The Electrocardiogram
Chapters 11 and 13
AUTUMN WEDAN AND NATASHA MCDOUGAL
The Normal Electrocardiogram
▪ P-wave
▪ Generated when the atria depolarizes
▪ QRS-Complex
▪ Ventricles depolarizing before a contraction
▪ T-wave
▪ Repolarization wave in the ventricles
The Normal Electrocardiogram
Figure 11-1
Depolarization Waves Versus Repolarization Waves
▪ During depolarization the negative potential inside the fiber reverses and becomes positive inside and negative outside
▪ Repolarization occurs when the fiber begins returning to a negative potential inside and a positive potential outside
▪ This occurs in approximately 0.30 seconds
Monophasic Action Potential
Figure 11-3
▪ Atria repolarizes about 0.15-0.20 seconds after termination of p-wave during the QRS complex
▪ Ventricular repolarization is the T wave
▪ Generally around 0.20-0.35 seconds
Voltage and Timing of Electrocardiograms
▪ Voltage is dependent on attachment and proximity to the heart
▪ Can be as high as 3-4 millivolts
▪ When recorded on two arms (or an arm and a leg) voltage for the ORS complex is about 1.0-1.5 millivolts
▪ P-Q or P-R interval
▪ Between the beginning of electrical excitation of the atria and excitation of the ventricles
▪ Normal interval is about 0.16 seconds
▪ Q-T interval
▪ Contraction of ventral
▪ 0.35 seconds long
▪ Heart rate as determined by electrocardiogram
▪ Reciprocal of the time interval between two successive beats
▪ Average heart rate is 72 beats per minute
Flow of Electrical Current
• The heart is surrounded by electrically conductive medium
• When depolarization occurs electrical current flows around the heart, inducing depolarization in the pattern shown
• Average current flow occurs with negativity toward the base of the heart and positivity toward the apex
Electrocardiographic Leads
▪ Lead 1: negative electrode on the right arm, positive electrode on the left arm
▪ Lead 2: negative electrode on the right arm, positive electrode on the left leg
▪ Lead 3:negative electrode on the left arm, positive electrode on the left leg
Cardiac Arrhythmias
▪ Abnormal rhythm of the heart
▪ Causes:
▪ Abnormal rhythmicity of pacemaker
▪ Shift of pacemaker from sinus node
▪ Blocks at different points in the spread of the impulse
▪ Abnormal pathways of impulse transmission
▪ Spontaneous generation of spurious impulse in almost any part of the heart
Abnormal Sinus Rhythms
▪ Tachycardia ▪ Fast heart rate
▪ > 100 beats/min
▪ Determined by time interval between QRS complexes
▪ Causes:
▪ Increased body temperature
▪ Stimulation by sympathetic nervous system
▪ Blood loss / shock
▪ Weakening of myocardium
▪ Bradycardia ▪ Slow heart rate
▪ < 60 beats/min
▪ Athletes
▪ Causes:
▪ Vagal Stimulation
Conduction System Blockage – SA block
▪ Rare
▪ Impulse blocked before entering atrial muscle
▪ Cessation of P waves
▪ Results in standstill of atria
▪ Ventricles pick up new rhythm
▪ Rate of Ventricular QRS-T complex is slowed but not altered
Conduction System Blockage – AV block
▪ Decrease the rate of impulse conduction through the bundle of His
▪ Ischemia of the AV node or AV bundle fibers
▪ Compression of the AV bundle
▪ Inflammation of the AV node or AV bundle
▪ Extreme stimulation of the heart by the vagus nerves
▪ First Degree
▪ Second Degree
▪ Third Degree
First Degree AV Block
▪ Incomplete heart block
▪ Delay of conduction from atria to ventricles
▪ Prolonged P-R interval
▪ Greater than 0.20 seconds
Second Degree AV Block
▪ Conduction through AV bundle is slowed
▪ P-R interval 0.25-0.45 second
▪ Atrial P wave without QRS-T wave
▪ “dropped beats” of the ventricles
Third Degree AV Block
▪ Complete AV Block
▪ Severe cases of AV node or AV bundle block
▪ Ventricles spontaneously establish their own signal
▪ P waves dissociated from the QRS-T complexes
Ventricular Fibrillation
▪ Most serious
▪ If not stopped within 1-3 minutes, will most likely result in death
▪ Cardiac impulses that go awry
▪ One portion of ventricular muscle is stimulated
▪ Then another, and another, etc.
▪ Eventually it feeds back onto itself to re-excite the same ventricular muscle over and over
▪ ECG is crazy looking with no rhythm
▪ Must intervene with Defibrillation to reset rhythm
Defibrillation
▪ A strong high-voltage alternating electrical current passed through the ventricles
▪ Stops fibrillation by throwing all the ventricular muscles into refractoriness simultaneously
▪ Hand pumping of the heart
▪ Aid to defibrillation
▪ Keeps blood moving to brain