Cardiac Conduction

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Cardiac Conduction
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Physiology of Cardiac Conduction

• The excitatory electrical conduction system of
  the heart is responsible for the contraction and
  relaxation of the heart muscle.
• The sinoatrial node (SA node) is the pacemaker
  where the electrical impulse is generated.

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Cardiac conduction pathway

• SA node fires to the AV node through gap jxns
• AV node delays 0.1 s for atrial diastole
• AV node fires to the AV bundle (HIS)
• AV bundle depolarizes through right left bundle
  branches
• Bundle branches carry impulses through Purkinje
  fibers to the ventricular myocardium for
  ventricular systole
• Total time of conduction 0.22 s

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Nodal Firing Rates

• Sa node 75 b.p.m.
• AV node 50 b.p.m.
• AV bundle 30 b.p.m.
• Purkinje fibers 30 b.p.m.

What would happen if the SA node could not
conduct an impulse to the AV node?
Heart block (no gap jxns found between atria
ventricles)
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How to Read an
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Electrocardiogram Paper

• A grid system where time is measured along the
  horizontal axis.
• Each small square is 1 mm in length
  represents 0.04 seconds.
• Each larger square is 5 mm in
  length represents 0.2 seconds.

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ECG Voltage

• Is measured along the vertical axis.
• 10 mm is equal to 1mV.

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Heart rate can be calculated from the EKG strip

• When the rhythm is regular, the heart rate is 300
  divided by the number of large squares between
  the QRS complexes.
• For example, if there are 4 large squares between
  regular QRS complexes, the heart rate is 75
  (300/475).

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Heart rate can be calculated from the EKG strip

• Can be used with an irregular rhythm to estimate
  the rate. Count the number of R waves in a 6
  second strip and multiply by 10.
• For example, if there are 7 R waves in a 6 second
  strip, the heart rate is 70 (7x1070).

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Quick Quiz

• True or false
• On a typical EKG grid, 5 small squares, or 1
  large square, represent 0.20 seconds of time

True
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PQRST wave
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Deflection waves

• P wave
• Lasts 0.08 s
• Results due to depolarization from SA node
  throughout atria
• Atrial systole
• Normal duration is not longer than 0.11 seconds
  (less than 3 small squares)
• Amplitude (height) is no more than 3 mm

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Deflection waves

• QRS complex
• Lasts 0.08 s (Normally not longer than 0.10 s in
  duration)
• Results due to depolarization of ventricles
• Ventricular systole atrial diastole
• R waves are deflected positively and the Q and S
  waves are negative
• T wave
• Results due to repolarization of ventricles
• Lasts 0.16 s
• Ventricular diastole

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Normal ECG Deflection Wave Pattern
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Abnormal ECG Deflection Wave Patterns
Sinus Bradycardia
Rate 40-59 b.p.m.
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Sinus Tachycardia

• Rate 101-160 b.p.m.
• Causes
• CHF, hypoxia, pulmonary edema
• Increased temperature
• Stress or response to pain

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Sinus Arrhythmia
Rate 45-100 b.p.m.
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Sinus Arrest
Causes Myocarditis MI Digitalis toxicity
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Atrial Flutter
Rate 250-350 b.p.m. Precipitates CHF
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Atrial Fibrillation (afib)
Causes COPD CHF
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AV block

• Causes
• Digitalis toxicity
• Acute infection
• MI
• Degeneration of the conductive tissue.

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Ventricular Tachycardia (V-tac)
Rate 100-220 b.p.m.

• Causes
• CAD
• Acute MI
• Digitalis toxicity
• CHF

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Ventricular Fibrillation (V-fib)
Causes Acute MI
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Asystole