The effect of applied sensor contact force on pulse transit time

Area of Science:

• Biomedical Engineering
• Physiological Measurement
• Cardiovascular Monitoring

Background:

• Pulse transit time (PTT) is a non-invasive measure of cardiovascular health, derived from electrocardiogram (ECG) and photoplethysmogram (PPG) signals.
• The accuracy of PTT is potentially influenced by external factors, including the contact force of the PPG sensor on the skin.
• Understanding these influences is critical for reliable PTT-based diagnostics.

Purpose of the Study:

• To investigate the effect of varying sensor contact force on PTT measurements.
• To determine how different characteristic points of the PPG waveform are affected by contact force.
• To provide recommendations for optimizing PTT measurement protocols.

Main Methods:

• Simultaneous ECG and PPG recordings were obtained from 30 healthy subjects.
• A range of 12 contact force magnitudes (0.1 N to 1.8 N) were applied to the PPG sensor.
• Three distinct points on the PPG waveform (PTT1, PTT2, PTT3) were analyzed to calculate PTT variations.

Main Results:

• PTT calculated at the foot (PTT1) and systole (PTT2) showed significant increases with contact force up to -0.1 N transmural force.
• Beyond -0.1 N, PTT1 and PTT2 plateaued, indicating a saturation effect.
• No significant trend was observed for PTT calculated near the PPG peak (PTT3).

Conclusions:

• Sensor contact force is a critical variable affecting PTT measurements, particularly PTT1 and PTT2.
• Inconsistent contact force can lead to inaccurate PTT values, potentially limiting its diagnostic utility.
• Standardized and controlled sensor contact force is recommended for PTT measurements to ensure reliability and validity.