Page 75 - Basic Monitoring in Canine and Feline Emergency Patients

of AV block, either 2nd degree (some P waves con- span of 30 large boxes (6 seconds), which corresponds
ducted, some nonconducted) or 3rd degree (no P to an average heart rate of 200 bpm. Instantaneous
VetBooks.ir waves conducted). heart rates range from ~115 bpm (13 small boxes within
the longest R-R interval) to ~300 bpm (5 small boxes
3. What is the degree of variation in R–R inter-
vals? If the bradyarrhythmia consists of normal P–
the rhythm can be classified as a tachyarrhythmia.
QRS–T complexes of sinus origin, then the ECG within the shortest R–R interval). With this heart rate,
diagnosis simply depends on the timing of R–R The next step in ECG interpretation is asking a
intervals. If R–R intervals are regular, the diagno- series of questions about the rhythm and P–QRS–T
sis is sinus bradycardia; if there is >10% variation waveforms (see Table 3.5). Asking these questions
in R–R intervals, the diagnosis is sinus arrhyth- sequentially yields the following information:
mia. Rhythm pauses greater than two normal R–R
intervals for that patient are termed ‘sinus arrest’ 1. How many rhythms are present? One rhythm
and are suggestive of sinus node dysfunction. 2. Are the R–R intervals regular or irregular? Irregular
3. How are P waves and QRS complexes related?
No consistent P waves present
4. Do P–QRS–T waveforms look normal or abnor-
3.7 Case studies: Using an ECG monitor mal? Normal (narrow and upright in lead II)
in Emergency Practice Using these answers in conjunction with the algo-
rithm shown in Fig. 3.14, this rhythm is identified as
Case study 1: Tachyarrhythmia atrial fibrillation. Note that simply recognizing this
in a Labrador Retriever
rhythm as a tachycardia with irregular R–R intervals
A 4-year-old male castrated Labrador Retriever should already suggest atrial fibrillation as the most
presents for a 1-week history of abdominal disten- likely ECG diagnosis. The cardiovascular ausculta-
sion and hyporexia and a single episode of syncope. tion findings are consistent with this rhythm (irregu-
Physical examination reveals a rapid, irregular lar tachycardia, weak and variable femoral pulses).
heart rhythm; mucous membranes are pale and Recall that in small animals, presence of atrial fibril-
capillary refill time is prolonged; and femoral lation generally suggests severe structural heart dis-
pulses are weak and variable. A grade III/VI right ease. This patient’s clinical signs (syncope) and physical
apical systolic heart murmur is auscultated. The examination findings (heart murmur, suspicion of
patient is panting but eupneic, with normal lung ascites) support this notion and suggest a diagnosis of
sounds bilaterally. The abdomen is pendulous with right-sided congestive heart failure. Given this signal-
a palpable fluid wave. The remainder of the physi- ment (young Labrador Retriever), prioritized differen-
cal examination is unremarkable. The ECG is tial diagnoses would include congenital tricuspid valve
shown in Fig. 3.20. dysplasia or dilated cardiomyopathy. Further diagnos-
The first step in ECG interpretation is calculation of tic testing is indicated, including noninvasive blood
the heart rate (see Table 3.4). This patient has approxi- pressure, thoracic radiographs, point-of-care thoracic
mately 10 QRS complexes within the span of 15 large and abdominal ultrasonography, point-of-care blood-
boxes (3 seconds) and 20 QRS complexes within the work (at minimum packed cell volume, total protein,

Fig. 3.20. Lead II ECG (25 mm/s, 10 mm/mV) from a Labrador Retriever with a history of abdominal distension and syncope.

Electrocardiography 67