Page 55 - Basic Monitoring in Canine and Feline Emergency Patients

placed on the right forelimb, left forelimb, and right hindlimb. When using standard right lateral
left hindlimb. Sometimes a fourth ‘grounding’ positioning for recording ECGs, a number of
VetBooks.ir electrode is placed on the right hindlimb to mnemonic devices can be used to recall proper
color configuration; these include ‘white on right,
reduce electrical interference. In this configura-
tion, the limbs act as volume conductors of elec-
‘snow (white) and grass (green) are on the
tricity, so placing the electrodes as distal as smoke above fire’ (black cranial to red), and
feasible generally improves the recording. By ground; Christmas (red and green) comes at the
convention in the United States, colored elec- end of the year (caudal end of the animal)’ (Fig.
trodes are placed as follows: white electrode on 3.3). Tips for electrode placement and reducing
the right forelimb; black electrode on the left artifact are listed in Box 3.1.
forelimb; red electrode on the left hindlimb; and Relationships between pairs of these electrodes
the optional green ‘grounding’ electrode on the are then used to generate the three bipolar leads.
Lead I represents electricity moving from the right
forelimb electrode (white) to the left forelimb elec-
trode (black). Lead II represents electricity moving
R 0.1 mV from the right forelimb electrode (white) to the left
0.02 sec
hindlimb electrode (red). Lead III represents elec-
tricity moving from the left forelimb (black) to the
0.5 mV
left hindlimb (red). These three bipolar leads are
often represented as an equilateral triangle showing
0.1 sec
electrical potential moving in three different direc-
tions, each separated by an angle of 60 degrees (see
Fig. 3.3). Advanced ECG machines are also capable
of comparing electric potential from one of these
three electrodes to an average of the other two elec-
ST trodes, generating augmented unipolar leads (aVR,
P segment T aVL, and aVF). Addition of augmented unipolar
leads to bipolar leads allows evaluation of electrical
potential at 30-degree intervals around the entire
heart.
S Although evaluation of multiple leads can be
Q useful to calculate the exact overall vector of ven-
tricular depolarization for a patient (termed the
mean electrical axis), in practice, most clinically
PR QT relevant information can be obtained from a sin-
interval interval
gle-lead tracing. Most ECG monitors used in
emergency settings or for anesthetic monitoring
Fig. 3.2. Labeling portions of the ECG waveform. The
following example ECG is recorded at a paper speed of display only one lead at a time. Lead II is most
25 mm/s with calibration 1 cm = 1 mV. Image courtesy commonly chosen for single-lead evaluation
of Dr. Melissa Tropf. because it evaluates electricity along a path that

Table 3.2. Common causes of sympathetic and parasympathetic activation that can influence rate of depolarization of
the SA and AV nodes.
Causes of SNS activation (increased ‘sympathetic tone’) Causes of PNS activation (increased ‘vagal tone’)
Pain Drugs/toxins (opioids, digoxin, general anesthesia)
Anxiety/excitement/exercise Ocular disease
Hypovolemia (or any cause of circulatory shock) Central nervous system disease
Congestive heart failure Respiratory disease
Drugs/toxins (methylxanthines, ketamine, methamphetamines, Gastrointestinal disease
anticholinergics)

Electrocardiography 47