Page 244 - Feline Cardiology

Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 249

first-line treatment for severe hyperkalemia in these plexes of many cats makes this finding of little value
patients. This way, both the hyperkalemia and hypocal- unless the change is observed in serial ECGs from the
cemia are addressed simultaneously. When infusing same patient. Even then, care must be taken to rule out
calcium intravenously, ECG changes attributed to hyper- technical factors, especially limb position and ECG
calcemia (notably shortening of the QT interval and filters, either of which can artifactually reduce R wave
sudden slowing of the sinus rate), lip-smacking, or rest- amplitude.
lessness can be used as markers of excessively rapid infu-
sion, and are grounds for stopping the infusion and P Pulmonale/P Mitrale
resuming later if necessary. Here, as with hyperkalemia, P wave enlargement is recognized as an indicator of
serial ECGs are particularly valuable because baseline atrial enlargement. An unusually tall P wave (>0.2 mV
ECG recordings obtained at admission can be compared in lead II in the cat), termed P pulmonale, or an unusu-
to ECG recording obtained during hospitalization, ally wide P wave (>0.04 sec in the cat), termed P mitrale,
allowing the clinician to determine whether the patient’s suggest the presence of right or left atrial enlargement,
parameters are changing during treatment compared to respectively. The mechanism consists of a larger myocar-
his or her own baseline. dial mass generating a greater amount of electrical activ-
ity toward the positive pole of a lead (P pulmonale), a
Hypercalcemia
larger mass of myocardium deviating the atrial axis and
Similarly, hypercalcemia generally is of greater concern requiring a longer period of time to complete depolar-
for its extracardiac effects than for any alterations in ization (P mitrale), or a combination of both. In cats, Arrhythmias
cardiac rhythm. Severe hypercalcemia raises the thresh- criteria for sensitivity and specificity of these findings
old of the cardiomyocyte, which should hinder depolar- are not established, such that the association between
ization. It also shortens early ventricular repolarization, such ECG findings and the corresponding change in
making the QT interval shorter. These consequences of atrial structure is loosely defined. In clinical practice, the
severe hypercalcemia are of secondary concern com- value of excessively tall or wide P waves in cats is that
pared to dystrophic mineralization of the kidneys and either finding should prompt the suspicion of atrial
other soft tissues, for example. enlargement, which justifies an echocardiogram.

MORPHOLOGIC ECG ABNORMALITIES
ST Segment Depression/Elevation
Axis The ST segment refers to the part of the ECG that
The mean electrical axis and its deviations are discussed extends from the end of the QRS complex (the junc-
in Chapter 9. tional or “J” point) to the beginning of the T wave. It
should be horizontal, flat, and at the same height as the
Electrical Alternans baseline and PR segment. A vertical shift of the ST
The pattern of alternating R wave height known as elec- segment of more than 0.1 mV away from baseline levels
trical alternans is classically associated with pericardial is referred to as ST segment shift, and specifically as ST
effusion. The alternating short-tall QRS complexes are elevation or depression depending on the direction of
attributed to the change in mean electrical axis associ- displacement of the ST segment (Figure 18.26).
ated with the swinging motion of the heart in a fluid- The basis for ST segment change is myocardial
filled pericardium. Differentials include ECG filtration, hypoxia, because a hypoxic state causes myocardium to
intraventricular conduction abnormalities, and the repolarize unevenly. Specifically
Brody effect(Côté 2010). In the cat, the occurrence of
pericardial effusion is less common than in the dog, and Under normal conditions, the ST segment is usually
the small feline QRS complex means that changes in nearly isoelectric because virtually all healthy myo-
amplitude need to be substantial to be visible. For these cardial cells attain approximately the same poten-
reasons, electrical alternans is rarely documented in cats. tial during early repolarization—that is, during the
plateau phase of the ventricular action potential.
Small QRS Complex Amplitude Ischemia, however, has complex time-dependent
A decrease in QRS amplitude may be noted as a result effects on the electrical properties of myocardial
of pleural or pericardial effusion, or in association with cells. Severe acute ischemia can reduce membrane
obesity, hypothermia, hemorrhage, hypothyroidism, potential, shorten the duration of the action poten-
and intrathoracic mass in other species (Côté 2010). tial in the ischemic area, and decrease the rate of
QRS complexes may indeed become smaller as a result rise and amplitude of phase 0. These changes cause
of such disturbances, but the normally small QRS com- a voltage gradient between normal and ischemic