Basic ECG Interpretation
An ECG reflects the electrical activity of cardiac muscle. The purpose of this guide is to
assist in basic lead II ECG interpretation and augment the ECG presentation that is
part of NURS4000-Applied Nursing Science 4.
Understanding the Graph Paper
The standard speed of graph paper printing is 25mm/min. On the paper you will notice
two different sizes of squares.
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The horizontal axis represents time:
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0.04 seconds = 1 small horizontal square
0.2 seconds = 5 small or 1 large horizontal square
1 second = 5 large horizontal squares
6 seconds = 30 large horizontal squares
The vertical axis represents voltage:
 0.1 mV = 1 small vertical square
 1 mV = 2 large vertical squares
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Understanding the ECG complex
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The electrical activity of a normal heart is predictable and results in a standard ECG
complex. Each complex of a normal ECG has a P-wave, a QRS complex, and a Twave. Sometimes a U-wave is also present. There are also normal intervals and
segments between the waves such as the PR-interval, the ST-segment, and the QTinterval. Each wave and interval has a normal range of time in which it occurs. The
dotted line in the diagram above shows the isoelectric line. In a normally beating heart,
each electrical event is followed by a physiological (mechanical) response in the heart
muscle.
Isoelectric line
 Represents the baseline of the ECG. When certain segments of the ECG
complex deviate or deflect from this line it indicates an abnormality. For example,
when the ST segment rises above or below the isoelectric line it usually indicates
cardiac ischemia.
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P wave
 Represents atrial depolarization, which is the conduction of an electrical impulse
through the atria.
 Normal P wave precedes each QRS and is rounded and upright.
PR interval
 Tracks the atrial impulse from the atria through the AV node. This is when the
atria contract.
 Normal PR interval is 0.12-0.20 seconds and is measured from the beginning of
the P wave to the beginning of the QRS complex.
 PR interval greater than 0.20 is a 1st degree AV Block (conduction delay through
the atria or AV junction).
QRS complex
 Represents depolarization of and impulse conduction through the ventricles,
after which the ventricles contract and blood is ejected and pumped through the
arteries, creating a pulse.
 Normally each QRS follows the PR interval and is 0.08-0.12 sec in duration.
Amplitude varies from 5 to 30mm(each small square vertically =1mm).
 Represents intraventricular conduction time, therefore a widened QRS may
indicate a ventricular conduction delay. A notched R wave (rabbit ears) may
indicate a bundle-branch-block.
ST segment
 Represents the end of ventricular conduction (depolarization) and the beginning
of ventricular recovery (repolarization).
 Starts at the end of the S wave to the start of the T wave.
 Normally is isoelectric (neither positive nor negative). Changes in the ST
segment may indicate myocardial damage, often elevation indicating infarct and
depression indicating ischemia. Depressed ST may also indicate digoxin toxicity.
T wave
 Represents ventricular recovery, or repolarization.
 Follows the ST segment.
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 Normally round and smooth, and in lead II is upright. Tall, peaked T waves can
indicate myocardial injury or hyperkalemia, inverted T in lead II may indicate
ischemia or infarct. Notched or pointed T in adults may indicate pericarditis.
Bumps in the T wave may be a hidden P wave.
U wave
 The U wave is a wave on an electrocardiogram that is not always seen. It is
typically small, and, by definition, follows the T wave. U waves are thought to
represent repolarization of the papillary muscles or Purkinje fibers.
QT interval
 It represents the time taken for ventricular depolarization and repolarization.
 It is measured from the beginning of the QRS complex to the end of the T wave
 The QT shortens at faster heart rates and lengthens at slower heart rates
 It should be less than half the R-R interval
RR interval
 This is the heart rate!
Remember: the presence of electrical activity does not guarantee that
there is any mechanical response or cardiac output. ECG monitoring
equipment can also fail; leads can fall off or it may pick up muscular
electrical activity. For any rhythm you see, you must assess your
patient to determine if the patient is compromised. For example, the
following rhythm looks alarming but it is actually “artifact”,
meaningless electrical activity brought on by patient movement (in
this case, brushing his teeth).
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STEPS TO INTERPRET A RHYTHM
It is important to interpret ECG’s in a systematic fashion to avoid making a
mistake and missing significant information. Here are the steps to follow:
1) Calculate the HR: Determine if it is tachycardic (>100) or bradycardic (<60).
There are a number of ways to calculate heart rate. Here are two ways:
 Count the number of complexes that occur within 6 seconds and multiply by 10
to get the heart rate.
 Count the number of big boxes in the RR interval and divide into 300 to get the
heart rate.
2) Check rhythm: look at R waves and the RR interval and determine if it is regular
or irregular (regularly or irregularly irregular).
3) Check P waves: do you have P waves and are they normally configured and
each one precedes a QRS? If the rhythm is regular and each P is followed by a
QRS then you have a normal sinus rhythm. If rate is >100, you have sinus tachy
etc.
4) Measure PR interval: if it is prolonged there is conduction delay through the AV
node, i.e. 1st degree AV block
5) Measure the QRS duration and amplitude: if it is widened it indicates a
delay i.e. Bundle Branch Block
6) Look at the ST segment: is there elevation or depression in relation to the isoelectric line. This may indicate ischemia.
7) Check the T wave: if it is inverted it could indicate ischemia/MI. Elevated, tall or
peaked could indicate hyperkalemia while a depressed ST segment could
indicate ischemia or hypokalemia. A bump in the T wave could be a hidden P
wave.
Common Heart Rhythms
Normal sinus rhythm: has all the waves and intervals/segments occurring
in the right order and within the normally accepted limits.
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Sinus bradycardia: has all the waves and intervals/segments occurring in
the right order and within the normally accepted limits EXCEPT the RR
interval, which is less than 60 beats per minute (bpm).
Sinus tachycardia: has all the waves and intervals/segments occurring in
the right order and within the normally accepted limits EXCEPT the RR
interval, which is more than 100 bpm.
Atrial Fibrillation (A. Fib): atrial and ventricular rhythms are irregular (irregularly
irregular ventricular response). There is no discernable P waves. They are replaced by
fine fibrillatory waves and the atrial rate almost indiscernable, usually >400
beats/minute; much greater than ventricular rate.
 ventricular rate under 100= a fib with controlled ventricular response
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 ventricular rate greater than 100= a fib with uncontrolled ventricular response
Think about the risks and treatments for this rhythm. Refer to the appropriate
PBL presentation on atrial fibrillation.
Atrial Flutter (A. Flutt): the electrical signal travels along a pathway within the
right atrium. It moves in an organized circular motion, or "circuit," causing the atria
to beat faster than the ventricles of your heart. It is similar to atrial fibrillation but
has a less chaotic atrial circuit. The result is a saw-tooth atrial pattern and may
result in a more regular looking ventricular response.
 ventricular rate under 100= a. flutt with controlled ventricular response
 ventricular rate greater than 100= a. flutt with uncontrolled ventricular response.
Very rapid rates may be indiscernible from a. fib.
Think about the risks and treatments for this rhythm. They are similar to those
for atrial fibrillation.
Premature Ventricular Contractions (PVC’s): caused by an ectopic cardiac
pacemaker located in the ventricle and characterized by premature and bizarrely
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shaped QRS complexes usually wider than normal, not preceded by a P wave, with
direction opposite the major deflection of the normal QRS. It’s clinical significance
depends on the frequency, complexity, and hemodynamic response, but they are often
insignificant.
Ventricular Tachycardia (VT): characterized by bizarrely shaped QRS
complexes usually wider than normal, not preceded by a P wave, with direction
opposite the major deflection of the normal QRS. More than 3 PVC’s in a row is
considered VT. If sustained (>30 seconds) there is a risk of immediate DEATH as
there is no time for ventricular filling or emptying. It requires immediate defibrillation if
sustained (confirm rhythm check LOC/VS). If the VT burst is short it may be well
tolerated, but has the potential to cause significant hemodynamic compromise,
particularly as it continues. Patient may initially have a pulse, patient must be
immediately assessed, if the patient has no pulse then defibrillate. If the patient has a
pulse, then cardioversion is generally performed. Essentially, there are differing
presentations to VT, therefore the patient must be assessed immediately. Both of the
following rhythms are VT.
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Ventricular Fibrillation: Continuous, rapid-rate discharges from numerous ectopic
foci in the ventricles, producing no ventricular contraction. This is a cardiac arrest.
There is no contraction, no cardiac output, no blood pressure. Requires immediate
initiation of CPR and defibrillation.
Asystole: Cardiac standstill with no cardiac output and no ventricular depolarization,
as shown in the image below; it eventually occurs in all dying patients.
Heart Blocks: These are not on the PBL exam but you may encounter
them in the clinical setting.
1ST degree AV Block: there are all normal P waves followed by QRS complexes,
but the PR interval is longer than normal (>.20 sec).
 Causes: idiopathic fibrosis, sclerosis of conduction system, ischemic heart
disease, or drugs (B blockers, Ca channel blockers, Digoxin, amiodarone),
increased vagal tone, congenital heart disorders
 Is rarely symptomatic, but needs further investigation if accompanied by other
heart disorders, or if thought to be caused by drugs
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2nd degree AV Block (Type 1: Mobitz 1, Wenckeback): the PR interval
increases with each beat until QRS dropped then AV nodal conduction resumes with
the next beat, and the sequence is repeated.
 treatment is usually unnecessary unless the block causes symptomatic
bradycardia and transient or reversible causes have been excluded. Treatment is
usually pacemaker insertion.
2nd degree AV Block (Type 2: Mobitz II): the PR interval remains constant,
beats are intermittently non-conducted and QRS complexes dropped, usually in a
repeating cycle of every 3rd (3:1) block or 4th (4:1 block) P wave. It is always pathologic,
block occurs at Bundle of His in 20% and the bundle branches in 805. The patient may
be asymptomatic or lightheaded, experience syncope, and are at risk of progressing to
complete AV block, therefore a pacemaker is required.
3rd degree AV Block: there is No AV conduction- impulses from the atria are
completely blocked at the AV node. The atria and the ventricles beat independently of
each other. When you map out the P-waves and the QRS complexes, they are often
regular but do not align in any way. You will see stable P-P intervals, with slow
ventricular escape rhythm. Cardiac function is maintained by a ventricular escape
rhythm. If escape rhythm is junctional (with rate of 40-60) then patient generally does
better than if escape rhythm is Purkinje system (ventricular, rate much slower). There
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are usually significant alterations in BP, CO and LOC. This rhythm is a complication
after MI, or can be due to certain drugs (calcium channel blockers, beta blockers,
digoxin toxicity). If it is due to drugs then temporary pacing may be required and the
drugs stopped; otherwise, most patients require permanent pacemaker insertion.
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