Diagnostic Features of ECGs

Question 1

Steps of cardiac conduction/depolarization

Answer 1

• sinoatrial (SA) node = pacemaker
• Electrical impulses initiated @ SA node –> internodal tracts –> wave of depolarization in the atrium –> atrioventricular (AV) node.
• @ AV node: brief delay –> impulses to Bundle of His and activate the ventricles through the right and left bundles
• bundles –> Purkinje fibers –> activate ventricular myocardial cell depolarization and contraction.

Question 2

Main clinically relevant components of EKG (6)

Answer 2

• P wave
• PR interval
• QRS complex
• QT interval
• T wave
• U wave

Question 3

P wave fxnl action

Answer 3

atrial depolarization

Question 4

PR interval fxnl significance

Answer 4

• measure of AV node conduction time
• normal = 0.12 - 0.20 secs

Question 5

QRS fxnl significance

Answer 5

• ventricular depolarization
• normal duration = 0.06 - 0.10 secs
• Q: negative
• R: positive
• S: late negative deflection

Question 6

QT interval fxnl significance

Answer 6

• lasts from begin of Q to end of T
• total duration of depolarization and repolarization

Question 7

T wave fxnl significance

Answer 7

ventricular repolarization

Question 8

U wave fxnl significance

Answer 8

• not always seen
• follows T wave
• possibly repolarization of purkinje fibers or papillary muscles

Question 9

Line measurements/speed of EKG paper

Answer 9

• paper speed = 25mm/sec
• vertical lines = 0.04 sec
• thick vertical lines = 0.2 sec

Question 10

Calculation of heart rate from EKG

Answer 10

HR=300/# of heavy lines between P waves

Question 11

EKG changes produced by ventricular hypertropy

Answer 11

• both left and right ventricular hypertrophy result in greater muscle mass –> greater voltage associated with depolarization and repolarization of the myocardium.
  
  • ecg ventricular hypertrophy is seen as a R wave with greater amplitude.
• Left ventricular hypertrophy: large positive deflections (R waves) in V5 and V6 and large n_egative deflections (S waves) in V1._
• Right ventricular hypertrophy: high voltage in V1 and V2.

Question 12

EKG changes caused by myocardial ischemia

Answer 12

• Ischemic changes 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.
  
  • In some patients a resting ekg is normal, but ST depression is only visible during exercise due to transient ischemia.
• Ischemia due to acute coronary artery obstruction during low oxygen demand can cause T wave inversion.
• Normally, T waves are in the same direction of the QRS complex.

Inversion of a T wave→myocardial ischemia

Question 13

EKG changes in acute myocardial infarction

Answer 13

• 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.
• 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.

Question 14

Evolution of transmural acute myocardial infarct

Answer 14

• 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.

Question 15

Transmural vs. subendocardial acute myocardial infarction

Answer 15

• 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 16

EKG changes in hypercalcemia

Answer 16

• shortened QT interval
• often associated with hyperparathyroidism.

Question 17

EKG changes in hypocalcemia

Answer 17

• lengthened QT interval
• may be associated with life threatening ventricular arrhythmias

Question 18

EKG changes associated with hyperkalemia

Answer 18

• 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.
• broad S wave often appears.
• At very high levels, a sinusoidal pattern appears without P or R waves.

Question 19

EKG changes associated with hypokalemia

Answer 19

• QT interval is generally prolonged
• prominent U waves are frequent
• T waves may be inverted