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Appendix C Electrocardiogram EKG or ECG Electrical Stimulation of the Heart: The electrical stimulus that is required before every heart beat causes sodium , potassium, and other ions to move in or out of every cell in the heart. This exchange of ions produces an electrical current that has three distinct phases: Electrical Phases in the Heart Polarization - ("ready phase") state of cellular rest when all ions are in their correct places and ready for stimulus Depolarization - state of cellular stimulation which precedes contraction Repolarization - ("recovery phase") state of cellular recovery which follows each contraction P wave The P wave represents the depolarization of the atria (atrial depolarization) The P wave contour is usually smooth entirely positive (Leads I, II, III, aVF, and V4 to V6) negative (aVR) (monophasic) in all leads except V1 The P wave duration is normally less than 0.12 sec. The P wave amplitude is normally less than 0.25 mV in all leads The P wave normally appears entirely upright PR Interval The PR interval measures the time required for the impulse to travel from the atria myocardium adjacent to the SA node to the ventricular myocardium adjacent to the fibers of the Purkinke network (atrial and ventricular depolarization) The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. Normal PR interval duration range is from 0.12 sec - 0.20 sec QRS Complex represents depolarization of the ventricles (ventricular depolarization) If the first deflection from the isoelectric line is negative it is a Q wave (not always present) The first positive deflection from the isoelectric line is an R wave The negative deflection following an R wave is an S wave Normal QRS interval range is from 0.04 sec - 0.12 sec Measured from the first deflection from the isoelectric line to the Jpoint (J-point is where the QRS complex ends and the ST segment begins) ST Segment represents the plateau (phase 2) of the action potential (ventricles in active state following depolarization, but NO electrical activity occurs at this time. Is normally isoelectric - no difference exists in electrical potential among the action potentials of the heart No current flow occurs because all cells are at zero potential T wave represents phase 3 of the action potential, when the ventricles are being rapidly repolarized (ventricular repolarization) Is normally rounded, slightly asymmetric, and the same polarity as the QRS complex The effective refractory period is present during the beginning of the T wave. QT interval represents the entire duration of ventricular depolarization and repolarization (ventricular refractory period is the time necessary for the ventricle to depolarize, then repolarize) The normal QT varies with age, gender, and heart rate Cardiac Rhythms SA node Normal Sinus or Regular Sinus Rhythm (NSR or RSR) P wave present and regular. Atrial rate (P waves) between 60 and 100 beats/min Each P wave is followed by a QRS complex Sinus Bradycardia P wave present and regular Atrial rate (P waves) < 60 beats/min Each P wave is followed by a QRS complex RX: May require no treatment Atropine IV May require temporary pacemaker or permanent pacemaker Sinus Tachycardia P wave present and regular Atrial rate (P waves) > 100 beats/min Each P wave is followed by a QRS complex RX: treat underlying cause Sinus Arrhythmia P wave present P-P interval - phasic shortening then lengthening of P-P interval, usually with respirations Impulse initation by SA node RX: usually none Common Arrhythmias Originating from Atria: Premature Atrial Contractions (PAC) initiated by ectopic focus in the atria premature P wave with a contour different from a sinus P wave (location of ectopic focus determines its shape) QRS may or may not be normal PAC is followed by a pause ~ equal to the sinus cycle (measured R to R) Premature Atrial Contractions (PAC) associated with use of caffeine, stress, or use of tobacco may be a precursor to developing uncontrolled AF RX: may require no treatment sedation quinidine Atrial Flutter rapid sawtooth P waves ventricular rate regular associated with CAD, pulmonary embolism, mitral valve disease, and thoracic surgical procedures. atria depolarize at a rate of 250 to 350 beats/min Atrial Fibrillation rapid irregular P waves > 350/min ventricular rate irregularly irregular ventricular rate varies, may increase to greater than 150 if untreated if rate > 100 beats/min referred to as uncontrolled AF if rate < 100 beats/min referred to as controlled AF Sick Sinus Syndrome (SSS) term to describe several disorders of the SA node tachycardia-bradycardia syndrome is the most common type of SSS complication associated with SSS is CHF and CVA resulting from thromboembolisms RX: stabilization of heart with perm pacemaker Atrial Tachycardia rate 150 to 250 beats/min P wave present but may be hidden QRS is generally normal ventricular rate is regular RX: usually none prolonged episodes may require carotid sinus pressure, vagal stimulation, verapmil, digitalis, or beta blocks Impulse Conduction Deficits A-V Blocks First Degree AV Block PR interval prolonged - > 0.20 sec May warn of impaired conduction Second Degree AV Block Mobitz I (Wenckebach) PR interval progressively lengthens until a P wave is not followed by a QRS complex Ratio of P waves to QRS complexes varies, i.e. can be 5:1, 4:1, 3:1, or 2:1 more often is a transient event seen with patients post Inferior MI, Digitalis toxicity, or postoperative Mobitz II more serious that Mobitz I less common than Mobitz I characterized by nonconducted sinus impulses despite constant PR intervals usually the QRS are widened because of a BBB, the dropped beat represents a form of intermittent blockage of both bundle branches the defect is found in either the bundle branches of the bundle of HIS occur more frequently with patients with acute anterior septal wall MIs often progress to CHB Third Degree AV Block (Complete Heart Block) atria and ventricles beat independently P waves have no relation to QRS ventricular rate may be as low as 20-40 beats/min Bundle Branch Block (BBB) same as normal sinus except QRS complex is > 0.12 sec depending on site of defect is labeled RBBB or LBBB - the LBBB has two main devision the anterior and posterior fascicles seen with severe CAD, acute anterior wall MI, hypertensive pts Ventricular Arrhythmias Premature Ventricular Contractions arise from an ectopic focus in the ventricles wide bizarre QRS greater than 0.12 sec no associated P wave T wave is in the opposite direction from the main QRS deflection ~ 50% of PVCS are followed by a compensatory pause (the interval from the beat preceding to the beat following the PVC is equal to two sinus cycles) Premature Ventricular Contractions the remaining 50% PVCs result in retrograde conduction to the atria causing the next sinus P wave to be early and a compensatory pause occurs associated with CHF, digitalis toxicity, electrolyte imbalances, and excessive caffeine intake Ventricular Tachycardia no P wave before QRS QRS wide and bizarre ventricular rate > 100 beats/min (usually 140-240) Ventricular Fibrillation chaotic electrical activity no recognizable QRS complex associated with MI, drug toxicity, electrocution, freshwater drowning no CO absent pulse or respirations -- CARDIAC ARREST