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25. Electric activity of cells of a myocardium.

In natural conditions of a cell of a myocardium are in a condition of rhythmic activity. Majority of cells have potential of rest nearby 90 mV and is defined by a concentration gradient of ions К+.

Autorhythmic activity is based on this mechanism of specified cardiac cells. Biological activity of these cells has also other important features: 1) a small steepness of rise of PA; 2) slow repolarization, smoothly passing in a phase fast repolarization. Ability of cells of a myocardium during human life be in a condition of continuous rhythmic activity is provided by effective work of ionic pumps of these cells.

26. The electrocardiogram. Main assignments of ecg.

Electrocardiography (ECG or EKG*) is the process of recording the electrical activity of the heart over a period of time usingelectrodes placed on the skin. These electrodes detect the tiny electrical changes on the skin that arise from the heart muscle'selectrophysiologic pattern of depolarizing during each heartbeat. It is a very commonly performed cardiology test.

In a conventional 12 lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall magnitude of the heart's electrical potential is then measured from twelve different angles ("leads") and is recorded over a period of time (usually 10 seconds). In this way, the overall magnitude and direction of the heart's electrical depolarization is captured at each moment throughout the cardiac cycle.[1] The graph of voltage versus time produced by this noninvasive medical procedure is referred to as an electrocardiogram (abbreviated ECG or EKG).

Usually use three so-called standard taps from extremities: I tap: the right hand - the left hand; II tap: the right hand - the left leg; III tap: the left hand - the left leg.

27. Basic peaks of ecg.

The P wave is the result of the atrial depolarization. This depolarization starts in the SA (sinoatrial) node. The signal produced by pacemaker cells in the SA node is conducted to the right and left atria. Normal atrial repolarization is not visible on the ECG (but can be visible during atrial infarction and pericarditis).

The QRS complex is the average of the depolarization waves of the inner (endocardial) and outer (epicardial) cardiomyocytes. As the endocardial cardiomyocytes depolarize slightly earlier than the outer layers, a typical QRS pattern occurs (figure).

The T wave represents the repolarization of the ventricles. There is no cardiac muscle activity during the T wave.

One heart beat consists of an atrial depolarization --> atrial contraction --> p-wave, ventricular depolarization --> ventricular contraction --> ORS-complex and the resting phase (including the repolarization during the T-wave) between two heart beats.

The origin of the U wave is unknown. This wave possibly results from "afterdepolarizations" of the ventricles.

The letters "Q", "R" and "S" are used to describe the QRS complex

  • Q: the first negative deflection after the p-wave. If the first deflection is not negative, the Q is absent.

  • R: the positive deflection

  • S: the negative deflection after the R-wave

  • Small print letters (q, r, s) are used to describe deflections of small amplitude. For example: qRS = small q, tall R, deep S.

  • R`: is used to describe a second R-wave (as in a right bundle branch block)

See figure for some examples of this.

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