Table 2.

Summary of studies about pulse transit time (PTT) based cuffless blood pressure (BP) measurement

First Author (year, country)	Indicator	Method (model)	Calibration (method/interval)/reference method	Subjects	Accuracy (mmHg)
					SBP	DBP
Young (1995, US) [71]	PTTPP	SBP = a1/PTT + b1 DBP = a2/PTT + b2	Oscillometric BP (5 min)/Invasive radial BP	35 patients	− 0.37 (− 29.0 to 28.2)	− 0.01 (− 14.9 to 14.8)
Chen (2000, Japan) [59]	PTTEPf	$\text{SBP}=\text{SBP}0-\frac{2}{\gamma {\text{PTT}}_0}\left(\text{PTT}-{\text{PTT}}_0\right)$	Intermittent BP from Invasive BP (5 min)/Invasive radial arterial BP	20 patients	RMSE: 3.70 ± 1.85
Fung (2004, Canada) [70]	PTTEPu	$\text{BP}=\frac{\text{a}}{{\text{PTT}}^2}+\text{b}$	Cuff Sphygmomanometer (10 min)/Cuff Sphygmomanometer	22 patients	− 0.08 ± 11.32
Poon (2005, China) [60]	PTTEPu	$\text{DBP}={\text{MBP}}_0+\frac{2}{\gamma }ln\frac{{\text{PTT}}_0}{\text{PTT}}-\frac{1}{3}{\text{PP}}_0·{\left(\frac{{\text{PTT}}_0}{\text{PTT}}\right)}^2$ $\text{SBP}=\text{DBP}+{\text{PP}}_0·{\left(\frac{{\text{PTT}}_0}{\text{PTT}}\right)}^2$	Cuff BP (Initial calibration)/Average of auscultatory and oscillometric BP	85 (39 hypertensive)	0.6 ± 9.8	0.9 ± 5.6
Muehlsteff (2006, Germany) [54]	PTTEPf	SBP = a * lnPTT + b	Cuff BP (Physical test)/Cuff BP	18 healthy	RMSE: 7.5
		SBP = a * L/PTT + b			RMSE: 6.9
		SBP = a*(L/PTT)2 + b			RMSE: 7.3
Wong (2009, China) [57]	PTTEPu	SBP = a1*PTT + b1 DBP = a2*PTT + b2	Oscillometric BP (exercise, half year)/Oscillometric BP	41 healthy (14 for 0.5 year)	0.0 ± 5.3 Half year: 1.4 ± 10.2	0.0 ± 2.9 Half year: 2.1 ± 7.3
Cattivelli (2009, US) [68]	PTTEPu	SBP = a1*PTT + b1*HR + c1 DBP = a2*PTT + b2*HR + c2	MIMIC Database (invasive BP, 1 h)/Invasive BP	25 ICU patients	− 0.41 ± 7.77	− 0.07 ± 4.96
Gesche (2011, Germany) [78]	PTTEPu	$B{P}_{\mathit{PTT}}={\text{P}}_1\times PWV\times e^{\left({\text{p}}_3\times PWV\right)}+P_2\times PW{V}^{P_4}-\left(B{P}_{PTT,cal}-B{P}_{\mathit{cal}}\right)$	Sphygmomanometer (13 subjects, exercise)/Sphygmomanometer	63	SD: 10.10 95% CI: − 19.8 to 19.8
Chen (2012, Singapore) [79]	PTTPP	$\text{SBP}=b_{\mathit{ij}}{e}^{-\frac{k_{\mathit{ij}}}{PW{V}_s}}$ $\text{DBP}=b_{\mathit{ij}}{e}^{-\frac{k_{\mathit{ij}}}{PW{V}_d}}$	Intra-arterial BP (9 subjects)/Intra-arterial BP	35 healthy	1.49 ± 6.51	2.16 ± 6.23
Choi (2013, Canada) [64]	PTTEPu	BP = a * PTT + b Recursive least square algorithm for recalibration	Cuff-based BP	25 patients	− 0.44 ± 3.85	− 0.93 ± 1.84
Kachuee (2015, Iran) [80]	PTTEP and PPG features	Support Vector Machine regression	Intra-arterial BP	–	MAE: 12.38	MAE: 6.34
Ding (2016, China) [81]	PTTEPu and PIR	$\text{SBP}={\text{DBP}}_0·\frac{{\text{PIR}}_0}{\text{PIR}}+{\text{PP}}_0·{\left(\frac{{\text{PTT}}_0}{\text{PTT}}\right)}^2$ $\text{DBP}={\text{DBP}}_0·\frac{{\text{PIR}}_0}{\text{PIR}}$	Finapres BP	27 healthy	− 0.37 ± 5.21	− 0.08 ± 4.06
Kachuee (2017) [82]	PTTEP and PPG features	Calibration free: Adaptive Boosting	Intra-arterial BP	942	MAE: 11.17	MAE: 5.35
		Calibration based		57	MAE: 8.21	MAE: 4.31
Buxi (2017) [83]	PTTEI	SBP = a1 * PTTEI + b1	Oscillometric BP	6 healthy	r (PTTEI − SBP) = 0.66	r (PTTIR − DBP) = 0.48
	PTTIR	DBP = a2 * PTTIR + b2
Huynh (2018) [84]	PTTIP and ICG indicator	$\text{SBP}={\text{DBP}}_0+\rho ·{\left(\frac{D}{\text{PTT}}\right)}^{2}ln\left(1+K\left(Z_{max0}-Z_{\mathit{max}}\right)\right)$ $\text{DBP}={\text{DBP}}_0+\rho ·{\left(\frac{D}{\text{PTT}}\right)}^{2}ln\left(1+K\left(Z_{max0}-Z_{\mathit{min}}\right)\right)$	Oscillometric BP	15 healthy	RMSE: 8.47 ± 0.91	RMSE: 5.02 ± 0.73
Liu (2018) [85]	PTTPP	MBP = HR*(K1*DRPPG TD + b1) PP = MBP*(k2*t/HP + b2) SBP = MBP + 2/3PP; DBP = MBP-1/3PP	Finapres BP	20 healthy	SD: 2.85	SD: 1.75
Ma (2018) [86]	PWV	BP = αPWV2 + β	–	–	–	–