Effect of Ambient Lighting and Skin Tone on Estimation of Heart Rate and Pulse Transit Time from Video Plethysmography.

Video-based photoplethysmography (vPPG) enables remote and contactless detection of the peripheral pulse of blood flow. This provides a potential mean to extract heart rate (HR) and pulse transit time (PTT) for the purpose of remote health monitoring. The accuracy of average HR and PTT extracted from a two-minute vPPG recording has been investigated at six different lighting conditions among participants with a range of Fitzpatrick skin scores. 12 healthy volunteers (6 females, 27 ± 6 years) were recruited. The video, electrocardiogram and finger PPG were acquired from immobile resting subjects. The vPPG signals from red, green and blue channels, and a combination of those were investigated. The vPPG signals were extracted from two regions of interest (ROIs): one on the forehead and one on the palm of the left hand. The estimated HR error (HR-error) was significantly lower for vPPG from green channels in both ROIs (ROI1 [p<0.001], ROI2 [p<0.05]). The signal from ROI1 demonstrated lower HR-error than ROI2 (p<0.001). HR-error from the darkest lighting conditions (Lumen 1 and 2) were significantly higher than the others (p<0.05). Furthermore, HR-error showed a positive correlation with skin tone scores in every lighting condition. However, at brighter lighting intensity, HR-error was independent of the skin tone score. PTT calculated from vPPG (vPTT) were compared between the 6 levels of lightings and the result was significantly different (p<0.05). In darker lighting conditions, the vPTT increased. Pulse arrival time measured from PPG (PAT-PPG) was calculated, and a positive correlation was found between the ratio of vPTT/PAT-PPG and skin tone score at six different lightings. However, this dependency decreases in brighter lighting intensity. These results suggest that HR-error and the ratio of vPTT/PAT increase with darker skins and at darker backgrounds. However, at brighter lighting conditions, the skin tone score is not a confounder of vPPG accuracy.

Progressive vascular remodelling, endothelial dysfunction and stiffness in mesenteric resistance arteries in a rodent model of chronic kidney disease.

Chronic kidney disease (CKD) and hypertension are co-morbid conditions both associated with altered resistance artery structure, biomechanics and function. We examined these characteristics in mesenteric artery together with renal function and systolic blood pressure (SBP) changes in the Lewis polycystic kidney (LPK) rat model of CKD. Animals were studied at early (6-weeks), intermediate (12-weeks), and late (18-weeks) time-points (n=21), relative to age-matched Lewis controls (n=29). At 12 and 18-weeks, LPK arteries exhibited eutrophic and hypertrophic inward remodelling characterised by thickened medial smooth muscle, decreased lumen diameter, and unchanged or increased media cross-sectional area, respectively. At these later time points, endothelium-dependent vasorelaxation was also compromised, associated with impaired endothelium-dependent hyperpolarisation and reduced nitric oxide synthase activity. Stiffness, elastic-modulus/stress slopes and collagen/elastin ratios were increased in 6 and 18-week-old-LPK, in contrast to greater arterial compliance at 12weeks. Multiple linear regression analysis highlighted SBP as the main predictor of wall-lumen ratio (r=0.536, P<0.001 n=46 pairs). Concentration-response curves revealed increased sensitivity to phenylephrine but not potassium chloride in 18-week-LPK. Our results indicate that impairment in LPK resistance vasculature is evident at 6weeks, and worsens with hypertension and progression of renal disease.

Quantification of peripheral and central blood pressure variability using a time-frequency method.

Systolic blood pressure variability (BPV) is associated with cardiovascular events. As the beat-to-beat variation of blood pressure is due to interaction of several cardiovascular control systems operating with different response times, assessment of BPV by spectral analysis using the continuous measurement of arterial pressure in the finger is used to differentiate the contribution of these systems in regulating blood pressure. However, as baroreceptors are centrally located, this study considered applying a continuous aortic pressure signal estimated noninvasively from finger pressure for assessment of systolic BPV by a time-frequency method using Short Time Fourier Transform (STFT). The average ratio of low frequency and high frequency power band (LFPB/HFPB) was computed by time-frequency decomposition of peripheral systolic pressure (pSBP) and derived central aortic systolic blood pressure (cSBP) in 30 healthy subjects (25-62 years) as a marker of balance between cardiovascular control systems contributing in low and high frequency blood pressure variability. The results showed that the BPV assessed from finger pressure (pBPV) overestimated the BPV values compared to that assessed from central aortic pressure (cBPV) for identical cardiac cycles (P<;0.001), with the overestimation being greater at higher power.

Potential effects of systematic errors in intraocular pressure measurements on screening for ocular hypertension.

AIM: Raised intraocular pressure (IOP) increases the risk of glaucoma. Eye-care professionals measure IOP to screen for ocular hypertension (OHT) (IOP>21 mm Hg) and to monitor glaucoma treatment. Tonometers commonly develop significant systematic measurement errors within months of calibration, and may not be verified often enough. There is no published evidence indicating how accurate tonometers should be. We analysed IOP measurements from a population study to estimate the sensitivity of detection of OHT to systematic errors in IOP measurements. METHODS: We analysed IOP data from 3654 participants in the Blue Mountains Eye Study, Australia. An inverse cumulative distribution indicating the proportion of individuals with highest IOP>21 mm Hg was calculated. A second-order polynomial was fitted to the distribution and used to calculate over- and under-detection of OHT that would be caused by systematic measurement errors between -4 and +4 mm Hg. We calculated changes in the apparent prevalence of OHT caused by systematic errors in IOP. RESULTS: A tonometer that consistently under- or over-reads by 1 mm Hg will miss 34% of individuals with OHT, or yield 58% more positive screening tests, respectively. Tonometers with systematic errors of -4 and +4 mm Hg would miss 76% of individuals with OHT and would over-detect OHT by a factor of seven. Over- and under-detection of OHT are not strongly affected by cutoff IOP. CONCLUSION: We conclude that tonometers should be maintained and verified at intervals short enough to control systematic errors in IOP measurements to substantially less than 1 mm Hg.

Non-symmetrical double-logistic analysis of 24 hour arterial stiffness profile in normotensive and hypertensive subjects.

OBJECTIVES: Mechanisms underlying the circadian profile of cardiovascular events (CE) are not totally understood. Whether circadian changes in arterial stiffness (AS) could be related to the circadian profile of CE remains to be investigated. As yet, there is no accepted way to measure circadian profiles or nocturnal-related and/or morning-related changes in cardiovascular variables. The aim of this study was to characterize the circadian pattern and day-night and night-day changes of AS in untreated hypertensive (HG) and healthy subjects (NG), using a recently developed non-symmetrical six-parameter double-logistic model. METHODS: Seven hypertensive and seven normotensive subjects underwent 24 hour ambulatory recordings of blood pressure (BP), heart rate (HR) and aorto-brachial pulse transit time (PTT(AB)) and pulse transit velocity index (PTV(AB)). PTT(AB) and PTV(AB) are inversely and directly related to AS, respectively. The circadian profile and transitional periods (day-night and night-day) were analyzed using a model described by a six-parameter double logistic equation. CONCLUSIONS: The model was adequate to characterize the circadian pattern of AS. We provide the first evidence that AS in humans follows an asymmetric circadian pattern and that this differs between NG and HG. In both NG and HG, AS had a circadian profile, with the highest levels in the night. HG showed larger levels of AS, larger BP variations and rate of change and minor changes in AS during transitional periods.

Characterisation of structural changes in the arterial elastic matrix by a new fractal feature: directional fractal curve.

A new fractal feature, the Directional Fractal Curve (DFC), defined over an arc of 180 and composed of 90 fractal dimensions determined at intervals of arc of 2, is developed to account for the anisotropic property of a fractal texture. The DFC algorithm is first applied to two images with different textural patterns, one without directional preference and one with a well-organised texture. The DFC of these images shows different patterns. The technique is then applied to quantify the structure of the elastic texture in the arterial wall where the elastic network was imaged by scanning electron microscopy following selective tissue digestion. The results suggest: (i) that images of the elastin matrix of the arterial wall exhibit fractal properties with directional preference, (ii) the DFC gives quantitative parameters which allow characterisation of structural changes in the elastin matrix of the arterial wall in terms of disorganisation and fragmentation of elastin fibres-conditions which are associated with medial degeneration due to normal ageing or presence of arterial disease.

Pressure wave propagation in a multibranched model of the human upper limb.

The influence of the large arteries and the peripheral load on pressure wave propagation in the human upper limb was investigated in an anatomically realistic multibranched model based on linear transmission theory. To mimic vascular changes seen in life, the viscoelastic properties of large arteries and the peripheral load properties (represented as modified windkessels) were altered as follows: Young's modulus (from 10.9 x 10(6) to 15.3 x 10(6) dyn/cm2) and phase (from 0 to 15 degrees) of the complex elastance, windkessel time constant (from 0 to 0.6 s), and peripheral reflection coefficient (from 0 to 0.95). The relationship between the central aortic and peripheral radial pressure waveforms was analyzed in the time and the frequency domain. Results indicate that the large arterial properties have less influence (peak systolic pressure changed by 3% and peak of transfer function changed by 29%) than the properties of the peripheral load (systolic pressure changed by 14% and peak of transfer function changed by 74%) on the pressure wave propagation in the upper limb.

Functional origin of reflected pressure waves in a multibranched model of the human arterial system.

The effects of wave travel and wave reflection were simulated in a mathematical model of the whole arterial tree consisting of 142 uniform transmission line segments. The arterial model was partitioned into three separate segments: upper limbs, trunk, and lower limbs. Aging was simulated by increasing average pulse wave velocities of these segments (10.9-12.9, 8.0-11.7, and 9.0-11.3 m/s for upper limbs, trunk, and lower limbs, respectively). Reflection coefficients at the terminal elements were altered to simulate vasodilation (0.0) and vasoconstriction (0.95). The impedance patterns and spatial distribution of pressure waveforms generated by the model simulating aging and vasoconstriction were similar to in vivo measurements by other investigators. Reflected pressure waves from each segment reached the ascending aorta and contributed differently to the late systolic peak on the aortic pressure wave. Aging does not alter the origin of these reflected pressure waves in the trunk. Aortic impedance and pressure wave changes induced by simulation of dilation of splanchnic bed were similar to those observed experimentally with nitroglycerin.

An analysis of the relationship between central aortic and peripheral upper limb pressure waves in man.

Amplification of the pressure pulse between central and peripheral arteries renders pressure values in the upper limb an inaccurate measure of ascending aortic (AA) pressure. Accuracy could be improved by allowance for such amplification. Transfer functions (TF) for pressures between AA and brachial artery (BA):(BATF) and between AA and radial artery (RA):(RATF) were derived from high-fidelity pressure recordings obtained at cardiac catheterization in 14 patients under control conditions, and after sublingual nitroglycerine 0.3 mg. There was no significant difference in BATF under control conditions and with nitroglycerine; hence results were pooled. Control and nitroglycerine results were also pooled to obtain a single RATF. BATF and RATF moduli peaked at 5 Hz and 4 Hz, reaching 2.5 and 2.8 times the value at zero frequency respectively. Frequency-dependent changes in modulus and phase of BATF and RATF were attributable to wave travel and reflection in the upper limb. BATF and RATF were compared to published transfer functions and those derived from analysis of aortic and brachial or radial pressure waves in previous publications. Results were similar. Our BATF and RATF were used to synthesize AA pressure waves from published peripheral pulses. Correspondence was close, especially for systolic pressure which differed by 2.4 +/- 1.0 (mean +/- SEM) mmHg, whereas recorded systolic pressure differed by 20.4 +/- 2.6 (mean +/- SEM) mmHg between central and peripheral sites. Results indicate that in adult humans a single generalized TF can be used with acceptable accuracy to determine central from peripheral pressure under different conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of aging on aortic morphology in populations with high and low prevalence of hypertension and atherosclerosis. Comparison between occidental and Chinese communities.

A comparative morphologic study of aortic changes with aging was conducted in different populations in an attempt to separate the effects of hypertension and atherosclerosis. Chinese and the occidental populations were chosen, as they are known to have a high prevalence of hypertension and atherosclerosis, respectively. Aortic tissue was collected from occidental (American and Australian) and Chinese populations from three geographic locations. Postmortem specimens were obtained from four fixed locations: ascending aorta (A), descending thoracic aorta (B), and abdominal aorta (suprarenal [C] and above the aortic bifurcation [D]). Histologic sections were used to measure aortic circumference, medial thickness, intimal thickness, and grade of atherosclerosis. Kidney sections were used to confirm the presence or absence of hypertension. A total of 302 cases (age range, 19 to 104 years; Male-to-female ration, 2:1) were studied: 112 Americans, 80 Australians, and 110 Chinese. Cases were divided into three age groups: 19 to 44; 45 to 64; and 65 years and older. The aortic circumference progressively decreased from sites A to D in all populations and age groups. The aortic circumference increased with age, and the increase was independent of the aortic location. When the populations were separated, however, the greater increase was at location A in the Chinese (P = .008) and locations D in the occidental (P = .13), a population contrast that was significant only in location A. Intimal thickness increased with advancing age and was maximal in the abdominal aorta. The population differences also were significant for intimal thickness and were significantly greater in the occidental population in B, C, and D locations, whereas for atherosclerosis significance was only seen in location D. Hypertension (as defined by the morphologic changes in the kidney) after adjusting for age, height, and weight resulted in no statistical significant effect on aortic circumference or on intimal thickness, but did show a significant increase in atherosclerosis score at locations B, C, and D. Also after adjusting for age, height, and weight, the Chinese had a significantly larger aortic circumference in location A compared with the occidental population, whereas in location D the occidentals with hypertension had a significantly larger circumference compared with Chinese, probably due to an interaction of atherosclerosis and hypertension. After similar adjustments, the medial thickness in locations A and C, the intimal thickness in B, C, and D, and atherosclerosis score in D were significantly greater in occidental than Chinese populations.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitroglycerin has more favourable effects on left ventricular afterload than apparent from measurement of pressure in a peripheral artery.

Nitroglycerin (0.3 mg) was administered sublingually to 14 patients undergoing cardiac catheterization, and pressure waves compared in the ascending aorta and brachial artery. After nitroglycerin, ascending aortic systolic pressure fell in all cases (by 6-44, average 22 mmHg) whereas brachial systolic pressure remained unchanged (in three) or fell to a lesser degree (4-33, average 12 mmHg). Diastolic pressure did not change significantly. Alterations in pressure and in wave contour were explained on the basis of arterial dilation, with reduction in wave reflection. Nitroglycerin reduces left ventricular afterload through arterial dilation as well as preload through venous dilation. This effect on afterload is not apparent from measurement of pressure in the brachial artery.

A comparison of brachial, femoral, and aortic intra-arterial pressures before and after cardiopulmonary bypass.

Following recent evidence that brachial and femoral artery pressures are more reliable than radial artery pressures after cardiopulmonary bypass, thirty-one adults had simultaneous pre- and post-bypass measurements of brachial, femoral, and ascending aortic pressures. Two minutes after cardiopulmonary bypass, brachial artery systolic pressure and mean arterial pressure fell significantly below corresponding pressures in the femoral artery and aorta. Five minutes after cardiopulmonary bypass, only brachial artery systolic pressure was still less than femoral and aortic systolic pressures. By ten minutes after bypass, all significant pressure differences had resolved except between brachial and femoral artery systolic pressures. Clinically significant (greater than or equal to 5 mmHg) aortic-to-brachial reductions in mean arterial pressures occurred in six (19%) patients at two minutes and in three (10%) patients at five and ten minutes after bypass. Equivalent aortic-to-femoral mean pressure diminution occurred in two (6%) patients at two minutes and one (3%) patient at five and ten minutes after bypass. Neither systemic vascular resistance nor body temperatures contributed significantly to post-bypass central-to-peripheral pressure reductions. Immediately following bypass, femoral artery pressures reproduce central aortic pressures more reliably than do radial or brachial artery pressures.

Effects of arterial dilator agents on central aortic systolic pressure and on left ventricular hydraulic load.

Recordings of pressure in the brachial or peripheral arteries fail to disclose the marked increase in systolic pressure that occurs in the proximal aorta and central arteries with increasing age and with hypertension. This systolic pressure boost is caused by wave reflection returning from the periphery of the body while the ventricle is still contracting. Such early wave reflection is caused in turn by increased pulse-wave velocity, attributable to stiffening of the aorta and major conduit arteries. Drugs have little effect on arterial stiffening, whereas wave reflection can be markedly reduced by agents that dilate peripheral arteries. Such reduction in wave reflection causes substantial decrease of systolic pressure in central arteries. Because of differential timing of wave reflection, however, such reduction is not apparent from pressure recordings taken in the brachial or other peripheral arteries. The sphygmomanometer, therefore, fails to show the favorable effects of reduced wave reflection in the proximal aorta and central arteries. Noninvasive tonometric pressure wave recordings can supplement the sphygmomanometer to assess the magnitude of beneficial effect.

Ascending aortic impedance patterns in the kangaroo: their explanation and relation to pressure waveforms.

This study seeks to explain mechanisms responsible for the peculiar ascending aortic pressure waveform and impedance spectral pattern in kangaroos. Pulsatile pressure and blood flow velocity were measured and input impedance calculated in the ascending aorta, descending thoracic aorta, and brachiocephalic artery of 15 rock kangaroos. Pressure and velocity waveforms and impedance spectral patterns were interpreted with the aid of an asymmetric uniform T-tube model of the systemic arterial tree. The ascending aortic pressure waveform displayed a very large secondary wave that began in late systole or early diastole and continued throughout most of diastole. The peak of this secondary wave (which almost always occurred in diastole) was often greater than peak systolic pressure and results from apparently intense wave reflections from peripheral vascular beds. This contention is supported by the configuration of the impedance spectral pattern that is explained on the basis of a single (or dominant) functionally discrete reflecting site in the lower part of the body. These findings are explicable on the basis of body size and shape and the extreme eccentric location of the heart within the body. Wave reflections from the diminutive upper body are so small that they are totally dominated by intensive wave reflections from the large muscular lower body. These conclusions are supported by results obtained from the asymmetric T-tube model.

Ventricular/vascular interaction in patients with mild systemic hypertension and normal peripheral resistance.

Total left ventricular external power and aortic input impedance spectra were calculated from recordings of pulsatile pressure and flow in the ascending aorta of 22 human subjects undergoing cardiac catheterization. Eleven subjects had increased aortic pressure (systolic 153 +/- 3.8[SEM] mm Hg, p less than .001; diastolic 91 +/- 2.4 mm Hg, p less than .03; mean 118 +/- 2.4 mm Hg, p less than .001) and constituted the group with mild hypertension (average age 50 +/- 1.9 years). The other 11 (age-matched) subjects had normal arterial pressures and constituted the control group. Cardiac output in the hypertensive group was abnormally high (6.9 +/- 0.3 liters/min, p less than .04) compared with that in control subjects (6.1 +/- 0.2 liters/min), so that peripheral resistance was similar. Characteristic aortic impedance (index of aortic elastance) was increased in the hypertensive group (142 +/- 19 vs 72 +/- 4.5 dyne-sec-cm-5, p less than .002), as was the fluctuation of impedance moduli and phase. These elevated pulsatile components of arterial load were associated with a significant (p less than .002) increase in pulsatile left ventricular external power (89%), and the increased cardiac output was associated with a significant (p less than .001) increase in steady flow power (31%). The ratio of pulsatile to total power was also increased (38%) in the hypertensive group (p less than .001). Increased characteristic aortic impedance in the hypertensive group suggests that the human aorta is stiffer, and fluctuations in the impedance spectra suggest increased or less dispersed wave reflections.(ABSTRACT TRUNCATED AT 250 WORDS)

The kangaroo as a model for the study of hypertrophic cardiomyopathy in man.

In experiments in 14 anaesthetised kangaroos, 10 developed unexpected ventricular fibrillation and died. In seeking a cause for this, similarities were noted with hypertrophic cardiomyopathy (HC) in man. Like patients with this condition, kangaroos have unexplained left ventricular hypertrophy and are known to be susceptible to sudden death with excitement and exertion. Retrospective analysis of all data showed other features of hypertrophic cardiomyopathy: unusually rapid left ventricular pressure development in systole (peak dP/dt: kangaroo 3602 (SEM472); HC 1947(SEM172) mmHg X s-1), unusually slow left ventricular relaxation (exponential time constant: kangaroo 54(SEM7); HC 63 (SEM5) ms; relaxation time: kangaroo 128(SEM7); HC 112(SEM7) ms), and inappropriately long duration of mechanical systole in relation to ventricular depolarisation (393(SEM21) ms and 214(SEM15) ms respectively). A disparity between the duration of mechanical systole and electrical activation caused a type of incomplete tetanus to develop with ventricular extrasystoles, a phenomenon previously seen in patients with hypertrophic cardiomyopathy. These findings suggest that the kangaroo may be a useful experimental model for studying the fatal rhythm disturbances and abnormal ventricular dynamics in human hypertrophic cardiomyopathy.

Improved arterial distensibility in normotensive subjects on a low salt diet.

Arterial pulse wave velocity (PWV), a noninvasive index of arterial distensibility, was measured in 57 normotensive subjects who followed a voluntary low salt diet for a period ranging from 8 months to 5 years (mean, 24.8 months). Subjects who followed a regular diet were matched for age and mean arterial pressure with the low salt (LS) sample and were used as controls (C). For both samples, subjects were divided into three age groups: Group 1 (aged 2 to 19 years, n = 16), Group 2 (29 to 44 years, n = 26), and Group 3 (45 to 66 years, n = 15). There was a marked increase in aortic PWV with age in the control sample but not in the LS sample. There was no significant difference in aortic PWV for Group 1, but in Groups 2 and 3, the LS subjects showed a decrease of 21.8% and 22.7%, respectively, compared to C subjects. Aortic PWV (cm/sec) was: Group 1: C = 581 (SE44), LS = 614 (SE31); Group 2: C = 942 (SE46); LS = 737 (SE27) (p less than 0.001); Group 3: C = 958 (SE77), LS = 741 (SE25) (p less than 0.05]. Arm and leg PWV were also significantly lower in the older age groups. These findings suggest that normotensive adult subjects who follow a low salt diet (mean intake, 44 mmol Na/24 hours) have reduced arterial stiffness and that the effect is independent of blood pressure. This is prima facie evidence that reduced salt intake has a beneficial effect in improving distensibility of the central aorta and large peripheral arteries, which is independent of its antihypertensive action.

Evidence of early degenerative changes in large arteries in human essential hypertension.

Noninvasive evaluation of brachial artery diameter (pulsed Doppler velocimetry) and pulse wave velocity (strain gauge mechanography) was performed in 23 normal subjects and 49 patients with uncomplicated essential hypertension. Pulsatile arterial function was described in terms of derived characteristic impedance and arterial compliance. Compared with normal controls, hypertensive patients had greater arterial diameter (p less than 0.01) and pulse wave velocity (p less than 0.001). Two nomograms obtained from normal subjects relating the product of age and diastolic pressure to diameter and pulse wave velocity were used for analysis of the hypertensive group; 35 patients were inside the 95% confidence limits of the diameter and pulse wave velocity nomograms (Group 1), while 14 patients were outside the pulse wave velocity nomogram (Group 2). Age and mean pressure were similar, but pulse wave velocity was higher (p less than 0.001), arterial compliance lower (p less than 0.001), and characteristic impedance higher (p less than 0.001) in Group 2 than in Group 1. The amplitude of pulse pressure was higher in Group 2 than in Group 1 (p less than 0.001), and a negative correlation was found between pulse pressure and arterial compliance in Group 2, but not in Group 1. Thus, in the majority of hypertensive patients, arterial modifications could be related to the normal influence of age and pressure. In contrast, other patients exhibited features suggesting excessive arterial stiffness, manifested by abnormally high pulse wave velocity, decreased arterial compliance, and increased characteristic impedance.