The electrocardiogram (ECG) is a recording of the electrical potentials produced by cardiac tissue. Formation of electrical impulses occurs within the conduction system of the heart. When excited, or depolarized, atrial and ventricular myocardial muscle fibers contract.
The electrical currents produced by these electrical impulses spread through the body and are recorded from the body surface by applying electrodes at various body surface points and connecting them to a recording apparatus.
[...] The normal QRS axis lies between 0 and +90 degrees; superior axis deviation (between -45 and -90 degrees) and right axis deviation (between +90 and ±180 degrees) are considered abnormal. Leftward deviation of the mean frontal plane QRS axis can occur with advancing age in the absence of clinically overt heart disease. The normal frontal plane P wave and T wave axes usually correspond to the normal QRS axis and point in the same general direction. The unipolar precordial leads approximate the electrical potentials (vectors) in the horizontal plane. [...]
[...] The U wave is the (usually positive) deflection following the T wave and preceding the subsequent P wave; it is thought to be due to repolarization of the intraventricular (Purkinje) conduction system and often is accentuated in left ventricular hypertrophy. In some circumstances, such as hypokalemia and hypomagnesemia, the U wave is thought to represent an oscillatory membrane potential, called an afterdepolarization. Negative U waves, best seen in leads can be seen in acute myocardial ischemia (where they are insensitive but relatively specific markers of left anterior descending coronary artery disease) and left ventricular hypertrophy from any cause. [...]
[...] the heart. When excited, or depolarized, atrial and ventricular myocardial muscle fibers contract. The electrical currents produced by these electrical impulses spread through the body and are recorded from the body surface by applying electrodes at various body surface points and connecting them to a recording apparatus. The ECG is a valuable diagnostic tool to evaluate conduction delay of atrial and ventricular electrical impulses, origin of arrhythmias, myocardial ischemia and infarction, atrial and ventricular hypertrophy, pericarditis, the effect of cardiac drugs (especially digitalis and certain antiarrhythmic agents), disturbances in electrolyte balance (especially potassium), the function of electronic cardiac pacemakers, and systemic diseases that affect the heart. [...]
[...] Determine the P wave axis, duration, and morphology to provide information about the focus of origin of the atrial rhythm and whether the atria are being depolarized antegradely or retrogradely. Conclusions If the atrial rhythm is sinus, the P wave morphology and duration can suggest the presence of atrial enlargement or hypertrophy or interatrial conduction delay. Identify the ventricular rate and whether it is regular or irregular. Conclusions Ascertain whether the ventricular rate is associated with the atrial rhythm and what their relationships are: Is there one P wave for each QRS complex? [...]
[...] Excitation and Threshold Potential Excitation of a cardiac cell occurs when a stimulus reduces the transmembrane potential to threshold potential (about -60 mV in atrial and ventricular muscle cells and about -40 mV in SA and AV nodal cells). If the resting membrane potential is raised toward the level of the threshold potential, a relatively weak stimulus can evoke a response. Conversely, if the resting potential is lowered away from the threshold potential, a relatively stronger stimulus is required to produce a response. [...]
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