Transthoracic impedance and impedance cardiography analysis

Electrical Bioimpedance (EBI) through transthoracic measurements offers a non-invasive manner to measure hemodynamic values. In the last decades, advances in the field of electrodes, electronic hardware and the development of textrodes have paved the ways for smaller, easier to use equipment for EBI measurements.

Impedance cardiography is the study of the bioimpedance of the human body but limiting measurement area to the chest cavity. The changes of the bioimpedance signal can be monitored to estimate cardiac parameters and assess the cardiac cycle. The monitoring can be performed using non-invasive measuring methods with skin electrodes.

Bioimpedance Plethysmography is a method of studying tissue volume changes in a living body by measuring the changes of electrical impedance at the body surface. Impedance Cardiography (ICG) is the Impedance Plethysmography measurement limited to the chest/thorax region.

Electrocardiography or EKG is the process of recording all the electric excitation phenomena of the heart. The EKG records with a good level of detail the electrical excitation in different parts of the heart muscle: which include the sinus and AV nodes, the atrial and ventricular muscles and nerve fibers in the ventricular muscle; it is crucial in the diagnosing of several heart diseases. As will be seen later on, the EKG signal shows a good relation with the ICG signal as some of the waves of the EKG are correlated with waves in the ICG measurement.

As with EKG, ICG measurements have the potential of being done in a non-invasive way. Several authors have tried to correlate invasive methods like thermo-dilution with ICG with skin electrodes with different degrees of success. Most of the authors agree that ICG requires more methodological investigation but are good enough to estimate changes of the Stroke Volume.

The impedance cardiography signal is obtained by injecting a current and then sensing the voltage in the thorax region to estimate the impedance using the Ohms law equation.

The typically used current to measure the impedance is a sinusoidal signal with a constant frequency between 20 and 150 kHz with a constant amplitude of 0.5-5 mA. The impedance usually consists of a Real component, with values between 15Ohm to 30 Ohm, and an often negligible and not used imaginary component.The changes of impedance due to the changes of fluid in the chest area are typically between 100-400 mOhm which results in a change of 0.1-0.4 mV in the signal received when applying 1mA to the patient.