Cardiac Axis

• In a normal heart, the average direction of the vector during spread of the depolarization wave through the ventricles, called the mean QRS vector, is about +59 degrees.

Abnormal Ventricular Conditions ThatCause Axis Deviation

Change in the Position of the Heart in the Chest. If the heart is angulated to the left, the mean electrical axisof the heart also shifts to the left. (1) at the end of deep expiration, (2) when a person lies down, because the abdominal contents press upward

against the diaphragm, and (3) In stocky, fat people (diaphragms normally press upward against the

heart all the time).Angulation of the heart to the right causes the mean electrical axis of the ventricles to shift to the right. This occurs (1) end of deep inspiration,(2) when a person stands up, and (3) normally in tall, lanky people whose hearts hang downward.

Hypertrophy of One Ventricle the axis of the heart shifts toward the hypertrophied ventricle for two reasons. 1st greater muscle mass excess generation of electrical potential on that side.

2nd more time is required for the depolarization wave to travel through the hypertrophied ventricle than through the normal ventricle. normal ventricle becomes depolarized considerably in earlier than the hypertrophied ventricle.

Vectorial Analysis of Left Axis Deviation Resulting fromHypertrophy of the Left Ventricle. Vectorial analysis demonstrates left axis deviation with mean electrical axis pointing in the -15-degree direction.

hypertension (high arterial blood pressure) left ventricle hypertrophyaortic valvular stenosis, aortic valvular regurgitation, orcongenital heart conditions in which the left ventricle enlarges while the right ventricle remains relatively normal in size.

Vectorial Analysis of Right Axis Deviation Resulting from Hypertrophy of the Right Ventricle.

ECG in Figure 12–13 shows intense right axis deviation, to an electrical axis of 170 degrees, which is 111 degrees to the right of the normal 59-degree mean ventricular QRS axis.

The right axis deviation demonstrated in this figure was caused by hypertrophy of the right ventricle as a result of congenital pulmonary valve stenosis.

Right axis deviation also can occur in other congenital heart conditions that cause hypertrophy of the right ventricle, such as tetralogy of Fallot and interventricular septal defect.

Bundle Branch Block Causes Axis Deviation. * lateral walls of 2 ventricles depolarize at same time.* Left and the right bundle branches of the Purkinje system transmit the cardiac impulse to the two ventricular walls at same time. •Potentials generated by the 2 ventricles (on the two opposite sides of the heart) almost neutralize each other. • if only one of the bundle branches is blocked, the impulse spreads through the normal ventricle long before it spreads through the other. depolarization of the two ventricles Does NOT occur same time depolarization potentials do not neutralize each other.

Vectorial Analysis of Left Axis Deviation in Left Bundle Branch Block. cardiac depolarization spreads through the right ventricle 2-3 times as rapidly as through the left ventricle. intense left axis deviation of about -50 degrees. prolonged QRS complex differentiates bundle branch block from axis deviation caused by hypertrophy.

Vectorial Analysis of Right Axis Deviation in Right Bundle Branch Block. When the right bundle branch is blocked, the left ventricle depolarizes far more rapidly than the right ventricle.Axis of about 105 degrees instead of the normal 59 degrees. prolonged QRS complex because of slow conduction.

Decreased Voltage of the ECGDecreased Voltage Caused by Cardiac Myopathies. Decreased voltage of the QRS complex is a series of old myocardial artery infarctions with resultant diminished muscle mass.

Depolarization wave to move throughthe ventricles slowly and prevents major portions of the heart from becoming massively depolarized all at once.

Prolongation of the QRS complex along with the decreased voltage.

Decreased Voltage Caused by Conditions Surrounding the Heart. fluid in the pericardium & Pleural effusion

Extracellular fluid conducts electrical currents with great ease, a large portion of the electricity flowing out of the heart is conducted from one part of the heart to another through the pericardial fluid.

Effusion effectively “short-circuits” the electrical potentials generated by the heart, decreasing the ECG voltages that reach the outside surfaces of the body.,

Pulmonary emphysema can decrease the ECG potentials.Conduction of electrical current through the lungs is depressed considerably because of excessive quantity of air in the lungs. Lungs act as an insulator to prevent spread of electrical voltage from the heart to the surface of the body.

Coronary Ischemia as a Cause of Injury PotentialInsufficient blood flow to the cardiac muscle depressesthe metabolism of the muscle for three reasons: (1) Lack of O2, (2) Excess accumulation of CO2, and (3) Lack of sufficient food nutrients.Repolarization of the muscle membrane cannot occur in areas of severe myocardial ischemia.Blood flow is sufficient to maintain life of the muscle eventhough it is not sufficient to cause repolarization of themembranes. As long as this state exists, an injury potential continues to flow during the diastolic portion (the T-P portion) of each heart cycle.Extreme ischemia of the cardiac muscle occurs aftercoronary occlusion, and a strong current of injuryflows from the infarcted area of the ventricles duringthe T-P interval between heartbeats.