1. Dysrhythmias/ECG Interpretation/Intervention. Hinkle p. 697
The ECG waveform reflects the function of the heart’s conduction system, which normally initiates and
conducts the electrical activity,
in relation to the lead. The ECG
offers important information
about the electrical activity of
the heart and is useful in
diagnosing dysrhythmias. ECG
waveforms are printed on
graph paper that is divided by
light and dark vertical and
horizontal lines at standard
intervals. Time and rate are
measured on the horizontal
axis of the graph, and
amplitude or voltage is
measured on the vertical axis.
When an ECG waveform moves
toward the top of the paper, it
is called a positive
deflection. When it moves
toward the bottom of the
paper, it is called a negative deflection. When an ECG is reviewed, each waveform should be examined and
compared with the others.
Waves, Complexes,
and Intervals
The ECG is composed of
waveforms (including the P
wave, the QRS complex, the
T wave, and possibly a U
wave) and of segments and
intervals (including the PR
interval, the ST segment,
and the QT interval).
The P wave represents
the electrical impulse
starting in the SA node and
spreading through the atria.
Therefore, the P wave
represents atrial
depolarization. It is normally 2.5 mm or less in height and 0.11 seconds or less in duration.
The QRS complex represents ventricular depolarization. Not all QRS complexes have all three
waveforms. The Q wave is the first negative deflection after the P wave. The Q wave is normally less than
0.04 seconds in duration and less than 25% of the R-wave amplitude. The R wave is the first positive
deflection after the P wave, and the S wave is the first negative deflection after the R wave . When a wave
is less than 5 mm in height, small letters (q, r, s) are used; when a wave is taller than 5 mm, capital letters
(Q, R, S) are used to label the waves. The QRS complex is normally less than 0.12 seconds in duration.
The T wave represents ventricular repolarization (when the cells regain a negative charge; also called
the resting state). It follows the QRS complex and is usually the same direction as the QRS complex. Atrial
repolarization also occurs but is not visible on the ECG because it occurs at the same time as ventricular
depolarization (i.e., the QRS).
The U wave is thought to represent repolarization of the Purkinje fibers; although this wave is rare, it
sometimes appears in patients with hypokalemia (low potassium levels), hypertension, or heart disease. If
present, the U wave follows the T wave and is usually smaller than the P wave. If tall, it may be mistaken
for an extra P wave.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and
represents the time needed for sinus node stimulation, atrial depolarization, and conduction through the
AV node before ventricular depolarization. In adults, the PR interval normally ranges from 0.12 to 0.20
seconds in duration.
, The ST segment, which represents early ventricular repolarization, lasts from the end of the QRS
complex to the beginning of the T wave. The beginning of the ST segment is usually identified by a change
in the thickness or angle of the terminal portion of the QRS complex. The end of the ST segment may be
more difficult to identify because it merges into the T wave. The ST segment is normally isoelectric (see
later discussion of TP interval). It is analyzed to identify whether it is above or below the isoelectric line,
which may be, among other signs and symptoms, a sign of cardiac ischemia.
The QT interval, which represents the total time for ventricular depolarization and repolarization, is
measured from the beginning of the QRS complex to the end of the T wave. The QT interval varies with
heart rate, gender, and age; therefore, the measured interval needs to be corrected (QT c) for these
variables through specific calculations. The QT c may be automatically calculated by the ECG technology,
or a nurse may use an ECG interpretation book or other resource that contains a chart of these
calculations. The QT interval is usually 0.32 to 0.40 seconds in duration if the heart rate is 65 to 95 bpm. If
the QT interval becomes prolonged, the patient may be at risk for a lethal ventricular dysrhythmia
called torsades de pointes. Although many hospitalized patients would benefit from QT monitoring, one
study indicated that nurses frequently do not have the skill to perform this assessment properly.
The TP interval is measured from the end of the T wave to the beginning of the next P wave —an
isoelectric period. When no electrical activity is detected, the line on the graph remains flat; this is called
the isoelectric line. The ST segment is compared with the TP interval to detect ST segment changes.
The PP interval is measured from the beginning of one P wave to the beginning of the next P wave. The
PP interval is used to determine atrial rate and rhythm. The RR interval is measured from one QRS complex
to the next QRS complex. The RR interval is used to determine ventricular rate and rhythm.
Determining Heart Rate from the Electrocardiogram
Heart rate can be obtained from the ECG strip by several methods. A 1-minute strip contains 300 large
boxes and 1,500 small boxes. Therefore, an easy and accurate method of determining heart rate with a
regular rhythm is to count the number of small boxes within an RR interval and divide 1,500 by that
number. If, for example, there are 10 small boxes between two R waves, the heart rate is 1,500/10, or 150
bpm; if there are 25 small boxes, the heart rate is 1,500/25, or 60 bpm.
An alternative but less accurate method for estimating heart rate, which is usually used when the
rhythm is irregular, is to count the number of RR intervals in 6 seconds and multiply that number by 10.
The top of the ECG paper is usually marked at 3-second intervals, which is 15 large boxes horizontally. The
RR intervals are counted, rather than QRS complexes, because a computed heart rate based on the latter
might be inaccurately high. The same methods may be used for determining atrial rate, using the PP
interval instead of the RR interval.
Determining Heart Rhythm from the Electrocardiogram
The rhythm is often identified at the same time the rate is determined. The RR interval is used to
determine ventricular rhythm and the PP interval to determine atrial rhythm. If the intervals are the same
or if the difference between the intervals is less than 0.8 seconds throughout the strip, the rhythm is
called regular. If the intervals are different, the rhythm is called irregular.
Sinus Node Dysrhythmias
Sinus Bradycardia
Sinus bradycardia occurs when the SA node creates an impulse at a slower-than-normal rate. Causes
include lower metabolic needs (e.g., sleep, athletic training, hypothyroidism), vagal stimulation (e.g., from
vomiting, suctioning, severe pain), medications (e.g., calcium channel blockers, amiodarone, beta -
blockers), idiopathic sinus node dysfunction, increased intracranial pressure, and coronary artery disease,
especially myocardial infarction (MI) of the inferior wall. Unstable and symptomatic bradycardia is
frequently due to hypoxemia. Other possible causes include acute altered mental status (e.g., delirium)
and acute decompensated heart failure.
Sinus bradycardia has the following characteristics:
Ventricular and atrial rate: Less than 60 bpm in
the adult
Ventricular and atrial rhythm: Regular
QRS shape and duration: Usually normal, but may
be regularly abnormal
P wave: Normal and consistent shape; always in
front of the QRS
, PR interval: Consistent interval between 0.12 and 0.20 seconds
P:QRS ratio: 1:1
All characteristics of sinus bradycardia are the same as those of normal sinus rhythm, except for the rate.
The patient is assessed to determine the hemodynamic effect and the possible cause of the dysrhythmia.
If the decrease in heart rate results from stimulation of the vagus nerve, such as with bearing down during
defecation or vomiting, attempts are made to prevent further vagal stimulation. If the bradycardia is
caused by a medication such as a beta-blocker, the medication may be withheld. If the slow heart rate
causes significant hemodynamic changes resulting in shortness of breath, acute alteration of mental
status, angina, hypotension, ST-segment changes, or premature ventricular complexes (PVCs), treatment is
directed toward increasing the heart rate. If the slow heart rate is due to sinus node dysfunction
(previously known as sick sinus syndrome), which most often occurs in people older than 50 years,
decreased exercise capacity, fatigue, unexplained confusion, or memory loss may result. Tachy-brady
syndrome is the term used when bradycardia alternates with tachycardia.
Medical Management. Management depends on the cause and symptoms. Resolving the causative factors
may be the only treatment needed. If the bradycardia is symptomatic (e.g., shakiness, hypotension,
syncope), 0.5 mg of atropine given rapidly as an intravenous (IV) bolus every 3 to 5 minutes to a maximum
total dose of 3 mg is the medication of choice. Rarely, catecholamines and emergency trans cutaneous
pacing are implemented when the rhythm is unresponsive to atropine.
Atropine blocks vagal stimulation, thus allowing a normal rate to occur. However, it should be avoided in
cardiac transplant patients because it may cause a paradoxical AV block (see later discussion on AV block).
Rather, theophylline 100 to 200 mg may be administered slowly IV to patients with bradycardia who have
had a cardiac transplantation, as well as those who have had an acute inferior MI or spinal cord injury.
Sinus Tachycardia
Sinus tachycardia occurs when the sinus
node creates an impulse at a faster-than-
normal rate. Causes may include the
following:
• Physiologic or psychological stress (e.g.,
acute blood loss, anemia, shock,
hypervolemia, hypovolemia, heart
failure, pain, hypermetabolic states,
fever, exercise, anxiety)
• Medications that stimulate the
sympathetic response (e.g.,
catecholamines, aminophylline, atropine), stimulants (e.g., caffeine, nicotine), and illicit drugs (e.g.,
amphetamines, cocaine, Ecstasy)
• Enhanced automaticity of the SA node and/or excessive sympathetic tone with reduced
parasympathetic tone that is out of proportion to physiologic demands, a condition called inappropriate
sinus tachycardia
• Autonomic dysfunction, which results in a type of sinus tachycardia referred to as postural orthostatic
tachycardia syndrome (POTS). Patients with POTS have tachycardia without hypotension within 5 to 10
minutes of standing or with head-upright tilt testing.
Sinus tachycardia has the following characteristics:
Ventricular and atrial rate: Greater than 100 bpm in the adult, but usually less than 120 bpm
Ventricular and atrial rhythm: Regular
QRS shape and duration: Usually normal, but may be regularly abnormal
P wave: Normal and consistent shape; always in front of the QRS, but may be buried in the preceding T
wave
PR interval: Consistent interval between 0.12 and 0.20 seconds
P:QRS ratio: 1:1
All aspects of sinus tachycardia are the same as those of normal sinus rhythm, except for the rate.
Sinus tachycardia does not start or end suddenly (i.e., it is nonparoxysmal). As the heart rate increases,
the diastolic filling time decreases, possibly resulting in reduced cardiac output and subsequent symptoms
of syncope and low blood pressure. If the rapid rate persists and the heart cannot compensate for the
decreased ventricular filling, the patient may develop acute pulmonary edema.
Medical Management. Medical management of sinus tachycardia is determined by the severity of
symptoms and directed at identifying and abolishing its cause. If the tachycardia is persistent and causing
hemodynamic instability, synchronized cardioversion is the treatment of choice (see later discussion).
Otherwise, vagal maneuvers or administration of adenosine may be considered. Beta-blockers and calcium
, channel blockers, although rarely used, may also be considered in a narrow-QRS tachycardia. If the
tachycardia has a wide QRS, then adenosine is considered only if the QRS is monomorphic (uniform shape)
and the ventricular rhythm is regular. Otherwise, procainamide, amiodarone, and sotalol are the options in
wide QRS tachycardia (see later discussion of all of these medications). Catheter ablation (discussed later
in this chapter) of the SA node may be used in cases of persistent inappropriate sinus tachyca rdia
unresponsive to other treatments. Treatment for POTS may include increased fluid and sodium intake and
the use of graduated compression stockings to prevent pooling of blood in the lower extremities.
Sinus Arrhythmia
Sinus arrhythmia occurs when the sinus node creates an impulse at an irregular rhythm; the rate usually
increases with inspiration and decreases with expiration. Nonrespiratory causes include heart disease and
valvular disease, but these are rare.
Sinus arrhythmia has the following
characteristics:
Ventricular and atrial rate: 60 to 100 bpm in
the adult
Ventricular and atrial rhythm: Irregular
QRS shape and duration: Usually normal, but
may be regularly Abnormal
P wave: Normal and consistent shape; always
in front of the QRS
PR interval: Consistent interval between 0.12
and 0.20 seconds
P:QRS ratio: 1:1
Medical Management. Sinus arrhythmia does not cause any significant hemodynamic effect and therefore
is not typically treated.
Atrial Dysrhythmias
Premature Atrial Complex
A premature atrial complex (PAC) is a single ECG complex that occurs when an electrical impulse starts in
the atrium before the next normal impulse of the sinus node. The PAC may be caused by caffeine, alcohol,
nicotine, stretched atrial myocardium (e.g., as in hypervolemia), anxiety, hypokalemia (low potassium
level), hypermetabolic states (e.g., with pregnancy), or atrial ischemia, injury, or infarction. PACs are often
seen with sinus tachycardia.
PACs have the following
characteristics:
Ventricular and atrial rate: Depends on the
underlying rhythm (e.g., sinus
tachycardia)
Ventricular and atrial rhythm: Irregular due
to early P waves, creating a PP interval
that is shorter than the others. This is
sometimes followed by a longer-than-
normal PP interval, but one that is less
than twice the normal PP interval. This
type of interval is called a non-
compensatory pause.
QRS shape and duration: The QRS that
follows the early P wave is usually
normal, but it may be abnormal
(aberrantly conducted PAC). It may even be absent (blocked PAC).
P wave: An early and different P wave may be seen or may be hidden in the T wave; other P waves in the
strip are consistent.
PR interval: The early P wave has a shorter-than-normal PR interval, but still between 0.12 and 0.20
seconds.
The ECG waveform reflects the function of the heart’s conduction system, which normally initiates and
conducts the electrical activity,
in relation to the lead. The ECG
offers important information
about the electrical activity of
the heart and is useful in
diagnosing dysrhythmias. ECG
waveforms are printed on
graph paper that is divided by
light and dark vertical and
horizontal lines at standard
intervals. Time and rate are
measured on the horizontal
axis of the graph, and
amplitude or voltage is
measured on the vertical axis.
When an ECG waveform moves
toward the top of the paper, it
is called a positive
deflection. When it moves
toward the bottom of the
paper, it is called a negative deflection. When an ECG is reviewed, each waveform should be examined and
compared with the others.
Waves, Complexes,
and Intervals
The ECG is composed of
waveforms (including the P
wave, the QRS complex, the
T wave, and possibly a U
wave) and of segments and
intervals (including the PR
interval, the ST segment,
and the QT interval).
The P wave represents
the electrical impulse
starting in the SA node and
spreading through the atria.
Therefore, the P wave
represents atrial
depolarization. It is normally 2.5 mm or less in height and 0.11 seconds or less in duration.
The QRS complex represents ventricular depolarization. Not all QRS complexes have all three
waveforms. The Q wave is the first negative deflection after the P wave. The Q wave is normally less than
0.04 seconds in duration and less than 25% of the R-wave amplitude. The R wave is the first positive
deflection after the P wave, and the S wave is the first negative deflection after the R wave . When a wave
is less than 5 mm in height, small letters (q, r, s) are used; when a wave is taller than 5 mm, capital letters
(Q, R, S) are used to label the waves. The QRS complex is normally less than 0.12 seconds in duration.
The T wave represents ventricular repolarization (when the cells regain a negative charge; also called
the resting state). It follows the QRS complex and is usually the same direction as the QRS complex. Atrial
repolarization also occurs but is not visible on the ECG because it occurs at the same time as ventricular
depolarization (i.e., the QRS).
The U wave is thought to represent repolarization of the Purkinje fibers; although this wave is rare, it
sometimes appears in patients with hypokalemia (low potassium levels), hypertension, or heart disease. If
present, the U wave follows the T wave and is usually smaller than the P wave. If tall, it may be mistaken
for an extra P wave.
The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex and
represents the time needed for sinus node stimulation, atrial depolarization, and conduction through the
AV node before ventricular depolarization. In adults, the PR interval normally ranges from 0.12 to 0.20
seconds in duration.
, The ST segment, which represents early ventricular repolarization, lasts from the end of the QRS
complex to the beginning of the T wave. The beginning of the ST segment is usually identified by a change
in the thickness or angle of the terminal portion of the QRS complex. The end of the ST segment may be
more difficult to identify because it merges into the T wave. The ST segment is normally isoelectric (see
later discussion of TP interval). It is analyzed to identify whether it is above or below the isoelectric line,
which may be, among other signs and symptoms, a sign of cardiac ischemia.
The QT interval, which represents the total time for ventricular depolarization and repolarization, is
measured from the beginning of the QRS complex to the end of the T wave. The QT interval varies with
heart rate, gender, and age; therefore, the measured interval needs to be corrected (QT c) for these
variables through specific calculations. The QT c may be automatically calculated by the ECG technology,
or a nurse may use an ECG interpretation book or other resource that contains a chart of these
calculations. The QT interval is usually 0.32 to 0.40 seconds in duration if the heart rate is 65 to 95 bpm. If
the QT interval becomes prolonged, the patient may be at risk for a lethal ventricular dysrhythmia
called torsades de pointes. Although many hospitalized patients would benefit from QT monitoring, one
study indicated that nurses frequently do not have the skill to perform this assessment properly.
The TP interval is measured from the end of the T wave to the beginning of the next P wave —an
isoelectric period. When no electrical activity is detected, the line on the graph remains flat; this is called
the isoelectric line. The ST segment is compared with the TP interval to detect ST segment changes.
The PP interval is measured from the beginning of one P wave to the beginning of the next P wave. The
PP interval is used to determine atrial rate and rhythm. The RR interval is measured from one QRS complex
to the next QRS complex. The RR interval is used to determine ventricular rate and rhythm.
Determining Heart Rate from the Electrocardiogram
Heart rate can be obtained from the ECG strip by several methods. A 1-minute strip contains 300 large
boxes and 1,500 small boxes. Therefore, an easy and accurate method of determining heart rate with a
regular rhythm is to count the number of small boxes within an RR interval and divide 1,500 by that
number. If, for example, there are 10 small boxes between two R waves, the heart rate is 1,500/10, or 150
bpm; if there are 25 small boxes, the heart rate is 1,500/25, or 60 bpm.
An alternative but less accurate method for estimating heart rate, which is usually used when the
rhythm is irregular, is to count the number of RR intervals in 6 seconds and multiply that number by 10.
The top of the ECG paper is usually marked at 3-second intervals, which is 15 large boxes horizontally. The
RR intervals are counted, rather than QRS complexes, because a computed heart rate based on the latter
might be inaccurately high. The same methods may be used for determining atrial rate, using the PP
interval instead of the RR interval.
Determining Heart Rhythm from the Electrocardiogram
The rhythm is often identified at the same time the rate is determined. The RR interval is used to
determine ventricular rhythm and the PP interval to determine atrial rhythm. If the intervals are the same
or if the difference between the intervals is less than 0.8 seconds throughout the strip, the rhythm is
called regular. If the intervals are different, the rhythm is called irregular.
Sinus Node Dysrhythmias
Sinus Bradycardia
Sinus bradycardia occurs when the SA node creates an impulse at a slower-than-normal rate. Causes
include lower metabolic needs (e.g., sleep, athletic training, hypothyroidism), vagal stimulation (e.g., from
vomiting, suctioning, severe pain), medications (e.g., calcium channel blockers, amiodarone, beta -
blockers), idiopathic sinus node dysfunction, increased intracranial pressure, and coronary artery disease,
especially myocardial infarction (MI) of the inferior wall. Unstable and symptomatic bradycardia is
frequently due to hypoxemia. Other possible causes include acute altered mental status (e.g., delirium)
and acute decompensated heart failure.
Sinus bradycardia has the following characteristics:
Ventricular and atrial rate: Less than 60 bpm in
the adult
Ventricular and atrial rhythm: Regular
QRS shape and duration: Usually normal, but may
be regularly abnormal
P wave: Normal and consistent shape; always in
front of the QRS
, PR interval: Consistent interval between 0.12 and 0.20 seconds
P:QRS ratio: 1:1
All characteristics of sinus bradycardia are the same as those of normal sinus rhythm, except for the rate.
The patient is assessed to determine the hemodynamic effect and the possible cause of the dysrhythmia.
If the decrease in heart rate results from stimulation of the vagus nerve, such as with bearing down during
defecation or vomiting, attempts are made to prevent further vagal stimulation. If the bradycardia is
caused by a medication such as a beta-blocker, the medication may be withheld. If the slow heart rate
causes significant hemodynamic changes resulting in shortness of breath, acute alteration of mental
status, angina, hypotension, ST-segment changes, or premature ventricular complexes (PVCs), treatment is
directed toward increasing the heart rate. If the slow heart rate is due to sinus node dysfunction
(previously known as sick sinus syndrome), which most often occurs in people older than 50 years,
decreased exercise capacity, fatigue, unexplained confusion, or memory loss may result. Tachy-brady
syndrome is the term used when bradycardia alternates with tachycardia.
Medical Management. Management depends on the cause and symptoms. Resolving the causative factors
may be the only treatment needed. If the bradycardia is symptomatic (e.g., shakiness, hypotension,
syncope), 0.5 mg of atropine given rapidly as an intravenous (IV) bolus every 3 to 5 minutes to a maximum
total dose of 3 mg is the medication of choice. Rarely, catecholamines and emergency trans cutaneous
pacing are implemented when the rhythm is unresponsive to atropine.
Atropine blocks vagal stimulation, thus allowing a normal rate to occur. However, it should be avoided in
cardiac transplant patients because it may cause a paradoxical AV block (see later discussion on AV block).
Rather, theophylline 100 to 200 mg may be administered slowly IV to patients with bradycardia who have
had a cardiac transplantation, as well as those who have had an acute inferior MI or spinal cord injury.
Sinus Tachycardia
Sinus tachycardia occurs when the sinus
node creates an impulse at a faster-than-
normal rate. Causes may include the
following:
• Physiologic or psychological stress (e.g.,
acute blood loss, anemia, shock,
hypervolemia, hypovolemia, heart
failure, pain, hypermetabolic states,
fever, exercise, anxiety)
• Medications that stimulate the
sympathetic response (e.g.,
catecholamines, aminophylline, atropine), stimulants (e.g., caffeine, nicotine), and illicit drugs (e.g.,
amphetamines, cocaine, Ecstasy)
• Enhanced automaticity of the SA node and/or excessive sympathetic tone with reduced
parasympathetic tone that is out of proportion to physiologic demands, a condition called inappropriate
sinus tachycardia
• Autonomic dysfunction, which results in a type of sinus tachycardia referred to as postural orthostatic
tachycardia syndrome (POTS). Patients with POTS have tachycardia without hypotension within 5 to 10
minutes of standing or with head-upright tilt testing.
Sinus tachycardia has the following characteristics:
Ventricular and atrial rate: Greater than 100 bpm in the adult, but usually less than 120 bpm
Ventricular and atrial rhythm: Regular
QRS shape and duration: Usually normal, but may be regularly abnormal
P wave: Normal and consistent shape; always in front of the QRS, but may be buried in the preceding T
wave
PR interval: Consistent interval between 0.12 and 0.20 seconds
P:QRS ratio: 1:1
All aspects of sinus tachycardia are the same as those of normal sinus rhythm, except for the rate.
Sinus tachycardia does not start or end suddenly (i.e., it is nonparoxysmal). As the heart rate increases,
the diastolic filling time decreases, possibly resulting in reduced cardiac output and subsequent symptoms
of syncope and low blood pressure. If the rapid rate persists and the heart cannot compensate for the
decreased ventricular filling, the patient may develop acute pulmonary edema.
Medical Management. Medical management of sinus tachycardia is determined by the severity of
symptoms and directed at identifying and abolishing its cause. If the tachycardia is persistent and causing
hemodynamic instability, synchronized cardioversion is the treatment of choice (see later discussion).
Otherwise, vagal maneuvers or administration of adenosine may be considered. Beta-blockers and calcium
, channel blockers, although rarely used, may also be considered in a narrow-QRS tachycardia. If the
tachycardia has a wide QRS, then adenosine is considered only if the QRS is monomorphic (uniform shape)
and the ventricular rhythm is regular. Otherwise, procainamide, amiodarone, and sotalol are the options in
wide QRS tachycardia (see later discussion of all of these medications). Catheter ablation (discussed later
in this chapter) of the SA node may be used in cases of persistent inappropriate sinus tachyca rdia
unresponsive to other treatments. Treatment for POTS may include increased fluid and sodium intake and
the use of graduated compression stockings to prevent pooling of blood in the lower extremities.
Sinus Arrhythmia
Sinus arrhythmia occurs when the sinus node creates an impulse at an irregular rhythm; the rate usually
increases with inspiration and decreases with expiration. Nonrespiratory causes include heart disease and
valvular disease, but these are rare.
Sinus arrhythmia has the following
characteristics:
Ventricular and atrial rate: 60 to 100 bpm in
the adult
Ventricular and atrial rhythm: Irregular
QRS shape and duration: Usually normal, but
may be regularly Abnormal
P wave: Normal and consistent shape; always
in front of the QRS
PR interval: Consistent interval between 0.12
and 0.20 seconds
P:QRS ratio: 1:1
Medical Management. Sinus arrhythmia does not cause any significant hemodynamic effect and therefore
is not typically treated.
Atrial Dysrhythmias
Premature Atrial Complex
A premature atrial complex (PAC) is a single ECG complex that occurs when an electrical impulse starts in
the atrium before the next normal impulse of the sinus node. The PAC may be caused by caffeine, alcohol,
nicotine, stretched atrial myocardium (e.g., as in hypervolemia), anxiety, hypokalemia (low potassium
level), hypermetabolic states (e.g., with pregnancy), or atrial ischemia, injury, or infarction. PACs are often
seen with sinus tachycardia.
PACs have the following
characteristics:
Ventricular and atrial rate: Depends on the
underlying rhythm (e.g., sinus
tachycardia)
Ventricular and atrial rhythm: Irregular due
to early P waves, creating a PP interval
that is shorter than the others. This is
sometimes followed by a longer-than-
normal PP interval, but one that is less
than twice the normal PP interval. This
type of interval is called a non-
compensatory pause.
QRS shape and duration: The QRS that
follows the early P wave is usually
normal, but it may be abnormal
(aberrantly conducted PAC). It may even be absent (blocked PAC).
P wave: An early and different P wave may be seen or may be hidden in the T wave; other P waves in the
strip are consistent.
PR interval: The early P wave has a shorter-than-normal PR interval, but still between 0.12 and 0.20
seconds.
