Cardiovascular
ECG Experimentando
Electrocardiograma: experimentando
1) Inspecting the ECG
  • While displaying Lead II, the P-wave, the QRS-complex, and the T-wave are identified.  In the program, experiment with different settings for the time scale and the hardware gain.
ecgdetail.gif (3092 bytes)
Time scale: 0.1 secs/division
ecgl2.gif (4014 bytes)
Time scale: 0.5 secs/division
2) Calibration
  • While displaying Lead II, the Cal (calibration) button on the amplifier is momentarily pressed, giving a pulse that is 1 mV in amplitude.  This pulse can be used to calibrate the amplitude of the ECG signal.  The calibration pulse must not saturate, therefore one must ensure that the amplifier gain is not set too high.
calpulse.gif (4116 bytes)
3) Identifying Waves and Intervals
  • The cm/sec switch is set to an appropriate value for displaying the overall shape of the P-QRS-T waveform.  The various complexes and intervals are examined.  The RR, PR, QRS and QT intervals are measured, and compared to the normal ranges given in the table to the right (in seconds).
Interval Min Max
RR 0.6 1.2
PR 0.12 0.20
QRS   0.10
QT   0.42
ecginterval.GIF (4648 bytes)
Time scale: 0.1 secs/division
4) Effect of Lead Placement
  • The RA electrode is moved from its position on the wrist to a new position somewhere above the elbow.
moveelec.gif (4292 bytes)
book_edit.gif (232 bytes) For convenience, the connections of the ECG electrodes are usually made at the ends of the limbs: at the wrists and ankles.  However, since the limbs act as conductors, they can be viewed as an extension of the patient cable lead, and so it makes no difference where the electrodes are placed along the limb length.
  • After returning the RA electrode to its original position, the subject extends the right arm outwards and holds it horizontally in mid-air away from the body.
armextend.gif (5182 bytes)
book_edit.gif (232 bytes) The above ECG trace appears very noisy, because the recording is also picking up the EMG activity from the muscles used in extending the arm outwards.
5) Effect of Respiration
  • The subject takes a deep slow breath, and then exhales slowly (inhaling for 5 seconds, and exhaling for five seconds).
inhalexhale.gif (4049 bytes)
book_edit.gif (232 bytes) In sinus arrhythmia, the heart rate varies with the phase of respiration.  The heart rate typically increases during inspiration and decreases during expiration.  Therefore, as observed, the R-R interval is longer during expiration.  These changes are mediated through vagal reflexes.  Sinus arrhythmia is more common in young healthy athletes.
6) The Timing of the Heart Sounds
  • One member of the group listens with the stethoscope to the subject's heartbeat to determine where the two well-separated heart sounds fall on the ECG trace.
ecgheartsound.GIF (3558 bytes)
For an avi video with sound, click here
For a QuickTime video with sound, click here
book_edit.gif (232 bytes)  The first heart sound is due to the closure of the mitral and tricuspid valves at the start of ventricular systole.  The second heart sound is due to the closure of the aortic and pulmonary valves. Click here for more on the heart valves
7) Changes in Morphology with Leads
  • The lead selector is switched to lead I.  The group should describe the changes seen with respect to lead II, and attempt to explain them.
  • The lead selector is switched to lead III.  The group should describe the changes seen with respect to lead II, and attempt to explain them.
ecglead2.gif (4076 bytes)
ecgl1.gif (3582 bytes)
ecgl3.gif (4422 bytes)
book_edit.gif (232 bytes) Recall that the R wave is due to the activation (depolarization) of the major portion of the ventricles.  From the sample data above, it is evident that the lead whose axis is most parallel to the direction of the subject's ventricular depolarization is lead II. (The R wave is largest in lead II.)   The R wave is very small in lead I.  We can therefore conclude that for this subject the direction of ventricular depolarization is more close to being perpendicular to lead I.