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Objective 12 Electrocardiograms Electrocardiogram: a graphical recording of the potential changes that occur in the heart during a cardiac cycle Electrocardiograph: a device used to measure these changes Lead: a combination of electrodes that are used to detect electrical changes; there are 12 standard leads (3 bipolar, 9 unipolar) William Einthoven Characteristic Waves and Intervals of a Typical ECG: ECG Event Represented Duration (Sec) P Wave atrial depolarization 0.08 – 0.10 P-R Interval time impulse is being transmitted through the atria and delay at AV node 0.12-0.2 Name QRS Complex Event Represented Duration (Sec) ventricular depolarization 0.06 – 0.10 ST Segment time between the end of 0.08-0.12 ventricular depolarization and ventricular repolarization T Wave ventricular repolarization 0.16-0.24 Objective 13 Common Arrhythmias Analysis of ECG tracings include an evaluation of these characteristics: 1. Overall rate and rate of individual wave formations 2. Rhythm 3. Shape, height, and duration of wave formations 4. Deviation from the baseline Common deviations include: Sinus bradycardia: impulses arise at the SA node at a slow rate < 60 bpm Note: in well conditioned athletes this may not be an abnormality Sinus Tachycardia impulses arise at the SA node at an accelerated rate > 100 bpm Causes may include: •Exercise •Excessive sympathetic stimulation •Ischemia •Increased temperature When ectopic foci suppress SA node inherent rate: Atrial Flutter a single ectopic pacemaker produces an atrial rate of 200-300 per minute Reentry causes many arrhythmias Usually an ectopic pacemaker or a reentry Atrial Fibrillation several ectopic foci cause an atrial rate of 350-600 per minute; some reach ventricle notice the presence of QRS complexes Ventricular Fibrillation several ectopic foci generate potentials independently; there is no effective contraction of the ventricular myocardium, and therefore, no pulse Atrioventricular block First degree AV block: all impulses are conducted from the atria into the ventricles, but with a greater than normal delay; P-R interval > 0.20 seconds Second degree: some, but not all impulses are conducted from the atria into the ventricles; further defined as 2:1, 3:1, 4:1, etc; there is a consistent P-R interval Third degree: called complete AV block; no impulses are conducted from the atria into the ventricles; the P-R interval varies because the atria and the ventricles are contracting independently Bundle Branch Block: conduction by one of the bundle branches is impaired; ECG shows a widening of the QRS complex RBBB LBBB