Impedance Cardiograph Measurement

Impedance cardiography is a safe, noninvasive way to measure and monitor the cardiac cycle.  It uses electrical impedance (the resistance of an alternating current circuit) to measure changes in blood volume.  Changes in blood volume are directly proportional to changes in blood pressure.

The impedance cardiograph signal is aquired via 8 electrodes, 4 on the neck and 4 on the thorax, as indicated in figure 1 above.  The resulting curves are shown in figure 2 below.  The top curve is the standard and familiar ECG curve. 

The middle curve, the impedance curve (IPG) is the signal picked up from the electrodes for this impedance measurement.  Note that it looks like an arterial pressure curve because this volume measurement is, in fact, proportional to the pressure.  Additionally, the impedance signal is measured over a larger segment (from the aortic valve down to the beginning of the abdominal aorta), which enables the stoke volume calculation that is not possible with standard pressure measurements.

The final curve in figure 2, the ICG curve, is most important for this discussion.  It is the mathematical first derivative of the IMP curve.  It can produce better results and more easily than the IMP.

Several studies have shown that the main signal source during systole is the blood volume change in the thoracic aorta. Another source is the vena cava, which affects the signal mainly during diastole. The heart itself has very little influence, possibly due to the electrical insulation of the pericardium.

Different points of the ICG, labeled with the letters in figure 2, indicate different parts of the cardiac cycle.  The B-Point, which marks the opening of the aortic valve, and the X-Point, which stands for its closing, are important.  The Y-Point marks the closing of the pulmonary valve, and O-Point indicates the time when the mitral valve is open to its maximum point during diastole.

The B-, C- and X-Points are used to calculate stroke volume and systolic time intervals.  The other points and waves have advantages in the visual curve shape analysis for detecting some cardiological diseases (such as valve stenosis, valve insufficiencies and increased central venous pressure due to cardiac failure).

The application of impedance cardiography is very widespread:

• Cardiological diagnosis, screening and monitoring

  Because impedance cardiography is a noninvasive technique and easy to use, it can by used by any doctor - not only for cardiologists - as a screening test for cardiac diseases.

• Pacemaker adjustments

  Various parameters (AV-delay, maximum pacing rate) of pacemakers require individual adjustment. Currently, many physicians optimize these parameters by trial and error--a time consuming process that usually does not reach the true optimum. The impedance cardiography enables easily optimized pacemaker settings by measuring cardiac output and examining the changes in curve shape.

• Long term monitoring in intensive care units

Some studies show that the monitoring of cardiac output by invasive catheter is not only expensive, but it also has a high infection risk for the patient. Impedance cardiography can often replace the catheter and is a more economical, more safe, and more comfortable way.

• Monitoring during surgery and anesthesia
• Stress test
• Pharmacological and physiological studies