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 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 Figure 11-3
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