Page 84 - Feline Cardiology

Chapter 9: Electrocardiography 81

A Diagnostic Testing
Figure 9.7. Cat instrumented with Holter monitor. With a light
but secure wrap, modern Holter monitors are sufficiently light
that they can be worn comfortably by cats. Photograph courtesy
of Dr. Fiona Campbell.

when they sense significantly low or elevated heart rates
(the triggering rates are chosen by the programmer).
Internal event monitors measure 0.5 × 2 × 7 cm, about
the size of a man’s thumb; have a battery life of approxi-
mately 7 months; and have a memory capacity of about
1 hour of ECG recording. The information they capture
B is retrieved after the episode is over, using a pacemaker
interrogator unit placed over the area of skin where the
Figure 9.6. Cardiac event monitors. (A) External event monitor monitor is located (requires a return trip to the veteri-
(left) and implantable event recorder (upper right) with its exter- nary hospital). The use of both types of event monitors
nal, owner-operated activator (lower right). (B) Cat instrumented is of proven diagnostic value in humans; both external,
with an external event monitor. The monitors are light (100 g; less and to a lesser extent so far, internal, monitors are in
than 1/4 lb.) and tolerated by most cats when lightly wrapped
using roll gauze and elastic wrap. Courtesy of Dr. Christopher accepted use in cats (Bright and Cali 2000; Côté 1999;
Rocchio, Cottonwood Images, and Veterinary Learning Systems. Krahn et al 1999). Each form of ambulatory ECG moni-
toring has its relative merits (Côté 2010). Holter moni-
tors are superior for screening the number of abnormal
and the monitors are programmed beforehand to save a heartbeats on the ECG in a fixed 24- or 48-hour period,
selected amount of ECG information (e.g., 45 seconds such as for assessing a cat’s arrhythmia before and during
preceding the push of the button and 15 seconds after it treatment with an antiarrhythmic drug (Figure 9.7).
for each triggered episode). They are available for use Conversely, external event monitors also have a number
from various sources, including many of those listed of advantages: they are smaller and lighter (3.5 oz [100 g],
previously for Holter monitors. compared with up to 14 oz [400 g] for a Holter monitor),
Internal event monitors (implantable loop recorders making them especially practical for cats; they have a
[ILRs], Reveal® monitors) are for extended periods of longer memory (at least 1 week, and up to 30 days in
ECG assessment. These units consist of two parts: the certain models), which is useful for episodes occurring
monitor and the patient (owner) activator (see Figure less frequently; they eliminate some uncertainty of
9.6). The monitor itself is implanted subcutaneously in interpretation by directly associating the ECG informa-
the patient, whereas the owner-activated remote control tion with an owner-observed event; and external event
is a handheld device that can be applied over the skin monitors can transmit the acquired ECG information
where the monitor lies in order to trigger the monitor transtelephonically, allowing the monitor to be reset and
at the time of a clinical event. These monitors can also a new monitoring period to begin without a return visit
be programmed so that they are triggered to record to the hospital.