Sunday, 22 November 2009

Technical problems (troubleshooting)

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.
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.