Guyton Kap.12- Ekg Abnormalities: Vector Analysis

Front 1

Mean QRS vector:

Back 1

The average direction of vector during spread of depolarization wave through the ventricles. About 59 degrees.

Front 2

Axis of leads:

Back 2

The direction from negative electrode to positive electrode. E.g. 60 degrees in lead II

Front 3

How to determine voltage in a vector?

Back 3

1. Draw the axis of the lead
2. Draw the vector with its length indicating the voltage of the potential
3. Draw a line perpendicular to the axis from the axis to the tip of the vector
4. Draw "projector vector" along the axis of the lead
5. If the arrow on this projected vector point towards the positive end of the lead the lead should record positively visa versa
6. The instantaneous vector voltage will be equal to the length of the projected vector divided by the length of the vector times 2 millivolts

Front 4

Q wave:

Back 4

The interventricular septal fascicle of the left bundle branch is depolarized 0.01 sec before the right bundle branch creating a vector from left to right for a short period. This cause the Q wave in some leads

Front 5

How long after ventricular depolarization does the repolarization start?

Back 5

0.15 seconds

Front 6

Why does repolarization start in epicardium at the apex of the heart and not in endocardium of base?

Back 6

Because of the high blood pressure inside the ventricles during contraction, which greatly reduces coronary blood flow on the endocardium, thereby slowing repolarization in endocardial areas

Front 7

Why is the T wave positive?

Back 7

Because repolarization starts at the apex of the heart meaning that the apex is positive on the outside relative to the depolarized base which is negative on the outside --> the overall vector is towards the positive apex ==> positive deflection in most leads

Front 8

Vector of atrial repolarization:

Back 8

Opposite of that of the ventricles (from apex to base). This is because the slow conduction through the atria`s compared to the Purkinje fibers causes the area around the sinus node to repolarize first, i.e becoming polar---> electric current goes towards the positive sinus nodal area

Front 9

Mean electrical axis of the ventricles:

Back 9

The predominant direction of the potential during depolarization. Of a normal ventricle it is 59 degrees.

Front 10

Causes of normal axis deviations:

Back 10

Anatomical differences in Purkinje conduction system or in the musculature itself.

Front 11

Causes of left shift of heart and mean electrical axis:

Back 11

Deep expiration
Lying in suspine position with abdominal organs pressing up against diaphragm
Adipose people where diaphragm is pressed up

Front 12

Causes of right shift of heart and mean electrical axis:

Back 12

Deep inspiration
Standing position
Tall skinny people

Front 13

Effect on hypertrophy of a ventricle on axis:

Back 13

Deviation towards the hypertrophied side because of larger quantity of muscle to conduct impulse and because it takes longer to depolarize more muscle mass

Front 14

Why is there left axis deviation in left bundle branch block?

Back 14

Because in that case the right ventricle depolarizes two- three times as fast as the left--> the left ventricle remain polarized 0.1 sec after the right ventricle is fully depolarized creating a vector from right to left about - 50 degrees

Front 15

How to separate axis deviation from bundle brach block from axis deviation from hypertrophy of a ventricle?

Back 15

Bundle branch block has prolonged QRS complex due to blocking of the Purkinje fibers combined with the axis deviation, hypertrophy does not have prolonged QRS.

Front 16

Right bundle branch block causes what axis deviation?

Back 16

Right axis deviation over 100 degrees and prolonged QRS complex

Front 17

Normal VS increased voltage of EKG:

Back 17

Normal: 0,5- 2 millivolts (R wave)

Increased: > 4 miillivolts
Due to increased muscle mass which generates larger quantities of electrical impulses around the heart

Front 18

Decreased voltage of EKG:

Back 18

Diminished muscle mass (myopathies or old MI`s)

Pericardial effusion also gives decreased voltage due to the easy conduction of the electricity through the fluid

Front 19

Prolongation of QRS due to block in the Purkinje system:

Back 19

Normal QRS: 0.06- 0.08

Block: usually up to 0.14 or greater (when it is greater then 0.12 it most certainly is from a block in the ventricular conduction system

Front 20

Causes of bizarre QRS complexes:

Back 20

1. Destruction of cardiac muscle tissue and replacement by scar tissue

2. Multiple small blocks in the Purkinje system

Front 21

Current of injury:

Back 21

Some of the heart muscle is depolarized all the time due to some pathology---> electrical charges from negative injured tissue to normal polarized tissue.

Front 22

"J" point:

Back 22

Zero reference potential for analyzing of current of injury. It is the point of the EKG at the end of the QRS complex when all of the cardiac muscle is depolarized and there is no current flow. If you draw a horizontal line from this point you get the "zero potential level" from which all currents caused by zero potential injury must be measured.

Front 23

What is the injury potential in each lead?

Back 23

The difference between the voltage of the EKG immediately before the onset of the P wave and the zero voltage level determined by the "J" point

Front 24

Why cannot severely ischemic cardiac tissue repolarize?

Back 24

Because injury depress metabolism because of lack of O2, increase of CO2 and lack of food nutrients (O2 and nutrients is required for the work of the Na+/K+ ATPase to restore membrane potential)

Front 25

Result of mild ischemia:

Back 25

Shortens depolarization of cardiac muscle due to increased current flow through potassium channels---> marked T wave changes

Front 26

Effect of digitalis on T waves:

Back 26

T wave inversions or biphasic T waves due to increased depolarization duration in some parts of the heart muscle.