Modalities in Cardiac Diagnostics
Electrocardiography, although the most widespread cardiac functional examination and the most known electrophysiological test, does not pretend to be the only examination in cardiology. The variety of modalities reflects almost the full range of diagnostic methods applied in medicine, and consequently the use of medical data varying in their origin, nature, and volume is common in the description of the heart. Particular modalities are based on different phenomena triggering the action, directly included in the action, resulting from the action or accompanying the action of the heart. Therefore, these techniques are used as complementary depending on patient status and history.
Electrocardiography is focused on the generation and conduction of electrical activity stimulating the heart muscle contraction and accompanying the subsequent refraction. All sources of the heart action and the stimulus conduction pathways are represented in the ECG. Conduction conditions are also reflected in the electrical relations in the cellular level (drugs or ischemiae) or by the muscle tissue alterations resulting from the infarction. The principal advantages of the ECG are its very low price, frequency of use, feasibility in home-care conditions by untrained personnel (or even the patient himself), low invasiveness, and high informative value.
Ultrasonography (USG) reveals a static image of moving tissues, and as a cardiology-oriented application it is useful in monitoring muscle contraction and valve functions. The average displacement of the muscle tissue yields a rough estimate of the stroke volume and the volume of blood flow per heartbeat. The impaired mobility is interpreted as heart tissue damage determined with a higher precision than in the ECG. The time-motion (TM) presentation in the USG is used to determine the heart rate, amplitude, and velocity of the moving valves. Doppler ultra-sonic examinations provide a direct insight into the blood flow and reveal anatomic defects or injury-caused leakages. The technology allows precise measurements of blood volume temporal relations and the spatial distribution of the flow in the vessel section.
Unfortunately, professional training is necessary for correct measurements, the equipment is rather expensive, and the examination conditions are limited to the cases of the resting subject. Functional imaging of the heart is the source of the most precise spatial information about the heart muscle tissue and its activity, and may reveal muscle diseases in the earliest phases. Despite its high resolution, the volume and price of the accompanying equipment as well as the professional skills required from the personnel significantly limit the possible applications to well-prepared patients. The resulting volume of data also limits tele-medical use of functional cardiac imaging.
Coronarography is the isotope radiation-based technology of monitoring the blood flow in the coronary arterias. The blood transports the marker throughout the vessels, and subsequent frames of recorded moving images are a background for calculations of the volume and speed of the blood flow. The unexpected deceleration of the blood is interpreted as flow obturation caused by a narrowing of the vessel or calcification impairing the transportation of oxygen and nutritive products to the heart muscle. Due to its invasiveness, the coronarography is currently rarely employed in functional heart imaging. The circulating isotope is influencing organs before it disappears.
Thorax-impedance measurements are based on modulation of electrical properties of the thorax, including the heart during the cardiac cycle. The drawback of this method is the influence of electrical property changes due to the respiration. The heart rate and global information about variations of properties of the heart muscle tissue are easily derived from thoracograms, however the precise location of the impaired regions is not possible to determine. The method is also suitable for rough estimations of heart volume stroke.
Rheography is a blood inertion-based mechanical technique based on turbulent blood flow during the heart contraction. Thanks to the known anatomy of the aorta arc, the sudden flow of a considerable amount of blood causes a compensatory motion in the body whose amplitude can be measured as a representation of the stroke volume. Mechanical rheography requires precise measurement equipment and patient positioning, but it is the only non-image-based methodology for measuring the circulatory effects of the heart activity. Due to its nature, the rheography requires the patient to be at rest, so the application area is limited.
Magnetocardiography is a non-invasive measure of the variation in magnetic field strength above the thorax and can be used to detect electromagnetic phenomena in the heart (Smith et al., 2006). The magnetic field sensors used to record magnetocardiograms (MCGs) are superconducting quantum interference devices (SQUIDs; Zimmerman, Theine, & Harding, 1970) that require liquid helium cooling (Hart, 1991; Cohen, Edelsack, & Zimmerman, 1970). The detectors are extremely sensitive and can measure the weak magnetic fields generated by the electrical activity of the heart. Because of their expense and the need for magnetic shielding, the diagnostic usefulness of MCG systems needs to be carefully assessed. Many studies have demonstrated the potential benefit of magnetocardiography over electrocardiography for some clinical applications (Fenici, Brisinda, & Meloni, 2005; Mori & Nakaya, 1988; Nomura et al., 1994). Magnetocardiograms have been found to be more accurate than ECGs for the diagnosis of right atrial hypertrophy and right ventricular hypertrophy, and have been used to determine the location of conduction pathways in the heart non-invasively, making MCGs potentially beneficial for the localization of arrhythmia sources for catheter ablation (Mori & Nakaya, 1988; Nomura et al., 1994). Magnetocardiography can also detect circular vortex currents, which give no ECG signal. As a result, MCGs may show ischaemia-induced deviations from the normal direction of depolarization and repolarization better than or in a different way than ECGs. The technique also offers a simple non-invasive method for examination of the foetal electrophysiological signal, which is difficult to obtain from the surface ECG and may be useful in antenatal assessment, identifying and classifying clinically relevant arrhythmias (Van Leeuwen et al., 1999; Kandori et al., 2002; Quartero, Stinstra, Golbach, Meijboom, & Peters, 2002; Wakai, Strasburger, Li, Deal, & Gotteiner, 2003; Van Leeuwen, Lange, Klein, Geue, & Gronemeyer, 2004).
Seven different basic techniques are briefly presented above as a review of cardiac functional reporting methods and their outputs as multi-modal records containing:
- •
a voltage time series representing an endogenous or artificially paced electrical stimulus;
- •
static images resulting from mechanical wave-based or radiation isotope-based imaging;
- •
motion images resulting from mechanical wave-based frequency-differential measurements, thorax impedance tomography, or radiation isotope-based serial imaging;
- •
a displacement time series representing the stroke blood mass of the body weight ratio; and
- •
magnetic fields measurement time series.