Chapter 12

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How is the current in heart flowing?

From area of depolarization to polarized areas

Electrical potentil generated can be represented by what?

A vector, with arrowhead pointing in positive direction

Length of vector is proportional to what?

To voltage of potential

Generated potential at any instance can be represented by what?

An instantaneous mean vector

What does QRS complex represent?

Ventricular depolarization beginning at ventricular eptum and proceeds toward apex of heart with an average direction of 59 degrees

What does ventricular T wave represent?

It represents repolarization of ventricle that begins near apex of heart and proceeds toward the base

What does the atrial P wave represent?

Depolarization of the atria that begins at sinus node and sprads in all directions

What is shifting the mean electrical axis of ventricles to left?1

During expiration or when a person is recumbent and abdominal contents press upward against the diaphragm

What is shifting the mean electrical axis of ventricles to left?2

Accumulation of abdominal fat, which presses upward on the heart

What is shifting the mean electrical axis of ventricles to left?3

Left bundle branch block, which is when the cardiac impulse spreads through the right ventricle two to three times as fast in the left ventricle, consequently, the left ventricle remains polarized much longer than right, and strong electrical vector points from right ventricle to left

What is shifting the mean electrical axis of ventricles to left?4

Hypertrophy of left ventricle, caused by hypertension, aortic valvular stenosis, or aortic valvular regurgitation

WHich are the factors that shift the mean electrical axis of ventricles to the right (Clockwise)?

Inspiration, standing up, lack of abdominal fat which allows heart to rotate clockwise compared with normal individual, right bundle branch block, right ventricular hypertrophy

What does hypertrophy of heart do?

It increases the voltage of QRS complex

What is the most common cause of highvoltage QRS complexes?

Right or left ventricular hypertrophy

Which are the conditions causing decrease in voltage of QRS complex?1

Hearts with old myocardial infarctions and the resultant decreased cardiac muscle mass

Which are the conditions causing decrease in voltage of QRS complex?2

Conditions surrounding heart that effectively ''short-circuit'' the cardiac electrical potential. FLuid in pericardium and pleural effusion both conduct currents form around the heart and prevent much of votlage from reaching the surface of body (Pulmonary emphysema also decreases conduction of cardiac potentials)

What is the most common cause of an extended QRS complex?

Prolonged conduction through ventricles (occuring in both hypertrophied and dilated hearts and increasaes the duration of QRS waves)

What is the most common cause of an extended QRS complex?2

Blockade of impulses in Purkinje system prolongs the QRS complex because the duration of ventricular depolarization increases in one or both ventricles

What is current of injury?

Several abnormalities cause a portion of heart to remain depolarized all of the time, and the current that flows from depolarized area to polarize areas of heart is called current of injury

Which are some of the abnormaliteis that can cause a current of injury?

Mechanical trauma, infectious processes that damage the cardiac muscle membrane, coronary ischemia

What can the axis of current of injury be determined with?

With electrocardiogram

How is the axis of current of injury determined?

1. Determine J point (point of zero potential at end of QRS wave) 2. Determine level of T-P segment with respect to J point on three standard leads 3. Plot the voltages on the coordinates of three leads to determine the axis of current of injury, and note that the negative end of vector originates in the injured area of ventricles

What is the current of injury useful for determining?

Whether an infarction is in anterior or posterior portion of heart

Which are the conditions that alter (change the electrical axis of T wave)?1

During bundle branch block, one of ventricles depolarizes before the other. The first ventricle to depolarize is also the first to repolarize, and this causes an axis deviation in the T wave. Therefore, a left bundle branch block causes a rightwar daxis deviation of T wave

What are the most common cause of shortened depolarization?

Mild ischemia of cardiac muscle in the base of ventricles

What does ventricular T wave represent?

Repolarization of ventricle

What does atrial P wave represent?

Depolarization of atria (beginning at sinus node and spreads in all directions)

What does left bundle branch block mean?

When cardiac impulse spreads through right ventricle faster than left ventricle, so left ventricle remains polarized much longer than right, and a strong electrical vector points from right ventricle to the left

What is hypertrophy of left ventricle caused by?

By hypertension, aortic valvular stenosis, or aortic valvualr regurgitation

How is Lead I recoreded?

From two electrodes placed respectively on the two arms (0 degrees)

What does vector A represent?

The instantaneous mean direction of curent flow in ventricles (can be 55 degrees)

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