Page 59 - Basic Monitoring in Canine and Feline Emergency Patients

Table 3.4. Instructions for calculating average and instantaneous heart rate at different paper speeds.

VetBooks.ir Paper speed (mm/s) Average heart rate Instantaneous heart rate
Count # of SMALL boxes between
@ 25 mm/s
Mark off 15 LARGE boxes (3 seconds)
Count # of QRS complexes
single R–R interval
Multiply by 20 = beats/minute 1500/(# small boxes) = beats/minute
@ 50 mm/s Mark off 30 LARGE boxes (3 seconds) Count # of SMALL boxes between
Count # of QRS complexes single R–R interval
Multiply by 20 = beats/minute 3000/(# small boxes) = beats/minute

Fig. 3.5. Example calculation of average and instantaneous heart rate. In this ECG recorded at 25 mm/s, the
average heart rate is equal to the number of QRS complexes occurring in 3 seconds (15 large boxes) multiplied by
20: 10 × 20 = 200 beats per minute (bpm). Instantaneous heart rate is equal to the number 1500 divided by
the number of small boxes between two consecutive QRS complexes, and ranges from a minimum of 1500/13
(115 bpm) to 1500/6 (250 bpm). The average heart rate for this irregular rhythm is between the maximum and
minimum instantaneous heart rates.

rhythm, P waves preced QRS complexes at consistent ECG waveforms can also be misleading if elec-
P–R intervals. trodes are placed on the incorrect limbs, or if the
Once waveforms are identified and labeled, the machine amplitude or paper speed settings are inap-
next step in ECG analysis involves asking a series propriate. Inappropriate amplitude settings can cause
of questions about the relationships between these complexes to appear either too small (e.g. P waves
waveforms (see Table 3.5). Answers to these ques- may not be visible) or too large (e.g. the top and bot-
tions will lead to a clear rhythm diagnosis in the tom of QRS complexes are ‘cut off’ from the monitor
majority of cases. view, causing waveforms to appear square or rectan-
gular). Unconventional paper speed settings can
cause complexes to appear too wide or too narrow.
3.5 Pitfalls of Electrocardiography: Most ECG machines have an automated function
When Can the Machine Lead You Astray? to calculate heart rate, and most will display this
machine-generated heart rate. Operators should
ECG machines can sometimes provide false or mis- never rely solely on an automated heart rate assess-
leading information. ECG artifacts are defined as ment, because ECG monitors are not calibrated to
deflections on the graph paper caused by a source sense dog and cat QRS complexes and thus often
other than actual electrical conduction activity calculate improperly. Based on the lead and ampli-
within the heart. Common causes of ECG artifact tude settings, ECG monitors may either fail to sense
include patient motion (panting, trembling, or cat small QRS complexes (leading to an underestima-
purring), extraneous metal touching leads (such as tion of heart rate) or oversense large P or T waves
the operator’s watch), or interference from nearby (leading to an overestimation of heart rate). For this
electronic devices. Cellular telephones have been reason, heart rate and ECG findings should be con-
reported to cause electrical interference if held firmed and correlated with the cardiovascular physi-
within 7.5 cm (3 inches) of the ECG electrodes, but cal examination, including auscultation of heart rate
not at longer distances. and palpation of femoral pulses (see Chapter 1).

Electrocardiography 51