The Electrocardiogram

1. Review textbook sections on the cardiac conduction system and the electrocardiogram.

2. Complete Part C of the laboratory report.

3. The laboratory instructor will demonstrate the proper adjustment and use of the instrument available to record an electrocardiogram.

Figure 42.1 The first sound of a cardiac cycle can be heard by placing the diaphragm of a stethoscope over the apex of the heart. The second sound can be heard over the second intercostal space, just left of the sternum.

Figure 42.1 The first sound of a cardiac cycle can be heard by placing the diaphragm of a stethoscope over the apex of the heart. The second sound can be heard over the second intercostal space, just left of the sternum.

Second Intercostal Space The Sternum

4. Record your laboratory partner's ECG. To do this, follow these steps:

a. Have your partner lie on a cot or table close to the electrocardiograph, remaining as relaxed and still as possible.

b. Scrub the electrode placement locations with cotton moistened with alcohol (fig. 42.2). Apply a small quantity of electrode cream to the skin on the insides of the wrists and ankles. (Any jewelry on the wrists or ankles should be removed.)

c. Spread some electrode cream over the inner surfaces of four plate electrodes and attach one to each of the prepared skin areas, using rubber straps (fig. 42.2). Make sure there is good contact between the skin and the metal of the electrodes. The electrode plate on the right ankle is the grounding system.

d. Attach the plate electrodes to the corresponding cables of a lead selector switch. When an ECG recording is made, only two electrodes are used at a time, and the selector switch allows various combinations of electrodes (leads) to be activated. Three standard limb leads placed on the two wrists and the left ankle are used for an ECG. This arrangement has become known as Einthoven's triangle* which enables the

*Willem Einthoven (1860-1927), a Dutch physiologist, received the Nobel prize for physiology or medicine for his work with electrocardiograms.

recording of the potential difference between any two of the electrodes.

The standard Leads I, II, and III are called bipolar leads because they are the potential difference between two electrodes (a positive and a negative). Lead I measures the potential difference between the right wrist (negative) and the left wrist (positive). Lead II measures the potential difference between the right wrist and the left ankle, and Lead III measures the potential difference between the left wrist and the left ankle. The right ankle is always the ground.

e. Turn on the recording instrument and adjust it as previously demonstrated by the laboratory instructor. The paper speed should be set at 2.5 cm/sec. This is the standard speed for ECG recordings.

f. Set the lead selector switch to Lead I (right wrist, left wrist electrodes) and record the ECG for 1 minute.

g. Set the lead selector switch to Lead II (right wrist, left ankle electrodes) and record the ECG for 1 minute.

h. Set the lead selector switch to Lead III (left wrist, left ankle electrodes) and record the ECG for 1 minute.

Figure 42.2 To record an ECG, attach electrodes to the wrists and ankles.

Figure 42.2 To record an ECG, attach electrodes to the wrists and ankles.

Normal Ecg Pattern

Figure 42.3 Components of a normal ECG pattern with a time scale.

1 1 1

ample)

Q

RS c

J

1

T

P

t

A

1

L

1

A

interval

i. Remove the electrodes and clean the cream from the metal and skin. j. Use figure 42.3 to label the ECG components of the results from Leads I, II, and III. The P-Q interval is often called the P-R interval because the Q wave is frequently small or absent. The normal P-Q interval is 0.12-0.20 sec. The normal QRS complex duration is less than 0.10 sec.

5. Complete Part D of the laboratory report.

Web Quest

Identify normal heart sounds and a murmur.

What is the purpose of stress electrocardiography? Compare normal and abnormal ECGs.

Search these at http://www.mhhe.com/biosci/abio/ martinlmwq.mhtml

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  • girma
    Where is the intercoastal space located?
    1 year ago

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