Page 76 - Basic Monitoring in Canine and Feline Emergency Patients

BUN/creatinine, and electrolytes), and eventually a Case 2: Bradyarrhythmia in a cat
complete echocardiogram. In addition to manage- An 8-year-old male castrated domestic shorthair cat
VetBooks.ir ment of the underlying structural heart disease and presents laterally recumbent after having been lost
congestive heart failure (abdominocentesis, furo-
outside for 2–3 days. Physical examination reveals dull
semide, pimobendan, and eventually an angiotensin-
converting- enzyme inhibitor ± spironolactone), this mentation, pale mucous membranes with prolonged
capillary refill time, and weak but synchronous femo-
patient will require treatment for this tachyarrhyth- ral pulses. Rectal temperature is 97.0°F (36.1°C).
mia, as rapid atrial fibrillation compromises cardiac Auscultation reveals a regular rhythm with no heart
filling time and thus decreases cardiac output. murmur and normal breath sounds. The patient is
Treatment for atrial fibrillation is directed at painful on abdominal palpation (growls and tries to
decreasing the ventricular response rate by slowing bite). The remainder of the triage physical examina-
AV node conduction. Drug options include cal- tion is unremarkable. The ECG is shown in Fig. 3.21.
cium-channel blockers (diltiazem), β-blockers, or The first step in ECG interpretation is calcula-
digoxin. This patient is suspected to be in conges- tion of the heart rate (see Table 3.4). This patient
tive heart failure and presumably has severe under- has approximately 6 QRS complexes within the
lying structural heart disease (likely involving span of 15 large boxes (3 seconds) and 12 QRS
decreased systolic function); therefore β-blockers complexes within the span of 30 large boxes (6
should be avoided due to their significant negative seconds), which corresponds to an average heart
inotropic properties. Diltiazem is very effective at rate of 120 bpm. This heart rate in a sick cat can be
slowing AV nodal conduction but is a weak nega- classified as a bradyarrhythmia.
tive inotrope; digoxin is less effective at slowing AV The next step in ECG interpretation is asking a
nodal conduction but is a weak positive inotrope. series of questions about the rhythm and P-QRS-T
An initial goal in the emergency setting is a moder- waveforms (see Table 3.5). Asking these questions
ate decrease in heart rate to 150–160 bpm (the sequentially yields the following information:
heart rate that maximizes cardiac output in dogs).
This can be accomplished either by diltiazem 1. How many rhythms are present? One rhythm
microboluses (0.05–0.2 mg/kg IV) or CRI (2–5 μg/ 2. Are the R–R intervals regular or irregular?
kg/min), or digoxin loading (2.5 μg/kg slow bolus Regular
repeated hourly for 4 hours, for a total of 10 μg/kg; 3. How are P waves and QRS complexes related?
see Table 3.6). For long-term management, this No P waves are present
patient will receive a combination of oral diltiazem 4. Do P–QRS–T waveforms look normal or abnor-
(extended-release formulation, Diltiazem ER, mal? All QRS complexes are abnormal (wide and
3−6 mg/kg PO q12h) and digoxin (3−5 μg/kg PO bizarre, negatively deflected in lead II, with large
q12h; see Table 3.6), with the goal of decreasing T waves of opposite polarity to QRS complexes)
heart rate to a more normal physiologic range
(average daily heart rate < 125 bpm). The combina- Using these answers in conjunction with the algo-
tion of digoxin and diltiazem has been shown more rithm shown in Figure 3.19, this rhythm is identi-
effective at heart rate reduction in atrial fibrillation fied as atrial standstill. This ECG diagnosis is
compared to either medication alone. critically important in the triage assessment of this

Fig. 3.21. Lead II ECG (25 mm/s, 10 mm/mV) from a domestic shorthair cat presenting with evidence of circulatory
shock and abdominal pain.

68 J.L. Ward