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| Year : 1986 | Volume
: 32
| Issue : 1 | Page : 1-3 |
Impedance plethysmography for screening vascular disorders.
Jindal GD
How to cite this article:
Jindal G D. Impedance plethysmography for screening vascular disorders. J Postgrad Med 1986;32:1-3 |
Impedance plethysmography (IPG), introduced by Nyboer,[8] is the indirect assessment of blood volume changes in any part of the body by measurement of its electrical impedance. Work carried out in our laboratory using this technique has been documented in several publications.[1],[2],[4],[5],[6],[7],[9],[10] Since blood is a good conductor of electricity, blood volume changes in any part of the body are reflected inversely in the electrical impedance of the body segment. A continuous or uniform flow of blood does not produce any significant change in the electrical impedance of the body segment. Pulsatile blood volume changes, however, are recorded as pulsatile electrical impedance changes as a function of time. Gross electrical impedance of the body segment (basal impedance Zo which is displayed on the panel), changes in the impedance as a function of time ? Z (t) and the rate of change of imedance (dZ/dt) are usually employed in this technique for the assessment of central and peripheral blood flow.
IPG waveform (dZ/dt) recorded from an extremity comprises a systolic wave (C-wave) and a diastolic wave (0-wave) [Fig.1]. C-wave occurs after the R wave of electrocardiogram (FCG), with the time interval RC linearly related to the distance of the body segment from the aortic root. It represents the arterial flow in the body segment. The O-point (the crest of the O-wave) is fixed with respect to R-point, irrespective of the site of the recording, The normalised amplitude of the O-wave (amplitude of 0wave/amplitude of the C-wave) has been observed to be dependent on the posture of the extremities. In normal subjects, it decreases significantly in the elevated position (0.25 ± 0.06) as compared to that in the supine position (0.43 ± 0.08). This decrease can probably be attributed to the change in the venous flow pattern from semi-pulsatile to semi-continuous as the venous return is assisted by gravity in the elevated position.
For the diagnosis of arterial occlusive disease (AOD), blood flow index (blood flow in ml per 1000 ml of body tissue per cardiac cycle) (BFI) and arterial pulse wave velocity (APWV) are assessed from the IPG wave forms recorded at the neck and various locations in the upper or lower extremities as follows: BFI = 500 x Ac x Zo, where Ac is the area under the C wave and APWV= (Ld-Lp)/(RCd - RCp), where Ld and Lp are the distances of the distal and proximal recording locations in meters and RCd and RCp are the corresponding RC time intervals in seconds.
A significant reduction in the BFI (normal range 1.50 ± 0.31) at and below a particular location indicates the presence of arterial occlusion at that location, APWV (normal range 7.53 ± 1.49) is also reduced at the site of occlusion and can be normal or reduced below the site of occlusion, depending upon the status of the distal arterial runoff. In our laboratory, the sensitivity and the specificity of this technique for the diagnosis of AOD have been found to be 96% and 98% respectively, in comparision to arteriography.
Deep vein thrombosis (DVT) of the lower extremities causes significant decrease (> 25%) in the basal impedance at the ankle location and in BFI at the calf and ankle locations, and causes significant increase in the normalised amplitude of 0-wave in supine as well as elevated positions. However, more than 8% increase in Zo and/or more than 50% increase in BFI at knee and ankle locations on elevation of the legs represents incompetence of the superficial venous system without involvement of deep veins. The sensitivity and specificity of this method for the diagnosis of DVT have been estimated to be 75% and 80% respectively in comparision to venography.
Occlusive impedance phlebography (OIP), a record of the changes in the electrical impedance of the calf region following a temporary occlusion of the proximal veins, gives an estimate of venous capacitance (VC) and maximum venous outflow (MVO) in the lower extremities. The sensitivity of this method is 96%, 50% and 38% for the diagnosis of acute DVT of the proximal, popliteal and distal veins respectively. The sensitivity of the same for the diagnosis of chronic DVT has been found to be low due to development of collateral circulation or recanalisation. However, IPG and OIP performed together are likely to give better diagnostic yield for deep venous thrombosis.
Impedance plethysmography is superior to other plethysmographic methods (volume displacement plethysmograph, strain gauge plethysmograph and photo plethysmograph), as it is directly related to the electrical property of the blood.[3] IPG also has wider clinical applications than these other plethysmographic methods.
Doppler ultrasonography, a popularly used method for the diagnosis of, AOD, has the advantage of locating the block anatomically but is insensitive to the deeper blood vessels. Three fold cost of the Doppler system and requirement of a skilled operator gives an edge to impedance technique over the Doppler.
Most of the diagnostic applications IPG mentioned here have been brought out by the post graduate students (Mrs. Padmashree Ashok, Mrs. Smita Nerurkar, Miss Sadhana Suraokar and Miss Kavita Masand) of Seth G.S. Medical College and K.E.M. Hospital Bombay under the guidance of Dr. M. D. Kelkar and Dr. G. B. Parulkar, in collaboration with Electronics Division of Bhabha Atomic Research Centre.
| 1. | Bharadwaj, Padmashree R., Jindal G. D., Dharani, J. B. and Parulkar, G. B.: Impedance cardiography in valvular diseases of the heart. Ind. Heart J., 35: 46-49, 1983.  |
| 2. | Dewoolkar, Smita, D., Jindal, G. D., Shenoy, S. G. and Parulkar, G. B.: Impedence plethysmography for the study of venous circulation. Ind. Heart J., 35: 9193, 1983. |
| 3. | Jaffrin, M. Y. and VanHoutte, C.: Quantitative interpretation of arterial impedance plethysmographic signals. Med. and Biol. Eng. Comput., 17: 2-10, 1979. |
| 4. | Jindal, G. D., Dharani, J. B. and Parulkar, G. B.: Vector impedance cardiography. Ind. Heart J., 34: 232-235, 1982. |
| 5. | Jindal, G. D., Kelkar, M. D., Bharadwaj, Padmashree, R., Dewoolkar Smita, D., Suraokar, Sadhana, P . , Babu, J. P. and Parulkar, G. B.: Editorial: Noninvasive diagnosis of aortic and arterial occlusive disease using on-line impedance. Clinic (Eur. J. Clin. and Exper. Med. and Surg.), 2: 3-14, 1984. |
| 6. | Jindal, G. D., Sanghvi, S. H., Haridasan, G., Reddy, Y. K., Gopalakrishnan, K. R. and Parulkar, G. B.: Impedance plethysmography-Bioelectric principles and historical review. Ind. Heart J., 34: 183-186, 1982. |
| 7. | Kelkar, M. D., Bharadwaj, Padmashree R., Jindal, G. D. and Parulkar, G. B.: Impedance plethysmography in arterial occlusive disease; Correlation with arteriography. Ind. Heart J., 35: 94-97, 1983. |
| 8. | Nyboor, J.: Regional pulse volume and perfusion flow measurements; Electrical impedance plethysmography. Arch. intern. Med., 105: 264-276, 1960. |
| 9. | Parulkar, G. B., Jindal, G. D., Bharadwaj, Padmashree, R., Suraokar, Sadhana B. and Dharani, J. B.: Impedance cardiography in mitral valve diseases. Ind. Heart J., 37: 37-42, 1985. |
| 10. | Parulkar, G. B., Padmashree, R. B., Bapat, R. D., Rege, R. V., Bhagtani, K. C. and Jindal, G. D.: A new electrical impedance plethysmogram: Observations in peripheral arterial occlusive diseases. J. Postgrad. Med., 27: 66-72, 1981. |
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