Impedance cardiography for cardiac output estimation in pacemaker patients: review of the literature.

Impedance cardiography permits noninvasive beat-to-beat determination of cardiac output, the product of the amplitude of the first derivative of thoracic impedance signal (dZ/dt), the ventricular ejection time, and heart rate corrected by the distance between the measuring electrodes. Its use is based on: (1) the dZ/dt signal that originates from the upper thorax; (2) the ventricular ejection period measured by the dZ/dt curve that occurs between the opening and closing of the aortic valve; (3) the dZ/dt curve is similar in morphology and timing to the aortic flow curve measured by an electromagnetic flowmeter with a significant linear correlation (r = 0.9) between dZ/dt and peak aortic flow; (4) similarity of the linear correlation between stroke volume, determined by the flowmeter and the impedance signal; and (5) significant reduction of the dZ/dt signal by 90% follows simultaneous occlusion of the aorta and the pulmonary artery. The rapid systolic portion of the impedance signal occurs only when blood is ejected into the aorta and is independent of right ventricular ejection. Most studies comparing impedance cardiography results with standard cardiac output determination have shown a correlation of 0.7-0.9. While the accuracy of impedance cardiography remains controversial and can be affected by the inherent limitations of the technique and by low cardiac output, intracardiac shunts, and valvular regurgitation, the high reproducibility of the method is established and may be comparable or superior to other commonly used techniques. When accurate determination of cardiac output is crucial, impedance cardiography may be used in conjunction with a standard technique to establish a baseline reference, thereby permitting further analysis. If only the trend need be followed, the high reproducibility of impedance cardiography measurements allows small changes in cardiac output to be detected on a frequent and ongoing basis. The ease and precision of this technique warrants its more widespread use in the assessment of pacemaker patients. Further use of this promising technique will allow a better definition of its role in the assessment of a wide range of cardiac patients.