Page 212 - WSAVA2018
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 25-28 September, 2018 | Singapore
Is there a P-wave for every QRS complex?
Where a QRS complex arises without a normal P-wave it implies that the atria did not depolarise normally prior to ventricular depolarisation. This can occur when the depolarisation resulting in the QRS complex arises in the wrong place, or the atria are unable to depolarise normally. The possibilities are therefore either
 · Ventricular depolarisation
· Junctional depolarisation (The junction refers to the
AV node and bundle of His)
· Atrial standstill
· Atrial fibrillation or
· Sinus arrest with escape complexes.
Where normal complexes are visible, i.e. those that appear to have arisen in the normal sequence from sino-atrial node to atrioventricular node to the ventricle, then measurement of the amplitude of the constituents of the complexes may be useful in providing information regarding the presence of chamber enlargement. There are a few patterns of chamber enlargement described.
· Right atrial enlargement (RAE) may be represented
· Left atrial enlargement (LAE) may be represented by the presence of a wide P-wave, i.e. longer duration than 0.06 seconds. The P-wave may also become notched or M shaped. This is sometimes described as P-Mitrale.
· Left ventricular hypertrophy (LVH) may be signified by the presence of tall R-waves, wide QRS complexes and a shift of the mean electrical axis to the left.
· Right ventricular hypertrophy (RVH) may be signified by the presence of deep S-waves in leads I, II and III. There may be a shift of the mean electrical axis to the right.
Unfortunately none of these changes is specific to chamber enlargement. There is considerable overlap between normal animals and those affected by hypertrophy. For this reason the chamber enlargement suggested by the EKG must be interpreted in the
light of other available information e.g. radiographs, echocardiography or post-mortem data.
What does it mean?
Electrocardiographic abnormalities are rarely significant in isolation. Abnormal findings should be interpreted in conjunction with physical examination findings and the results of other diagnostic tests. The EKG is a relatively poor indicator of changes in size of cardiac chambers but is an excellent indicator of the presence of cardiac arrhythmias.
Is there a QRS complex for every P-wave?
If a P-wave is visible on the EKG and it is not followed
by a normal QRS complex then there has been failure
of conduction of the atrial depolarisation through the atrioventricular node in the normal way. This is described as atrioventricular block and can occur in many forms. There are three types commonly recognised and these are described as first, second and third degree AV block.
First degree AV block is a prolongation of conduction through the AV node. Second degree AV block is
an occasional failure of conduction through the AV node and third degree AV block is complete failure
of conduction through the AV node. In the latter case there must be an escape focus beneath the AV node to maintain ventricular depolarisation, albeit at a lower rate than normal.
Are the P-waves and QRS complexes consistently and reasonably related?
P-waves and QRS complexes may arise concurrently and yet not be related. This tends to show as an inconsistent relationship between the two and implies the presence of separate ventricular and atrial rhythms. This is described as atrioventricular dissociation.
Are the QRS complexes and the P-waves all the same?
Variation in the appearance of P-waves or QRS complexes may imply that they have originated from a different site or been conducted differently. This would normally suggest an abnormality of rhythm however some variation in P-wave amplitude can be normal in dogs and is described as a wandering pacemaker.
Is the heart rhythm regular or irregular?
If the rhythm is irregular is it regularly irregular or irregularly irregular. Normal rhythms tend to be either regular, or regularly irregular. An irregularly irregular rhythm is almost always abnormal. The most common rhythm of this type is atrial fibrillation; this sounds chaotic. Auscultation is a more sensitive way of determining the regularity of a rhythm.
• Determination of magnitude of deflections.
by the presence of a tall P-wave, i.e. greater than 0.4 mV. This is sometimes also described as P-pulmonale

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