A new electrical impedence plethysmogram : observations in peripheral arterial occlusive diseases.
Arteriography which is routinely used for investigating cases of arterial disease is an invasive procedure which supplies information about the anatomical status of the arterial tree but not about the haemodynamics of the circulation in the limb. During recent years significant progress has been made in developing new methods of investigation of noninvasive character, to study the peripheral circulation. These methods ha -cluded Doppler Ultrasonogram,[l] Straingauge plethysmogram, Pulse volume recorder, Electrical impedance plethysmogram,, ,  ,  etc.
Impedance plethysmography had been used to assess peripheral blood flow in the limbs by using venous occlusion principle.,  This technique, however, had certain drawbacks, such as being an offline and lengthy procedure.,  Moreover, the technique did not yield beat to beat information of blood flow because of its low signal to noise ratio. In order to eliminate these limitations a new impedance plethysmogram with a high signal to noise ratio was developed at the Electronics division of Bhabha Atomic Research Centre, Bombay and clinically assessed at the Department of Cardiovascular Surgery, Seth G.S. Medical College and K.E.M. Hospital, Bombay. The present communication is a preliminary report on the assessment of this impedance plethysmogram as an aid for the diagnosis and treatment of patients suffering from peripheral arterial occlusive diseases.
Purpose of this study was to assess the capability of the modified model of impedance plethysmogram to investigate patients with peripheral arterial occlusive disease for obtaining following information:
(1) To locate the level or levels of arterial obstruction.
(2) To determine the extent of the arterial obstruction.
(3) To decide the status of the collateral circulation.
(4) To determine the flow in the artery proximal, and distal to the block (arterial run off).
(5) To study the effect of various drugs on the peripheral arterial circulation.
(6) To select patients for arteriographic study.
(7) To provide aid in the selection of modality of therapy.
(8) To study the benefits derived from surgical procedures such as sympathectomy, vascular bypass operations, etc.
(9) To carry out clinical follow up of patients with peripheral arterial occlusions.
Impedance plethysmogram used for this investigation employs an isolation transformer to pass a 3 mA r.m.s. constant current at 100 kHz through the body with the help of current electrodes (prepared from braided wire in the form of loop around the body segment). The voltage developed across any two points along the current path is sensed with the help of voltage electrodes (similar to current electrodes). This signal is transformer coupled to an amplifier and a precision rectifier so as to display the basal impedance (Zo) in ohms. The small changes in the impedance caused by the blood flow, respiration etc. are further amplified (16-160 Hz 3-dB points). The output is fed to a strip chart recorder, to display the time derivative of impedance, referred to as dz/dt waveform or impedance plethysmographic waveform.
Fifty normal healthy volunteers (nonsmokers-age 15 to 55 years), 15 patients with superior extremity arterial block and 35 patients with inferior extremity arterial block were investigated with the impedance plethysmogram.
The subjects were asked to lie in a supine position. Current electrodes were connected around the proximal and distal end of the upper and lower limbs. Voltage electrodes were placed at any 2 points between the current electrodes on the segment of the limb to be studied. The distance between the voltage electrodes were kept constant (5 cm) at every level. The blood flow per beat per 1000 ml of tissue was computed at various levels of the limb.
[Table - 1] gives the findings of average blood flow measurement at different levels in 50 normal individuals.
Plethysmographic observations were made in 50 cases of arterial occlusion (15 in upper extremity and 35 cases in lower extremity). In all these cases arteriography was also performed. Study was performed in 3 patients with subclavian blocks, 5 patients with axillary block, 5 patients with brachial block and 2 patients with radial block.
The values of the computed average flow in the upper arm, elbow and lower arm of patients having block in the subclavian, axillary, brachial and radial regions are given in [Table - 2].
In the 35 cases of lower extremity studied, there were 10 patients with Leriche's syndrome aorto-iliac block, 5 patients with high femoral block, 10 patients with femoropopliteal block and 10 patients with distal arterial block.
The values of the computed average blood flow, in the upper thigh, lower thigh, leg and foot of patients having arterial blocks at the sites mentioned above are given in [Table - 3].
Plethysmophic study was performed on 20 patients on admission and after administration of 500 ml of low molecular weight dextran intravenously. Plethysmograph revealed a marked increase in blood flow in 5 patients, moderate increase in 12 patients and no increase in 3 patients. The increase in blood flow after lomodex therapy depended on;
(1) Level of block-Patients with proximal block (aorto-iliac) did not show any significant increase after low molecular weight dextran administration.
(2) Status of collateral vessels.
(3) Condition of distal arterial run off. The computed average blood flow values before and after lomodex therapy in the series where the response was good in 17 cases, are given in [Table - 4].
Plethysmographic study was performed on 10 patients with lower extremity block before lumbar sympathectomy and on the 3rd, 6th and 10th day after lumbar sympathectomy. It was found that there was some increase in the total blood flow on the 6th day and then a gradual decrease in the blood flow reaching the preoperative value by the 10th day after sympathectomy. Only total blood flow to a segment of a limb could be measured with plethysmogram but it was unable to detect any readjustment in the circulation from one tissue to another.
The computed average blood flow values of the patients in this series with femoro-popliteal block is given in [Table - 5]
In all the 50 patients with arterial block, the impedance was observed to increase below the site of occlusion. In all the cases, the plethysmographic observations correlated accurately with arteriographic findings as regards the level of obstruction, status of the collateral circulation and the condition of the distal run off but it was not possible to locate accurately the extent of arterial obstruction particularly in the ilio-femoral level.
[Fig. 1] shows a typical dz/dt waveform recorded from the upper arm (axillary), elbow (brachial) and forearm (radioulnar) regions of the upper extremity and those recorded from the abdomen, upper thigh (femoral), lower thigh (popliteal), leg (tibial) and foot (dorsalis pedis) regions of the lower extremity in a normal individual. The average blood flow at each region is given on the tracing.
[Fig. 2a] shows plethysmographic tracings of a patient with subclavian block of the right upper limb. The plethysmographic tracings of the normal left upped limb of the same patient are available for comparison.
[Fig. 2b] shows arteriographic finding of the same patient.
[Fig. 3a] shows plethysmographic tracing of a patient with aorto-iliac block before and after aorto-iliac bypass operation. In the pre-operative tracings, goad distal run off is evident. The tracings of the same patient after the bypass show excellent result of surgery.
[Fig. 3b] shows the arteriographic findings before bypass operation in the same patient confirming the plethysmographic finding.
[Figs. 4a] and [Fig. 4b] show the plethysmoigraphic observations and arteriographic findings in a patient with popliteal block. The plethysmographic tracings show reduced blood flow in the calf region indicating accurately the level of obstruction.
[Fig. 5] shows the plethysmographic tracing of a patient with femoro-popliteal block before and after administration of low molecular weight dextran. The tracings show that the blood flow to the thigh and leg increased after lomodex was administered intravenously.
[Fig. 6] shows the plethysmographic tracing of a patient with femoro-popliteal block before and after lumbar sympathectomy (on the 3rd, 6th and 10th post-operative days). The tracings show slight increase in the total blood flow on the 3rd and the 6th day but on the 10th day the blood flow is reduced and is comparable to the pre-operative levels.
[Fig. 7] shows a comparison between the tracing obtained on a Doppler ultrasonogram and that obtained on an impedance plethysmogram. The tracings are found to have almost similar waveforms. The larger peak of plethysmograph represents the arterial flow and the smaller peak represents the venous return. Whereas in Doppler ultrasonogram unless a directional Doppler system is employed, simultaneous arterial and venous signals may be summed in the output display.
The impedance plethysmogram has been modified in the present study so as to give a high signal to noise ratio so that even the minimum changes in the blood flow could be detected. In the Doppler ultrasonogram, however, there is a considerable background noise when the blood flow is decreased significantly.
`On line' measurement using impedance plethysmogram has yielded significant in formation in 50 patients suffering from arterial occlusive disease of the upper and lower extremities. This information correlated accurately with the findings on arteriography.
Plethysmographic study was useful in confirming the diagnosis in early cases with arterial block. In 5 patients with minimal arterial block, pedal pulsations were well palpable. In these patients the arterial block was suspected only after plethysmography study and was later confirmed by arteriography. The plethysmographic observations were found to be useful to determine the condition of the distal run off and the status of the collateral circulation which is an essential information for planning the management.
Thus, in patients with proximal arterial block and good distal run off, bypass operation would be the method of choice. As against this, in a patient with poor distal run off, bypass operation should not be advised.
The technique of electrical impedance plethysmography is superior to other noninvasive investigations, such as pulse volume recorder because there is no necessity of temporary occlusion of the venous return as plethysmography yields blood flow velocity rather than pressure variations. The plethysmographic observation, however, could not distinguish between the common iliac, external iliac and very high femoral blocks.
The present study on 50 normal subjects and 50 patients suffering from arterial occlusive disease reveals the following observations:
(1) The impedance plethysmogram provides a noninvasive and simple method for studying arterial circulation and the findings correlate well with radiological observations.
(2) Plethysmography provides accurate information regarding the status of distal run off thus providing a reliable method to select the modality of treatment.
(3) Plethysmography provides a method to assess the extent of the benefit after various methods of treatment such as intravenous lomodex therapy, sympathectomy and bypass operations.
The authors are thankful to Shri C. Ambasankaran, Director, E. & I. Group of B.A.R.C. and Dr. C. K. Deshpande, Dean, Seth G.S. Medical College and K.E.M. Hospital, Bombay for permission to collaborate during this study and to communicate this paper; to Shri V. A. Pethe, Head, Electronics Division, Shri K. R. Gopalkrishnan, Head, Medical Instruments Section and Dr. G. Haridasan, for providing necessary facilities and having encouraged us right through the studies.