Session 4: ECG

Session 4: ECG Electrocardiogram


Properties of cardiac tissue

  • Automaticity – tissue that can initiate an impulse without an outside force acting on it.
  • Contractility – the ability to respond to an impulse through a mechanical force
  • Conductivity – transmit an impulse
  • Excitability – the ability to be stimulated by the electric impulse.


  • Polarized is the resting state of the heart
    • Na out and K in the cell
  • Depolarized is the working of contraction phase of the heart
    • Na inside and K moves outside
    • If depolarization does not happen in a synchronized way then the chamber will not be able to contract effectively
  • Repolarization – the recovery phase moving from depolarization to polarized
  • Absolute refractory phase – the muscle cannot be depolarized under any physiologic stimuli. This occurs after the start of depolarization, during repolarization.
    • From the beginning of the QRS to the peak of the T
  • Relative refractory phase – after the absolute refractory phase where the muscle will respond to strong stimulus.
    • A vulnerable phase during the t wave, where we do not want to shock.
  • Nodes
    • SA node –  located in the right atrium and automatically fires 60-100 times per minute.
    • AV node – on the right side of the heart between the right atria and the right ventricle, this can fire at 40-60 beats per minute if needed,
    • Bundle of His – central heart in between all of the chambers.
    • Right and left bundle branches –  in between the ventricles
    • Purkinje fibers – in the bottom sides of the heart. This can fire at 15-40  

Getting an ECG

  • Electrodes are placed on the chest and the electrical activity reflects the activity in the heart.
  • The electrodes are placed at specific locations.
  • Common forms of ECGs are 3, 5, and 12 lead. The more leads the better “picture” of the heart we can get.

Interpreting the graph

  • Horizontal lines represent voltage (up and down)
  • Vertical lines represent time (left to right)
  • One small box represents 0.04 second from left to right, and 0.1mV up and down
  • One large box represents 0.2 seconds from left to right, and 0.5mV up and down
    • One large box is 5×5 small boxes.


  • The waveform is made up of peaks and valleys that are designated as the letters P through U. Each letter corresponds to a specific part of the Waveform.
  • P wave – Atrial depolarization, rounded and shorter than the QRS complex
  • PR interval – Measurement from the beginning of the p wave to the beginning of the QRS
    • Normal is 0.12-0.2 seconds
    • This is the time is takes for the impulse to travel from the SA node through the AV node and into the ventricles.
  • QRS – ventricular depol, less than or equal to 0.12 seconds
  • ST segment – end of V Depol to the beginning of V Repol
    • Elevation correlates with myocardial infarction
    • Depression correlates with ischemia
  • T – ventricular repolarization
  • QT interval – total duration of depol and repol

ECG interpretation

  1. Determine regularity
    1. Regular? Regularly irregular? Irregularly irregular?
  2. Calculate the heart rate
    1. Count the R waves on a 6 second strip and multiply by ten
    2. Do the atrial and ventricular waves match?
  3. Assess the P waves
    1. Are they present?
    2. Are the a consistent shape
    3. Is the ratio of P:QRS 1:1?
  4. Measure the PR interval
    1. Is is consistent? Is it normal?
    2. Can it be measured?
  5. Measure the QRS
    1. Do the complexes look the same?
    2. Is the measurement normal?


  • sinus tach – BPM over 100
    • Beta blockers will reduce rate
    • Adenosine given in a FAST 4 second push will slow or stop the heart to restart in a better rhythm
  • Sinus Brady – normal rhythm less than 60 bps
    • All Trained Dogs Eat
    • Atropine
    • Transcutaneous pulsing
    • Dopamine
    • Epinephrine
  • Atrial dysrhythmias
      • Likely to form a clot in the atrial these people need to be on anticoagulants for life.
      • AV node controls the  number of impulses that pass and then depolarize the ventricles. Needs a good AV node to control rate.
      • Controlled is a HR less than 100 uncontrolled is over 100
      • Unstable will need Synchronized Electrocardioversion, beta blockers, Vagal maneuver
      • Decreased CO and increased chance for clots.
    • Atrial flutter – saw tooth
      • One ectopic foci that is discharging an impulse at 250-400 times per minute
      • F waves no P waves
    • Atrial Fibrillation
      • Multiple ectopic foci that are discharging at 250-40 times per minute
      • Most common clinically significant dysrhythmia
      • Most common cause of a ischemic stroke.
  • Synchronized cardioversion – synchronized shock on the R wave
    • Need an R wave
    • Can convert them out of A flutter or fib
    • May keep them in sinus for 3-6 months
    • Amiodarone Is used to prolong the time in sinus
    • May throw a clot that was in the atrium once the rhythm converts back to sinus.
    • Get a transesophageal echocardiogram to look for clots.
  • Heart blocks
    • 1st degree
      • Consistent Long PR interval over 0.20
      • Do not treat usually do to lack of signs and symptoms
    • Second degree type one, or wenckebach, or Mobitz 1
      • AV node conduction issue
      • Progressively increasing PR interval until a QRS is missed and the arrhythmia start over
      • Usually asymptomatic
    • Second degree type II, or Mobitz II
      • Normal PR interval with a dropped QRS every so often
      • Monitor for a third degree heart block
    • Third degree heart block
      • No communication between the atria and ventricles
      • Need a pacemaker, from the cathlab
  • Ventricular dysrhythmias
    • PVC – Premature ventricular contraction
      • Can be normal and untreated
      • Bigeminy – on pvc for one normal
      • Trigeminy – two PVC for every normal PQRST
      • Causes – hypoxia
      • Amiodarone is the drug of choice
    • V Tach –
      • Can cardiovert
      • Can easily turn into V fib
      • Pulseless we will defibrillate
      • Large sharp “QRS” complexes one immediately after another with no breaks
    • V Fib
      • Defibrillate to hope the SA node takes over
      • Smooth rapid continuous electrical activity with no breaks
      • Cannot cardiovert
      • AICD is an internal defibrillation device
  • PEA
    • Pulseless electrical activity
    • Give fluids and epi
  • Asystole
    • Straight line on the ECG

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