Electrocardiography


Electric heart activity measured from the body surfaces over time is called the electrocardiogram (ECG). Electrocardiograph is the tool to measure it. Electrocardiogram is the significant means for the proper heart diagnosis such as myocardial infarction or ischemia, [1] and, etc. Heart disease or heart deterioration could be referred to particular type of arrhythmia [22]. However, ECG interpretation and diagnosis requires experienced knowledge. Therefore, advanced algorithms can serve only as a supportive system, even though several studies showed relatively good results. Based on thresholding technique, P. Langley and others discriminated ischemic from non-Ischaemic ST changes and get reasonable good results: sensitivity 99.0%, specificity 88.8% and accuracy 91.1% [23]. M.G. Tsipouras and colleagues employed Hamilton Tompkins's algorithm for QRS complex detection and classified MIT-Arrythmia DB, they get 98% accuracy for arrhythmic beat classification and 94% accuracy for arrhythmic episode classification [24]. Moreover, already exist commercial devices like Biolog™ Handheld Electrocardiogram (see appendix I, Fig 15.31), which is based on Louvaine algorithm and capable achieve accuracy to 82.1% for the myocardial infarction detection [25].

Electrocardiogram waveforms and intervals (normal ECG wave) Most popular in the practical medicine is 12-lead ECG system composed of nine electrodes placed around the body. Three limb electrodes (or augmented aVL, aVR and aVF leads) and six chest electrodes (V1 – V6). During the ECG wave measurement patient normally is at the supine position. Later, electrocardiogram analyzed by physician for disease treatment. Time duration to record ECG depend on the test type. For ambulatory or home environment 12 lead ECG is cumbersome, though Holter ECG monitors traditionally have 5-12 ECG leads or can be other configurations like 3-5-7 lead systems. One study showed that 12 lead systems can be replaced by single lead system; they get close correlation between single and 12 lead ECG systems, sensitivity 83% and specificity 75 % [113]. Thus it let to conclude that single lead ECG system would be sufficient for daily time use.

Universally accepted that ECG is recorded on special paper, moving at the constant 25mm/s speed, where one large square is 5 mm or 0.2 s, and small square equal to 1 mm or 0.04 s (Figure above).

Electrical heart activity can be represented as a cardiac vector expressed as a dipole. Electrode placement on the body describes the view of the vector as a time function. It could be said that lead configuration characterizes where lead is placed, thus positive or negative deflections at the same time can be observed at the different type of electrodes. Most basic electrode placement based on Einthoven’s triangle, which is expressed as theoretical triangle drawn around the heart. Einthoven’s law states that any value of the triangle can be derived, if the other two point values are known (see appendix F for more details).

describe the ECG wave deflections, which are different in different type of leads. Fig 3.3 summarizes the concept of cardiac vector, which basically describes the motion of depolarization (increasing negative charge) wave over the body, thus magnitude and direction describe the type of electrode. Positive deflection is observed, when depolarization current moves from the body to the electrode. In the electrocardiogram observed positive wave from the base line. Likewise, when a depolarization current flows from the electrode to the body, then negative deflection is observed. During a re-polarization period negative deflection observed when re-polarization current flows from the body to the electrode, and positive deflections are observed when re-polarization current flows from the electrode to the body. According to hexaxial reference system [1-3] positive or negative deflection can be expressed.

Cardiac vector and different deflection waves in normal ECG. Deflections at the Lead II and aVR are
inverse at the same time. [source: 3] data can be acquired. For instance, in particular, cases it is hard to see P waves; therefore, additional leads are required. For the daily time use, it is preferable fewer leads as possible, thus lead I configuration is the most common.

Any abnormal heart electric behavior is due to specific type of arrhythmia, which can bring serious health damage. Accepted arrhythmias classify into particular behavior types (see appendix G). Bates’ guide [19] proposed arrhythmia classification algorithm, where ECG rhythm classified into normal and irregular, and later to existing ECG pattern (see appendix D). Thus proposed algorithm further can serve as a good guide.


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