R. Skidmore

Analysis of femoral artery Doppler signals by LaPlace transform damping method

The LaPlace transform damping (LTD) method of common femoral artery Doppler waveform analysis is a new method for assessing aortoiliac stenosis. A microcomputer was used to analyze the waveform to determine the value of the damping parameter, which is related to inflow stenosis. The study included 60 limbs with arterial disease and 20 limbs in normal control subjects. With a damping value of 0.60 used as the cutoff point between nonocclusive lesions and iliac stenoses of greater than 50% diameter reduction, we found a sensitivity of 100% and a specificity of 93% (overall accuracy 95%). Unlike Doppler waveform analysis by the pulsatility index method, the accuracy of the LTD method is not affected by occlusion of the superficial femoral artery. The Doppler signal processor and the computer currently used for the LTD analysis are simple to operate and fully suitable for routine use in a clinical vascular laboratory. Our experience indicates that the LTD method of waveform analysis provides the best noninvasive method for detection of significant inflow disease.

A DOPPLER FLOWMETER FOR USE IN THEATRE

We have developed a Doppler flowmeter based on a 10 MHz pencil probe and mean frequency estimator which overcomes many of the limitations of existing electromagnetic and ultrasonic flowmeters. The output of the flowmeter, which is proportional to the first moment of the Doppler power spectrum and hence mean blood velocity is linear from 1.3 to over 50 cm s-1 for pulsatile flow. Variation in vessel diameter and angle of insonation, which are the common sources of error in Doppler flowmetry, are minimised by constraining the vessel in a plastic cuff which fixes the probe angle at 50 degrees. A simple gauge is used to compress the vessel flat, before the cuff is applied, to measure the wall thickness to within 0.25 mm. The vessel internal diameter and hence blood flow can then be calculated using an experimentally determined calibration factor to compensate for non-even insonation. A range of sterilizable cuffs from 3-12 mm diameter have been built and the flowmeter is now being used routinely during all arterial reconstructive surgery. The accuracy and reproducibility of the system was tested for range of different sized silastic tubes on a hydraulic model and found to be less than 12% for vessels greater than 2 mm internal diameter. Satisfactory signals were easily obtained from all prosthetic materials with the exception of PTFE. The instantaneous output was compared to an electromagnetic flowmeter using a fast Fourier transform algorithm; the moduli of the harmonics were virtually identical but the Doppler system produced a smaller phase shift with increasing harmonics.(ABSTRACT TRUNCATED AT 250 WORDS)

Physiological interpretation of Doppler shift waveforms: the femorodistal segment in combined disease

A new method is presented for assessing the femorodistal segment in multisegmental arterial disease, using the Laplace transform technique of Doppler waveform analysis. Blood velocity/time waveforms were obtained at femoral and ankle levels in three groups of limbs--50 without arterial disease, 12 with isolated aortoiliac stenoses, and 32 with femoropopliteal occlusions, with and without proximal disease. The waveforms were analysed for Laplace transform and pulsatility index values. The omega 0 coefficients of the Laplace transform analysis at femoral and ankle levels were compared in each subject, as the omega 0 gradient (femoral/ankle omega 0): and pulsatility index damping factor (femoral/ankle P1) was also calculated. The omega 0 gradient was shown to detect femoropopliteal occlusion in the presence of multisegmental arterial disease and to give some indication of its haemodynamic significance. The diagnostic accuracy of the omega 0 gradient was superior to that of pulsatility index damping factor. When combined with its existing ability to detect aortoiliac stenosis, this new application of the Laplace transform method offers the possibility both of a system for complete localisation of significant arterial lesions, and potential for follow-up of vascular surgical procedures in the lower limb, from two simple Doppler recordings.

Variability and reproducibility of arterial Doppler waveforms

The variability and reproducibility of arterial Doppler waveforms are influenced by factors such as operator experience, physiological changes in the patient, and the type of Doppler processing system. Some aspects of these were studied using Laplace transform analysis of waveforms from the femoral and posterior tibial arteries of normal subjects and arteriopaths. Five consecutive waveforms were obtained from the femoral and posterior tibial arteries of normal subjects during about 1 hr of supine rest. Group 1 (n = 20) was studied 9 months before Group 2 (n = 11), early in the operators experience of waveform recording. Variability (root mean square) of femoral and posterior Laplace delta decreased from 16.9 to 12.5%, and 23.3 to 17.9% respectively (P less than 0.05 in each case). In arteriopaths (n = 8) there was a trend to more variable results at femoral level (only significant in the case of Laplace omega 0 - P less than 0.05), while posterior tibial results tended to be less variable. Normal subjects examined in the longer term (n = 4), with a mean interval of 28 days between recordings, did not show significantly greater variability than those examined over 1 hr. There was no significant difference in variability of results in normal subjects (n = 30) using instantaneous average Doppler processing compared with maximum frequency processing, except in the case of posterior tibial Laplace omega 0 (maximum frequency more variable - P less than 0.05).

LaPlace transform analysis of femoral artery doppler signals: The state of the art

A followup study was conducted to validate our previous experience with the LaPlace Transform Analysis (LTA) method for processing Doppler ultrasound signals from the common femoral artery to detect significant stenosis of the aorto-iliac segment. The first phase used the same instantaneous mean velocity signal processor as used in the prior study. A comparison of the Doppler examinations with angiograms in 98 legs yielded a sensitivity = 92% and sensitivity = 94% in the identification of 50% or greater stenosis of the aorta-iliac segment, results almost identical to the last study. Because of theoretical disadvantages of using the instantaneous mean velocity signal we carried out a second phase using a peak velocity detector. In 148 limbs sensitivity = 87% and specificity = 98%. The presence or absence of superficial femoral artery occlusion did not affect the accuracy of the waveform analysis in the detection of proximal disease. The LTA parameter related to distal resistance, G, was not found to have clinical value in the assessment of the femoral-popliteal segment.