PHYS - Intro to EKGs

Question 1

PRINCIPLES OF EKGS

Answer 1

• EKG is a powerful, non-invasive tool to assess macroscopic behavior of the heart
  
  • Requires little time to acquire and analyze
  • Possible because the heart is large and synchronous
    
    • Brain signals are not, so only get small signals from an EKG
  • EKGs are recorded by special electrodes connected to sensitive amplifiers
  • Records extracellular potential differences (mV) and reports them as a function of time
    
    • By convention, sensing electrode (or lead) records upward deflections
      
      • Depolarization, positive charges, moving toward electrode

Question 2

THE HEART IS A SIMPLE DIPOLE/VOLUME CONDUCTOR

Answer 2

• Dipole = asymmetrical charge distribution within a volume conductor
  
  • (-) and (+) charges in a line outward from a central point
  • Electrodes placed perpendicular (orthogonal) to that line of charge = net charge/potential of 0
  • Line between electrodes closer to being in line with the dipole (180 or 0 degrees) = greater the potential difference between them
• Volume conductor = moves charge through an ionic solution (plasma, extracellular fluid)
• Creates an oriented electric field through which charge propagates in a specific direction
• Each cardiac cell creates it’s own dipole, the impulse propagates along the vectoral sum, this is what you want to line your electrodes up with

Question 3

BIPOLAR RECORDING

Answer 3

• Place two charged (-) and (+) electrodes and measure potential difference between them as an impulse moves across them.
• Potentials change as charge moves farther/closer to electrodes shifting the amount of depolarization and repolarization…results in the squiggles of the graph as potentials increase and decrease through a single impulse’s movement
• Advantages
  
  • Can determine velocity based on distance between electrode placement and time of depolarization -> repolarization
  • Change in length of conduction –> indication of pathology
  • Can also determine direction of propagation

Question 4

EKG DEFLECTIONS

Answer 4

• Positive inflection, current is moving toward positive electrode
• Negative deflection, current is moving away from positive electrode
• SMALL positive and negative deflection is movement of current perpendicular to electrodes

Question 5

EKG SEGMENTS AND INTERVALS

Answer 5

• P wave: Atrial Depolarization
  
  • PR interval = important clinically, time between atrial and ventricular depolarization (120-210ms)
    
    • Close to 0 mV because AV node and Bundle of Hiss are firing, but are very small and hard to detect their potentials
• QRS complex: Ventricular Depolarization
  
  • QT interval = Ventricle impulse propagating
  • ST interval = isoelectric, caused by plateau phase of ventricle cells
• T wave: Ventricular Repolarization

Question 6

EINTHOVEN’S TRIANGLE

Answer 6

• Three leads (electrodes) –> one at each wrist, and left ankle
• Determine the angle (direction) and strength of ventricular excitation
• LEAD I
  
  • (-) on R arm
  • (+) on L arm
  • Index of impulse in horizontal (0 degrees) direction
• LEAD II
  
  • (-) on R arm
  • (+) on L foot
  • Index of impulse at 60 degree angle
• LEAD III
  
  • (-) on L arm
  • (+) on L foot
  • Index of impulse at 120 degree angle

Question 7

LIMB LEADS

(3 unipolar and 3 bipolar, records in frontal plane)

Answer 7

• aVf = R and L arm connected
  
  • L leg = (+)
  • Index of impulse at 90 degree angle
  • QRS positive = moving in same direction as impulse
• aVr = L arm and foot connected
  
  • R arm = (-)
  • Index of impulse at 210 degree angle (-150)
  • QRS negative = moving in opposite direction of impulse
• aVl = R arm and L foot connected
  
  • L arm = (+)
  • Index of impulse at 330 degree angle (-30)
  • QRS very small/gone = perpendicular to impulse
• WOW, FIGURED OUT THE ANGLE OF PROPOGATION, COOL
  
  • Follows Lead II toward L foot, at 60 degrees

Question 8

CHEST LEADS

(6 unipolar, records in horizontal plane)

Answer 8

• Second electrode is grounded by combining the 3 limb leads
• Locations
  
  • V1 4th intercostal, R of sternum
  • V2 4th intercostal, L of sternum
  • V3 Between V2 and V4
  • V4 5th intercostal, L midclavicular
  • V5 L anterior axillary line, horizontally L of V4
  • V6 Midaxillary line, horizontally L of V4 and V5

Question 9

DETERMINING HR FROM EKG

Answer 9

• Paper on which the EKG is printed is calibrated
• Every 5 large boxes = 1 second, note 3 sec/6 sec lines are marked
• Height of each little square is 0.5 mV
• P wave to P wave

Question 10

ARRYTHMIA

Answer 10

• SA node abnormality
  
  • Atrial flutter
    
    • Reentry of SA impulse
    • Lots of waves
  • Atrial fibrillation
    
    • Many foci of excitation, not a nice wave
    • QRS inverted
• AV junction abnormalities
  
  • 2nd degree AV block
    
    • Long refractory period between T and P waves
    • 2:1 impulse
  • 3rd degree AV block (heart block)
    
    • 3:1 impulse
    • Ventricles are depolarized by another pacemaker
    • AV node is not depolarizing at all
• Ventricle abnormalities
  
  • Ventricular tachycardia
    
    • Lots of waves, with one notable larger and wider
  • Ventricular fibrillation
    
    • Lots of tall, skinny cray waves
• Try to correct arrhythmia by polarizing all cells in the heart and allowing SA (or any pacemaker) to reset rhythm

Question 11

MEAN ELECTRICAL AXIS

Answer 11

• Adding the information from the unipolar chest and limb leads –> 3D representation of the impulse propagation

Question 12

APPLICATIONS OF EKGs

Answer 12

• Determine HR
• Assess normal/abnormal cardiac rhythms
• Analyze intervals, duration of electrical complexes and segments
• Determine the mean electrical axis of the heart (to understand if there is R or L ventricular hypertrophy)
• Detect acute and chronic ischemic heart disease