Page 57 - Basic Monitoring in Canine and Feline Emergency Patients

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Fig. 3.4. Relationship between length (mm) and time (s) on the graph paper used for ECGs. One ‘small box’ always
measures 1 mm in length, while one ‘large box’ always measures 5 mm. ‘Slower’ paper speed (25 mm/s) makes
ECG complexes appear narrower, since more heartbeats occur within a given length of paper. ‘Faster’ paper speed
(50 mm/s) makes ECG complexes appear wider, since fewer heartbeats occur within the same length of paper. The
figure shows two ECGs taken from the same patient at the same time, shown at 25 mm/s and 50 mm/s.

Changing paper speed will change how complexes 3.3 Indications: What Can
appear on an ECG: ‘faster’ paper speed will make an ECG Monitor Tell You?
complexes appear wider (think of the paper being
‘pulled out’ of the machine more quickly while ECGs depict how electricity moves through the
deflections are being recorded), while ‘slower’ heart. This provides a number of different pieces of
paper speed will make complexes appear narrower information that differ in clinical relevance and
(Fig. 3.4). The most commonly used paper speed whether the ECG is the ‘best’ tool to obtain that
for patient monitoring and rhythm diagnosis is information. Potential uses of ECGs are outlined in
25 mm/s, while the standard paper speed for meas- Table 3.3. Fundamentally, ECGs are most useful
uring ECG complexes is 50 mm/s. At a paper speed for calculation of heart rate and determination of
of 25 mm/s, each small box represents 0.04 s heart rhythm, both of which provide critically
(40 ms), and each large box represents 0.2 s important clinical information in the triage assess-
(200 ms). At a paper speed of 50 mm/s, each small ment of emergent veterinary patients. ECG assess-
box represents 0.02 s (20 ms), and each large box ment of heart rate and rhythm can prompt
represents 0.1 s (100 ms). emergency treatment and intervention, and no
A recent advancement in heart rate and rhythm other diagnostic tests perform better than an ECG
monitoring in veterinary patients is the existence for these purposes. Additional information pro-
of smartphone applications to analyze and record vided by ECGs includes mean electrical axis (over-
ECG tracing (e.g. Alivecor, Kardia Mobile). A all vector of ventricular depolarization) and
small rectangular device consisting of two metal presence of conduction disturbances (such as bun-
electrodes is placed directly over the chest wall dle branch block). Although intraventricular con-
(using alcohol ± coupling gel) oriented with one duction abnormalities can be clinically useful (e.g.
electrode on either side of the heart. An ECG trac- they can suggest presence of underlying heart dis-
ing is transmitted wirelessly to a nearby smart- ease), they are rarely relevant in an emergency set-
phone. The tracing can be viewed in real time or ting and do not require directed treatment.
recorded/downloaded as a PDF file that can subse- Historically, ECG complex measurements (e.g. P
quently be emailed to veterinarians or other car- wave or QRS complex amplitude) were commonly
egivers. Because of the electrode configuration, used as a screening tool for cardiac chamber
smartphone ECGs project only a single ‘base– enlargement. However, ECGs are relatively insensi-
apex’ lead (rather than the bipolar leads I–III). tive indicators of cardiac chamber size.
These tracings cannot be used to calculate mean Echocardiography is a vastly superior modality for
electrical axis or perform P-QRS-T complex meas- assessment of cardiac structure and function, and is
urements, but smartphone ECG apps have shown now widely available. For these reasons, ECG
good agreement with traditional ECGs for calcula- analysis in an emergency patient should focus on
tion of heart rate and determination of heart assessment of heart rate and rhythm.
rhythm. Similar to other ECG systems, the smart- There are a number of indications for ECG
phone user interface allows manipulation of paper monitoring in the emergency setting. An ECG is
speed and calibration. certainly indicated for any patient with an arrhyth-

Electrocardiography 49