Page 58 - Basic Monitoring in Canine and Feline Emergency Patients

Table 3.3. Applications of electrocardiography in cardiovascular assessment of veterinary patients.

VetBooks.ir What information can How good / reliable is the How many ECG How difficult is it to use How useful is this
information clinically?
ECG at telling you this?
leads do you need? an ECG to get this?
an ECG tell you?
Heart rate
****
****
Lead II only
Heart rhythm / **** Lead II only Easy ****
Medium
Arrhythmias
Conduction **** Lead II required, Medium **
disturbances others helpful
Mean electrical *** At least two Hard *
axis orthogonal
leads, ideally
six
Chamber * Lead II only Hard *
enlargement
The number of asterisks represent a subjective assessment of reliability or clinical utility, with (*) representing the least reliable or
useful, and (****) representing the most reliable or useful.

mia (including tachycardia or bradycardia) aus- R–R intervals are all the same), then heart rate
culted on triage examination. Continuous ECG calculated by the two methods will be identical; if
monitoring is useful for patients with trauma and/ the heart rhythm is irregular (R–R intervals vary),
or hemodynamic instability (shock) to assess then the heart rate calculated by the two methods
response to treatment (fluid resuscitation, blood may differ.
products, analgesics, etc.). ECGs should also be The ‘ballpoint pen method’ has been described
performed to monitor cardiovascular stability in as a shortcut to calculating average heart rate on
heavily sedated or anesthetized patients, since these an ECG. A standard ballpoint pen (with cap on) is
drugs can have cardiodepressant actions. In criti- approximately 15 cm (150 mm) long; therefore,
cally ill or anesthetized patients, heart rate trends the length of the pen represents approximately 6
act as a surrogate for autonomic nervous system seconds of ECG recording at a paper speed of
activity; an increase in heart rate can signal 25 mm/s. Heart rate (at 25 mm/s) can thus be esti-
increased SNS stimulation that may indicate hypo- mated as the number of QRS complexes occurring
tension, pain, or inadequate anesthetic plane (see within the length of a pen multiplied by 10 (‘pen
Table 3.2). times ten’).
The next step in ECG analysis is determination of
cardiac rhythm. Rhythm analysis requires first labe-
3.4 Interpretation of the Findings:
How to Analyze an ECG ling individual waveforms to identify P, QRS, and
T waves. For some ECGs these distinctions are easy,
The first step to analyzing an ECG is calculation of while other cases are less obvious (see Fig. 3.6).
heart rate. There are two general methods of heart Remember that every heart beat (ventricular contrac-
rate calculation: average heart rate and instantane- tion) must be preceded by a QRS complex (ventricular
ous heart rate (see Table 3.4 and Fig. 3.5). Average depolarization). Every time the ventricle depolarizes
heart rate represents the mean heart rate over a (QRS), it must repolarize (T wave); however, depend-
defined period of time (generally 3 or 6 seconds is ing on the lead and size of the patient, T waves may
chosen as a representative sample); this method is not always be visible. Depending on the rhythm,
typically preferred for clinical assessment. P waves may or may not be present. Therefore, it is
Instantaneous heart rate represents the heart rate typically easiest to start by identifying QRS complexes,
between two QRS complexes only; this method can since these are the waveforms most consistently pre-
be useful for transient tachyarrhythmias (e.g. sent. Usually (but not always), QRS complexes are the
instantaneous rate between two VPCs) or bradyar- largest (highest amplitude) waveform identified on the
rhythmias (e.g. instantaneous rate during a period ECG. When visible, T waves always occur after QRS
of sinus arrest). If heart rhythm is regular (i.e. if complexes at a consistent Q–T intervals. In sinus

50 J.L. Ward