Telemetry Biopotential Signals

Telemetry Biopotential (ECG, EMG, EEG) Data 

Biopotential telemetry measures the electrical potential between two electrodes/leads.  Depending on where the leads are placed, biopotential telemetry can be used to measure long-term electrocardiogram (ECG), electroencephalogram (EEG) and electromyogram (EMG) in rats and mice. The Millar Rat Telemetry System offers solutions for the long term recording of a single biopotential signal using our TR50B or simultaneous recording of two biopotential signals (e.g. EEG+EMG) with our TR50BB telemeters. For mice, Millar Mouse Telemetry offers the MT10B telemeter for the long term recording of a single biopotential signal.

Electrocardiography – ECG or EKG

ECG records the net change in electrical activity across the heart with each heartbeat over time.  Under normal conditions, the mechanical activity is temporally linked with the electrical activity.  Changes in the electrical potential from the depolarization of the myocardium result in muscular contraction in the region of depolarization while repolarization results in the relaxation.   The wave of depolarization in each heart beat originates from the sinoatrial node, travels through the atria to the atrial-ventricular node, then travels down the ventricular septum and up walls of the ventricles. 

The P, QRS and T waves recorded in an ECG represent the change in electrical potential during the depolarization of the atria, the depolarization of the ventricles (and the simultaneous repolarization of the atria), and the repolarization of the ventricles respectively. 

 ECG telemetry data for rats

Clinically, ECG is recorded using 12 lead positions to generate information about how the electrical activity propagates in 3 dimensions.  In rats and mice, telemetry ECG is often recorded using two electrodes recording a single lead configuration.  Depending on the position of the electrodes and lead configuration, different ECG profiles are seen.  The modified lead II position of an ECG in a rat is shown in the figure below. The lead wire positions have been adjusted slightly from the standard lead II position, which is most in line with the ventricles, to improve signal quality when the rat or mouse is moving around.  

ECG telemetry data and telemeter (transmitter) placement

Example ECG mouse telemetry

ECG Mouse Telemetry Data

Adjusting the position of the electrodes to record from different ECG lead configurations changes the relative size of the P, QRS and T waves, which allows detailed focus on one event. Good muscle contact will be required for large clean telemetry ECG signals as electrical conductance through connective tissue is poor.  It is also

important to keep the lead wires together as far as practical and avoid curling the leads into a loop to minimize interference from ambient electrical noise.  

The most common use for ECG is the measurement of heart rate, which can be calculated by the number of R waves in a given period of time.  In addition to the detection of the heart rate, the ECG can be used to record: 

  • heart rate variability (R-R intervals)
  • duration of depolarization of the atria (P wave duration) or ventricles (QRS duration)
  • arrhythmias and counts of P, QRS, T waves
  • electrical conductance across the heart and waveform abnormalities  
  • relative amplitudes of the waveforms can also be calculated

These measurements can be useful in basic cardiovascular physiology, behavioral or pharmacological/toxicology studies. 

Download a 5-minute segment of rat arterial pressure and ECG (modified lead II) acquired using Millar Rat Telemetry here. The data file is in LabChart format, to open the file, LabChart Reader is available as a free download at the ADInstruments webpage. For more information on the surgical implantation procedure, please visit our Knowledge Center for detailed instructions and surgical videos.

Electromyography - EMG

EMG records the electrical potential and the change in potential (electrical activity) across the skeletal muscle.  The electrical potential changes during contraction and relaxation of the skeletal muscle as the muscle depolarizes and then repolarizes. EMG can be used to assess the muscle contraction, mechanics or inferring movement rates across the muscle.  For example, attaching the lead wires to the diaphragm allows for the measurement of respiration rate, while attachment of the lead wires to the gluteus maximus muscle can be used to assess walking or movement.  Other potential applications include measurement of gastrointestinal peristaltic movements and movement of specific muscles such as nuchal EMG. 

Below is an example of the EMG signal recorded from the gluteus maximus muscle in a walking rat.  

EMG telemetry system data

EMG can be recorded from a muscle or muscle groups, provided that the size of the muscle is large enough for the attachment of the two electrodes with a gap to prevent contact between the electrodes.  It is important that the electrodes do not contact as this results in electrical shortening, and a signal will not be detected.  It is also important that the electrodes are tied securely to the muscle that is being recorded from to minimize movement artifacts. 

Telemetry is advantageous over tethered systems for measuring EMG as the rats and mice are allowed to move freely in their home cage eliminating behavioral adaptations to tethering. The battery of the Millar rat telemeters also allows recording of EMG away from the home cage and tethers, opening up opportunities such as EMG recordings during behavioral tests. Millar rat biopotential telemeters provide up to 4 hours of continuous data transmission powered by a rechargeable battery when the rat telemeter is not actively being powered by the SmartPad.

Electroencephalography - EEG

EEG records the electrical activity over an area of brain tissue. Electrical activity in the brain is generated by depolarization of neurons.  As the electrical activity provided by a single neuron is too small to be detected, recorded telemetry EEG represents the summation of electrical activity from a large number of neurons in an area.  

Electrodes can be placed on the surface of the brain for cortical surface EEG, or deeper into the tissue to measure from a particular area of interest. EEG can be applied to the study of neurological disorders, such as seizure detection or epilepsy studies and pharmacological studies into neurostimulating drugs. EEG can also be used in behavioral research to study brain activity patterns in response to stimuli or in studies of sleep-wake cycles. Both the Millar Rat and Mouse Telemetry Systems allow recording of EEG from animals living in their home cages free from the stress and restriction of using tethers.

Below is an example of EEG recorded in a rat using the Millar TR50BB telemeter with simultaneous nuchal EMG recordings in a study of the sleep-wake cycle.  

EEG and EMG telemetry data

In addition to EMG, it is possible to record EEG in conjunction with brain oxygen (TR50B + TR57Y) or intracranial pressure (TRM54PB) using the Millar Rat Telemetry System.