An ECG complex represents the electrical events occurring in one
cardiac cycle. A complex consists of five waveforms labeled with
the letters P, Q, R, S, and T. The middle three letters —Q, R, and
S—are referred to as a unit, the QRS complex. ECG tracings represent
the conduction of electrical impulses from the atria to the
ventricles.
The P wave
The P wave is the first component of a normal ECG waveform.
It represents atrial depolarization—conduction of an electrical
impulse through the atria. When evaluating a P wave, look closely
at its characteristics, especially its location, configuration, and deflection.
A normal P wave has the following characteristics:
• location—precedes the QRS complex
• amplitude—2 to 3 mm high
• duration—0.06 to 0.12 second
• configuration—usually rounded and upright
• deflection—positive or upright in leads I, II, aVF, and V2 to V6;
usually positive but variable in leads III and aVL; negative or inverted
in lead aVR; biphasic or variable in lead V1.
If the deflection and configuration of a P wave are normal—
for example, if the P wave is upright in lead II and is rounded and
smooth—and if the P wave precedes each QRS complex, you can
assume that this electrical impulse originated in the sinoatrial
(SA) node. The atria start to contract partway through the P wave,
but you won’t see this on the ECG. Remember, the ECG records
electrical activity only, not mechanical activity or contraction
The PR interval
The PR interval tracks the atrial impulse from the atria through
the AV node, bundle of His, and right and left bundle branches.
When evaluating a PR interval, look especially at its duration.
Changes in the PR interval indicate an altered impulse formation
or a conduction delay, as seen in AV block. A normal PR interval
has the following characteristics (amplitude, configuration, and
deflection aren’t measured):
• location—from the beginning of the P wave to the beginning of
the QRS complex
• duration—0.12 to 0.20 second.
These characteristics are different for pediatric patients. (See
Pediatric rates and intervals, page 46.)
The short and long of it
Short PR intervals (less than 0.12 second) indicate that the impulse
originated somewhere other than the SA node. This variation
is associated with junctional arrhythmias and preexcitation
syndromes. Prolonged PR intervals (greater than 0.20 second)
may represent a conduction delay through the atria or AV junction
due to digoxin toxicity or heart block—slowing related to ischemia
or conduction tissue disease.
The QRS complex
The QRS complex follows the P wave and represents depolarization
of the ventricles. Immediately after the ventricles depolarize,
as represented by the QRS complex, they contract. That contraction
ejects blood from the ventricles and pumps it through the
arteries, creating a pulse.
Not necessarily mechanical
Whenever you’re monitoring cardiac rhythm, remember that the
waveform you see represents the heart’s electrical activity only. It
doesn’t guarantee a mechanical contraction of the heart and a subsequent
pulse. The contraction could be weak, as happens with
premature ventricular contractions, or absent, as happens with
pulseless electrical activity. So before you treat the strip, check
the patient.
It’s all normal
Pay special attention to the duration and configuration when
evaluating a QRS complex. A normal complex has the following
characteristics:
• location—follows the PR interval
• amplitude—5 to 30 mm high but differs for each lead used
• duration—0.06 to 0.10 second, or half of the PR interval. Duration
is measured from the beginning of the Q wave to the end of
absent.
• configuration—consists of the Q wave (the first negative deflection
after the P wave), the R wave (the first positive deflection
after the P wave or the Q wave), and the S wave (the first negative
deflection after the R wave). You may not always see all three
waves. The ventricles depolarize quickly, minimizing contact time
between the stylus and the ECG paper, so the QRS complex typically
appears thinner than other ECG components. It may also
look different in each lead. (See QRS waveform variety.)
• deflection—positive in leads I, II, III, aVL, aVF, and V4 to V6 and
negative in leads aVR and V1 to V3.
Crucial I.D.
Remember that the QRS complex represents intraventricular conduction
time. That’s why identifying and correctly interpreting it
is so crucial. If no P wave appears with the QRS complex, then the
impulse may have originated in the ventricles, indicating a ventricular
arrhythmia. (See Older adult ECGs.)
Deep and wide
Deep, wide Q waves may represent myocardial infarction. In this
case, the Q-wave amplitude is 25% of the R-wave amplitude, or
the duration of the Q wave is 0.04 second or more. A notched R
wave may signify a bundle-branch block. A widened QRS complex
(greater than 0.12 second) may signify a ventricular conduction
delay. A missing QRS complex may indicate AV block or ventricular
standstill.
The ST segment
The ST segment represents the end of ventricular conduction or
depolarization and the beginning of ventricular recovery or repolarization.
The point that marks the end of the QRS com plex and
the beginning of the ST segment is known as the J point.
Normal ST
Pay special attention to the deflection of an ST segment. A normal
ST segment has the following characteristics (amplitude, duration,
and configuration aren’t observed):
• location—extends from the S wave to the beginning of the T wave
• deflection—usually isoelectric (neither positive nor negative);
may vary from –0.5 to +1 mm in some precordial leads.
Not so normal ST
A change in the ST segment may indicate myocardial damage.
An ST segment may become either elevated or depressed. (See
Changes in the ST segment.)
The T wave
The T wave represents ventricular recovery or repolarization.
When evaluating a T wave, look at the amplitude, configuration,
and deflection. Normal T waves have the following characteristics
(duration isn’t measured):
• location—follows the S wave
• amplitude—0.5 mm in leads I, II, and III and up to 10 mm in the
precordial leads
• configuration—typically round and smooth
• deflection—usually upright in leads I, II, and V3 to V6; inverted
in lead aVR; variable in all other leads.
Why is that T so bumpy?
The T wave’s peak represents the relative refractory period of ventricular
repolarization, a period during which cells are especially
vulnerable to extra stimuli. Bumps in a T wave may indicate that a
P wave is hidden in it. If a P wave is hidden, atrial depolarization
has occurred, the impulse having originated at a site above the
ventricles.
Tall, inverted, or pointy Ts
Tall, peaked, or tented T waves indicate myocardial injury or hyperkalemia.
Inverted T waves in leads I, II, or V3 through V6 may
represent myocardial ischemia. Heavily notched or pointed T
waves in an adult may mean pericarditis.
The QT interval
The QT interval measures ventricular depolarization and repolarization.
The length of the QT interval varies according to heart
rate. The faster the heart rate, the shorter the QT interval. When
checking the QT interval, look closely at the duration.
A normal QT interval has the following characteristics (amplitude,
configuration, and deflection aren’t observed):
• location—extends from the beginning of the QRS complex to the
end of the T wave
• duration—varies according to age, sex, and heart rate; usually
lasts from 0.36 to 0.44 second; shouldn’t be greater than half the
distance between consecutive R waves when the rhythm is regular.
The importance of QT
The QT interval shows the time needed for the ventricular
depolarization-repolarization cycle. An abnormality in duration
may indicate myocardial problems. Prolonged QT intervals indicate
that the relative refractory period is longer. A prolonged QT
The U wave
The U wave represents the recovery period of the Purkinje or ventricular
conduction fibers. It isn’t present on every rhythm strip.
The configuration is the most important characteristic of the U
wave.
When present, a normal U wave has the following characteristics
(amplitude and duration aren’t measured):
• location—follows the T wave
• configuration—typically upright and rounded
• deflection—upright.
The U wave may not appear on an ECG. A prominent U wave
may be due to hypercalcemia, hypokalemia, or digoxin toxicity.
No comments:
Post a Comment