Today, clinical practice needs echo-Doppler cardiography as an important adjunct to the plain ECG of early days, even for intensive care immobile patients. Regular Doppler B-scan instruments for this purpose are unwieldy and are not locatable at the bedside. If a less complicated instrument which combines ECG on-line displays along with at least a continuous-wave (CW) Doppler spectrogram view is available, it will help periodic observations on intensive-cared cardiac patients. For this purpose, a microprocessor based ECG CRT Display instrument with combined Doppler spectrum view was developed.
This system employs a 6502 microprocessor along with a BASIC interpreter as part of the firmware, so that the calculations particular for Doppler spectrum evaluation could be handled. A CW ultrasound probe (2โข5 MHz) and associated receiver circuitry are used to provide the two Doppler-shift signals which are in the audio range. An Analog-digital converter of 1โข2 s time is interfaced for scanning the ECG and Dopplershift signals, for a time period of about 2 seconds, after probe positioning by the physician. Digitally, the separation of the forward and reverse flows is done using a complex fast Fourier transform (CFFT) method. The short-time (6โข4 ms) discrete Fourier transform is calculated using a BASIC routine called by the overall firmware. The results are displayed, along with the stored ECG as two waveforms on the CRT display.
The Doppler spectrogram is shown as the maximum curve with intensity modulated segments, so that the estimate of relative amplitudes can also be made. While working as a normal ECG on-line monitor, continuous non-fading ECG display and heart rate are both shown.
Additionally, programs are available for assessment of cardiac parameters such as pressure half-time. Such a unit, being simpler and combining CW-Doppler, is likely to be of greater use than plain ECG monitors at the bedside.