Cardiology - ECG abnormalities Flashcards
What causes changes to the shape of the P wave? In what leads is this best viewed?
The P wave shape is altered by atrial enlargement and arrhythmias. The normal P wave is the sum of the right and left atrial depolarization vectors and is best examined in leads II and V1.
What effect does it have on the ECG?
The ECG is not reliable in diagnosing right atrial enlargement. When the RA is enlarged depolarization takes longer (more distance to travel) and involves greater current flows (depolarizing atrial myocytes let more ions in, increasing current flow).
The vector directed towards lead II becomes larger, so the p wave in lead II becomes taller. V1 is less affected by atrial enlargement.
What is the effect of left atrial enlargement on the ECG?
In LA enlargement, the LA depolarization vector is prolonged and increased:
- in lead II there is a long late high voltage positive deflection after the initial RA P wave resulting in a bifid shape (called P mitrale from rheumatic mitral stenosis)
- as the vector of LA depolarization proceeds away from lead V1, after a small initial positive deflection arising from the normal right atria, the P wave is dominated by a late negative deflection which is a sensitive marker of LA enlargement
Causes of right atrial enlargement
COPD related pulmonary hypertension (often the cardiac axis is right shifted)
What causes left atrial enlargement?
Hypertension: look for ECG left ventricular hypertrophy
Aortic and mitral valve lesions: listen for murmurs
Previous MI
Cardiomyopathy: non specifically abnormal ECG or conducting tissue disease
What feature of the P wave shape would suggest a low atrial or AVN ectopic pacemaker?
If the pacemaker is situated other than in the sinus node then the P wave shape is altered. Inverted P waves in leads II, III and aVF can suggest ectopic pacemaker activity.
What determines the size of the QRS complex?
1) The number and activity of myocytes. Myocytes may become less numerous with age and are more electrically active in youth (< 40 years) and in ventricular hypertrophy.
2) The insulation between the heart and the observing electrodes. A pericardial effusion or obesity, diminishes the amount of electrical activity reaching the electrodes.
How does right ventricular hypertrophy affect the ECG?
Right ventricular hypertrophy results in greater voltages from the right ventricle, resulting in:
1) Right axis deviation
2) Leads looking at the right ventricle show a positive deflection. In particular, in lead V1 the size of the R waves increases - instead of the normal situation where there are small R waves followed by large negative S waves. The R wave can be the same height as the S wave and remains narrow
3) In severe RVH there can be T wave changes
4) Left sided chest leads are unchanged by RVH (the bulk of the right ventricle even when hypertrophied does not overshadow the influence of the left ventricle)
5) Signs of right atrial enlargement may be present, the the P wave becoming leaked early on in lead II and V1.
How does left ventricular hypertrophy affect the ECG?
In LVH greater voltages are generated by the left ventricle which results in:
1) Left axis deviation
2) Leads looking at the left ventricle show an increased deflection - i.e. leads I, II, aVL, and V5 and V6
3) Leads looking away from the left ventricle show increased negative deflections - i.e. increased size of the S wave in leads V2 and V3
4) May be signs of left atrial enlargement (bifid P wave in lead II and late negative deflection in V1)
What are pathological Q waves?
These are a key finding as they indicate cardiac damage. They must be distinguished from physiological Q waves which occur in:
- left sided chest leads where they reflect left to right septal depolarization
- lead aVR which “looks” directly through the AV valve at the endocardial surface of the heart
What is the mechanism underlying pathological Q waves?
Pathological Q waves indicate an “electrical window” in the part of the heart directly facing the electrode which allows current from the opposite wall to influence the electrode in an unopposed fashion. As depolarization proceeds from the endocardium to the epicardium, the electrode looking directly into the heart through this electrical window sees a Q wave rather than an R wave .
What causes pathological Q waves?
These indicate:
- an old transmural infarct; the Q wave distribution reflecting which artery has been occluded
- less commonly another pathology such as LVH (increased Q waves in left sided leads in association with large R waves) or occasionally hypertrophic cardiomyopathy (large Q waves in inferior leads without substantial R waves)
- myocarditis or dilated cardiomyopathy (rare)
What causes pathological loss of R wave height?
Damage insufficient to cause Q waves, but sufficient to cause death of some cells in the heart (i.e. sufficient numbers survive so that some electrical activity continues) leads to a decrease in R wave height without Q wave formation. Pathologically small R waves can be difficult to diagnose as there is normal variation in R wave height.
Pathological loss of R wave height usually follows regional distribution in coronary arteries.
What is meant by the term QRS axis?
The overall QRS vector shows the direction of depolarization of the bulk of ventricular mass. As such, it is mainly directed towards the main muscle mass being depolarised - i.e. in health towards the left ventricle.
What causes left axis deviation?
There are 2 main interpretation of left axis deviation:
1) More left ventricular muscle mass to depolarize - i.e. LVH is present which usually also causes left sided prominent R waves and deep right sided S waves
2) Left ventricular depolarization is delayed as the conducting system is damaged. The left anterior fascicle of the left bundle branch supplies much of the anterior part of the left ventricle. If this is damaged, the left anterior part of the left ventricle depolarizes late, which then predominates the depolarization vector, resulting in left axis deviation
What causes right axis deviation?
1) More right ventricular mass - i.e. RVH
2) The posterior lying bulk of the left ventricle is depolarized late due to disease in its conducting tissue - the posterior fascicle of the left bundle
What is the normal PR interval and what affects it?
The PR interval reflects the time between the start of the P wave and the first QRS deflection. On ECG paper it is <3 small squares. The normal PR interval is lengthened by high vagal tone/ low sympathetic tone and low heart rates and shortened by exercise and high heart rates.
PR interval prolongation can also result from diseases affecting the AVN, the bundle of His or both bundle branches
What is bundle branch block?
Disruption of the conducting system at any point from the SAN to the bundle of His can prolong the PR interval. Disruption below this level broadens the QRS complex resulting in bundle branch block.
What is right bundle branch block (RBBB)? How does it affect the ECG?
RBBB causes the right ventricle to be activated late, so right sided leads see unopposed activity late on resulting in a late deflection in lead V1. QRS complexes in leads looking directly at the left ventricle (i.e. I, II, aVL and V4-6) appear mostly normal.
An aide memoire is that RBBB causes a “MaRRoW” type pattern in the chest leads. The QRS complex is mostly above the isoelectric line in lead V1 and below it in lead V6.
What is left bundle branch block? How does it affect the ECG?
Full block of the left bundle gives rise to delayed and slowed activation of the left ventricle, so broadening the QRS complex in the left sided leads. Partial blockage of the left bundle results in axis deviation without QRS broadening.
The septum is depolarized right to left (rather than left to right) so there are:
i) no physiological Q waves in left sided chest leads and
ii) delayed activation of the left ventricle, resulting in a large late positive deflection in left ventricle (LV) leads
