Subcutaneous bioimpedance recording: Assessment of a method for hemodynamic monitoring by implanted devices

Subcutaneous Bioimpedance. Background: Diagnostic evaluation of patients with suspected symptomatic arrhythmias is limited by inability to assess the hemodynamic impact of a detected rhythm. Objective: To address this limitation, we utilized closely spaced subcutaneous electrodes, small enough to incorporate within an implantable monitor, to detect blood flow-induced pectoral muscle bioimpedance (Z) changes in a swine model of hemorrhage-induced hypotension. Methods: In seven anesthetized and ventilated adult pigs, small ring electrodes (current electrodes 5 cm apart; voltage electrodes 3.5 cm apart) were positioned on the left pectoral muscle. Z signals (Biopac® system) and invasive arterial blood pressures were recorded. Hypotension was induced by hemorrhage (50% blood volume reduction). Mean arterial pressure (MAP) and pulse pressure (PP) with corresponding pulse Z (ΔZ) and base Z (Z_o) were measured. A longitudinal mixed model with a first-order autoregressive error structure was used to test for associations (change in ΔZ vs change in MAP and change in ΔZ vs change in PP) taking into account within pig correlation. Results: During bleeding-induced hypotension, Z_o increased. Changes of ΔZ correlated with both a change in MAP (coefficient = 1.17, P < 0.0001) and change in PP (coefficient = 0.98, P < 0.0001). A change in ΔZ of 1-2 orders of magnitude corresponded to an approximate 40-70% drop in MAP and PP in a porcine model in which the baseline MAP was 69-70 mmHg. Conclusion: Our findings suggest that closely spaced subcutaneous electrodes identify changes in local tissue/vascular bioimpedance that correlate well with direct invasive measures of induced hypotension in a porcine model.