuming, however, that a B arrow is found, the processor then recalls and positions within margin 10 the entire complex of which that B arrow forms part.

This is shown in Figure 4.

The processor may allocate a time of capture to the stored complex and preferably includes sufficient memory to store a number of complexes. This permits clinicians to scroll through the stored complexes, display each against data being received in real time, and readily observe changes in cardiac function.

It will thus be appreciated that the present invention, at least in respect of the preferred form of apparatus described herein, provides a simple yet effective means of determining changes in cardiac function over time, whatever the cause.