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Unformatted text preview: ssive depolarization of the rest of the heart muscle cells.
The depolarization wave moves from the right atrium to the left atrium rapidly enough that they occur
essentially at the same time. The atria at the top of the heart and the ventricles at the bottom of the
heart (see Fig. 1(b)) are essentially electrically insulated from each other. The atrioventricular (AV) node
conducts the depolarization wave from the atria down the Bundle of His to the ventricles. The Purkinje
3 ﬁbers, located in the muscle wall of the ventricles, cause all parts of the ventricles to depolarize nearly
simultaneously. In this way, the electrical signal wave spreads throughout the entire heart, and from there
also spreads through the body .
When the atria contract, blood is forced into the ventricles. The ventricles then contract, pushing blood
out through the aorta and the pulmonary artery, pumping the blood throughout the body. The EKG does
not measure the mechanical contraction of the heart but rather represents the summation of all of the action
potentials as detected on the surface of the body. 2.5 EKG Figure 2: Example of an EKG trace, showing all ﬁve PQRST components.
Figure 2 shows a single trace of a healthy heart. In it, there are 5 signals visible, arising from the
isoelectric line (the relatively ﬂat baseline). Any deviation from the isoelectric line denotes electrical
activity. The ﬁve signals in one heartbeat are designated by P, Q, R, S, and T . One heart cycle begins with
the P wave, then the QRS wave complex, and ends with the T wave.
The P wave is associated with the contraction of the atria. This is the electrical pulse that initiates the
rest of the heart’s contraction.
The P − Q interval represents the depolarization across the atria. This signal is typically small in
amplitude since the atria are smaller than the ventricles.
The QRS complex shows the depolarization and contraction of the ventricles. Often this is the largest
and most deﬁnitive peak in the EKG trace. Q is a negative deﬂection, followed by a positive R deﬂection
and a negative S deﬂection. The atria repolarize while the ventricles depolarize, so their repolarization is
not visible in the trace.
The T wave indicates the repolarization of the ventricles, and is often a positive peak. This repolarization prepares the heart for the next heartbeat.
4 Typical times  for portions of the EKG are:
Resting heart rate 70-80 beats/min P − R interval 0.12 to 0.20 s QRS interval less than 0.1 s Q − T interval less than 0.38 s start of P to peak of Q
peak of Q to where S
returns to isoelectric line
peak of Q to where T
returns to isoelectric line Table 1: Typical values, as stated in . DO NOT BE ALARMED if your values fall outside of these numbers. These are typical averages and
many healthy hearts can fall outside of these parameters. Effective reading and analysis of an EKG takes
much skill and practice and should be left to medical professionals. Figure 3: A test patient’s EKG trace using lead I conﬁguration.
Figure 3 shows a sample EKG. First, we will look at the heart rate by noting the time of the R peaks in
the trace. A sample analysis follows:
There are four peaks visible. The time at which the R signal occurs is listed in the table, as are the
differences between the peaks. The average difference between peak times is (0.75s + 0.82s + 0.85s)/3 =
0.81s. Therefore, the averag...
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- Spring '14