RESUMO
UNLABELLED: The transthoracic impedance (T) and its variations may be estimated through the measurement of the electrical impedance between the can and the right ventricular coil of a defibrillation lead. This method may allow the monitoring of fluid overload before a heart failure attack. Aim of this study was to validate in vitro a method to calculate T in case of a standard bipolar pacing lead, by performing 3 measurements: standard unipolar impedance from the tip (Zuni-tip); unipolar impedance from the ring (Zuni-ring); standard bipolar impedance (Zbip). The formula we used is derived from the standard equivalent circuit of a pacing system: [Formula: see text] T represents the tissue impedance between the can and the electrodes of the lead. To validate the method we used a saline solution and 3 different pacing leads manufactured by Vitatron (Vitatron BV, Arnhem, The Netherlands): Impulse II (high impedance lead), Crystalline ActFix (screw-in lead), Brilliant S+ (VDD single-lead). The measured values of the saline solution impedance were compared to the values calculated through the formula. RESULTS: The calculated impedance of the solution, evaluated through the proposed formula, is reliable independently of the electrode used and highly correlated to the corresponding measured values (R>0.9). CONCLUSION: Tissue impedance may be calculated from standard unipolar and bipolar impedance measurements with a standard bipolar pacing lead.