Page 83 - Feline Cardiology

80 Section B: Diagnostic Testing

-90° mitter pack are attached to the cat and the cat can move
-120° -60° and behave normally in his/her cage in the intensive care
unit.
A superior alternative is Holter monitoring. Modern
+aVR +aVL
-150° -30° Holter monitors weigh 300–400 grams (<1 lb), and they
Diagnostic Testing +180° 0° +I 24 hours (most commonly) up to 6 days, usually on
record the ECG continuously for periods ranging from
multiple leads simultaneously (Côté 2010; Petrie 2005).
The information is stored digitally, although in older
-180°
models an audiocassette inside the monitor may be used
0°
for data recording. Veterinarians can rent Holter moni-
tors from various suppliers, including such online
sources as www.vetheart.com, www.albamedical.com,
+150° +30° www.labcorp.com, www.vetcardio.com, www.idexx.com,
and www.pdsheart.com. Owners need to maintain a
diary of the patient’s activities—and lack thereof—
+120° +60° during the entire monitoring period to provide a context
+III +II for analysis of the tracing. For example, a heart rate of
+90°
+aVF 90 bpm is normal for a sleeping cat, but it would be an
Figure 9.5. Hexaxial lead system diagram. important finding if it occurred during physical activity.
A feline patient’s Holter report, if produced through
human cardiology software, should be reviewed care-
Robertson et al. 1976) and are listed in the inside covers fully. Automated reviewing is attractive because manual
of this book. review of Holter studies in cats has been documented
to take up to 2 hours of interpretation per hour of
NORMAL EVOLUTION OF THE ECG recording. However, in automated review, over- and
THROUGH LIFE underinterpretation are common, because software
designed for automated analysis of feline Holter record-
At birth, a kitten’s heart rate is slightly higher (247 beats/
minute) than at 30 days of age (237 beats/minute). As ings does not exist at this time. It remains the veterinar-
ian’s obligation to assess the abnormal ECG segments
might be expected with postnatal changes of the pulmo-
nary circulation during the first 30 days of life, the mean shown in the Holter report and to decide whether the
interpretation correlates well with the rest of the patient’s
electrical axis shifts leftward toward a more normal
adult position, from the −150° to ± 180° segment to the information.
The third type of telemetric ECG monitor is the
+120° to +150° segment (Gomes-Lourenço and Ferreira
2003). cardiac event monitor (Figure 9.6). Event monitors
continuously receive ECG information into a loop
memory system, which can only retain the information
HOLTER AND EVENT MONITORING
temporarily. When an episode of clinical signs is
If an arrhythmia occurs only intermittently, it may not observed, the owner needs to trigger the event monitor.
be apparent on an ECG even though it is detected on Doing so saves an exact segment of ECG information
physical examination, is suspected to be responsible for in the monitor’s memory. Several episodes can thus
a cat’s clinical signs, or both. In these instances, telemet- be recorded over a number of days and the ECG can
ric (wireless) monitoring may be useful. Three forms of then be printed out either at the veterinary hospital or,
telemetry are available for cats: in-hospital cage-side in the case of external event monitors (Figure 9.6),
ECG, Holter monitoring, and cardiac event recording. transtelephonically.
In-hospital cage-side ECG monitoring, typically per- External event monitors are approximately the size of
formed for 24 or 48 hours, introduces the element of a a deck of cards (1 × 5 × 10 cm) and provide 7 to 30 days
foreign environment, which may cause sympathetic of monitoring time (Bright and Cali 2000; Côté 2009).
stimulation that abolishes bradycardias (false negative External event monitors are connected to patients using
result). However, it is valuable for the patient whose two or three wires attached to cutaneous ECG electrode
clinical signs or severity of episodes indicate a possible patches, one on each side of the chest over the heart
life-threatening recurrence and the risk of death at (modified precordial lead). The memory capacity of
home. Ideally, ECG patch electrodes and a small trans- external event monitors ranges from 5 to 18 minutes,