Enhanced BBI Measurement

Dec 18, 2023

The Fitrockr Health Solution platform implements “Enhanced BBI” which highly improves accuracy of measuring the time intervals between heartbeats.

The Fitrockr platform has been extended with a new feature called “Enhanced BBI”. The feature preserves the bandwidth of the signal and provides higher accuracy for high BBI variations. It is a unique feature in the industry because it provides a confidence metric for every heartbeat to filter out noisy or unreliable data.

What is BBI (beat-to-beat-interval)?

Heart rate variability (HRV) is a very broad term that refers to variations in the time intervals between heartbeats. There are many ways to measure HRV, both in the time domain and frequency domain.

The basic measurement used to compute HRV metrics, such as the root mean square of successive differences (RMSSD) and the standard deviation of NN intervals (SDNN), is a time series of inter-beat intervals (IBI). Two ways to measure IBI are using electrocardiogram (ECG) and photoplethysmography (PPG) signals. ECG measures the electrical signals that drive the functioning of the heart, while PPG relies on measuring the waveform of the arterial pressure wave. IBIs obtained using ECG measurements are called R-R Intervals (RRI) as they are measured by obtaining the time interval between the R wave of the QRS complex in two successive beats. IBIs obtained using PPG measurements are called Beat-to-Beat Intervals (BBI) in that the PPG signal measures the time interval between two successive pulse pressure waves of the capillary arterial blood flow in the sensor field of view (which in the case of Garmin wearables, is the wrist).

Compatible Garmin devices measure the BBI directly using the PPG sensor on the bottom of the device. Fitrockr imports the time series of BBI measurements directly from the Garmin device and additionally computes HRV metrics such as RMSSD, SDNN, PNN50, SD1/SD2, VLF, LF, HF, LF/HF, TP.

What is Enhanced BBI?

Standard BBI measurement emphasizes availability even during periods of marginal signal quality. This necessitates extra filtering of the PPG signal which, in turn, limits the bandwidth with the consequence of sometimes affecting the accuracy of higher frequency HRV markers such as RMSSD.

Enhanced BBI uses a different signal processing chain which preserves the bandwidth of the signal and provides higher accuracy for high BBI variations. In addition to this higher bandwidth, a confidence metric (a binary indicator of ONE for high confidence and ZERO for low confidence) was added to the BBI time series to indicate whether any particular beat might be of higher error due several reasons (described further below).

Enhanced BBI is unique in the industry in that it is providing a confidence metric for every heartbeat (outside of areas of extremely high motion or poor signal, where no beat information is provided). This allows to interpret the data and perform outlier rejection.

BBI Confidence Explained

Generally speaking, beats can be marked as low confidence for three reasons:

Motion: By far the most common reason to mark a beat as low confidence is due to motion artifact. Garmin wearables contain a three-axis accelerometer sampled simultaneously with the PPG signal. When the accel detects motion above a predetermined threshold, the beat is marked as low confidence. If device motion exceeds an even higher threshold, the device will decline to provide a beat measurement at all, and the time series will indicate a gap until the motion falls beneath the threshold again.

Signal Quality: There are many reasons for low signal quality. The most common is simply a fit issue. If the optical sensor is not seated against the skin on the back of the wrist, there may be no signal. Low blood perfusion in the tissue in the field of view of the optical sensor can also create reduced signal quality. Further, the signal can be almost extinguished if the wearer lays on the limb and puts a lot of pressure on the watch.

Abnormally High HRV: Even in cases where the sensor signal quality is good, the PPG signal can still occasionally contain errors for a variety of reasons. To combat this, Enhanced BBI contains an algorithm to estimate the BBI variance in real time, and mark as low confidence any beats that are thought to be unlikely due to being abnormally short or abnormally long. A large amount of data was processed to tune the Enhanced BBI algorithm, including many recordings of heart rhythms besides sinus rhythm, including atrial fibrillation and frequent PVCs.

How Accurate Is Enhanced BBI?

As shown below, Enhanced BBI approaches the accuracy of an ECG-based RRI measurement system with far less user friction – all a user has to do is wear a Garmin watch or band properly on either wrist. Below is an analysis of a single sleep interval (i.e. one night) using Enhanced BBI.

An analysis compared Enhanced BBI against RRI obtained from a reference ECG monitor. In this case, the subject wore a Garmin Venu 2 Plus on the left wrist, and a Firstbeat Bodyguard 2 ECG monitor was worn as the reference.

The BBI and RRI time series, when only high confidence beats are included, are extremely similar. This means that when only high confidence beats are compared to the reference beats, a high level of accuracy is attained.

Summary

– Enhanced BBI is a more accurate way of measuring the time intervals between heartbeats using the optical PPG signal.

– It also gives a confidence indicator for each beat measurement, which helps Fitrockr users to filter out noisy or unreliable data.

– Enhanced BBI preserves the bandwidth of the signal and provides higher accuracy for high BBI variations which are of great analysis interest.

– Enhanced BBI is unique in the industry because it provides a confidence metric for every heartbeat, so Fitrockr users can easily curate BBI data.

Available on all Fitrockr subscriptions!

Enhanced BBI is available in all Fitrockr platform subscriptions for compatible Garmin devices. Enhanced BBI confidence factor can be retrieved in the Fitrockr BBI data export.

The following Garmin devices are compatible: all models with Gen 4 or greater sensor and Vivosmart 5.