[Design of an impedance plethysmograph for reactive hyperemia evaluation in the forearm]

Impedance pletismography is based on the evaluation of the voltage change that occur as a consequence of blood flow variations in a particular tissue section. Current applied for the procedure should be alternating, weak and of high frequency to avoid electrically excitable cells stimulation, such as muscles and nerves. Blood volume changes can be measured by this method and has been applied in the diagnosis of deep venous thrombosis of limbs. An impedance meter was designed and built to be used in the forearm of patients. It consists of a voltage stabilizer TPS 76150 with simultaneous ECG recording (DII lead). Injected signal had 50 KHz (Wien's bridge) and registered through two operational amplifiers LM 3080 (1 mA). Signal was demodulated and amplified. Electrodes used were made of silver strips with conductance improved by conductivity gel, fixed by elastic rubber strips. Circuit wiring and equisition software was developed at the Bioengineering Department of Simón Bolívar University. Fifteen healthy subjects, age range 18-30 years old, were submitted to noninvasive forearm blood flow evaluation with the already described electronic device. Brachial artery occlusions were made with a cuff at subdiastolic, supradiastolic and suprasystolic blood pressures, for 60 and 90 seconds and then this occlusion was abruptly released. The curve was displayed and recorded in a portable PC (laptop). During suprasystolic occlusion the impedance observed curve showed a progressive increased until stabilization. When occlusion was released a sudden peak appeared which corresponds to reactive hyperemia of the forearm. This peak has been associated to the secretion of endothelial vasodilatory substances. In conclusion, this device is suitable to be used in clinical settings for the evaluation of reactive hyperemia and potentially useful in diagnosis of deep veins occlusions.