Heart disease continues to be the leading cause of death in the modern world. Monitoring pump function of the heart through thermodilution for stroke volume or cardiac output has become the most commonly employed procedure in the clinic, but is both costly and invasive. The low-cost impedance cardiography method can provide a low-cost, noninvasive alternative. With the accuracy of modern systems much improved over earlier versions, the method could find greater use in-hospital, in-transit and during routine cardiac diagnosis in the clinic.
by Dr Hongjun Zhang and Dr John K-J. Li
Stroke volume has long been recognised as an important index for assessing the pump function of cardiology patients. Heart failure and coronary arterial disease patients are generally characterised with low cardiac output. There are several available methods that can be used for measuring stroke volume, such as catheterisation thermodilution, Fick’s method, ultrasound, angiography and magnetic resonance
imaging based techniques. These methods however, require the patients to be immobilised
and are often costly, particularly if repeated measurements are required.
Interest in impedance cardiography for stroke volume measurement, which was developed a few decades ago [1], has recently been renewed. The method provides a low-cost, noninvasive approach to recording of stroke volume. In the past, the method did not become popular in the clinical setting because of questions
concerning its accuracy. With the improved technology now available, recent evaluations of the accuracy of the bio-impedance-based noninvasive method, compared to the established invasive thermodilution method, justify its re-emergence as a viable tool for heart patient diagnosis and evaluation [2-4].
Impedance cardiography principles and cardiac output measurement
The principle behind impedance cardiography hinges on bio-impedance changes due to the movement of conductive blood during various phases of cardiac contraction. This is because the cardiac chambers, in general, function as conductivity cells of varying geometric dimensions. Since the resistivity of cardiac muscle is several times that of blood, the principal impedance changes are due to the movement of blood [5, 6]. Thus, the recorded impedance decreases with filling and increases with ejection. For the recording of the impedance changes
either a two-electrode, bipolar system, or a
four-electrode, tetra-polar system can be used. The variable placement and separation of electrodes are important in determining the accuracy of the measured stroke volume, particularly in the case of neck electrode placements. Figure 1 schematically illustrates the impedance method for the determination of stroke volume in a human subject. The electrodes
are silver-silver chloride electrodes used for electrocardiogram (ECG) measurement.
The incremental amount of resistance changes due to an increase of entering blood volume. Assuming that impedance can be substituted for the resistance, we then have the familiar equation of blood volume change, ∆Vb, as a function of impedance change, ∆Z, in terms of the resistivity of blood, ρb, the basal impedance Zo, and the distance between the measuring electrodes, L:
(1) 
The entering blood volume increases conductivity and decreases impedance, hence the negative sign.
The above equation can be modified for impedance cardiographic recording of stroke volume as:
(2) 
where T is the ejection time and dZ/dtmax is the maximum value of the first derivative of impedance. This equation is the formula that is applied most widely for the estimation of stroke volume from impedance cardiography.
A constant current source injects a 20KHz 4mA peak-peak sine wave current into the body through the outer two electrodes, exciting the cardiac impedance (modulating process). At this frequency (or up to 100KHz), tissue is not excitable, except possibly at very high current levels, so the risk of physiological effects is minimised. The instrument amplifier picks up the modulated voltage signal through the inner two electrodes; this signal is then demodulated and filtered to obtain Z, Z0 and ∆Z. A standard lead ECG is also normally used to synchronise the measurement of ejection time T.
Typical data derived from impedance cardiography are illustrated in Figure 2, which shows three traces, namely impedance, its first derivative and the ECG obtained in a normal human subject during steady state. Application of the equations, with L=30cm, L/Zo=1.22 cm/ohm and T=0.21 sec, gives the calculated stroke volume as 74 mL. This translates to a cardiac output of about 5 litres/min when the heart rate is about 70 beats/min. With L=25cm, L/Zo=1.26 cm/ohm and T=0.22 sec, and at different electrode placement positions, the calculated stroke volume is still a reasonable 66 mL and the cardiac output is 4.6 litres/min.
Discussion and conclusion
Impedance cardiograhy is mostly employed to measure stroke volume and hence cardiac output noninvasively. The pumping function of the heart can adjust rapidly to perfusion and metabolic demand [7]. For this reason, the ability to obtain transient changes in impedance noninvasively can be particularly desirable in monitoring dynamic function of the heart, as in the case of arm exercise [8], treadmill exercise [9] or dynamic postural changes [3]. The principle of impedance cardiography can be used for continuous, noninvasive monitoring of cardiac function in terms of beat-to-beat stroke volume changes. Since the sensor can be worn on the body, the technique can be incorporated for use in a multi-functional body sensor network system to effectively monitor rapidly changing physiological and pathological conditions in the clinical setting.
References
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6. Newman G, Callister R. The non-invasive assessment of stroke volume and cardiac output by impedance cardiography: a review. Aviat Space Environ Med 1999; 70: 780-789.
7. Li JK-J. The Arterial Circulation. Physical Principles and Clinical Applications. Humana Press, Totowa, NJ. 2000.
8. Miles DS, Sawka MN, Wilde SW, Doerr BM, Frey MAB, Glasser RM. Estimation of cardiac output by electrical impedance during arm exercise in women. J Appl Physiol 1981; 51:1488-11492.
9. Zhang Y, Qu M, Webster JG, Tompkins W, Ward BA, Bassett DR. Cardiac output monitoring by impedance cardiography during tradmill exercise. IEEE Trans Biomed Eng 1986; 33: 1037-1042.
The authors
Hongjun Zhang, Ph.D.
and John K-J. Li, Ph.D.
Cardiovascular Engineering Lab
Department of Biomedical engineering
Rutgers University
599 Taylor Road
Piscataway, NJ 08854, USA.
Tel. +1 732 445-4500x6305
Fax +1 732 445-3753
e-mails:
hongjun@rci.rutgers.edu
johnkjli@rci.rutgers.edu
