Calibrated impedance plethysmograph.

The accuracy of quantitative pulse-volume measurement with a calibrated electrical impedance plethysmograph was determined on a laboratory limb-segment model. Changes in electrical impedance detected via a tetraprolar electode configuration were related to pulse-volume changes by the parallel-impedance model described by Nyboer et al. (The Impedance Plethysmograph: An Electrical Volume Recorder. Natl. Res. Council, Comm. Aviation Med. 149, 1943). The instrument employed in this study calculated pulse volume assuming two standard conditions: distance between voltage electrodes, l, is 15 cm, and resistivitiy of blood, rho b, is 150 omega x cm. The effects of varying tissue and blood resistivity, pulse frequency, and electrode distance were investigated. Measurements under three ionically distinct conditions gave an overall accuracy of 96.6% with correlation coefficients of 0.99 for each condition. Measurement accuracy was maintained with a pulse-frequency variation in the range of 15-150 pulses/min and with application of correction factors for electrode distance and blood resistivity other than the standard values.

The calibrated impendance plethysmograph: laboratory and clinical studies.

By careful experimental studies a calibrated impedance plethysmograph has been shown to be highly accurate (standard error = 3.8%). Its ability to measure the rate of pulsatile volume change has been extremely useful in the diagnosis of arterial insufficiency of the lower extremities. By helping in the objective definition and documentation of arterial perfusion, this instrument can greatly improve our selectivity of patients for angiography and subsequent revascularization.

Limb salvage and selectivity.

In a group of properly prepared poor risk patients with advanced atherosclerosis of both the aortoiliac and femoropopliteal segments, a combined procedure, extra-anatomic bypass with an extended profundoplasty, was used for successful salvage of their ischemic--mean calf pressure=27 millimeters of mercury--limbs. Prediction of success versus failure could be determined on neither clinical nor angiographic-anatomic grounds but was possible using two simple measurements: a large aortoiliac differential pressure--deltaP--between inflow pressure obtained from a standard arm cuff and outflow pressure measured directly from the femoral artery, 89.3 millimeters of mercury versus 33.2 millimeters of mercury, p less than 0.001, and a potentially adequate outflow bed as determined by the percentage drop in outflow [(deltaPo/Po) per cent] upon papaverine injection, 36.3 versus 11.3 per cent, p less than 0.001. These values became an important part of the selection scheme developed in the approach to 24 ischemic limbs.

Patient and operation selectivity. The peripheral vascular laboratory.

Careful use of the data from the vascular laboratory has resulted in better patient and operation selectivity in peripheral arterial disease of the lower extremities. Using the combination of segmental pressure measured with an ultrasonic Doppler and flow as approximated by the measurement of the rate of pulsatile volume change with a calibrated plethysmograph, objective criteria were established for arterial insufficiency. Using a two cuff technique, resistance indices were established to distinguish femoropopliteal from aortoiliac disease. The ratio of angiograms to femoropopliteal and aortoiliac operations has been reduced from an earlier value of almost 3/1 to the present value of approximately 1/1.