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Advan. Physiol. Edu. 30: 128-a, 2006; doi:10.1152/advan.00008.2006
1043-4046/06 $8.00
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ADV PHYSIOL EDUC 30:128, 2006
© 2006 American Physiological Society

ILLUMINATIONS

Demonstration of the Origin of ECG Waves

Anand Bhaskar and Arati Vinod

Department of Physiology
Christian Medical College
Vellore, Tamil Nadu 632002, India
E-mail: anandbhaskar{at}cmcvellore.ac.in

Introduction

ECG is taught extensively for undergraduate medical students through lectures and practical demonstrations on healthy volunteers. The fact that the P wave is contributed by the atria and that the QRS and T waves are contributed by the ventricle is memorized by the students without seeing the actual contribution. An experimental setup where the students can have a hands-on experience will definitely help them understand and retain easily some of the basic concepts in ECG. Keeping this in mind, we developed a novel technique of recording the frog ECG using a basic human ECG machine. This will help students to study the origin of ECG waves by manipulating the structure of the frog heart.

Experimental setup. The experimental animal used is the frog Rana hexadactyla. The frog is pithed after ether anesthesia. The chest wall is excised so as to fully expose the heart. The ECG is recorded on ECG paper using a basic human ECG machine (Cardiart 308, BPL). The experimental setup is illustrated in Fig. 1. Silver needle electrodes (Grass Instruments) are used to pick up ECG signals from the frog heart. The right arm electrode is kept in the thoracic cavity close to the right side of the heart. The left arm electrode is kept in the thoracic cavity close to the left side of the heart. The left foot electrode is kept in the peritoneal cavity close to the left leg. The right foot electrode is kept in the peritoneal cavity close to the right leg. The chest electrode is placed in contact with the lower part of the ventricle. The other ends of these electrodes are attached to the corresponding electrodes of the ECG machine.


Figure 1
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  		Fig. 1 Experimental setup. RA, right arm electrode; LA, left arm electrode; C, chest electrode; RF, right foot electrode; LF, left foot electrode.

 
Experiment 1: recording of normal ECG. The ECG machine is calibrated such that 1-cm deflection in the ECG paper represents 1 mV. The speed of the ECG paper is kept at 25 mm/s. These are the normal settings for the recording of ECG in human beings. After calibration, the lead selector switch is turned to that of the chest lead. The normal ECG is recorded as in Fig. 2, which shows the P, R, and T waves.


Figure 2
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  		Fig. 2 Normal frog ECG showing P, R, and T waves.

 
Experiment 2: effect of removal of the ventricle. The ventricle is removed by excision at the atrioventricular junction. The chest electrode is kept on the lower end of the atria. The recording then shows only P wave (Fig. 3). Therefore, the R and T waves are contributed by the ventricle.


Figure 3
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  		Fig. 3 ECG recorded after the removal of the ventricle showing only the P wave.

 
Experiment 3: effect of removal of the atria. The atria are also removed and the recording electrode is kept on the sinus venosus. The ECG recorded shows no P wave (Fig. 4). Therefore, the P wave is contributed by the atria.


Figure 4
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  		Fig. 4 ECG recorded after the removal of the atria shows no waves.

 
We have demonstrated this experiment to other staff members in the Department of Physiology, and they felt that this experiment will be definitely appreciated by the students. A basic human ECG machine is available in all teaching institutions, and hence this experiment can be easily demonstrated by others.


   Acknowledgments
 
We acknowledge the help and support rendered by the staff members of the Department of Physiology, Christian Medical College (Vellore, India).





This Article
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