ECG Quiz 57 – Discussion – Incognito Paced Rhythm
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At one look it looks like left bundle branch block (blue arrows – lead I and aVL). But V5 shows negative complexes which is not typical of LBBB (violet arrow). There is left axis deviation as indicated by negative complexes in the inferior leads (green arrows). PR interval can be seen as varying on scrutiny of multiple beats. But PP interval is constant (red arrows). Atrial rate is higher than the ventricular rate (PP interval shorter than RR interval).
Hence the four criteria for complete heart block are satisfied:
- Regular PP interval
- Regular RR interval
- Totally varying PR interval
- Atrial rate higher than ventricular rate
But there is a catch here – it should be an infrahisian block as the QRS is wide. We would expect the ventricular escape rate to be around 40 per minute. But here it is 60 per minute. This should make us suspect that it is right ventricular apical paced rhythm in a person with complete heart block. Then where is the pacing artefact? It is hard to locate any pacing artefact even on close scrutiny. Why? Look at the low pass filter setting – 40 Hz (yellow arrow) at the bottom of the ECG.
Low pass filter setting and pacing spike/artefact detection
Pacing spike/artefact is a high frequency signal, which needs a low pass filter setting of 100 Hz for detection. Here since the low pass filter is set at 40 Hz, pacing signals are not displayed in the ECG. This is a common problem with digital ECGs. If the person/caregivers are not mentioning the presence of a pacemaker when in an emergency department, it is easy to mistake the ECG for just left bundle branch block. Of course a routine chest X-ray will very well show the pacemaker. Changing the low pass filter to 100 Hz or more will show the pacing artefacts well. An alternate option in most modern ECG machines and monitors is to switch on pacemaker detection, which will produce a very prominent pacing artifact in the tracing, often of very large amplitudes.
High pass and low pass filters
What are these high pass and low pass filters. Here the high pass filter is at 0.5 Hz. It means that only signals above this frequency level will be detected. This filter is meant to filter out the baseline fluctuation due to respiratory thoracic movements, which is in the low frequency range. Low pass filter is meant to filter out high frequency signals, mostly electromyographic in origin, which is of high frequency. In addition to this a notch filter of 50 or 60 Hz to filter out line voltage alternating current (AC) interference is also routinely used. Switching of the line voltage filter often produces sine wave artefacts in the ECG baseline corresponding to the line voltage in use (50 Hz or 60 Hz).
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