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Poor Agreement Between Bioreactance and Echocardiographic Assessment of Fluid Responsiveness During Passive Leg Raising in Patients With Shock: A Prospective Study

This prospective study of 30 critically ill patients with shock found that bioreactance-based monitoring demonstrates poor agreement with transthoracic echocardiography in assessing fluid responsiveness during passive leg raising, indicating that the two methods are not interchangeable for this clinical purpose.

Original authors: Antonious Anis, Juliann Shih, Vidhani Goel², Kavita Batra, Irene Huang⁴, Mutsumi J. Kioka

Published 2026-07-05

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CC BY 4.0

Original authors: Antonious Anis, Juliann Shih, Vidhani Goel², Kavita Batra, Irene Huang⁴, Mutsumi J. Kioka

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are a captain steering a ship (the patient's body) through a storm (shock). Your most critical job is to decide whether to add more water to the ship's ballast tanks (giving fluids) to keep it stable. If you add too little, the ship sinks; if you add too much, it capsizes. To make the right call, you need to know if the ship will actually rise when you add water. This is called testing for "fluid responsiveness."

In the hospital, doctors use two different tools to answer this question during a simple maneuver called the Passive Leg Raise (PLR). Think of the PLR as a quick, temporary "test drive" where the patient's legs are lifted up to see if blood rushes back to the heart, simulating what would happen if you gave them fluids.

The two tools being compared in this study are:

The Echocardiogram (The Expert Eye): This is like a high-definition, real-time video camera looking directly inside the heart. A skilled technician uses ultrasound to watch the heart pump and measure exactly how much blood it pushes out. It's accurate but requires a trained expert and clear images.
Bioreactance (The Electrical Guess): This is a non-invasive monitor that uses electrodes stuck to the patient's chest. It sends tiny electrical signals through the body and guesses how much blood is moving based on how those signals change. It's easy to use and gives a continuous readout, like a dashboard gauge.

The Experiment

The researchers at the University of Nevada wanted to see if these two tools tell the same story. They took 30 patients in the Intensive Care Unit (ICU) who were in shock and needed strong medications (vasopressors) to keep their blood pressure up. They performed the "leg raise" test on everyone and measured the heart's output with both the camera (Echo) and the electrical gauge (Bioreactance) at the exact same time.

What They Found

The results were surprisingly messy. The two tools were essentially speaking different languages.

The "Tale of Two Lists": When the expert camera looked, it found that 12 out of 30 patients would benefit from fluids. When the electrical gauge looked, it said 16 out of 30 would benefit. They didn't even agree on who needed help.
No Connection: When the researchers tried to see if the numbers moved together (correlation), they found almost no relationship. It was as if one tool was reading the weather in New York while the other was reading the weather in Tokyo. The numbers didn't match up at all.
The "Wide Gap": Using a statistical map called a Bland-Altman plot (which checks how close two measurements are), they found the gap between the two tools was huge. The difference was so wide that the two methods could not be used interchangeably. If you trusted the electrical gauge, you might be completely wrong compared to what the camera saw.
The "Coin Flip": When they tested how well the electrical gauge could predict what the camera saw, the results were no better than flipping a coin. It had no real skill at identifying the right patients.

Why Did They Disagree?

The paper suggests a few reasons why the electrical gauge struggled in this specific group of patients:

The Stormy Seas: These patients were in severe shock and on powerful drugs that change how their blood vessels behave. This "electrical noise" and changing body chemistry likely distorted the signals the gauge was trying to read, much like static on a radio during a thunderstorm.
Timing: The leg raise test is very fast. The camera can freeze-frame the heart's movement instantly, but the electrical gauge might be slightly slower to react to these rapid changes.

The Bottom Line

The study concludes that for patients in shock who need strong blood-pressure medications, you cannot swap the electrical gauge for the expert camera.

If a doctor relies only on the bioreactance monitor to decide whether to give fluids, they might make the wrong call. The paper advises that until we have better proof, the "Expert Eye" (echocardiography) remains the preferred method for this specific, high-risk group of patients because the electrical "guessing" tool is simply not accurate enough to be trusted on its own.

Technical Summary: Poor Agreement Between Bioreactance and Echocardiographic Assessment of Fluid Responsiveness During Passive Leg Raising in Patients With Shock

Problem Statement
In the management of critically ill patients with shock, assessing fluid responsiveness is essential to avoid both under-resuscitation and fluid overload. The passive leg raising (PLR) maneuver, combined with stroke volume (SV) measurement, is a standard bedside method for this assessment. While transthoracic echocardiography is frequently used as the reference standard, it requires significant operator expertise and adequate image acquisition, limiting its routine application. Bioreactance-based monitoring has emerged as a promising noninvasive, continuous alternative that does not require image acquisition. However, a critical knowledge gap exists regarding the agreement between bioreactance-derived and echocardiography-derived SV changes during PLR. It remains uncertain whether these two modalities are interchangeable for assessing fluid responsiveness, particularly in hemodynamically unstable patients requiring vasopressor support.

Methodology
This was a prospective observational study conducted in a medical intensive care unit (MICU) involving adult patients with shock requiring vasopressor support.

Study Population: 30 patients were enrolled after screening 37 candidates. The cohort had a mean age of 60.0 years, with septic shock being the predominant subtype (76.7%). All patients required vasopressor support.
Intervention: A PLR maneuver was performed, elevating legs to 45 degrees from a semi-recumbent position.
Measurements: Stroke volume was measured before and approximately two minutes after PLR using two simultaneous methods:
1. Transthoracic Echocardiography: Performed by a single experienced operator (NBE diplomate) using a portable ultrasound system. SV was calculated from the left ventricular outflow tract (LVOT) diameter and velocity–time integral (VTI).
2. Bioreactance Monitoring: A noninvasive system (Starling SV) using surface thoracic electrodes.
Definitions: Fluid responsiveness was defined as an increase in SV of ≥10% following PLR.
Statistical Analysis: Agreement was evaluated using correlation analysis, Bland–Altman analysis (bias and limits of agreement), responder classification agreement (Cohen's kappa), and receiver operating characteristic (ROC) curve analysis. Bootstrap methods were used for confidence intervals.

Key Contributions

Direct Comparative Analysis: This study provides a direct, paired comparison of bioreactance and echocardiography specifically during the PLR maneuver in a population of vasopressor-dependent shock patients, a group often excluded from previous validation studies.
Multi-Metric Evaluation: The study employed four distinct analytical approaches (correlation, Bland–Altman, classification agreement, and diagnostic performance) to comprehensively assess interchangeability, rather than relying on a single metric.
High-Quality Reference Standard: By utilizing a single, highly experienced echocardiographer for all measurements, the study minimized inter-operator variability, ensuring consistency in the reference data.

Results

Responder Classification: Echocardiography identified 12 patients (40.0%) as responders, while bioreactance identified 16 patients (53.3%). The overall agreement in classifying responders was only 53.3%, with a Cohen's kappa coefficient of 0.079 (95% CI, -0.264 to 0.422), indicating agreement no better than chance.
Correlation: The correlation between the percentage change in SV ( $\Delta$ SV%) derived from echocardiography and bioreactance was weak and non-significant ( $r = -0.072, p = 0.705$ ).
Bland–Altman Analysis: The analysis revealed a mean bias of -6.8 percentage points with extremely wide limits of agreement (-66.5% to +52.8%), indicating clinically unacceptable agreement. The percentage error for absolute stroke volume comparisons was 84.8% at baseline and 102.5% post-PLR, both far exceeding the 30% threshold for clinical interchangeability.
Diagnostic Performance: ROC analysis showed that bioreactance-derived $\Delta$ SV% had poor discrimination for identifying echocardiography-defined responders (AUC 0.542, 95% CI 0.318–0.765; $p = 0.715$ ). Even at an optimized cutoff, sensitivity was low (41.7%).
Statistical Significance of Change: While bioreactance detected a statistically significant increase in SV after PLR ( $p = 0.003$ ), the echocardiographic increase was not statistically significant ( $p = 0.223$ ).

Significance and Claims
The study concludes that bioreactance-derived stroke volume changes during PLR demonstrate poor agreement with echocardiographic assessment of fluid responsiveness in critically ill patients with shock. The authors assert that the two methods should not be considered interchangeable for this specific clinical application.

The paper highlights that while bioreactance offers advantages in ease of use and continuous monitoring, its inability to reliably track echocardiographic measurements in this population suggests significant limitations. The authors caution clinicians against relying solely on bioreactance measurements to guide fluid administration in patients with shock requiring vasopressor support. Instead, when operator expertise and image quality are adequate, echocardiographic assessment remains the preferred approach. The findings suggest that the discrepancy may stem from the intrinsic limitations of bioreactance in the setting of altered vascular tone and complex circulatory physiology associated with shock, as well as differences in the temporal response characteristics of the two modalities.

The Experiment

What They Found

Why Did They Disagree?

The Bottom Line

Technical Summary: Poor Agreement Between Bioreactance and Echocardiographic Assessment of Fluid Responsiveness During Passive Leg Raising in Patients With Shock

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