Page 54 - Basic Monitoring in Canine and Feline Emergency Patients

the autonomic nervous system. Stimuli that increase 3.2 The Monitor: How Does
sympathetic nervous system (SNS) activity (causing an ECG Machine Work?
VetBooks.ir increased β1-adrenergic stimulation to the heart) The ideal way to measure cardiac conduction is to
will increase the rate of SA node depolarization and
place catheters inside the heart that can measure
AV nodal conduction (see Table 3.2). In contrast,
stimuli that increase parasympathetic nervous sys- electrical activation at each point within the heart.
tem (PNS) activity (causing increased vagal stimu- Obviously, this is not practical in most settings.
lation to the heart) will decrease the rate of SA Instead, metal electrodes are placed on the outside
node depolarization and AV nodal conduction (see of the body to measure electric potentials moving
Table 3.2). The two arms of the autonomic nervous between them. The pattern of electric potentials
system thus have a major influence on heart rate measured on the body surface can be used to infer
and occurrence of certain ECG abnormalities dis- how electricity is moving through the heart. For
cussed in this chapter. example, if electricity is measured moving from an
electrode placed on the right forelimb to an elec-
trode on the left forelimb, then one can infer that
the electricity moved from right to left within the
heart.
SA node An ECG lead is created by placing two elec-
trodes on opposite ‘sides’ of the heart in some
configuration (for example, on the right and
AV node left forelimbs, or on the left forelimb and left
RA LA
hindlimb). The ECG machine arbitrarily labels
one electrode in the pair as the ‘positive’ electrode
Bundle and one as the ‘negative’ electrode. When the ECG
of His detects electricity moving towards the positive
electrode, the machine creates a positive deflec-
RV LV
Bundle tion on the screen or paper for that lead. When the
branches ECG detects electricity moving away from the
positive electrode (towards the negative elec-
trode), the machine creates a negative deflection
on the screen or paper for that lead. Finally, if
Purkinje
fibers electricity is moving exactly perpendicular to the
two electrodes, no deflection is recorded for that
lead (termed isoelectric).
Having multiple leads allows the ECG to ‘look’
Fig. 3.1. Schematic depiction of the cardiac conduction
system. See text for details. RA, right atrium; RV, right at electricity moving through the heart from dif-
ventricle; LA, left atrium; LV, left ventricle; SA, sinoatrial; ferent angles. The most common electrode and
AV, atrioventricular. lead configuration involves three electrodes

Table 3.1. Events in cardiac conduction as depicted on an ECG.
Depolarization of this structure Causes this waveform on an ECG
Sinoatrial node Before P wave
Atria P wave
Atrioventricular node PR interval
His bundle / bundle branches PR interval
Ventricles QRS complex
Q = first negative deflection
R = first positive deflection
S = first negative deflection after R wave
Ventricular repolarization T wave

46 J.L. Ward