Can someone explain to me what the difference between the default 12 lead filter, 0.05-150Hz 4x3, and the filter I typically use, 0.05-40Hz 4x3 is on our Zoll E series?
I have found i get a better, cleaner ECG if i use the latter, but really have no functional understanding of what the difference is.
The filters applied by cardiac monitors fall into 3 types:
- High pass filter
- Low pass filter
- Notch filter (optional)
How to read a filter specification:
High - Low ~ Notch
ex: 0.05 - 150 Hz ~ 60 Hz
High Pass Filter
The high pass filter (so named because it allows
higher frequency components thru) is the lower number. In
Diagnostic Quality mode it will be 0.05 Hz. This means frequency components above that will pass through and lower frequency components will not (roughly speaking). ST-segment components are low frequency, and high pass analog filters above 0.05 Hz cause too much distortion. Digital filters can use higher frequency filters, but aren't seen in any cardiac monitor I'm aware of.
Baseline wander is a low frequency component, lower than the heart rate, and thus can be filtered out by applying a typical 0.5-0.67 Hz high pass filter as seen in
Monitor Mode. Consider the trouble which could be caused since a heart rate of 60 beats per minute has a frequency of 1 Hz! Filter design is a big part of ECG research.
Low Pass Filter
The low pass filter (allows
lower frequency components thru) is the higher number. In
Diagnostic Quality mode it is recommended to be set to 150 Hz by the AHA/ACC. This provides minimal attenuation/distortion of the higher frequency components of an ECG. However, this lets through 50/60 Hz interference ("line frequency" interference) and a lot of muscle artifact. So, many monitors will provide a cap of 40 Hz, which conveniently is lower than the standard line frequencies and lower than most muscle artifact. This leads to a "cleaner" ECG that remains diagnostic quality.
In
Monitor Mode some vendors choose to leave the 40 Hz low pass filter in place, although some vendors utilize filters in the 21-30 Hz range. For ambulatory monitoring, this is more than adequate to remove most artifact present, however, it provides noticeable attenuation of the signal.
Notch Filter
Some cardiac monitors provide a notch filter (filters a
specific frequency) set to the "A/C line frequency" in the country of operation. This number is typically given after the high/low pass definition, and will be either 50 or 60 Hz. This is enabled to filter out "50/60 cycle artifact" due to A/C circuits.
