Challenges Presented by Cuffless Measurement of Blood Pressure if Adopted for Diagnosis and Treatment of Hypertension.

The global health burden presented by hypertension is providing increased motivation for improved means of collection of blood pressure (BP) data. A growing area of research and commercial activity is the use of wearable devices to provide BP data using non-invasive cuffless techniques. The accelerated progress in recent years, particularly relating to connectivity of smartphone technology, has promoted the availability of consumer devices that provide values of BP. The main types of devices are wrist-worn, watch-type devices with sensors that typically record a photoplethysmography (PPG) signal, sometimes also with an electrocardiography (ECG) signal. The general underlying concept of the cuffless BP measurement in most device types is the association of BP and the travel time of the arterial pulse between two locations, determined from the time delay between the ECG and PPG signals. Other methods may involve additional analysis of the PPG waveform features. Experimental data are presented to illustrate the challenges presented by cuffless BP techniques in obtaining reliable BP measurements when the change in BP is caused by different stimuli affecting cardiac and vascular mechanisms. These effects influence the association of the measured and physiological BP change, thus presenting significant challenges and potential limitations in the use of cuffless BP devices for the diagnosis and treatment of hypertension.

Contactless video-based photoplethysmography technique comparison investigating pulse transit time estimation of arterial blood pressure.

BACKGROUND: Non-contact measurement of physiological vital signs, such as blood pressure (BP), by video-based photoplethysmography (vPPG) is a potential means for remote health monitoring. However, the signal-to-noise ratio of cardiovascular signals within the vPPG is very low. OBJECTIVE: This study investigates the potential of BP estimation from vPPG. METHODS: In 10 healthy volunteers (4 females, 28 ± 7 years), continuous electrocardiogram, finger BP and video of the face and palm of the hand were recorded. BP was varied by isometric hand grip exercise and leg ischemia. Four vPPG methods were compared: (i) averages of the green (GREEN) color intensity; (ii) the best linear combination of color channels using independent component analysis (ICA); (iii) a linear combination of chrominance-based (CHROM) signal by standardizing the skin color profile; (iv) plane orthogonal to the skin tone (POS) as vPPG signal. These were applied to 14 regions of interest (ROIs) on the face and 5 ROIs on the palm. Pulse transit time (PTT) between ROIs, for all permutations, were calculated and the correlation with BP quantified. RESULTS: A significant, negative PTT-BP correlation was defined as success. A maximum success rate of 80% was achieved, occurring for the GREEN, POS and ICA methods only for specific ROIs within the face, but not for any permutation using the hand. CONCLUSIONS: These results indicate that the use of vPPG for estimation of BP will be challenging. A combination of different vPPG methods and within-face ROIs may yield useful information.

Blood pressure-independent neurogenic effect on conductance and resistance vessels: a consideration for cuffless blood pressure measurement?

BACKGROUND: Pulse transit time (PTT) and pulse arrival time (PAT) are promising measures for cuffless arterial blood pressure (BP) estimation given the intrinsic arterial stiffness-BP relationship. However, arterial stiffness (and PTT) is altered by autonomically-driven smooth muscle tension changes, potentially independent of BP. This would limit PTT or PAT as accurate BP correlates, more so in resistance vessels than conductance arteries. OBJECTIVE: To quantify if there is a measurable neurogenic effect on PAT measured using photoplethysmography (PPG) (path includes resistance vessels) and radial artery tonometry (path includes only conductance vessels) during physiologically induced BP changes. METHODS: PATs were measured continuously in participants (n=15, 35±15 years, 9 male) using an electrocardiogram and, simultaneously, a Finometer® PRO finger sensor, a finger PPG sensor and radial artery tonometer during seated rest, cold pressor test, cycling and isometric handgrip (IHG) exercise. ΔBP/ΔPAT was calculated for each sensor and each condition. RESULTS: All interventions significantly increased BP. A significant difference was observed in ΔBP/ΔPAT between cycling and both the cold pressor test and IHG exercise (p<0.05). ΔBP/ΔPAT did not differ whether measured via PPG or tonometry. CONCLUSIONS: Under the conditions tested, autonomic function does not have a BP-independent effect on PAT where the path includes resistance vessels (PPG signal), likely due to the speed of the wave and the short path length of resistance vessels. Autonomic function therefore does not limit the ability for use of PPG as a signal for potentially estimating BP without a cuff.

Sensitivity of Video-Based Pulse Arrival Time to Dynamic Blood Pressure Changes.

Estimating blood pressure (BP) from pulse arrival time (PAT) by image-based (skin video) photoplethysmography (iPPG) is of increasing interest due to the non-contact method advantage (over cuff-based methods) and potential for BP measurement to be built into portable devices such as mobile phones. The relationship between pulse transit time extracted from iPPG has been investigated during stable BP. The sensitivity of beat-to-beat iPPG-PAT to dynamic changes in BP has not been explored. This study investigated the correlation between iPPG-PAT and diastolic BP (DBP) during 1-minute seated rest and 3-minute isometric handgrip exercise. 15 healthy participants (9 female, 34±13 years) were recruited. Video was recorded from subjects' faces at 30 frames per second using a standard web-camera with simultaneous measurement of the electrocardiogram and noninvasive finger BP. The iPPG waveform was from the averaged green channel intensity of regions of the forehead or cheek. PAT was calculated from the R-wave ofthe electrocardiogram to the foot of the iPPG or finger BP waveform respectively for direct comparison. Handgrip exercise caused a steady increase in DBP (75±9 to 87±13 mmHg, p<0.001). Beat-to-beat iPPG-PAT and DBP was negatively correlated (mena ±SE -1.33±1.70 ms/mmHg, P=0.0024) as was finger-PAT (mean ±SE -0.5S ±0.39 ms/mmHg, P<0.001). The proportion of individual significant negative regression slopes between DBP and finger-PAT and between DBP and iPPG-PAT was not significantly different. Despite high variability of the correlation between iPPG-PAT and DBP among subjects, iPPG-PAT can track dynamic changes in BP.

Effects of instructed meditation augmented by computer-rendered artificial virtual environment on heart rate variability.

Previous research has supported the use of virtual reality (VR) to decrease stress, anxiety, perceptions of pain, and increase positive affect. However, the effect of VR on blood pressure (BP) and autonomic function in healthy populations have not been explored. This study quantifies the effect of instructed meditation augmented by a virtual environment (VE) on BP and heart rate variability (HRV) during rest and following physical (isometric handgrip) or mental (serial sevens subtraction) stress. Sixteen healthy participants underwent all conditions, and those that responded to the stress tests were included in the analysis of stress recovery. Results showed that under resting conditions, VE had no significant effect on BP or HRV when compared to seated rest and the VE video on a 2D screen. Following serial sevens, VE maintained the increased low frequency (LF) power of HRV $( 66 \pm 4$ normalized units (n.u.)) compared to seated rest $( 55 \pm 5\mathrm {n}$.u., $\mathrm {p}=0.0060)$; VE maintained the decreased high frequency (HF) power of HRV $( 34 \pm 4\mathrm {n}$.u.) compared to seated rest $( 44 \pm 5\mathrm {n}$.u., $\mathrm {p}=0.014)$; and VE maintained the increased LF/HF ratio $( 2.4 \pm 0.5)$ compared to seated rest $( 1.6 \pm 0.3$, $\mathrm {p}=0.012)$. Hence, after mental stress, VE sustains the increased sympathetic drive and reduced parasympathetic drive. VE may act as a stimulatory driver for autonomic activity and BP. Further studies are required to investigate the effect of different types of VE on BP and autonomic function.

Cuffless Estimation of Blood Pressure: Importance of Variability in Blood Pressure Dependence of Arterial Stiffness Across Individuals and Measurement Sites.

OBJECTIVE: Measuring arterial pulse transit time (PTT) to estimate blood pressure (BP) without conventional brachial cuff-based measurement is not new, but is a focus of current wearable technologies research. Much research pertains to efficient, accurate sensing of artery-related waveforms, yet the relationship between PTT and BP receives less attention despite being key for accurate BP estimation. This study investigated BP/PTT calibration by quantifying anatomical site variability (n = 10, 3 female, age 30 9 years) and individual variability ( n = 103, 50 female, age 53 22 years). METHODS: BP and pulse wave velocity (PWV) were measured in both seated and supine. Carotid-femoral PWV (cfPWV), carotid-radial PWV (crPWV), and carotid-finger-volume PWV (cvPWV) were measured with the wrist and hand positioned at the level of the upper thigh to achieve the same hydrostatic pressure effect across all measurements. RESULTS: The postural change invoked a small (4 7 mmHg) change in brachial diastolic BP with an additional 27 2 mmHg change in hydrostatic pressure. cfPWV decreased in the supine position (1.75 0.17 m s-1, p < 0.001), but crPWV and cvPWV were more variable. The calibration term (BP/PWV) across the sample population varied from 6.6 to 98.3 mmHg sm-1 (mean 22 14 mmHg sm-1) and was correlated with age, heart rate, diastolic and pulse pressure, and weight. These variables did not explain the majority of the variability (R2 = 0.248). CONCLUSION: There is anatomical site and between-individual variability in the calibration term for BP estimation from PTT. SIGNIFICANCE: Using and accurately calculating hydrostatic changes in BP within the individual may be one method to increase the accuracy of this calibration term.

Characterisation of cardiac autonomic function in multiple sclerosis based on spontaneous changes of heart rate and blood pressure.

BACKGROUND: Prevalence of cardiovascular autonomic dysfunction (CAD) in multiple sclerosis (MS) varies between studies. Cardiac autonomic function is usually assessed by cardiovascular reflex tests. We hypothesized that MS is associated with CAD, quantifiable by non-invasive means including quantification of baroreceptor sensitivity (BRS) and heart rate variability. METHODS: In this study a comprehensive suite of cardiovascular autonomic tests based only on the spontaneous changes of heart rate and blood pressure was applied to 23 MS patients and age and gender-matched controls. From 5-min continuous non-invasive recording of the electrocardiogram and blood pressure, heart-rate, blood pressure, and autonomic function variables were calculated. Analysis included heart rate variability in the time domain, heart rate and blood pressure variability in the frequency domain, and baroreceptor sensitivity in both the time and frequency domain. RESULTS: BRS measured by the frequency technique in high frequency band was found to be significantly lower in MS (16 ±â€¯9 ms/mmHg) compared to controls (29 ±â€¯17 ms/mmHg) (p < 0.05). Also mean of BRS modulus in MS averaged 15 ±â€¯8 ms/mmHg which is significantly lower compared to controls (25 ±â€¯15 ms/mmHg) (p < 0.05). Systolic blood pressure variability in the high frequency band (0.15-0.5 Hz) was found to be significantly higher in the MS compared to controls (5.8 ±â€¯16.7 mmHg2 vs. 1.3 ±â€¯0.8 mmHg2) (p < 0.05). CONCLUSIONS: The results, using techniques novel to MS investigation, showed diminished baroreceptor reflex and impaired sympathetic function using frequency domain systolic blood pressure variability analysis.