Page 65 - Small Animal Internal Medicine, 6th Edition

CHAPTER 2 Diagnostic Tests for the Cardiovascular System 37

Front limb electrodes are placed at the elbows or slightly is 0.04 second in duration (from left to right). At 50 mm/
below, not touching the chest wall or each other. Rear limb sec recording speed, each small box equals 0.02 second. A
VetBooks.ir electrodes are placed at the stifles or hocks. With alligator deflection from baseline (up or down) of 10 small boxes
(1 cm) equals 1 mV at standard calibration (0.1 mV per
clip or button/plate electrodes, copious ECG paste or (less
ideally) alcohol is used to ensure good contact. Communica-
2.4) are representative of most normal animals, although
tion between two electrodes via a bridge of paste or alcohol small box). ECG reference ranges for cats and dogs (Table
or by physical contact should be avoided. The animal is complex measurements for some subpopulations can fall
gently restrained in position to minimize movement arti- outside these ranges. For example, endurance-trained dogs
facts. A relaxed and quiet patient produces a better quality can have ECG measurements that exceed the “normal”
tracing. Holding the mouth shut to discourage panting or range, probably reflecting the training effects on heart size.
placing a hand on the chest of a trembling animal may be Such changes in nontrained dogs suggest pathologic cardiac
helpful. enlargement. Manual frequency filters, available on many
A good ECG recording produces minimal artifact from ECG machines, can markedly attenuate the recorded volt-
patient movement, no electrical interference, and a clean ages of some waveforms when activated, although baseline
baseline. The ECG complexes should be centered and totally artifact is reduced. The effects of filtering on QRS ampli-
contained within the background gridwork so that neither tude may complicate the assessment for ECG chamber
the top nor bottom of the QRS complex is clipped off. If the enlargement criteria.
complexes are too large to fit entirely within the grid, the
calibration should be adjusted (e.g., from standard [1 cm SINUS RHYTHMS
= 1 mV] to ½ standard [0.5 cm = 1 mV]). The calibration The normal cardiac rhythm originates in the sinus node and
used during the recording must be known to accurately produces the P-QRS-T waveforms previously described. The
measure waveform amplitude. A calibration square wave P waves are positive in caudal leads (II and aVF), and the PR
(1 mV amplitude) can be inscribed manually during record- (also called PQ) intervals are consistent. Regular sinus
ing if this is not done automatically. The paper or digital rhythm is characterized by less than 10% variation in the
recording speed and lead(s) used also must be evident for timing of the QRS to QRS (or R to R) intervals. Normally
interpretation. the QRS complexes are narrow and upright in leads II and
A consistent approach to ECG interpretation is recom- aVF. However, an intraventricular conduction disturbance
mended. First the recording speed, lead(s) used, and calibra- or ventricular enlargement pattern may cause them to be
tion are identified. Then the heart rate, heart rhythm, and wide or abnormally shaped.
MEA are determined. Finally, individual waveforms are Sinus arrhythmia is characterized by cyclic slowing and
measured. The heart rate is the number of complexes (or speeding of the sinus rate. This usually is associated with
beats) per minute. Unless otherwise specified, this means respiration; the sinus rate tends to increase on inspiration
QRS complexes (rather than P waves) are counted to provide and decrease with expiration as a result of fluctuations in
the heart (ventricular) rate. Heart rate can be calculated by vagal tone. There also may be a cyclic change in P-wave
counting the number of complexes in 3 or 6 seconds and configuration (“wandering pacemaker”) with the P waves
then multiplying by 20 or 10, respectively. If the heart rhythm becoming taller and spiked during inspiration and flatter in
is regular, 3000 divided by the number of small boxes (at expiration. Sinus arrhythmia is a common and normal
paper/trace speed 50 mm/sec) between successive RR inter- rhythm variation in dogs. It occurs in resting cats but rarely
vals equals the instantaneous heart rate. Because variations is seen in the clinical setting. Pronounced sinus arrhythmia
in heart rate are so common (in dogs especially), determin- can be associated with chronic pulmonary disease, especially
ing an average heart rate over several seconds is usually more in dogs.
accurate and practical than calculating an instantaneous “Brady-” and “tachy-” are modifying terms that describe
heart rate. abnormally slow or fast rhythms, respectively, without
Heart rhythm is assessed by scanning the entire ECG identifying intracardiac origin. Both sinus bradycardia and
recording for irregularities and identifying individual wave- sinus tachycardia are rhythms that originate in the sinus
forms. The presence and pattern of P waves and QRS-T node and are conducted normally; however, the heart rate
complexes are determined. The relationship between the of sinus bradycardia is slower than normal for the species,
P waves and QRS-Ts is then evaluated. Calipers are often whereas that of sinus tachycardia is faster than normal. Some
useful for evaluating the regularity and interrelation- causes of sinus bradycardia and tachycardia are listed in
ships of the waveforms. Estimation of MEA is described Box 2.3.
on p. 45. Sinus arrest is the absence of sinus activity lasting at least
Individual waveforms and intervals are, by convention, twice as long as the animal’s longest expected QRS to QRS
measured using lead II. Complex amplitude is recorded interval. An escape complex usually interrupts the resulting
in millivolts and duration in seconds (or msec). Only pause if sinus activity does not resume soon enough. Long
one thickness of the inscribed pen/trace line should be pauses can cause fainting or weakness. Sinus arrest cannot
included for each measurement. At 25 mm/sec record- be differentiated with certainty from SA block on the surface
ing speed, each small (1 mm) box on the ECG gridwork ECG. Fig. 2.27 illustrates various sinus rhythms.