Table 1.
Previous PPG force studies performed assessing sensor contact force to the quality of measurements made using PPG. A Assumption here is that the PPG probe was no larger than the force-sensing probe reported in the study; B Straight conversion kPa to mmHg (kPa times 7.501); C Straight Conversion hPa to mmhg (hPa divided by 1.333); D Same as assumption in A, but also assuming researchers used the smallest version of the sensor reported available to account for the nature of the study (paediatrics, index finger). 1 [4]; 2 [5]; 3 [6]; 4 [7]; 5 [8]; 6 [9].
| Study (1st Author) | Sensor Location | Optimum Pressure/Force Reported | Conversion to mmHg | Research Question | Findings |
|---|---|---|---|---|---|
| Teng 1 | Finger | 0.2–1.0 N | 21–105 mmHg A | Change in AC/DC ratio with change in contact force as an important metric when calculating blood oxygen saturation | PPG does have an optimum or “peak” value for the contact force applied. Hence careful sensor design consideration is required. |
| Grabovskis 2 | Posterior Tibial A., Femoral A., Popliteal A., | 10.9, 11.8, 15.2 kPa | 81, 88, 114 mmHg B | The effect of probe contact pressure (CP) on the PPG signal for assessing arterial stiffness | Wrong contact pressure would adversely affect the AC PPG 2nd derivative peak ratio (known as the b/a ratio), a measurement index to assess arterial function. Also, suggests an optimal contact pressure. |
| Shimazaki 3 | Forearm, Wrist | 40–50 hPa at both locations | 30–37 mmHg C | The effect of fastening or applying contact pressure in wearable devices such as wristwatches which employ PPG to measure heart rate. Motion artefact reduction during exercise, elevated respiration artefact and accuracy of heart rate prediction were the key parameters investigated | All these studies reported that CP has a significant impact (i) reducing the noise introduced by motion artefact during exercise, (ii) increasing respiration related modulations in PPG, and (iii) increasing error in heart rate calculation up to ±11 beats per minute. Additionally, studies confirmed that further optimisation of the CP is indeed needed to reliably calculate physiological parameters. |
| Kasbekar 4 | Forehead, Wrist | 12 kPa (Forehead) | 90 mmHg (Forehead) B | ||
| Lee 5 | Index Finger (Paediatric Study, mean age = 4.1 y) | 0.4–0.6 N | 5.9–8.8 mmHg D | ||
| Scardulla 6 | Wristband | 54 mmHg | NA |