cal pattern, including ectopic beats. Their place in
the cardiac cycle and their frequency determine
whether ectopic beats are part of an ARRHYTHMIA
that is potentially harmful. When this is the case,
the cardiologist will conduct further diagnostic
testing to determine the underlying causes and
guide treatment decisions.
Though ectopic beats are really not preventa-
ble, reducing factors that contribute to irregulari-
ties in the heartbeat, such as caffeine
consumption, often help significantly reduce their
occurrence.
See also ATRIAL FIBRILLATION; MEDICATIONS TO TREAT
CARDIOVASCULAR DISEASE; PREMATURE VENTRICULAR
CONTRACTION(PVC).
electrocardiogram (ECG) A noninvasive diag-
nostic procedure that converts the heart’s electri-
cal activity into patterns of signals typically
recorded on paper or displayed on a screen. The
ECG is the cornerstone of cardiovascular diagno-
sis. The normal HEARTgenerates a consistent elec-
trical pattern; nearly anything that goes wrong
with the heart shows up on an ECG. A normal
heart rhythm produces five predictable fluctua-
tions, called waves, that doctors identify by the
letters P, Q, R, S, and T. The main thrust of cardiac
activity, ventricular contraction, is the QRS com-
plex.
ECG TRACING
P wave sinoatrial (SA) node’s pacing impulse initiates
the CARDIAC CYCLE
Q wave pacing impulse arrives at the ventricular apex
R wave main ventricular contraction
S wave completion of ventricular contraction
T wave heart’s return to readiness for the next cardiac
cycle
Reasons for Doing This Test
ECG is a common procedure to assess the function
of the heart. It can be baseline, diagnostic, or
monitoring. A baseline ECG is generally part of a
ROUTINE MEDICAL EXAMINATION and establishes a
record of the heart’s activity when the heart is
presumably healthy. A baseline ECG provides a
standard for comparison should there be cardio-
vascular symptoms the cardiologist needs to eval-
uate. The cardiologist does a diagnostic ECG to
examine the heart’s electrical activity as it may
correlate to symptoms the person is experiencing.
The most common symptoms for which doctors
conduct diagnostic ECGs are CHEST PAINand PALPI-
TATIONS. A monitoring ECG checks the heart’s
rhythm as a means of evaluating whether medica-
tions are working effectively to treat ARRHYTHMIA
or to determine whether the heart’s function is
stable following heart surgery or a cardiac crisis
such as HEART ATTACK.Preparation, Procedure, and Recovery
It is a good idea to avoid CAFFEINEand cigarettes
for an hour or so before a scheduled ECG. Doctors
generally prefer for ECG to show the heart at rest
and prefer people not engage in strenuous exer-
cise within four hours of ECG. Otherwise, ECG
requires no preparation and may take place in the
doctor’s office, at a cardiovascular testing facility,
or a hospital. The person lies quietly on a gurney
or procedure bed and the ECG technician places
about a dozen electrodes on the chest, back, arms,
and legs. Talking or moving during the ECG can
produce electrical “static” from the muscles. A typ-
ical ECG takes about five minutes to complete.
Though the reading is immediately available, a
cardiologist must interpret it and usually it takes
a day for the doctor to report the results back for a
routine ECG. The person may go home after the
ECG recording is finished.
Variations on the standard ECG procedure
include- Holter monitor or ambulatory ECG, which uses
a small, battery-operated unit the person wears
on a shoulder strap or belt to monitor the
heart’s electrical activity over a period of time,
typically 24 hours (an ECG technician places
the electrodes on the person’s chest and back
and connects them to the unit) - Exercise ECG, in which the person walks at
varying paces on a treadmill or rides a station-
ary bicycle while the ECG records the heart’s
changes in rhythm - Event ECG, in which the person wears elec-
trodes attached to a small, battery-operated
unit that the person turns on during a cardiac
event such as palpitations
52 The Cardiovascular System