Technical Problems

From ECGpedia
Jump to: navigation, search
Author(s) J.S.S.G. de Jong
Moderator J.S.S.G. de Jong
Supervisor
some notes about authorship

Contents

Lead reversals

A Normal ECG
right arm - right leg reversal
left arm - left leg reversal
left leg - right leg reversal.
v3 - v2 reversal
left - right reversal

Lead switches are a common mistake when ECGs are made and can lead to wrong diagnoses. Common mistakes are:

  • Left-right arm reversals lead to a negative complex in lead I with a negative P wave in lead I. They are one of the most common causes of right axis deviation on the ECG!
  • Arm-foot switches lead to a very small or 'far field' signal in leads II or III.
  • Chest lead reversals lead to inappropriate R wave progression (increase-decrease-increase) and are often easily recognized.

Therefore any right axis or small signal in an extremity lead should be reason enough to check lead positioning. A previous ECG can be very helpful.

More specific patterns with every lead reversal:

  • right and left arm electrodes:
    • inversion of lead I
    • reversal of leads II and III
    • reversal of leads aVR and aVL
  • right leg and right arm:
    • diminished signal in lead II
  • left arm and left leg:
    • reversal of leads I and II
    • reversal of leads aVL and aVF
    • inversion of lead III
  • right arm and left leg:
    • inversion of leads I, II and III
    • reversal of leads aVR and aVF

It is possible to distinguish lead reversal and dextrocardia by watching the precordial leads. Dextrocardia will not show any R wave progression in leads V1-V6, whereas lead reversal will.


Artifacts

Artifacts (disturbances) can have many causes. Common causes are:

  • Movement
  • Electrical interference

Filter settings

Although not a technical problem, filter settings influence the interpretation of the ECG.[1]

Filter settings can influence the interpretation of ST elevation as these examples show. On the left side an ECG with normal settigns (0.01-40Hz), on the right side a rhythm strip from the same patient with a different high-pass filter setting (0.32-40Hz). Also, the P wave morphology is different between the two ECGs.

To reduce electrical interference ECG machine use two filters:

  • A high-pass filter reduces low frequency noise. This filter reduces base line drift on the ECG.
  • A low-pass filter reduces high frequency noise, such as produces by chest and extremity muscles and electrical interference from the power grid.

Depending on the purpose of the ECG these filters can be adjusted.

  • In the monitor mode the high-pass filter can be set higher at 0.5-1.0 Hz and the low-pass filter on 40 Hz. This is the strongest filter setting (only a narrow frequency range is passed by the filter). This setting is especially useful for rhythm monitoring where noise can be distracting and ST segment interpretation is not very important. In this mode, pacemakerspikes are sometimes invisible while filtered out.
  • In the diagnostic mode the high-pass filter is set at 0.05 Hz and the low-pass filter at 40, 100 or 150 Hz. This improves the diagnostic accuracy of the ST semgent. On the downside a base line drift occurs more easily.



References

  1. García-Niebla J, Llontop-García P, Valle-Racero JI, Serra-Autonell G, Batchvarov VN, and de Luna AB. Technical mistakes during the acquisition of the electrocardiogram. Ann Noninvasive Electrocardiol 2009 Oct; 14(4) 389-403. doi:10.1111/j.1542-474X.2009.00328.x pmid:19804517. PubMed