The formal procedure for obtaining
an ECG is given in Chapter 13, Electrocardiogram.
Every ECG should be approached in a systematic, stepwise way. Many
automated ECG machines can give a preliminary interpretation of
a tracing; however, all automated interpretations require analysis
and sign-off by a physician. Determine each of the following:
- Standardization. With the
ECG machine set on 1 mV, a 10-mm standardization mark (0.1 mV/mm)
is evident (Figure 19–1).
- Axis. If the QRS is upright
(more positive than negative) in leads I and aVF, the axis is normal.
The normal axis range is –30 degrees to +105 degrees.
- Intervals. Determine the PR,
QRS, and QT intervals (Figure 19–2).
Intervals are measured in the limb leads. The PR should be 0.12–0.20
s, and the QRS, < 0.12 s. The QT interval increases with decreasing
heart rate, usually < 0.44 s. The QT interval usually does not
exceed one half of the RR interval (the distance between two R waves).
- Rate. Count the number of QRS
cycles on a 6-s strip and multiply that number by 10 to roughly
estimate the rate. If the rhythm is regular, you can be more exact
in determining the rate by dividing 300 by the number of 0.20-s
intervals (usually depicted by darker shading) and then extrapolating for
any fraction of a 0.20-s segment.
- Rhythm. Determine whether each
QRS is preceded by a P wave, look for variation in the PR interval
and RR interval (the duration between two QRS cycles), and look
for ectopic beats.
- Hypertrophy. One way to detect
LVH is to calculate the sum of the S wave in V1 or V2 plus
the R wave in V5 or V6. A sum > 35 indicates LVH.
Some other criteria for LVH are R > 11 mm in aVL or R in I + S
in III > 25 mm.
or Ischemia. Check for ST-segment elevation or depression,
Q waves, inverted T waves, and poor R-wave progression in the precordial
leads (see Myocardial Infarction).
Examples of 10-mm standardization mark and time marks
and standard ECG paper running at 25 mm/s.
Diagram of the ECG complexes, intervals, and segments.
The U wave is normally not well seen.
- Lead I: Left arm to right arm
- Lead II: Left leg to right arm
- Lead III: Left leg to left arm
V1 to V6 across the chest (see Figure
With the ECG machine set at 25 mm/s, each small box
represents 0.04 s, and each large box 0.2 s (see Figure
19–1). Most ECG machines automatically print a standardization
Note: A small amplitude in the Q,
R, or S wave ...