Page 56 - Basic Monitoring in Canine and Feline Emergency Patients

VetBooks.ir RA Lead I LA

Lead II Lead III

LL

Fig. 3.3. Electrode placement and directionality for three bipolar leads (lead I, II, III). RA, right atrium; LA, left atrium,
LL, left hind limb.
abnormal ventricular conduction (e.g. ventricular
Box 3.1. Tips for performing a high-quality origin of QRS complexes or bundle branch block).
ECG. Evaluation of lead II is sufficient to diagnose heart
rate and heart rhythm, which are the most critical
Patient positioning: lateral recumbency preferred; components of ECG analysis in the emergency
right lateral recumbency is the standard for setting.
ECG complex measurements Recall that ECGs are simply lines on graph
Electrode placement: clip or attach metal paper that show electrical potential (on the
electrodes to skin folds on the appropriate
limbs, as distally as feasible (based on skin Y-axis) plotted over time (on the X-axis) for a
tautness) given lead (direction between two electrodes).
Electrical contact: apply isopropyl alcohol or Once an ECG tracing is generated, the operator
conductive paste to the electrode–skin inter- can manipulate several aspects of how the ECG
face (avoid alcohol if defibrillation may be appears visually, based on how electrical signals
required) are translated into linear deflections. All ECGs
Reducing artifact: avoid crossing/touching wires; are printed on graph paper standardized such
avoid contact between electrodes and other that one small box measures 1 mm in length and
metal; avoid nearby electronic devices; reduce height (and therefore one large box measures
patient trembling/panting/purring
5 mm); but the operator can control how these
measures of length (mm) are translated into either
electrical potential (mV) on the Y-axis or time (s)
on the X-axis. First, the operator can adjust the
most closely approximates normal ventricular ECG calibration or amplitude, meaning how tall
conduction. Recall that in a normal heart, the the Y-axis deflections will be for a given amount
ventricles depolarizes cranial to caudal and right of electrical potential. Using ‘standard’ ECG cali-
to left. This occurs because the wave of ventricular bration, detection of 1 mV of electrical potential
depolarization comes from the AV node in the results in a deflection that is 1 cm ‘high’ on the
right atrium, and terminates with individual myo- graph paper (1 cm = 1 mV). However, for patients
cyte depolarization within the more massive left with very small QRS complexes (especially cats),
ventricle. Among bipolar leads, lead II (right fore- it may be useful to change calibration such that
limb to left hindlimb) most closely approximates 1 mV of electrical potential results in a deflection
this path. QRS complexes should normally be posi- that is 2 cm ‘high’ (2 cm = 1 mV), or vice versa
tively deflected in lead II. Negatively deflected for patients with very large QRS complexes
QRS complexes in lead II are abnormal in cats (0.5 cm = 1 mV).
and dogs, and suggest abnormal ventricular myo- Second, operators can adjust paper speed, mean-
cyte mass (e.g. right ventricular hypertrophy) or ing how quickly time passes along the X-axis.

48 J.L. Ward