Some Basic Interpretation Of The Ecg Trace

The ECG waveform and mean electrical axis are quite useful in the clinical setting. The ECG is considered one of the most important monitors of a patient's cardiovascular status and is commonly used for measurements as basic as the heart rate. Most ECG monitoring devices used today include automated systems that detect changes in durations between subsequent QRS complexes (i.e., the duration of one cycle).

Simply, determination of the PR intervals provides information regarding whether a patient may have heart block. Elongated PR intervals (longer than ~0.21 s) serve as a good indication that conduction through the atrioventricular node is slowed to some degree (first-degree heart block). Conduction in the atrioventricular node may even intermittently fail, which would elicit a P wave without a subsequent QRS complex before the next P wave (second-degree heart block). An ECG trace showing P waves and QRS complexes beating independently of each other indicates the atrioventricular node has ceased to transmit impulses (third-degree heart block).

net of zero lead HI

Cross Section The The Heart
Fig. 9. (A) A cross-section of the chest shows the relative position of the six precordial leads in the traverse plane, along with a typical waveform detected for ventricular depolarization. (B) An anterior view of the chest shows common placement of each precordial lead, V1 through V6.

Prolonged Q-T intervals (which are normally no more than 40% of the cardiac cycle length) are normally an indication of delayed repolarization of the cardiomyocytes, possibly caused by irregular opening or closing of sodium or potassium channels. More important, though, is the elevation of the S-T segment, which typically indicates a regional ventricular ischemia. The S-T segment elevation (or depression) can also be used as an indication of many other abnormalities, including myocar-dial infarction, coronary artery disease, and pericarditis.

The electrical axis is also a helpful diagnostic tool. For example, in the case of left ventricular hypertrophy, the left side of the heart is enlarged with greater tissue mass. This could cause the dipole during ventricular contraction (and therefore the mean electrical axis) to point more towards the left.

Much more can be said about specific interpretations of the intervals and segments that make up the ECG waveform. For more details on changes in ECG patterns associated with various conditions or abnormalities, refer to Chapter 9.

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