Diagnositc Features of the ECG

Question 1

Identify the anatomy and describe the function of the cardiac structures responsible for generation and spread of cardiac depolarization which produce the normal heart beat

Answer 1

The sinoatrial (SA) node is the pacemaker of the heart. Electrical impulses initiated by the SA node proceed through the internodal tracts which activates a wave of depolarization in the atrium that converges on the atrioventricular (AV) node. Here, there is a brief delay, after which impulses are sent rapidly towards the Bundle of His and activate the ventricles through the right and left bundles (the left bundle also further splits into anterior and posterior segments). The impulses then diverge into Purkinje fibers which activate ventricular myocardial cell depolarization and contraction.

Question 2

P wave:

Answer 2

atrial depolarization

Question 3

PR interval (from the beginning of P to beginning of Q):

Answer 3

measure of AV node conduction time. Normal PR interval is 0.12 to 0.20 seconds.

Question 4

QRS complex:

Answer 4

ventricular depolarization. Duration is normally 0.06-0.10 seconds.
o Q: negative
o R: positive
o S: late negative deflection

Question 5

QT interval (from beginning of Q to the end of T):

Answer 5

total duration of depolarization and repolarization

Question 6

T wave:

Answer 6

ventricular repolarization

Question 7

U wave:

Answer 7

not constantly present

Question 8

Paper speed is __ mm/second. Heart Rate: ___/# of heavy lines.

Answer 8

• 25

- 300

Question 9

EKG changes produced by Ventricular Hypertrophy:

Answer 9

both left and right ventricular hypertrophy result in greater muscle mass. Greater muscle mass results in a greater voltage associated with depolarization and repolarization of the myocardium. Therefore, on an ecg ventricular hypertrophy is seen as a R wave with greater amplitude.
o Left ventricular hypertrophy: large positive deflections (R waves) in V5 and V6 and large negative deflections (S waves) in V1.
o Right ventricular hypertrophy: high voltage in V1 and V2.

Question 10

EKG changes produced by Myocardial Ischemia:

Answer 10

Ischemia occurs when blood supply is insufficient to meet oxygen demand in the ventricles. Ischemic changes in the EKG alter ventricular repolarization and affect the ST segment and the T wave. Ischemia due to sudden high oxygen demand in the presence of a fixed coronary obstruction causes depression of the ST segment. Ischemia due to acute coronary artery obstruction during low oxygen demand can cause T wave inversion.
o In some patients a resting ekg is normal, but ST depression is only visible during exercise due to transient ischemia.
o Normally, T waves are in the same direction of the QRS complex.
 Inversion of a T wave→myocardial ischemia

Question 11

EKG changes produced by Myocardiac Injury or Infarction:

Answer 11

o ST elevation is a sign of transmural injury in an acute coronary syndrome, usually with a clot due to platelet aggregation obstructing a coronary artery.
 Acute myocardial infarction
o Sizeable (>0.04 s) Q waves can be a sign of transmural necrosis. Infarcts usually involve only the left ventricle.
 Inferior leads (II, III, aVF): inferior infarcts
 V1-V4: anterior wall infarct
 I, aVL and V5, V6: lateral wall infarcts.
o A transmural acute myocardial infarct evolves over time:
 Giant upright “hyperacute” T wave
 T wave inverts and ST segment rises.
 Sometimes, ST elevation precedes of occurs simultaneously with T inversion.
 Q waves are usually the last to develop.
o Transmural vs. subendocardial
 Transmural—involves the entire thickness of the LV
 Subendocardial—localized to the inner layer of the LV wall.
 Subendocardial infarcts do not have Q waves or ST elevation. They do have persistent ST depression

Question 12

EKG changes produced by Electrolyte Disorders:

Answer 12

o Hypercalcemia: shortened QT interval. Often associated with hyperparathyroidism.
o Hypocalcemia: lengthened QT interval. May be associated with life threatening ventricular arrhythmias.
o Hypokalemia: QT interval is generally prolonged, prominent U waves are frequent and T waves may be inverted.
o Hyperkalemia: increased T wave voltages with a distinctive peaked, symmetrical appearance. At higher levels, the P waves may be flattened and the QRS and T waves widened. A broad S wave often appears. At very high levels, a sinusoidal pattern appears without P or R waves.