Can You Trust IVY®? Evaluating Its Optical Heart Rate Tracking Technology

Objective

The IVY® wrist-worn tracker is setting a new standard in cardiovascular monitoring for women by utilizing advanced optical heart rate monitoring technology. With an incredible 92.61% accuracy in detecting inter-beat intervals, this device outshines many competitors by delivering reliable heart rate variability insights. Its innovative algorithms not only ensure data accuracy but also adapt dynamically to various physiological conditions, making it ideal for sleep monitoring. Backed by extensive validation against gold-standard ECG systems, the IVY® promises consistent and actionable health insights.

The objective of this article is to evaluate the accuracy and reliability of the optical heart rate monitoring technology embedded in the IVY® wrist-worn tracking device for women. Its performance is analyzed with a focus on the precision of inter-beat interval (IBI) estimation, a key element in heart rate variability (HRV) analysis. This evaluation addresses the central question: Can IVY® wearable be trusted to provide consistent and accurate cardiovascular insights?

Introduction

Photoplethysmography (PPG) has become a fundamental technology in modern health-tracking devices, enabling optical heart rate monitoring (oHRM) to measure cardiovascular activity and other health-related metrics. When paired with motion sensors, oHRM provides additional insights into users’ activity levels and sleep patterns, offering a comprehensive view of their overall well-being. More recently, oHRM has shown promise in stress assessment and management by analyzing heart rate variability (HRV), relaxation, and sleep quality. However, accurately monitoring these parameters requires the precise estimation of inter-beat intervals (IBI) — the time between consecutive heartbeats seen in an electrocardiogram (ECG). High-quality PPG signals, combined with advanced algorithms, are essential to achieve this level of precision.

Bellabeat has made significant advancements by integrating oHRM technology into the compact and stylish IVY® device, achieving this without compromising accuracy or functionality. A focus on optimizing optomechanical system design has ensured the delivery of reliable PPG signals, achieved through innovative approaches such as improving skin contact, fine-tuning LED currents, and reducing overall power consumption. These advancements enhance the quality of data collected while also addressing the practical demands of wearable devices, such as battery life and user comfort.

The oHRM algorithm library integrated into IVY® relies on intelligent state-based systems that enable efficient, low-power processing of physiological metrics. By dynamically adjusting operational modes, the algorithms ensure that resources are allocated effectively — for example, reducing sampling frequency during periods of inactivity or detecting when the device is not being worn. This not only extends battery life but also enhances measurement accuracy by excluding unreliable data when conditions are suboptimal for precise IBI estimation, such as during significant user movement or device misplacement.

Bellabeat’s oHRM algorithm has been developed and thoroughly validated with data from several hundred users and thousands of data points, enabling precise estimation of heart rate (HR), heart rate variability (HRV) metrics like SDNN, RMSSD, LF and HF. In addition to these classical measures, a new metric was developed to capture the coupling between sympathetic and parasympathetic nervous systems: the cardiac coherence. This metric is particularly effective in assessing stress and relaxation, making it ideal for calculating daily Readiness Score. It offers the advantage of extracting valuable information from signal durations as short as a few breathing cycles, while classical HRV analysis typically requires around 5 minutes, which impacts battery life.

How Accurately Does the IVY® Device Measure Heart Rate and HRV During Sleep?

The accuracy of the IVY® device for inter-beat interval (IBI) measurement was validated through a comparative analysis against the gold-standard SCHILLER medilog® AR Holter System, which combines the advanced medilog® AR recorder with DARWIN2 software for highly precise ECG analysis (32,000 Hz sampling rate, 5 leads, 3 channels). Since heart rate (HR), heart rate variability (HRV), and cardiac coherence are derived from IBIs, accurate IBI detection is crucial for reliable cardiovascular assessment.

The analysis focused on nighttime recordings during sleep, a period ideal for IBI tracking due to minimal motion artifacts and stable physiological conditions.

A Closer Look: Single-Night IBI Measurement with Near-Perfect Agreement

To highlight the accuracy of the IVY® device, let’s take a closer look at a single-night IBI recording from an individual with near-perfect agreement between IVY® and ECG. The IBIs measured by the IVY® device closely followed the ECG RR intervals (the time between two consecutive R-wave peaks), demonstrating a near-perfect correlation throughout the night.

Over the course of approximately 5 hours and 38 minutes of recorded sleep, the Schiller ECG detected a total of 17,979 IBIs, serving as the reference for evaluating the IVY® device. Of these, 16,650 IBIs (92.61%) were successfully detected by IVY® with high confidence (100% confidence level), while 1,003 IBIs (5.58%) were marked as faulty (0% confidence level).

Additionally, 326 IBIs (1.81%) were entirely missing and had to be interpolated to maintain alignment with the ECG reference. These missing IBIs occurred during periods where the IVY® device was unable to detect a reliable PPG signal, likely due to motion artifacts, variations in skin contact, or low perfusion.

After incorporating the interpolated IBIs, the total high-confidence detection rate was 92.61%, reaffirming the device’s robust capability for accurate IBI tracking during sleep. The presence of interpolated IBIs serves as a reminder of the importance of optimal sensor placement and signal quality for continuous heart rate tracking.

The first plot presents the complete dataset, while the second offers a zoomed-in view, highlighting more detailed aspects of the signal.

The plots below illustrate the performance of the IVY® wrist-worn device in detecting inter-beat intervals (IBIs) with a 100% confidence level, ensuring that only the most reliable measurements are considered for analysis. During signal processing, the IVY® device automatically discards low-confidence IBIs. By focusing exclusively on high-confidence IBIs, the device ensures that only accurate input data is used for heart rate (HR) and heart rate variability (HRV) calculations.

The first plot displays the complete dataset of high-confidence IBIs, revealing the close alignment between IVY® and the Schiller ECG reference over thousands of samples. Despite variations in the signal and the presence of large dynamic changes in RR intervals, the IVY® device reliably tracks the ECG reference signal with remarkable consistency.

The second plot provides a zoomed-in view, showcasing finer details of the signal. This detailed perspective highlights the IVY® device’s ability to maintain a near-perfect correlation with the ECG reference, even during rapid fluctuations in IBI values. The overlapping signals confirm the robustness of the IVY® algorithm, particularly for capturing physiological dynamics with precision.

By filtering out low-confidence IBIs and leveraging only 100% confident measurements, the IVY® device demonstrates its capability to deliver accurate and reliable input for downstream calculations, reaffirming its potential for precise cardiovascular monitoring.

Summary of Key Metrics

The IVY® device demonstrated exceptional agreement with the Schiller ECG reference when only high-confidence (100%) IBIs were used, with minimal differences in all calculated metrics.

Entire Recording Comparison

When comparing the entire recording (including all ECG data and only high-confidence IVY® IBIs):

Even across the entire recording, the IVY® device demonstrated strong agreement with the Schiller ECG reference, showing its ability to perform consistently, even when some portions of the signal were excluded due to lower confidence.

Even Better Accuracy in the  accompanying apps

While the full-session comparison shows strong accuracy, the results are even better when viewed in Bellabeat’s accompanying apps and compared over time segments. Since IVY® processes and saves HR and HRV continuously, analyzing shorter periods provides an even clearer picture of heart activity.

Additionally, adaptive algorithms compensate for missing IBIs, ensuring smooth and accurate HR and HRV estimations. This allows the device to maintain high precision, even when some data points are unavailable.

By looking at HR and HRV in smaller time segments, the app reflects more precise and real-time trends, reducing the impact of temporary fluctuations. This means the insights users see in Bellabeat’s accompanying apps are often more accurate than a single long-term average.

Strong Correlation Between IVY® and ECG Measurements

To further assess the accuracy of the IVY® device, a correlation analysis was performed between its inter-beat interval (IBI) measurements and the gold-standard Schiller ECG reference. The results indicate a very high agreement, confirming that the IVY® device reliably tracks heart activity.

Correlation Analysis Results

What this means:

• Pearson’s correlation (0.977) indicates a strong linear relationship between the IVY® device and ECG IBIs, meaning the two datasets follow the same pattern with minimal deviation.
• Spearman’s correlation (0.982) confirms that even if there are slight variations, the ranking order of IBIs remains highly consistent between IVY® and ECG.
• Kendall’s correlation (0.919) further supports this agreement, showing that paired IBIs from IVY® and ECG match exceptionally well.
• R² of 0.9998 suggests that over 99.9% of the variance in IVY® IBI measurements can be explained by the ECG reference, demonstrating a near-perfect fit.

Visualizing the Agreement

A regression plot was created to visualize the relationship between IVY® and ECG IBIs. The trendline closely follows the data points, reinforcing the strong correlation and high accuracy of the IVY® device.

The correlation analysis confirms that the IVY® device delivers IBI measurements closely aligned with ECG data, achieving an exceptional R² value of 0.9998. This highlights the device’s ability to provide accurate real-time insights for HR and HRV analysis.

The Krouwer version of the Bland-Altman plot further highlights the strong agreement between the IVY® wrist-worn device and the Schiller ECG reference for inter-beat interval (IBI) measurements. The plot demonstrates a minimal mean bias (red dashed line) and clinically acceptable limits of agreement (green dashed lines), with only a small number of outliers. This reinforces the device’s ability to deliver accurate and consistent IBI tracking by filtering out noisy or low-confidence intervals.

• Mean Difference (red dashed line): The IVY® device demonstrates minimal bias, with the mean percentage difference centered near 0%, indicating high accuracy in IBI estimation.
• Limits of Agreement (LoA) (green dashed lines): The majority of data points fall within the clinically acceptable range, confirming the device’s reliability across varying IBI values.
• Outliers: While a few outliers are observed below -50% and above 30%, they are 
rare and have negligible impact on overall agreement.

Conclusion

The results highlight that the IVY® device maintains strong agreement with ECG across the full range of IBIs, making it a reliable tool for cardiovascular monitoring. Furthermore, the device’s ability to filter out noisy or low-confidence intervals ensures robust performance, even under challenging conditions.

While the results discussed so far highlight the exceptional performance of the IVY® device under conditions of near-perfect agreement, it is equally important to consider its accuracy in typical agreement scenarios, which better reflect the experience of an average user. In everyday use, missing IBIs and low-confidence IBIs are more prevalent, often caused by motion artefacts, variations in skin contact, or environmental factors. As a result, the device’s accuracy is slightly reduced compared to near-perfect agreement conditions.

Despite these real-world variations, the IVY® device continues to demonstrate high reliability for nocturnal cardiovascular tracking. Whole-night HR measurements remain highly accurate (R² ≥ 0.998), and HRV (RMSSD) maintains a strong correlation with ECG (R² ≥ 0.982). The mean bias for HR (-0.42 bpm) and HRV RMSSD (-6.39 ± 7.89 ms, range: -14.28 to +1.5 ms) indicates only minor deviations from ECG reference values, ensuring the device remains a valuable and practical tool for sleep-based cardiovascular monitoring. Bland-Altman analysis confirms that, while the limits of agreement (LoA) widen in typical usage conditions, the majority of data points remain within clinically acceptable ranges, reinforcing the device’s consistency and reliability, even under non-ideal conditions.

Final Verdict: Can IVY® Be Trusted?

Yes, IVY® can be trusted to provide consistent and accurate cardiovascular insights. The device’s exceptional agreement with ECG, combined with its ability to adapt to real-world conditions, ensures precise, reliable HR and HRV tracking. Whether used for daily wellness monitoring or more advanced physiological assessments, IVY® delivers high-quality, clinically relevant insights, making it a top-tier wrist-worn tracking solution.