04: EKG Reading

Question 1

What order should ECGs be read?

Answer 1

1. Calibration
2. Heart rate
3. Heart rhythm
4. Intervals (PR, QRS, QT)
5. Mean QRS axis
6. P-Wave abnormalities
7. QRS abnormalities
8. ST-segment/T-wave abnormalities

Question 2

How should an ECG be calibrated?

Answer 2

Speed = 25mm/sec

Voltage = 10 mm/mV

One large box (5x5mm) = 200ms, 0.5mV

One small box (1x1mm) = 40ms, 0.1mV

Question 3

Identify the various components of an ECG tracing.

Answer 3

• P-wave: depolarization of atria
• QRS complex: depolarization of ventricular muscles/(repolarization of atria)
• T-wave: Repolarization of ventricles
• PR interval: P-wave to onset of QRS complex
• QT interval: beginning of QRS to end of T-wave

Question 4

Describe electrocardiogram deflections.

Answer 4

Depolarization current toward (+) electrode: upward deflection recorded

Question 5

How do you calculate heart rate?

Answer 5

Count off boxes between two QRS complexes: 300-150-100-75-60-50

If irregular rhythm (e.g., AFib): Count $ of complexes during 6 seconds and multiply by 10. (ECGs have time markers spaced 3 sec apart.)

Question 6

Describe sinus heart rhythm.

Answer 6

• Every P wave followed by QRS
• Every QRS preceded by P wave
• P-wave upright in leads I, II, III
• PR interval greater than 0.12 sec (3-5 small boxes)
• If HR <60bpm, sinus bradycardia
• If HR >100bpm, sinus tachycardia

Question 7

What is the Corrected QT (QTc)?

Answer 7

• Divide the measured QT by the square root of the R-R interval (in seconds)
• If measured QT interval <50% in two consecutive QRS complexes, likely normal

Question 8

For the PR interval, identify:

1. Normal
2. Decreased
3. Increased

Answer 8

1. 120-200 ms (3-5 small boxes)
2. Pre-excitation/WPW, junctional rhythm
3. First-degree AV block

Question 9

For the QRS interval, identify:

1. Normal
2. Decreased
3. Increased

Answer 9

1. = 100ms (= 2.5 small boxes)
2. Atrial flutter
3. Bundle branch block, ventricular ectopy, drug effect (e.g., antiarrhythmics), or ↑K⁺

Question 10

For the QTc interval, identify:

1. Normal
2. Decreased
3. Increased

Answer 10

1. = 450 (male) or 460 (female) ms
2. ↑Ca, Short QTc (2/2 hypercalcemia, congenital, digoxin)
3. ↓K, Ca, ischemia, congenital, toxic drug effect (e.g., antiarrhythmics), Long QTc

Question 11

Describe the types of AV blocks.

Answer 11

• 1st Degree: Large PR interval (>200ms or 1 large box).
• 2nd Degree, Mobitz I (Wenckebach): Progressive prolongation of PR interval culminating in a non-conducted P wave.
• 2nd Degree, Mobitz II: Intermittent non-conducted P waves *without *progressive prolongation of the PR interval.
• 3rd Degree: Complete absence of AV conduction – none of the supraventricular impulses are conducted to the ventricles (rhythm is maintained by a junctional or ventricular escape rhythm).

Question 12

Describe the mean QRS axis.

Answer 12

• Mean direction of electrical forces in the frontal plane (limb leads) as measured from Lead I
• Normal = -30° to 90° (positive in lead I, II)
• Right axis deviation = 90° to 180° (**negative in I)
  
  • ​Increased RV mass/electrical impulse
  • DDx: RVH, pulmonary embolus, LPFB
• Left axis deviation = -30° to -90° (negative in II)
  
  • ​Increased LV mass/electrical impulse
  • DDx: Inferior MI, LVH, LAFB

Question 13

Describe normal and abnormal P waves.

Answer 13

• Look in leads:
  
  • II: 1 (RA) & 2 (LA) both upward
    
    • Enlargement: >2.5sbox high, peaked (RA) or wide, bifid (LA)
  • V1: 1 up, 2 down
    
    • Enlargement: >1sbox

Question 14

Describe normal and abnormal Q waves.

Answer 14

• Under normal circumstances, Q waves are not seen in the right-sided leads (V1-3); (waves represent absence of electrical forces from infarcted cells)
• Abnormal: width >/= 1 small box wide & depth >25% of QRS height
• Pathological Q waves usually indicate current or prior myocardial infarction (must be in at least 2 consecutive leads; in presence of LBBB, Q waves not helpful in diagnosis of MI)

Question 15

Describe the localization of myocardial infarctions by leads.

Answer 15

Question 16

Describe the localization of myocardial infarctions to coronary arteries.

Answer 16

1. Inferior (II, III, aVF): RCA
2. Anteroseptal (V1, V2): LAD
3. Anteroapical (V3, V4): LAD (distal)
4. Anterolateral (V5, V6, I, aVL): CFX
5. Posterior (V1, V2; tall R wave, not Q wave): RCA

Question 17

What are the indicators of left ventricular hypertrophy?

Answer 17

• Larger-than-normal electrical forces generated by LV
• Left lateral leads (V5, V6, I, aVL) have taller-than-normal R waves +/- T-inversions
• Right lateral leads (V1, V2) have opposite– deeper-than-normal S wave

Question 18

What are the indicators of bundle branch blocks?

Answer 18

• 2/2 ischemia or degenerative damage
• Slow myocyte-to-myocyte spread of electrical activity: delay prolongs depolarization/widens QRS
  
  • QRS duration 0.10-0.12sec (2.5-3sbox) = incomplete BBB
  • QRS duration >0.12sec (>3sbox) = complete BBB

Question 19

What are the indicators of a right branch block?

Answer 19

• 2/2 right atrium strain (pulmonary embolism, rate-related aberration)
• R’ (secondar R wave) in V1 (‘M’ pattern)
• Slurred S in V6 (‘W’ pattern)

Question 20

What are the indicators of left bundle branch block?

Answer 20

• 2/2 degenerative disease, cardiomyopathy, myocardial infarction
• Deep S waves in right precordials (V1-3)
• Broad/notched R wave in lateral leads (I, V5-6)

Question 21

Describe the progression of ECG abnormalities in myocardial ischemia/infarction.

Answer 21

1. Minutes-hours: ST elevation
2. Hours: Q-waves
3. Hours-days: T inversions
4. Days: ST-T changes resolve +/- continued Q waves

Question 22

What are other ST/T-wave abnormalities?

Answer 22

• Short QT: hypercalcemia, hyperkalemia
• Long QT: hypocalcemia, genetic, drugs
• Inverted T waves: ischemia, LVH, digoxin
• Peaked T waves: hyperkalemia
• U-waves (image below): hypokalemia, hypomagnesium