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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
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