Running a base-apex lead
There are four steps to run a base-apex Lead recording in the horse:
1) The right-arm electrode (frequently the white color-coded one) is clipped to the skin over the right shoulder or right
2) The left-arm electrode (black) is placed over the apex of the left heart slightly above the left elbow.
3) The left-leg (red) electrode serves as a ground and usually is placed on the right side of the neck.
4) The recorder is set to lead I. This will make the left-arm electrode positive and the right arm negative.
If you place your surface electrodes parallel to the main sweep of depolarization (negative charge moving from SAN to AVN
to ventricular apex) and set the positive electrode at the left apex of the heart, you will optimize the ECG recording.
This is exactly what happens in lead I. The "base" electrode (top of right heart) is negative and the "apex" electrode (apex
of left heart) is positive.
The P-wave represents atrial depolarization. Atrial repolarization is not seen as a separate event on the ECG recording. The
QRS complex represents ventricular depolarization.
Technically, the Q-wave is the first negative deflection of the trio, the R-wave is the first positive deflection and the
S-wave is the next negative deflection generated by ventricular depolarization. The T-wave represents ventricular repolarization.
The large atria in horses often results in slightly asynchronous depolarization, so the P-wave often appears as biphasic positive
deflection in the base-apex recording (i.e., has two small humps). The QRS in horses is depicted mostly as a downward deflection
of the ECG recording in the base-apex mode (i.e., mostly the S-wave). The T-wave usually is represented by an upward deflection,
but may also be biphasic or negative.
Intrepreting an ECG
There are several steps to interpreting an ECG, but a key step is identification of the QRS complexes.
1) Make sure the horse is standing still and the leads are secure.
2) Find the largest deflections. These should be the QRS complexes or the S-waves.
3) If there was a QRS complex (i.e., ventricular depolarization), there must be ventricular repolarization. So look immediately
behind the QRS complex to identify the T-wave.
4) Look in front of the QRS complexes. Is there a P-wave for every QRS? Is there a QRS for every P-wave? If there is no P-wave,
it could represent atrial fibrillation or an ectopic junctional or ventricular contraction. If there is a P-wave without a
QRS, then the AVN was blocked.
5) Study the intervals from QRS to QRS complex. Are they regular or irregular?
This will help identify normal rhythm, premature beats, and blocked or escaped beats.
6) Do the QRS complexes come earlier (premature) or later (escape or blocked) than expected?
7) Do the configurations of all the QRS complexes and P-waves look the same? This can help you tell if the origin of a complex
is normal or ectopic and unifocal or multifocal.
8) Determine the heart rate.When the ECG recorder is set to a paper speed of 25 mm/sec, each of the smallest boxes on the
ECG paper grid is equivalent to 0.04 sec. There are five of these boxes in the next larger box on the grid, which is 0.2 seconds.
So if you count the number of QRS complexes over 30 of the larger boxes (six seconds) and multiply by 10, you have the number
of ventricular contractions per minute. Some recorders place a large "tick" on the top of the paper that is generated every
three seconds. So again, the heart rate can be determined by counting the number of QRS complexes generated over six seconds
and multiplying by 10, when the paper speed is 25 mm/sec (Figure 1).
Many newer ECG units will give a heart rate based on the number of QRS deflections, but be careful: Because the equine P-
and T-waves are bigger than many other species, these automated counts often are erroneously high, as they mistakenly include
the P-and T-waves, as well as the QRS complexes.
9) Are the durations of the complexes normal? In general, this information usually is not critical in making the diagnosis
of an arrhythmia in horses, but here are reference intervals: