autonomic nervous system Archives

TREV Q&A: Unpacking Trans-radial Electrical Bioimpedance Velocimetry

As researchers look for cleaner, more direct ways to capture sympathetic nervous system (SNS) activity, Trans-radial Electrical Bioimpedance Velocimetry (TREV) has emerged as a powerful new approach for measuring beat-by-beat changes in cardiac contractility to index sympathetic drive. To explain how the technique works and where it fits alongside more established measures, we sat down with Viktoriya Babenko, PhD, BIOPAC’s in-house TREV expert and research specialist. Babenko helps researchers implement advanced cardiovascular and autonomic measurements, combining her extensive hands-on experience with bioimpedance technologies and a strong grounding in the interpretation of physiological signals. In the Q&A below, she describes the physiological principles behind TREV, how it differs from traditional electrodermal activity (EDA), electrocardiogram (ECG), and impedance cardiography (ICG) approaches, and why it offers a uniquely direct and practical window into sympathetic drive for both laboratory and neuroimaging research.

At a high level, what is Trans-radial Electrical Bioimpedance Velocimetry (TREV), and what does it measure?

TREV is a noninvasive method for estimating cardiac contractility as a continuous, beat-by-beat index of SNS activity. Rather than measuring bioimpedance across the chest like traditional ICG, TREV measures impedance changes across the radial and ulnar arteries of the forearm.

As the heart contracts, it generates a pressure wave that travels through the arterial system and causes red blood cells to momentarily align. This alignment reduces electrical resistance in the blood, which TREV continuously tracks. The impedance signal measured by TREV provides velocity, acceleration, and ultimately a contractility index, where stronger contractions reflect greater sympathetic drive.

Why is cardiac contractility such a useful index of SNS activity?

Cardiac contractility reflects the vigor with which the heart contracts during each heartbeat and is strongly modulated by sympathetic tone, either through direct neural innervation or circulating catecholamines like epinephrine. Increased sympathetic activation leads to stronger, faster contractions, while reduced sympathetic drive results in weaker contractile force.

Because of this close physiological relationship, contractility is considered one of the most direct and objective noninvasive measures of SNS activity. Unlike heart rate alone, which reflects both sympathetic and parasympathetic influences, contractility provides a more targeted window into sympathetic drive specifically.

How does TREV differ from traditional ICG- or ECG-based approaches?

Traditional approaches for indexing sympathetic drive, such as combining ECG and thoracic ICG to extract the pre-ejection period (PEP), are powerful but complex. They typically require 10 electrodes placed across the neck, chest, and torso, along with careful preprocessing and time-based feature extraction between both ICG and ECG to get PEP.

TREV dramatically simplifies this process by using just four strip electrodes placed on the forearm. The resulting contractility waveform produces a single sharp peak per heartbeat, where the amplitude directly reflects contractile strength. Analysis is as simple as extracting peak amplitudes, rather than identifying precise timing relationships across ECG and ICG signals, making TREV faster to set up and easier to analyze.

What is the physiological rationale for using the forearm and radial and ulnar arteries as the measurement site?

The radial and ulnar arteries provide a relatively simple and accessible vascular model that is well-suited for bioimpedance measurements. When the heart contracts, a pressure wave propagates rapidly through the arterial tree and reaches the forearm, aligning red blood cells even before bulk blood flow arrives.

This pressure-wave-driven alignment is sufficient to cause measurable drops in impedance, allowing TREV to estimate cardiovascular dynamics without the anatomical and physiological complexity of the thorax. Measuring at the forearm also reduces sensitivity to respiratory and pulmonary effects that complicate thoracic impedance measurements.

What cardiovascular or hemodynamic parameters can be derived from TREV?

TREV provides estimates of blood velocity, acceleration, and the integral of velocity (blood position) derived from the impedance waveform. Most importantly, the second derivative of the impedance signal yields a contractility index (blood flow “jerk”) that reflects sympathetic modulation of the heart on a beat-by-beat basis.

While multiple parameters are available, BIOPAC primarily recommends TREV as a tool for tracking cardiac contractility over time, particularly in studies interested in stress, arousal, motivation, and cognitive or emotional processes linked to sympathetic activation.

What does a typical TREV measurement session look like, from setup to data output?

Once the arm is clean and dry, four hydrogel strip electrodes (EL526) are placed on the forearm and connected via the CBL246 adapter to the NICO100D system. I recommend watching the TREV Webinar titled “A Simple Next-Gen Method for Stress Research: TREV” for step-by-step guidance on electrode placement.

During data collection, the participant’s arm must remain fully supported and relaxed, ideally with the elbow and forearm stabilized through the wrist to minimize muscle activity. When collected under proper conditions, TREV produces a clean, continuous contractility waveform with one peak per heartbeat, allowing for straightforward beat-by-beat analysis of sympathetic drive.

What advantages does TREV offer compared to ECG, EDA, or other stress-related measures?

Many commonly used physiological signals, such as heart rate, blood pressure, or heart rate variability, are influenced by multiple factors, including hydration, temperature, respiration, and both branches of the autonomic nervous system (parasympathetic and sympathetic nervous systems). EDA provides a sympathetic index, but it is relatively slow, delayed, and indirect, reflecting eccrine sweat gland activity modulated by the SNS rather than cardiovascular dynamics.

TREV fills an important gap by offering a fast, dynamic, and more direct measure of sympathetic drive through cardiac contractility. It combines the physiological specificity of ICG-derived measures with a much simpler setup and analysis pipeline, making it especially attractive for experimental and applied research settings

What are the main challenges in acquiring clean, interpretable TREV data?

The primary challenge with TREV is its sensitivity to motion artifact. Muscle activity in the forearm generates electromyography (EMG) signals that are much larger than the impedance changes of interest and can easily overwhelm the TREV signal.

For this reason, careful experimental control is essential. The participant’s arm should be fully supported, relaxed, and free from movement or strain, while other body movements are generally permissible. When these conditions are met, TREV data quality is excellent, but the method does demand attention to posture and comfort.

In what research or applied settings is TREV showing the most promise?

TREV is particularly well-suited for research at the intersection of stress, cognition, emotion, and autonomic physiology. It has been successfully used in laboratory studies examining sympathetic responses to cognitive and emotional stressors, as well as in MRI environments to enable the simultaneous study of neural activity and SNS dynamics.

Across both standard laboratory settings and neuroimaging environments, TREV provides a continuous, beat-by-beat index of sympathetic activity, making it especially valuable for event-related experimental designs where precise timing and dynamic changes in autonomic responses are of interest. In the MRI specifically, this temporal resolution allows researchers to directly link moment-to-moment fluctuations in sympathetic drive with neural processes measured using functional MRI (fMRI), offering a powerful framework for studying how brain activity and cardiovascular sympathetic responses interact during task performance, affective processing, or stress-related appraisals. As with most MRI-based physiological measurements, specific hardware configurations and safety considerations apply, and researchers should contact the BIOPAC support team before data collection to ensure proper setup and compliance.

How does TREV perform in dynamic or real-world conditions, such as movement or exercise?

At present, TREV performs best in controlled conditions where the forearm can remain fully supported and at rest. Simple movements while seated may be permissible, provided that the arm containing the TREV electrodes remains immobile and relaxed. Due to its sensitivity to motion, TREV is not currently recommended for continuous data collection during active movement or exercise.

That said, TREV can still be extremely valuable in movement- or exercise-related research by comparing pre- and post-task states. This approach allows researchers to quantify sympathetic changes associated with physical or psychological challenges without collecting data during movement itself.

Looking ahead, how do you see the TREV method evolving over the next five to ten years?

As validation studies continue and more researchers adopt the method, TREV is likely to become a standard tool for indexing sympathetic drive in both basic and applied research. Ongoing work, including peer-reviewed publications, will further refine interpretation, best practices, and experimental design considerations.

In the longer term, improvements in signal processing, hardware integration, and multimodal data collection may expand TREV’s use into more naturalistic settings, helping bridge the gap between controlled laboratory studies and real-world physiological monitoring.

If you are planning a study and would like to learn more about integrating TREV, our team of experts can help guide you to the right equipment and strategies.

A System of Systems: Network Physiology Connects the Dots

In the field of physiology, it is tempting to see the human body as a collection of independent systems performing a range of functions—converting food to energy, protecting us from harm and disease, and helping us react to and interact with our environments, to name a few. It is easy to overlook that these systems often work together and are interconnected. The field of network physiology does just that—look at how these systems (nervous, cardiovascular, muscular, etc.) work together, seeing them as part of a dynamic network rather than a collection of independent parts.

The author of the article linked above, Boston University research professor of physics Plamen Ivanov, first coined the term “network physiology” in 2006. Through his work as director of the Keck Laboratory of Network Physiology, Ivanov and his team pioneered research in this new and rapidly growing field. The latest research methods, breakthroughs, and ideas in network physiology will take center stage this summer at the Fourth International Summer Institute on Network Physiology (ININP) hosted at the Lake Como School of Advanced Studies in Italy.

Researchers are exploring various ways to apply the concepts of network physiology, including how stress affects both the brain and the body. Medical researchers are looking into how to identify early warning signs of future health problems in how various physiological systems interact when developing treatments that address multiple bodily systems. While this may sound like the multimodal approach to physiology—collecting data simultaneously and synchronously from multiple systems—the two terms are not synonymous. Network physiology uses multimodal methods to collect data, but goes further by connecting the dots between the various systems.

A study by a team of researchers from the University of New York and the University of Houston demonstrated how combining data from different body systems can give a more complete picture of how the brain and body work together, specifically in response to music and mental effort. The study explored how different types of music—either calming or exciting—affect a person’s working memory (the ability to retain and use information for short periods) and overall brain and body arousal. During the experiment, participants completed memory tasks while listening to either calming or exciting music. Researchers collected detailed data from the body using a set of BIOPAC sensors and electrodes, including general-purpose electrodes for gathering electrocardiogram (ECG) signals, a respiratory effort transducer, a wireless skin temperature sensor, a wireless pulse transducer for photoplethysmogram (PPG) data, and electrodes to measure electrodermal activity (EDA), blood flow in the finger, and muscle activity in the shoulders. These multimodal signals were gathered using a BIOPAC data acquisition and analysis system. The data was collected at 2 kHz to capture even small changes in the body’s responses. The research team found that while the difficulty of the memory tasks didn’t significantly impact arousal levels, the type of music did. Exciting music helped participants feel more alert and even improved their performance on some tasks.

A 2024 South Korean study investigated how the autonomic nervous system (ANS) responds differently to social pain (such as emotional distress from losing a loved one) versus physical pain (like discomfort from a pressure cuff). Researchers used a BIOPAC 16-channel data acquisition and analysis system running AcqKnowledge software to collect a range of signals from 73 healthy young adult participants. The following signals were gathered synchronously using a variety of sensors and transducers: ECG electrodes placed on both wrists and the left ankle to monitor heart activity, EDA sensors attached to the index and middle fingers to assess skin conductance (indicating stress levels), PPG sensor on the thumb to measure blood volume changes, chest-wrapped sensor to track breathing patterns, and a temperature sensor on the ring finger to monitor peripheral temperature changes. Participants experienced social pain via a 60-second grief-themed film clip and physical pain through a blood pressure cuff inflated to 250 mmHg. Social pain increased heart rate and skin conductance, while lowering blood volume pulse, pulse transit time, breathing rate, and finger temperature, reflecting a mix of sympathetic and parasympathetic nervous system activity. Physical pain, on the other hand, raised heart rate variability and skin conductance, reduced blood volume pulse and pulse transit time, but left finger temperature unchanged, suggesting activation of the body’s stress response system. These distinct patterns show how different types of pain trigger unique responses across interconnected body systems.

A multimodal approach was employed in a study by researchers from several institutions including the University of Southern California, University of California, Irvine, Rutgers University, Vanderbilt University, and Wayne State University School of Medicine to examine how daily heart rate variability (HRV) biofeedback training influences brain function, emotional regulation, and physiological responses in both younger and older adults. In the study, 193 participants were randomly assigned to either increase or decrease their heart rate oscillations through daily biofeedback sessions over five weeks. Before and after the intervention, participants underwent various assessments, including MRI scans, psychological evaluations, and sleep monitoring using wristband sensors. During the MRI sessions, physiological data such as blood pressure, pulse, respiration (using a BIOPAC MRI-safe respiration transducer and module), and end-tidal CO₂ were continuously recorded. During stress tasks, blood pressure and hemodynamic measurements were taken using a noninvasive blood pressure monitor and transferred to a BIOPAC data acquisition and analysis system. The study’s findings suggest that HRV biofeedback can positively impact both brain function and physiological health, highlighting the interconnected nature of bodily systems, a central concept in network physiology.

These are just a few examples of how network physiology is being applied in research today. For more information on the topic, including links to related webinars, citations, and events, see our Network Physiology applications page.

Are you planning to integrate a range of signals for your next network physiology study? Our customer service specialists can help identify the best equipment and strategies for the synchronous collection of high-quality multimodal data.

Silent Partner: Autonomic Function Testing in Research

Like the inner workings of an intricate machine, most of our bodily functions operate with little or no conscious effort. That task falls to the autonomic nervous system, a sort of control system that regulates a wide range of processes, including digestion, heart rate, respiration, urination, and sexual arousal, to name a few. The autonomic nervous system also controls physiological responses to stimuli—the fight-or-flight response, and critical reflex actions like vomiting, swallowing, coughing, and sneezing.

The ability to effectively and accurately test autonomic function allows clinicians to diagnose and treat a range of ailments. Such tests can non-invasively evaluate the severity and distribution of autonomic failure, for example. Beyond these clinical applications, autonomic function testing provides valuable insight into how the body regulates and manages various critical functions and how the autonomic nervous system affects our physical and psychological health.

Autonomic function testing has been employed to examine the impact childhood trauma has on cognitive function as it relates to adolescent depression. A 2020 study published in Development and Psychopathology looked at whether economic deprivation and threat influenced cognitive processes and autonomic nervous system function connected to the severity of depression. Researchers measured study participants’ electrodermal activity (EDA), respiratory sinus arrhythmia (RSA), and electrocardiogram (ECG) data via an MP150 system. EDA was gathered using a BioNomadix wireless BN-PPGED amplifier set, while ECG and respiration (RSP) recordings were obtained with a BioNomadix wireless BN-RSPEC amplifier set. The study’s findings supported the assumption that a history of trauma and economic deprivation can lead to risk factors for youth depression.

Continuing with the theme of applying autonomic function testing to measure the effects of trauma, a 2021 study publish in Neurobiology of Stress examines autonomic function related to sex differences among individuals with posttraumatic stress disorder (PTSD). Skin conductance, blood pressure (BP), heart rate (HR), and high-frequency heart rate variability (HF-HRV) were recorded with an MP150 data acquisition system among male and female emergency personnel following trauma exposure. The results, which showed notable sex differences in autonomic functioning among trauma-exposed individuals, have implications for predicting cardiovascular risk in those with PTSD.

These are just a few examples of how autonomic function testing is being used outside of the clinical setting to shed new light on several research areas. Check out Part 1 and Part 2 of our webinars demonstrating how to use BIOPAC equipment and software in autonomic function research.

For additional information on how BIOPAC can assist you with a wide range of research and educational applications, contact your local sales representative.