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