Monitors vs ECG machines

  Monitors are used to display (and sometimes record) heart's rhythm and the ST segment deviation

      for a long time.

      On the other hand, prolonged monitorization of cardiac rhythm and possible ST segment deviation

      are not the primary objectives of ECG machines.

  Arrhythmia detecting algorhythms may vary among commercially available monitors.

  Most monitors are defibrillator-proof.




For good ECG signal acquisition

  The skin is a poor conductor of electricity.

      Before monitorization, you must prepare the skin for good skin-electrode contact.

      Skin oils and cutaneous debris must be removed.

      Clean the skin with soap and water. Do not use ether or pure alcohol.

      Ether or pure alcohol will dry the skin, increasing its resistance.

  If hair is present on the skin, shave it before applying the electrodes on the surface.

  If you will use
snap leadwires, then it will be better if you attach the leadwires to the electrodes first,

      then apply the electrodes to the patient.

  When placing the electrodes on the body, choose non-muscular and flat sites

      to avoid interference by the muscular movement.

  Unlike the standard 12-lead ECG, bedside cardiac monitoring
limb electrodes are placed on the torso.

      This reduces muscle artifact due to limb movement and avoids tethering the patient.

  The right arm (
RA) electrode is placed on the infraclavicular fossa close to the right shoulder.

      The left arm (
LA) electrode is placed on the infraclavicular fossa close to the left shoulder.

      The left leg (
LL) electrode is placed below the rib cage on the left side of the abdomen.

      This type of electrode placement is the oldest and simplest type of cardiac monitoring, and

      permits only the monitorization of leads I, II and III. However, it does not permit

      ST segment monitorization or differentiation of Ventricular Tachycardia from Supraventricular Tachycardia.

      Alternatively 5- or 10-electrodes may be used for monitoring.

  Change the electrodes every 24 hours.

  The monitor must have proper grounding.




Which lead to choose for monitoring?

  Leads with monophasic QRS complexes (either completely positive or completely negative)

      should be chosen.

      Leads showing biphasic QRS complexes are not preferred.

  To prevent double heart rate counting, leads with
tall and narrow QRS complexes

      but with
small P and T waves must be preferred for monitoring.

  If
ASYSTOLE developes during monitorization of a patient with tall P waves,

      the monitor may erroneously count the P waves as QRS complexes.

      This may result in failure to detect the ASYSTOLE.

      Therefore
choose leads with small P waves.



Monitoring during surgery

  To minimize the interference from the electrosurgery unit, electrocautery, etc. the electrodes

      should be placed so that the current from the surgical burning area to the return electrode

      does not go through the measured ECG lead.




Commenting on the ECG data being displayed on the monitor

  Some monitors can follow ST segment changes to detect silent ischemia.

      However this necessitates good ECG signal acquisition to prevent artifacts.

  ST segment may show fluctuations due to changes of the body position. Unlike true ischemia,

      this type of
positional ST segment fluctuations are usually accompanied by QRS changes.

  Increasing the size of ECG waves on the monitor display
DOES NOT increase diagnostic capability.

  When monitoring patients with atrial fibrillation, abrupt changes may be observed in heart rate.

      This is normal for an irregularly irregular rhythm in which RR intervals change abruptly.

      Since monitors derive the instantaneous heart rate from the RR interval, it is normal for a monitor

      to depict abruptly changing heart rates in accordance with the abruptly changing RR intervals.

      A similar observation may be seen in patients with atrial flutter and abruptly changing levels of AV block.




Monitoring patients with cardiac pacemakers

  When monitoring pacemaker patients,

            -
PACE PROGRAM must be activated in some commercially available monitors.

            - the lead with the largest pacemaker spike must be chosen.

  If a pacemaker patient developes
ASYSTOLE, the monitor may count pacemaker spikes

      as QRS complexes.

      This may result in
failure to detect ASYSTOLE. Such patients must be kept under close observation.

  If a pacemaker patient developes arrhythmias, the rate meter of the monitor may reflect the pacemaker rate.

      In such patients, do not rely only on the rate meter alarms.

 
Heart rate double counting may be seen in some pacemaker patients

      due to separate counting of the pacemaker spike and the following QRS complex.

  Pacemaker spike's size and/or shape may vary from patient to patient.

      Do not use pacemaker spike's size and/or shape for diagnosis.

  In some patients with
UNIPOLAR pacemakers, repolarization tail may result in

      double heart rate counting. Leads with prominent
repolarization tail should not be chosen for monitoring.

  If pacemaker spikes are not detected by the monitor, try alternative electrode placement.




Consistent lead placement is necessary for ST segment monitoring

  Some patients neeed monitorization for several days.

  During this time, the electrodes may have to be changed (during echocardiography, etc.).

  In such patients, electrode locations must be marked with indelible ink.

  Replacing the electrodes at the same site will result in reliable ST segment analysis.




Which patients are NOT SUITABLE for ST segment monitoring?

  Patients with left bundle branch block (LBBB).

  Patients with right ventricular pacing (have LBBB morphology).

  Patients with atrial fibrillation or flutter.

      (fibrillation or flutter waves deform the ST segment).

  Agitated patients with noisy signal due to movement.




References

  Circulation 2004;110:2721-2746.

  U.S. PATENT: 4934376.






Figure 1. Above is a set of suction cup electrodes for recording of the precordial ECG leads.
These electrodes are not suitable for longer cardiac monitorization.





Figure 2. Above are snap electrodes used for bedside cardiac monitoring.
A hard plastic cover under the electrode protects the underlying gel (left).





Figure 3. The hard plastic cover is removed before applying the snap electrode to the body.
The innermost black circle and the
inner white layer encircling it are covered by transparent gel which eases
the conductivity of the electrode. The outer and wider white circle contains the sticky surface.





Figure 4. On the side view, the snap electrode's backside protective plastic cover has a
camber.
The reason is to protect the underlying conductive transparent gel.
Otherwise, the underlying gel would be dispelled when touched.





Figure 5. First connect the
snap leadwire to the snap electrode, then remove the plastic back cover.
Now the electrode is ready for application on the body.
If you first apply the snap electrode to the patient's body and then try to connect the snap leadwire to the snap electrode, then
the force applied by your hand to connect the snap surfaces will dispell the underlying gel making it ineffective.
This will in turn decrease the signal quality.






Figure 6. The basic cardiac rhythm monitorization is done with 3 electrodes.

The
right arm (RA) electrode is placed on the infraclavicular fossa close to the right shoulder.
The
left arm (LA) electrode is placed on the infraclavicular fossa close to the left shoulder.
The
left leg (LL) electrode is placed below the rib cage on the left side of the abdomen.





Figure 7. Incorrect placement of the electrodes is seen above.





ECG 1. When monitoring, select the leads with
low amplitude P and T waves but with monophasic QRS complexes.
The QRS complexes may be positive (as above) or negative.





ECG 2. When monitoring, select the leads with
low amplitude P and T waves but with monophasic QRS complexes.
The QRS complexes may be positive or negative (as above).





ECG 3. When monitoring, the leads with high amplitude
P and/or T waves are not preferred.
Leads with
biphasic QRS complexes as above are not preferred.




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