ECG

Question 1

How a vector is compared to a lead?

Answer 1

• Depolarization
  
  • toward a (+) pole of a lead = Upward deflection
  • toward a (-) pole of a lead = Downward deflection
• Magnitude of deflection
  
  • proportional to how parallel the vector is to the lead
    
    • parallel = greatest deflection
    • perpendicular = smallest deflection (zero)
      
      • flat line

Question 2

Explain the peaks of and ECG related to the Heart Electrical State

Answer 2

PR Interval = Atrial depolarization

QRS Complex = Ventrical depolarization

T wave = Ventrical repolarization

Note:

• Atrial repolarization = masked by QRS complex

Question 3

What is the 12-lead snapshot?

Answer 3

Basics:

• ECG looks at 3 beats simultaneously from 12 angles of electricity

High Lateral wall leads

• look at hear from left side

Septal leads

• look at septal wall of LV

Anterior leads

• look at anterior wall of LV

Inferior Wall leads

• looking up from bottom

Lower lateral leads

• look at inferior wall of LV

Clinical Use:

• to determine location of MI
• assess LV & RV hypertrophy
• calculate mean QRS axis

Question 4

What does the P wave & PR interval look like in an ECG?

Answer 4

P wave = Atrial depolarization

• Electrical vector: SA –> AV node
• Vector = uprigt in leads II & aVF
  
  • electrical activity is going toward + electrode
• Vector = inverted in lead aVR
  
  • electrical activty is going away from + electrode

Clinical Relevance:

• P wave = crucial for normal sinus rhythm

PR Interval:

• Atrial depolarization
• Delay in AV junction
  
  • ​can lead to heart blocks

Clinical Relevance:

• AV nodal conduction is susceptible to blocks

Question 5

What do the QRS waves look like during an ECG?

Answer 5

QRS waves = Ventricular Depolarization

• Normal ventricular activation starts w/ septal depolarization
  
  • left –> right direction
  • endocardium –> epicardium
• Q wave = inital (-) deflection from the baseline
• R wave = first (+) deflection from baseline
• S wave = (-) deflection

Clinical Relevance:

• Normal QRS = hearts ability to pump blood following normal excitation
  
  • contraction coupling

Question 6

What does the ST segment look like on an ECG?

Answer 6

ST segment = end of ventricular depolarization

• Vector = isoelectric
  
  • all depolarized

Clinical relevance:

• Can be altered during ischemia & MI

Question 7

What does the T wave look like on an ECG?

Answer 7

T wave = Ventricular Repolarization

• begins at the apex –> progresses superiorly
• when comple, heart = ready for next cycle

Clinical Relevance:

• Time from QRS complex –> T wave = absolute refractory period!!!
  
  • ​​no additional APs

Question 8

What does the QT Interval look like on an ECG?

Answer 8

QT Interval

• electrical depolarization/repolarization of left & right ventricles
• varies based on HR

Question 9

What is the Mean Electrical Axis?

Answer 9

Mean Electrical Axis

• temporally summated vector over the entire process of ventricular depolarization
  
  • QRS
• Variation due to:
  
  • difference in anatomical distribution of purkinje fibers

Examples of Axis Deviations:

• Ventricular hypertrophy
  
  • MEA deviates toward hypertrophied ventricle
    
    • ex: right axis deviation in severe pulmonary HTN
      
      • R.V. hypertrophy
    • ex: left axis deviation in aortic valvular disease
      
      • L.V. hypertrophy