Connector sensors for permittivity-based thrombus monitoring in extracorporeal life support

Bioimpedance Analysis as Early Predictor for Clot Formation Inside a Blood-Perfused Test Chamber: Proof of Concept Using an In Vitro Test-Circuit

Clot formation inside a membrane oxygenator (MO) due to blood-to-foreign surface interaction represents a frequent complication during extracorporeal membrane oxygenation. Since current standard monitoring methods of coagulation status inside the MO fail to detect clot formation at an early stage, reliable sensors for early clot detection are in demand to reduce associated complications and adverse events. Bioimpedance analysis offers a monitoring concept by integrating sensor fibers into the MO. Herein, the feasibility of clot detection via bioimpedance analysis is evaluated. A custom-made test chamber with integrated titanium fibers acting as sensors was perfused with heparinized human whole blood in an in vitro test circuit until clot formation occurred. The clot detection capability of bioimpedance analysis was directly compared to the pressure difference across the test chamber (ΔP-TC), analogous to the measurement across MOs (ΔP-MO), the clinical gold standard for clot detection. We found that bioimpedance measurement increased significantly 8 min prior to a significant increase in ΔP-TC, indicating fulminant clot formation. Experiments without clot formation resulted in a lack of increase in bioimpedance or ΔP-TC. This study shows that clot detection via bioimpedance analysis under flow conditions in a blood-perfused test chamber is generally feasible, thus paving the way for further investigation.

Simultaneous electrical online estimation of changes in blood hematocrit and temperature in cardiopulmonary bypass

Two equations have been developed from multi-frequency measurements of blood impedance Z_b for a simultaneous electrical online estimation of changes in blood hematocrit ΔH [%] and temperatures ΔT [K] in cardiopulmonary bypass (CPB). Z_b of fixed blood volumes at varying H and T were measured by an impedance analyzer and changes in blood conductivity σ_b and relative permittivity ε_b computed. Correlation analysis were based on changes in σ_b with H or T at f = 1 MHz while H and T equations were developed by correlating changes in ε_b with H and T at dual frequencies of f = 1 MHz and f = 10 MHz which best capture blood plasma Z_p and red blood cell cytoplasm Z_cyt impedances respectively. Results show high correlations between σ_b and H (R² = 0.987) or σ_b and T (R² = 0.9959) indicating dependence of the electrical parameters of blood on its H and T. Based on computed ε_b, changes in blood hematocrit ΔH and temperature ΔT at a given time t are estimated as ΔH(t) = 1.7298Δε_b (f = 1 MHz) - 1.0669Δε_b (f = 10 MHz) and ΔT(t) = -2.186Δε_b (f = 1 MHz) + 2.13Δε_b (f = 10 MHz). When applied to a CPB during a canine mitral valve plasty, ΔH and ΔT had correlations of R² = 0.9992 and R² = 0.966 against H and T respectively as measured by conventional devices.