Page 78 - Small Animal Internal Medicine, 6th Edition

50 PART I Cardiovascular System Disorders

I aVR V 3 I aVR V 3
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II aVL V 6 II aVL V 6

III aVF V 10 III aVF V 10

12/16
12/14
A B
FIG 2.36
+
ECGs recorded in a female Poodle with hypoadrenocorticism at presentation (A) (K =
+
+
10.2; Na = 132 mEq/L), and 2 days later after treatment (B) (K = 3.5; Na =
+
144 mEq/L). Note absence of P waves, accentuated and tented T waves (especially in
chest leads), shortened QT interval, and slightly widened QRS complexes in A compared
with B. Leads as marked, 25 mm/sec, 1 cm = 1 mV.

300 beats/min have been recorded with excitement or activ- weakness or collapse that may not recur often enough to be
ity. Episodes of bradycardia (<50 beats/min) are common, captured during a single 24-hour continuous ECG record-
especially during quiet periods and sleep. Sinus arrhyth- ing. However, because it cannot store prolonged, continuous
mia, sinus pauses (sometimes for more than 5 seconds), ECG activity, the recorder must be manually activated when
and occasional 2° AV block appear to be common in dogs, an episode is observed. The recorder then stores the ECG
especially at times when mean heart rate is lower. Heart from a predetermined time frame (e.g., from 30-45 seconds
rates in normal cats also vary widely over 24 hours, some- before activation to 15-30 seconds after) for later retrieval
times ranging from about 70 to 290 beats/min. Regular and analysis.
sinus rhythm predominates in normal cats, although sinus Implantable (subcutaneous) ECG loop recorders also can
arrhythmia is evident at slower heart rates. Ventricular pre- be used in veterinary patients and allow intermittent ECG
mature complexes occur only sporadically in normal dogs monitoring over a period of months. The ECG is recorded
and cats; VPC prevalence appears to increase only slightly from two electrodes on the device’s casing and held in a
with age. temporary memory buffer, similar to external event record-
ers, until manual or automatic activation triggers storage
Intermittent Event Recording of a brief segment into more permanent memory. A com-
Cardiac event recorders are small loop recorders. When patible pacemaker programmer is used to set param-
activated, brief periods of a single modified chest lead ECG eters for activation and recording, and to download the
are stored within their microprocessor’s memory. Event recorded ECG segments. Automatic activation is triggered
recorders are used most often to determine whether epi- by preset determinants for “bradycardia,” “tachycardia,”
sodic weakness or syncope is caused by a cardiac arrhyth- and “asystole.”
mia. The event recorder typically is worn for periods of a Intermittent ECG acquisition using a handheld recording
week or longer. This is helpful in patients with intermittent device (Kardia Mobile, AliveCor.com; Veterinary iPhone