Right axis deviation Electrophysiology of Cardiac Contractions 251 I II III QRS complexes point together like letter R Lead I S = R implies +90° Lead I S > R implies > +90° I III QRS complex – Normal if width <0.12 second (three small squares). – If >0.12 second — bundle-branch block. – An apparently wide QRS complex, <0.12 second wide — partial bundle-branch block or interventricular conduction defect. – Left bundle-branch block (LBBB) is usually associated with some form of heart disease. – RBBB is often a normal variation, especially in athletes. Immedi- ately after a myocardial infarction the development of RBBB may be serious. Left bundle-branch block – M pattern in V 6 . – Throughout ECG, slurred ST segment and T wave inversion opposite to major deflection of QRS. – Lead V 6 – depolarization of septal muscle from right bundle gives positive deflection – right heart depolarization gives negative deflection – left heart depolarization gives positive deflection 252 Chapter 12:The 12-Lead Electrocardiogram 1 3 2 2 1 3 Lead V 6 R 1 R V6 – Standard leads – left axis deviation as impulse spreads from right bundle up to left ventricle – also occurs if only anterior fascicle of left bundle blocked – left anterior hemiblock Right bundle-branch block – M pattern in V 1 . – Lead V 1 – depolarization of septal muscle from left bundle gives positive deflection – left heart depolarization gives negative deflection – right heart depolarization gives positive deflection – Standard leads – axis usually normal, as depends on large muscle mass of left ventricle – if RBBB is associated with left axis, there is block of anterior fascicle of left bundle — bifascicular block All heart is being excited via remaining posterior fascicle of left bundle. Arrhythmias – sinus arrhythmia – ectopics – tachycardias – bradycardias Sinus arrhythmia Normal variation with respiratory rate — increase rate on inspiration. Electrophysiology of Cardiac Contractions 253 1 3 2 2 1 3 Lead V 1 R 1 R V1 Ectopics Atrial ectopics Ectopic focus anywhere in atria. Depolarization spreads across atrium to AV node like any normal beat: – P wave is abnormal shape – normal QRS complex The atrial ectopic focus must fire early — or would be entrained by normal excitation: – appears early on rhythm strip – followed by compensatory pause — waiting for normal SA node cycle 254 Chapter 12:The 12-Lead Electrocardiogram P P PP Compensatory pauseAtrial ectopic Ectopic focus Sinus node Atrial ectopic—an inverted P wave Atrial ectopics Junctional or nodal ectopics Ectopic at AV node; no P wave. Electrophysiology of Cardiac Contractions 255 P Nodal ectopic TT PT TP Ventricular ectopics Ectopic anywhere in ventricles. Depolarization occurs first in that ventricle then spreads to other ventricle: – no P wave – wide complex – bundle branch-block pattern – left focus — RBBB pattern – right focus — LBBB pattern Atrial and junctional ectopics are invariably innocent when picked up on a random ECG.The majority of ventricular ectopics are also innocent except after a myocardial infarction. Ventricular ectopics picked up on routine monitoring of healthy patients are approximately proportional to age, i.e. 30% of 30-year-olds, 50% of 50-year-olds and almost 100% of 70- year-olds. Innocent ventricular ectopics usually disappear on exercise. Tachycardias Classification of tachycardias ° Tachycardias are divided into: – Narrow-complex regular — QRS complex up to 0.08 seconds — two little squares on ECG – sinus tachycardia 256 Chapter 12:The 12-Lead Electrocardiogram P Ventricular ectopic T – supraventricular tachycardia, atrial tachycardia, atrial flutter – Narrow-complex irregular – atrial tachycardia with varying block, atrial fibrillation – Broad-complex — QRS complex about 0.12 seconds — three small squares – ventricular arrhythmias and occasionally supraventricular with aberrant (delayed) conduction ° Deciding whether a tachycardia is atrial or ventricular is not easy. Here are some pointers. – Narrow-complex tachycardias are usually atrial and broad- complex usually ventricular, but not always. – When acute ischaemic heart disease is present, tachycardias are usually ventricular. In the absence of ischaemic heart disease tachy- cardias are usually atrial, but not always. – If there is independent atrial activity (random appearance of p values), the tachycardia is ventricular. – Look at the patient’s preceding ECGs or rhythm strip. If the tachy- cardia looks like a previous ectopic beat in shape, it will be that type of tachycardia. – Vagal stimulation (rubbing carotid, etc.) will only be effective in atrial rhythms. – The regularity or irregularity is not helpful in distinguishing ventricular from atrial arrhythmias. Atrial fibrillation The electrical impulse and contraction travel randomly around the atria: – ‘bag-of-worms’ quivering atria – irregular little waves on ECG — best seen V 1 Electrophysiology of Cardiac Contractions 257 Sinus tachycardia When it first develops, often 150+,fibrillation waves are difficult to see: – AV node fires irregularly – normal QRS complexes If irregular rate, no P waves, normal QRS — likely to be atrial fibrillation. Digoxin is still the drug of choice — it decreases transmission of impulses down the bundle of His. Atrial flutter Atria contract very rapidly, 200–250 beats/min, giving a sawtooth pattern, but the ventricles only respond to every second or third or fourth contraction (2:1, 3:1, 4:1 block). 258 Chapter 12:The 12-Lead Electrocardiogram Treated with digoxin, normally changes to atrial fibrillation. Supraventricular tachycardia (SVT) – Arises near AV node, 170 beats/min or more, regular. – Complexes are identical, normal width or wide if also bundle- branch block. – Common in young patients (20–30 years). – Rarely represents heart disease. – Sudden onset and finish. – Last few minutes to several hours. – May be tired, light-headed, uncomfortable. – In older patients SVTs more likely to represent heart disease. Electrophysiology of Cardiac Contractions 259 2 : 1 atrial flutter Re-entry mechanism Vagal stimulation (rubbing carotid sinus) can terminate attack. Re-entry is the most common mechanism for tachycardias (Fig. 12.5). Assumes two conduction pathways lead to ventricles. Normally conduc- tion passes equally quickly down both pathways. Problems arise when one pathway recovers more slowly than the other. When this happens the next conduction passes down only one pathway. Conduction subsequently passes retrogradely up the other pathway, which is no longer refractory. This pathway then becomes refractory while the first pathway conducts again and the impulse races round the pathways to give a tachycardia. Wolff–Parkinson–White syndrome This is the classic re-entry arrhythmia. There are two separate pathways from the atria to the ventricles. In the resting ECG the early entry, by the aberrant conduction pathway bypassing the bundle of His, is seen as a delta wave. 260 Chapter 12:The 12-Lead Electrocardiogram Non-refractoryRefractory RefractoryNon-refractory Non-refractoryRefractory Fig. 12.5 The mechanism of re-entry tachycardias. [...]... short-acting – medium-acting – long-acting Ethanol (physiological . healthy patients are approximately proportional to age, i.e. 30% of 30-year-olds, 50% of 50-year-olds and almost 100% of 7 0- year-olds. Innocent ventricular ectopics usually disappear on exercise. Tachycardias Classification. delta wave. 260 Chapter 12:The 12-Lead Electrocardiogram Non-refractoryRefractory RefractoryNon-refractory Non-refractoryRefractory Fig. 12.5 The mechanism of re-entry tachycardias. Electrophysiology. R Lead I S = R implies +90 ° Lead I S > R implies > +90 ° I III QRS complex – Normal if width <0.12 second (three small squares). – If >0.12 second — bundle-branch block. – An apparently