Text 7. Cardioscope

Cardioscopes are found in almost every ward and operating theatre in the hospital. They are used to show that the heart is beating, that the ECG waveform is approximately correct, and that the heart rate is reasonable. More specialized use is made of the information in coronary care units and in intensive therapy units.

In its basic form the device consists of a cathode ray tube display with associated drive circuitry, a high quality differential amplifier, a switch for selecting a small number of the standard ECG lead configurations, and a screened lead and electrodes for connection to the patient. In addition to these basic elements it may also have a small computer-type memory to retain two or three lines of the display which can be frozen and examined and also to provide a non-fading view of the last ten seconds or so of the recording. Other refinements might include a display of the average heart rate over the last few seconds, or details of the rate trend over the last few minutes or hours presented in computer-generated lettering on the same screen as the ECG. This type of computer-aided display is becoming commonplace.

Older types of cardioscope used long persistence cathode ray tubes which provided a slow moving spot with a 'comet's tail', enabling a clear view of the last few seconds of the tracing, provided the ambient light level was low.

Cardioscopes usually have a smaller bandwidth than electrocardiographs (which are used for diagnostic purposes). A typical high-frequency cut-off is 40 Hz, being sufficient to see that the main elements of the ECG waveforms are present but not sufficient for demonstrating all the possible variations from the normal. This restricted bandwidth improves noise immunity and places less demand on the quality of the electrode connections. It should be remembered here that the electrodes for cardioscopes are normally applied by the ward nursing staff, not by specialized technicians. The electrodes may be applied according to standard lead positions, but are often applied without accurate positioning, since the actual waveforms are not important.

Cardioscopes may be included in more complex diagnostic systems where heart rate and variability on a beat-to-beat basis is important. Missed beats and extra (ectopic) beats can be identified by watching the screen for every second of the day, but some cardioscopes can automatically recognize these events. Such devices might be called ectopic beat detectors, or beat-to- beat heart-rate analysers.

Cardioscopes are also used in conjunction with defibrillators, pacemakers and diathermy apparatus. In these cases the input circuitry must be capable of withstanding the relatively high voltages which may be applied directly to it. A good quality cardioscope should be able to remain connected during defibrillation or surgical diathermy without damage and with almost instant recovery of the correct display. This facility is crucial in synchronized defibrillation