Page 60 - Basic Monitoring in Canine and Feline Emergency Patients

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Fig. 3.6. Examples of labeling P, QRS, and T waves on ECGs. In the first example, QRS complexes are easily
identified as the tallest waveforms; P waves precede the QRS at consistent P–R intervals, and T waves follow the
QRS at consistent Q–T intervals. The rhythm diagnosis is sinus tachycardia. In the second example, there are two
different ECG complex morphologies, each of which requires identification of waveform elements. For the normal
sinus complexes (labeled in black), P–QRS–T waveforms are all present at consistent intervals. For the ventricular
premature complexes (labeled in red), QRS complexes are identified as the initial large positive deflections. For
abnormal depolarizations (wide bizarre QRS complexes), the T waves are the large deflections of the opposite polarity
(negative deflections) following the QRS complexes.

Most ECG pitfalls can be avoided by use of (Fig. 3.7). Sinus arrhythmia is mediated by variations
appropriate electrical coupling medium (isopropyl in PNS activation (vagal tone) of the SA node. This
alcohol), connecting electrodes to the appropriate rhythm is often associated with the cyclical altera-
limb, adjusting paper speed and calibration set- tions in vagal tone associated with breathing (respiratory
tings, and minimizing electrical interference and sinus arrhythmia), but can also occur independently of
patient motion. When considering possible ECG breathing pattern. Presence of a sinus arrhythmia
artifact of any kind, the operator should auscult the suggests relatively high vagal tone (see Table 3.2),
patient’s heart and palpate femoral pulses to con- which can help to rule out significant hemodynamic
firm whether ECG findings match the patient’s instability (shock, congestive heart failure, etc.).
actual heart rate and rhythm. Sinus bradycardia is defined as a sinus rhythm occur-
ring at a rate below the expected heart rate for that
patient and context (typically <60 bpm for an awake
3.6 Common Cardiac Rhythms dog or <140 bpm for an awake cat). Sinus bradycardia
in Veterinary Medicine is caused by a decreased firing rate of the SA node, and
is typically a manifestation of either increased vagal
Sinus rhythms
tone (see Table 3.2) or drug administration (especially
Sinus rhythm is characterized electrocardiographi- sedatives such as opioids or α-2 agonists). In contrast,
cally by the presence of normal P–QRS–T morphol- sinus tachycardia reflects accelerated normal automa-
ogy for each beat with consistent P–R, Q–T, and R–R ticity of the SA node (Fig. 3.8). Sinus tachycardia gener-
intervals. Sinus rhythm implies that cardiac depolari- ally occurs secondary to stimuli that increase SNS
zation utilized the ‘normal’ conduction pathway as activity, such as pain, anxiety, or hypovolemia (see
shown in Fig. 3.1: the impulse began in the SA node, Table 3.2). Less common causes include drugs or tox-
was conducted through the atria to the AV node, and ins (chocolate, β-adrenergic agonists, or anticholiner-
traversed the AV node before depolarizing the ventri- gics such as atropine). Heart rate in sinus tachycardia
cles. Sinus rhythm is the normal expected rhythm in rarely exceeds 240–250 bpm in the dog or 260–
dogs and cats. Sinus arrhythmia is a variant of sinus 280 bpm in the cat. Onset and termination of sinus
rhythm that is also normal in dogs. It is defined elec- tachycardia typically occur as gradual increase or
trocardiographically as a sinus rhythm with greater decrease in rate (‘warm-up’ or ‘cool-down’), rather
than 10% variation in R–R interval between beats than abrupt start or end of tachycardia.

52 J.L. Ward