Applications involving sampling of electrical signals like ECG and EEG require telemetry implants with adequate technical specifications to accurately acquire and analyze data. The purpose of this document is to provide definitions of key terms and clearly outline what telemetry implants are appropriate for certain applications.

Important principles covered by these guidelines:

• Only signal frequency content within the specified channel bandwidth of the telemetry implant is acquired accurately
• In certain transmitters with multiple biopotential channels, channel #1 is not recommended for ECG if EEG is being sampled by another channel
• Delta EEG wave and therefore sleep cannot be properly acquired by certain telemetry implants

Definitions:

Channel Bandwidth:

• The frequency range between the lowest and highest attainable frequency, measured in Hertz (Hz)
  • For DSI products, this focuses on the range of frequencies for which the reported signal amplitude remains within an error power band of 3 dB
• The two values of a published channel bandwidth indicate the low and high frequencies where the response has fallen by no more than 3dB
  • 3dB is the “half-power” point where signal power (dB) has fallen to 50% or where the amplitude (Volts) has dropped to 70.7%
  • 3 dB is an accepted standard in physics and electrical engineering (see Appendix A)

Nominal Sampling Rate:

• Nominal implant sampling rate: The rate at which data points are sampled by the implant. Higher sample rates enable accurate response to higher signal frequencies, but at the cost of shorter battery life
• Aliasing: Arises when a signal is discretely sampled at a rate that is insufficient to capture the changes in the signal (see Appendix B)

Nyquist Sampling Theorem:

• Definition: The sampling frequency should be at least twice the highest frequency contained in the signal to avoid aliasing
• For DSI implants, we choose to sample sometimes higher than a factor of 2. We typically sample 4-5x higher than highest targeted frequency content

Software Sampling Rate:

• The rate at which the raw data is reconstructed in the application software (e.g. Ponemah) for plotting and feature extraction
• Higher software sample rates (relative to nominal sampling rate of implant) will include more points along an interpolated representation of the raw telemetry data
• Interpolation: or upsampling, is the insertion of additional data points between points collected via the nominal sampling rate of the implant
• Interpolation allows for a more continuous representation of the physiological data, but it cannot increase the accuracy of the signal reconstruction
• Select a software sampling rate that is at least equal to the nominal implant sampling rate

Best Practices per Application Type

Best Practices - ECG sampling:

• Input voltage range must accommodate full amplitude of ECG signal
  • Minimum input voltage for mouse ECG is ±2.5 mV, rat is ±5 mV, large animal is ±10 mV
• For the following implants, it is not recommended to use channel #1 for ECG, EMG, or EOG, if you also desire to sample EEG in the other available biopotential channels: 4ET, HD-X02, HD-S02, F20-EET, and F40-EET
  • Channel #1 is not recommended because its negative lead is connected to the internal ground reference of the implant. As a result, if a high amplitude signal type (e.g. ECG) is collected on Channel #1, a small ECG signal artifact may be seen in the other channels.
• For the L03/L04 implants
  • The L03 6-lead configuration has no common leads.  Each pair of leads is coupled into an instrumentation amplifier (differential inputs).  The common-mode specs apply to each input.  Our specification is that each biopotential channel shall have a Common Mode Rejection Ratio (CMRR) of -40dB or better.  This common-mode response test shall be met for each biopotential channel at test frequencies of 0.5Hz and 10Hz, and the common mode signal applied to the biopotential channel under test will be generated with respect to the implant housing connection. 
  • it is not recommended to use intravenous negative lead placement (solid tip lead) combined with diaphragmatic positive lead placement when EEG data will be collected concurrently with ECG data due to the concern over cross-talk between the biopotentials channel (i.e. ECG artifact on the EEG channels)
  • Channel 4 (physical channel on the device) has a reduced bandwidth and isn't recommended for ECG use.  

Best Practices – EEG sampling:

• DSI’s recommends the use of IROX EEG screws (Appendix C) if you desire to use a single biopotential channel telemetry implants or the D70-CCTP to record EEG.
  • These implants were designed to work best with ECG, the lowest attainable frequency is 3 Hz for EEG recordings if not used with IROX screws
• Source input impedance is most consistent across multiple cortical recordings by anchoring the biopotential electrodes to screws and making contact with the dura (See Appendix C)
  • EEG electrodes have a higher source impedance than ECG electrodes because the contact area between electrode and target tissue tends to be smaller than ECG electrodes
    • Impedance: the measure of the opposition that a circuit presents to a current when a voltage is applied
  • The source impedance of EEG electrodes is inversely proportional to the electrode contact between the target tissue and electrode material; a high source impedance may attenuate the lower EEG frequencies
• Input Voltage Range
  • Rodents: recommended input voltage range of ±1.25 mV or ±2.5 mV to keep noise levels low
  • Large Animals: recommended input voltage range of ±2.5 mV to keep noise levels low

Application Summary