Table 4.

Classification of modern BCG systems.

Modern BCG System	Measuring Axis	Key Benefits/Successes	Challenges/Constraints
Accelerometer in center of weight [82]	3-axis	- Characterized 3D BCG vector - Measured 3D BCG in microgravity	- the need for weight reduction—either in space or with dry immersion
Bed and chair [70,83,84]	Longitudinal or out of the plane	- Minimal movement artifacts (usually) - Easy to integrate into home and everyday life	- Changes in sleep position may affect signal quality and morphology - It is difficult to pair BCG with other physiological measurements (e.g., ECG)
Monitor vital signs on the ears [85]	Primarily longitudinal	- ECG can be measured simultaneously - Miniature, potentially cheap system	- Head position may affect signal integrity - Repeatability to be assessed
Weight [35,36]	Longitudinal	- Correlation with CO/contractility changes - obtaining multiple physiological signals in addition to BCG	- Postural differences between measurements may affect signal integrity - Motion artifacts must be automatically detected and mitigated
Vest MagIC [37,86]	Primarily longitudinal	- Correlation with CO changes - Obtain multiple physiological signals in addition to BCG	- Signal variation based on sensor position - Motion artifacts must be automatically detected and mitigated