Month: February 2015

Electrocardiogram Waveshape – Pt 1

When recording ECG, it’s important to consider the impacts of electrode placement and amplifier filters on the measured biopotential. Electrode placement is important because different orientations of biopotential surface electrodes with respect to the heart’s position will result in different signal shapes.

By Alan Macy, BIOPAC Systems, Inc.

When recording ECG, it’s important to consider the impacts of electrode placement and amplifier filters on the measured biopotential. Electrode placement is important because different orientations of biopotential surface electrodes with respect to the heart’s position will result in different signal shapes. As the heart contracts and expands, during the course of pumping blood, a time-varying, voltage signal is propagated over the volume of the heart. This voltage signal conducts to the surface of the body and is detectable between any two points on the body that straddle the heart. The signal can look quite different depending on whether the signal is collected from left hand to right hand or from right hand to left leg. When the ECG signal is recorded far from the heart, electrode placement is not critical to obtain similar-looking ECG waveforms when recording over time. As electrodes are placed increasingly close to the heart, then electrode placement becomes quite specific to recreate similar-looking ECG signals. This is because the body is a volume conductor and the heart is not a point-source of electrical signal activity.

The ECG waveform has been specified to consist of frequency components in the range of 0.05 Hz to 150 Hz, even though the characteristic ECG waveform shape is largely composed of components from 0.05 Hz to 35 Hz. However, even if the ECG’s primary waveshape is preserved, bandwidth limits affect usability of the data. For a high-resolution ECG recording, the signal bandwidth should be maximized. Although the Association for the Advancement of Medical Instrumentation (AAMI) standard recommends an ECG bandwidth of 0.05-150 Hz, surface electrodes measurements can show ECG components up to 500 Hz. [1]

As with any physiological measurement from a subject, the balance lies in attempting to isolate the signal of interest from residual noise sources. In the case of ECG recording, interfering noise is often generated by skeletal muscle that lies in the superposition path defined by electrode placement. This noise (recorded as the electromyogram) can be especially problematic if the subject is intentionally moving during the ECG measurement. Other noise sources include interference resulting from mains-generated displacement currents (50/60 Hz hum).

Depending on the nature of the ECG measurement requirements, filtering methodology can be a helpful ally to reduce the effects of interfering noise sources.  The critical aspects of filtering are a function of the filter’s magnitude and phase characteristics.  Choice of filter topologies can have a significant effect on the shape of the ECG.  Given that AAMI guidelines are primarily concerned with ECG filter flatness from 0.05-150 Hz, typically, second-order (or higher) Butterworth filters are used to define these limits.

The discussion of ECG waveshape is continued in Electrocardiogram Waveshape Pt. II.

[1] http://circ.ahajournals.org/content/115/10/1306.full

For more information on BIOPAC’s ECG solutions, visit BIOPAC’s ECG-Cardiology page or view BIOPAC’s full line of electrodes, amplifiers, and wearable, wireless transmitters and loggers.

For more information on BIOPAC Systems, visit us at www.biopac.com.

Electrical Stimulators for Physiological Research: Compliance

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source,

By Alan Macy, BIOPAC Systems, Inc. 

All stimulators incorporate a fundamental characteristic known as “compliance.” Compliance comes in two flavors, namely voltage compliance and current compliance. Voltage stimulators emulate a voltage source, up to a certain current compliance. Current stimulators emulate a current source, up to a certain voltage compliance. The stimulator compliance limit is important information. Here are examples:

Current Compliance

A voltage stimulator is being used to establish a field stimulation in a tissue bath. The voltage pulse is 100 volts, with a width of 1 ms. The bath establishes a 1000 ohm impedance between stimulation electrodes. Therefore, 100 volts/1000 ohms is 100 ma. In this situation, the voltage stimulator must be able to provide a current compliance up to 100 ma for at least 1 ms. If the stimulator’s current compliance is limited to 50 ma, then the output voltage will be limited to 50 ma * 1000 ohms or 50 volts, even if the stimulator is being instructed to output 100 volts.

Voltage stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance current. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then voltage stimulators may exhibit a quality known as voltage droop. Voltage droop is the quality when the stimulator output voltage can’t be maintained over a specific time interval, due to lack of sufficient current compliance. If the above stimulator can only be current compliant to 100 ma for 0.5 ms, then the output voltage pulse will be 100 volts for 0.5 ms and then will decay in direct proportion to the current compliance limit.

Voltage Compliance 

A current stimulator is being used for neuromuscular stimulation between two small electrodes. The current pulse is 10 ma, with a width of 1 ms and there is a 10,000 ohm impedance between stimulation electrodes. Therefore, 10 ma * 10,000 ohms is 100 volts. In this situation, the current stimulator must be able to provide a voltage compliance up to 100 volts for at least 1 ms. If the stimulator’s voltage compliance is limited to 50 volts, then the output voltage will be limited to 50 volts / 10,000 ohms or 5 ma, even if the stimulator is being instructed to output 10 ma.

Current stimulators commonly operate with the help of high voltage DC-DC converters and storage capacitors to provide the needed compliance voltage. If the storage capacitors can’t store enough charge or are not sufficiently supplied, then current stimulators may exhibit a quality known as current droop. Current droop is the quality when the stimulator output current can’t be maintained over a specific time interval, due to lack of sufficient voltage compliance. If the above stimulator can only be voltage compliant to 100 volts for 0.5 ms, then the current pulse will be 10 ma for 0.5 ms and then will decay in direct proportion to the voltage compliance limit.

Example

In the case of transcranial Direct Current Stimulation (tDCS), a constant current of up to 2 ma (Is) may be applied through the cranium. Assuming a combined series electrode / skin junction resistance of 20 Kohms, a cranium resistance of 100 ohms and other loop series resistances considered negligible, then the total stimulation loop impedance (Zs) is 20.1 Kohms. Accordingly, in this situation, the voltage compliance (Vs) of the tDCS current stimulator must be at least:

Is * Zs = Vs
0.002 amp * 20,100 ohms = 40.2 volts

Given the variable nature of the combined series resistances in the stimulation loop, it’s generally helpful to double the expected compliance voltage requirement of the stimulator. Accordingly, in this case, a stimulator compliance voltage of 80 volts or higher is recommended. Because the series resistance in the loop is largely a function of the electrode / skin interface junctions, the choice of electrode type and contact area has a great effect on the voltage compliance requirements of the current stimulator. Other factors contributing to the voltage compliance requirement are the equilibrium potentials and over-potentials associated with the specified electrode / electrolyte / skin junctions. Furthermore, if high resistance electrode leads are used, the potential across the leads may become material.

 

BIOPAC Systems, Inc. provides life science researchers and educators with data acquisition and analysis systems that inspire people and enable greater discovery about life. BIOPAC lets you measure physiology anywhere, with innovative, compatible wireless, wearable solutions that can be used by anyone for meaningful discovery. We make high-quality scientific tools for physiology measurement and interpretation with superior compatibility and world-class customer service and support. Visit us at www.biopac.com.

error: Content is protected !!