Electrocardiogram of the snake: Effect of the location of the electrodes and cardiac vectors

The electrocardiogram of the snake can be reliably recorded with two leads: one longitudinal, the other transverse. The longitudinal lead consists of a cephalad electrode located at 10% of the body length and a caudad electrode at 50% of the body length; both electrodes are on the ventral surface of the animal. The transverse electrodes are located bilaterally at the level of the heart (approximately 24% of the body length for the species reported here). The most appropriate calibration for adequately large deflections of the recording pen is 2cm=1mV or, in some cases, 3cm=1mV. A longitudinal and a transverse distribution of the cardiac potentials have been obtained for each of the three components of the ECG. They would agree qualitatively with the predictions of the dipole theory. With the two orthogonal leads described above, a reasonable estimate of the cardiac vectors can be obtained. Changes in the position of the heart, either by handling or by artificial respiration, may introduce significant changes in the magnitude and orientation of the vectors. When transverse leads are recorded, rotation around the YY-axis usually produces large variations in the amplitudes of the ECG waves. Spontaneous changes in T-amplitude are relatively common in the snake.