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Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 1 of 6 ECG 2 Vectorial analysis of ECG (cont) Vector of ventricular depolarization (-) electrode on right arm (+) electrode on left arm Horizontal axis produced by the limb lead I system o Determining Normal depolarization vector (A) resolved into a horizontal component: --draw a line perpendicular to axis to touch the end of the vector (A) --deflection will be B-units in this limb system –use other limb leads to determine resultant vector A Axis System Axes system for each limb lead: I – horizontally II – downward to the left III – downward to the left (clockwise to axis system) ANGLES downward is positive Actual depolarization process is A o what is measured is the 3 limb lead deflections: B (limb lead I), C (limb lead II), and D (limb lead III) o polarity of deflections are also measured: B is (+), C is (+), D is (+) o determine A by drawing perpendicular lines from the magnitude of each limb lead (I, II, III) to determine magnitude of A Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 2 of 6 Change of waveform in QRS complex 1. 2. 3. 4. 1. Atrial depolarization begins in SA node and spreads over atria and to the AV node produces an upright deflection = P wave 2. Wave enters interventricular septum depolarization extends from left to right predominantly = Q wave Q wave tells us that there is a depolarization away from the positive electrode in limb lead I -away from left arm and to the right arm – thus negative deflection 3. Depolarization thru the ventricles greater left ventricular muscle mass = upright deflection of R wave as depolarization becomes complete, the R wave rises, and then goes down R wave is at peak when half of the ventricle is depolarized 4. Final left ventricular depolarization little bit of depolarization away from (+) electrode (rt arm) = small negative S wave Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 3 of 6 Limb Lead Systems 3 limb lead system augmented limb lead system Horizontal plane depolarization ECG is a voltmeter with (+) and (-) input augmented limb lead R (+) electrode placed on right arm (-) input to ECG is the electrical sum of the left leg and left arm thus direction of axis is upward and toward right arm augmented limb lead L (+) electrode placed on left arm (-) input to ECG is the electrical sum of the left leg and right arm thus, direction of axis is upward and toward left arm augmented limb lead F (+) electrode placed on left leg (-) input to ECG is the electrical sum of the left arm and right arm thus, direction of axis is down and left toward left leg Augmented and 3-lead system use same leads, but different machines -these 2 systems provide info about depolarization in the vertical plane Horizontal plane depolarization process chest leads = VI – V6 (know where they go – in picture) (+) input is from each lead VI – V6 (-) input is from combination of the the 3-limb lead positions V6 – on left side of chest produces large upright deflections (depolarization down and left in ventricles) V1 – on right side of chest produces a net negative QRS complex (depolarization away from (+) V1) Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 4 of 6 Augmented Lead System difference in the orientation of the QRS complex in different lead systems aVR depolarization away from right arm (-) thus, net negative complex Axes of 3 limb leads I=0 II = 60 III = 120 Augmented limb leads (bisect limb leads) Determine direction of depolarization with augmented limb leads: lead with largest deflection is most parallel with direction of depolarization ECG and Cardiac Myopathies 1. Ventricular Hypertrophy Magnitude and Orientation of Ventricular Depolarization -used to determine if a ventricle has hypertrophied -more muscle mass to depolarize -greater sum of depolarization processes in area of hypertrophy -larger amplitudes of QRS complexes Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 5 of 6 eg/ right ventricular hypertrophy due to pulmonary artery stenosis – pulmonary valve inadequate right ventricle must generate greater pressure for normal output detected on ECG with: large negative wave in limb lead I axial deviation to the right eg/ left ventricle hypertrophy o due to hypertension (greater pressure in aorta) o left ventricle must generate greater pressure for output o detected on ECG with: left axis deviation 2. Ischemia Ischemia’s effect on S-T segment “S-T segment elevation or depression indicates myocardial ischemia” -clinical lingo, but REALLY: during diastole, part of heart is depolarized (with ischemia) diastolic period (T P) will be shifted away from baseline baseline (isoelectric line) is set at 0, therefore appears as S -T deviation 3. Cardiac Arrhythmias Tachycardia = fast heart rate >100 beats / min Bradychardia = slow heart rate <60 beats / min see examples of ECG in handout 4. Sinus Arrhythmia Cardiac rhythm changes with time and affects heart rate Deeper respiration exaggerates sinus rhythm Sinus rhythm controlled by sympathetic and parasympathetic systems and by amount of blood entering atria (stretches pacemaker cells) 5. Nodal Block SA block No P wave or ventricular waves on ECG -ventricular depolarization begin later, but still no P waves AV block prolonged P-R interval = 1st degree AV block 2nd degree AV block failure to conduct excitation thru AV node dropped beat – no QRS with a P wave rd 3 degree AV block P waves with very few QRS Cardiopulmonary I Tues., 01/28/03, 11am Dr. Downey Corey Fischer Page 6 of 6 6. Premature Contraction - beats that occur earlier than expected Atrial atrial contraction earlier than expected Ventricular o PVC (Premature ventricular contraction) o Different characteristics than normal QRS complex o Arises from ectopic location in ventricle, and is conducted in different pattern thru ventricles -took more time without Purkinje system o longer QRS duration, and greater amplitude o multifocal PVCs occur in different places in ventricle 7. One-Way Conduction -causes reentry o enlarged ventricle that conducts in only one direction (ischemic) o reentry = tissue that has been previously excited is excited again with same action potential becomes a continuous circular process of depolarization types of reentry: atrial and ventricular flutter o regular, circular reentry atrial and ventricular fibrillation o less organized reentry o ventricular fibrillation – ventricle can’t contract treated with defibrillator