Impedance cardiography is a noninvasive assessment strategy that can be used to estimate stroke volume and cardiac output (Mohapatru, 1981). Basically, the process involves transmitting a small constant electrical current through the thorax from the neck to the abdomen using a series of electrodes that encircle the body.
Resistance to this electrical current is influenced by the volume of blood being ejected during each cardiac cycle. When a large amount of blood is ejected into the circulatory system, resistance to the electrical current temporarily decreases, as blood is a good conductor of electricity.
The impedance cardiograph then detects these alterations in electrical resistance, and through a formula initially proposed by Kubicek et al. (1966), the measure of resistance can be converted into an index of stroke volume and consequently cardiac output.
There are several advantages to using impedance cardiography for obtaining estimates of hemodynamic functioning. In addition to its obvious advantage of being noninvasive, measures of stroke volume are obtained continuously, so that immediate acute responses to various environmental stimuli (stressors) can be determined. Furthermore, the impedance method permits concurrent assessment of other measures of interest, including indices of cardiac contractility and systolic time intervals, like pre-ejection period, that have proven useful in assessing autonomic nervous system influences upon the heart.
Several dozen studies have compared measures of stroke volume and cardiac output obtained via impedance cardiography with measures of these same parameters by means of a variety of invasive strategies (see Fuller, 1992; Mohapatru, 1981). Correlations between impedance-derived and invasive estimates of these hemodynamic parameters are typically very high, with the vast majority above .80, providing evidence that both are measuring the same hemodynamic process.
But although the two are highly intercorrelated, there is evidence that impedance cardiography tends to overestimate stroke volume consistently, occasionally more than 10 percent, leading to its infrequent adoption and use in hospital settings where more precise measures of hemodynamic functioning are needed (Sherwood, 1993). Because investigations of the transition from early-stage elevated blood pressure associated with increased cardiac output to sustained high blood pressures maintained by increased total peripheral resistance rely upon accurate measurement of absolute levels of hemodynamic functioning, they have typically used invasive methods of assessing hemodynamic functioning (for example, Lund-Johansen, 1991).
However, despite these limitations associated with impedance cardiography???derived absolute levels of stroke volume and cardiac output, it is generally recognized that relative changes in stroke volume and cardiac output can be determined quite accurately using the impedance cardiograph (Sherwood et al., 1990).
Therefore, impedance cardiography has been shown to be very useful in determining the hemodynamic processes that are responsible for specific increases or decreases in blood pressure in response to environmental events. To extend its usefulness beyond the scope of the clinic or laboratory setting, an ambulatory version of the impedance cardiograph has been devised and validated to assist researchers in determining the hemodynamic foundations associated with blood pressure elevations that occur during daily life (Nakonezny et al., 2001).