Diagnosis of peripheral arterial occlusive diseases using impedance plethysmography.
Impedance plethysmographic observations have been compared with arteriographic findings in 216 patients with peripheral arterial occlusive diseases. Impedance plethysmographic diagnosis in these patients was obtained by Parulkar's method without apriori knowledge of arteriographic diagnosis. But for minor discrepancy in the anatomical location of the block in few patients, impedance plethysmographic observations correlated very well with arteriographic findings. Impedance plethysmographic diagnosis was found to be correct in 312, wrong in 53, false negative in 8 limbs respectively, yielding a sensitivity of 97.5% and specificity of 98.1% of this technique.
Conventionally venous occlusion principle, is employed for the assessment of peripheral arterial blood flow by impedance plethysmography (IPG). This method, however, does not yield sufficient information to the clinicians for the management of peripheral arterial occlusive disease (PAOD). For instance, it does not reveal approximate anatomical location of the block, status of collateral circulation and patency of distal arteries. Furthermore, the estimated values of blood flow by this method are often misleading in oedematous limbs and also patient cooperation is necessary for recording satisfactory plethysmograms. These limitations allowed the supercession of volume displacement plethysmography over IPG technique.
Estimation of peripheral blood flow as per Parulkar et al, has been reported to yield approximate anatomical location of the block, status of collateral circulation and distal arterial runoff,,. We have used this method for investigating more than 2000 patients suspected of PAOD at Non-invasive Vascular Laboratory, King Edward Memorial Hospital, Mumbai during the past four years. Two hundred and sixteen of these patients have subsequently undergone arteriography. In this paper, we present the comparison between impedance plethysmographic observations and arteriographic findings in these 216 patients.
Two hundred and sixteen patients (188 male, 28 female) in the age group of 18 to 80 years were subjected to impedance plethysmographic investigation at Non-invasive Vascular Laboratory. King Edward Memorial Hospital, Mumbai. These patients presented to various surgical OPDs of King Edward Memorial Hospital with symptoms suggestive of peripheral arterial occlusive diseases. [Table - 1] summarises the clinical profile in these patients.
Impedance plethysmographic data was (4) recorded from neck, thorax and various locations in the upper and lower extremities in all the patients as described by Bhuta et al and values of blood flow index (BFI) and differential pulse arrival time (DPAT) were calculated. IPG diagnosis was made from this data as follows:
1) Value of an IPG parameter was said to be decreased (or increased) if it was one standard deviation less (or more) than the control value 2 or if it was 20% less (or more) than the corresponding value in the opposite extremity.
2) Bilateral decrease in BFI either at upper arm location or at thigh location indicated an aortic occlusion and the diagnostic criteria described by Deshpande et al 2 was used to arrive at IPG diagnosis.
3) Decrease in the value of BFI with increase in the value of DPAT at and below a location in an upper or lower extremity suggested an arterial occlusion in the proximity of that location provided there was no singnificant increase in the value of BM on elevation of the limb.
4) Decrease in the value of BFI as well as differential pulse arrival time (DPAT) in a limb indicated narrowing of the arteries due to generalised atherosclerosis.
5) Collateral circulation was said to be good, moderate or fair in a diseased limb provided the value of BFI at distal location was more than 50%, between 35% and 50% and between 20% and 35% of the control value respectively.
6) Normal values of DPAT in distal segments of a diseased extremity represented patent distal arteries.
7) Normal values of BFI in a lower extremity in elevated position ruled out possibility of an arterial occlusion irrespective of the value of BFI in supine position.
8) Arteriography in all the patients was carried out at the Department of Radiology, King Edward Memorial Hospital using standard procedure.
Impedance plethysmographic data is illustrrated in [Table - 2]. Arteriographic findings of 216 patients revealed unilateral PAOD in upper and lower extremities in 17 and 86 patients respectively and bilateral PAOD in upper and lower extremities in 7 and 105 patients respectively. Arteriographic observations were normal in one patient.
The IPG observations were compared with artriographic findings in 207 out of 216 patients subjected to this study. Out of remaining 9 patients angiography revealed arterio-venous malformation in 5 patients and thoracic outlet syndrome in 4 patients, which are desecribed elsewhere in this volume. 5,8 As can be seen from the Table 2 there is very good agreement between IPG diagnosis and angiographic diagnosis in most of the patients. One patient with non-healing ulcer in the left foot having decreased BFI at calf and ankle level in the left leg, however had normal femoral angiogram and contributed one false positive diagnosis by IPG. Atherosclerotic narrowing upto 60% in the arteries has not been detected by IPG and has contributed 7 false negative diagnoses to this study. One patient with kink in the left subclavian artery causing turbulent flow recorded normal impedance plethysmogram and added one more false negative diagnosis by IPG. Minor discrepancy about the anatomical location of the occlusion has been observed in 2 patients, however the IPG diagnosis in these cases has been taken as true positive.
[Figure - 1] and [Figure - 2] illustrate the IPG waveforms and the aortogram respectively in a patient (RKP-30-M) with femoral artery block in the left leg. [Figure - 3], [Figure - 4], and [Figure - 5] and 146 F similarly illustrate the IPG data in patients with iliac artery block (DRR-52-M), vasculitis of calf vessels (SJB30-M) and axillary artery block (SPS-22-F) respectively. Good correlation between IPG data and arteriographic findings is evident from these illustrations.
Decrease in BFI in left upper extremity with increase in DPAT at elbow shows axillary artery block. Since BFI at wrist is equal to that at elbow, good distal arterial run-off is indicated. Aortogram (VR 2965 dated 2.7.87) in this patient shows hypoplastic left subclavian and proximal axillary arteries. Thrombotic occlusion of axillary artery distal to left circumflex humeral artery is seen with reformation of brachial artery via collaterals with good distal arterial run-off.
Comparison of IPG data with arteriograms in 374 limbs in this study has shown IPG diagnosis to be true positive in 312 limbs, true negative in 53 limbs, false positive in 1 limb and false negative in 8 limbs. The sensitivity and specificity of IPG technique in the diagnosis of PAOD can therefore be given as follows.
True positive 312
Sensitivity = ---------------------------------------- ----- = 97.5%.
True positive + false negative 320
True negative 53
Specificity = ---------------------------------------- ----- = 98.1%.
True negative + false positive 54
The only limitation of this technique is its inability to give exact anatomical location of the block. This limitation makes pre-operative arteriography essential in patients undergoing surgical correction.
The authors are thankful to Indian Council of Medical Research, New Delhi and Research Society, Seth GS Medical College & King Edward Memorial Hospital, Mumbai, for providing financial aid. The authors are thankful to Shri MK Gupta, Assoc. Director, E & I Group, BARC, Shri BR Bairi, Head, Electronics Division, BARC and Shri KR Gopalakrishnan, Head, Nuclear Instruments Section, BARC for encouraging right through this study. The authors are also thankful to Shri SP Agrawal, Scientific Officer, DRP, BARC and Shri Hari Singh, Scientific Officer, NUMAC, BARC for their valuable help in the analysis of the results.