Biomedical Engineering Reference
In-Depth Information
Lead-I
Lead-II
Lead -III
-
-
-
+
(a)
+
+
aVR
aVL
aVF
+
+
NP
-
NP
-
NP
-
(b)
+
(c)
Figure 3.9
Lead locations in a standard 12-lead ECG: (a) standard leads, (b) augmented leads, and (c)
precordial leads.
which are unipolar leads and record the electrical variations that occur directly
under the electrode look to the six leads. The most commonly used clinical ECG
system, the 12-lead ECG system, consists of the following 12 leads: six leads (I, II,
III, aVR, aVL, and aVF) record the electrical activity in the vertical plane (head to
foot), and the other six leads (V1, V2, V3, V4, V5, V6), record in the horizontal
plane (across the chest). Understanding the precordial leads and how they align
with the heart is critical to get an accurate lead recording on the ECG. For ex-
ample, normally the R wave becomes progressively taller moving from V1 to V5.
However, misplacement of electrodes could lead to poor R wave progression as
electrical potential reading is a function of the inverse of the square of the distance.
This perturbation also occurs in some pathological conditions in which the R wave
in leads V1-V4 either does not become bigger, or increases very slowly in size.
Hence for proper diagnosis, the location of electrodes is the key aspect apart from
other noises in the signal (discussed later). The American Heart Association man-
dates protocols for standard identification techniques for placing the electrodes.
Nevertheless, 12-lead EKG may not be sufficient in assessing certain abnormalities
and a number of additional leads are used to get a better assessment of electrical
variations.
3.4.5 Biopotential Recording Practicalities
To measure biopotentials, a metal electrode (discussed in Chapter 9) is placed in
contact with the tissue. Consider the case of EMG where either surface electrodes
on the skin or a needle electrode inserted into a muscle record the time variations
in electric potential. Needle electrodes probe a single muscle fiber and give a char-
acteristic time record of electric potential with variations of several millivolts. Such
recordings of voluntary muscle activity check for normal functioning of nerve stim-
ulation of muscle. More detailed information is obtained with external electrical
