Photoplethysmography for Assessing Microcirculation in Hypertensive Patients After Taking Antihypertensive Drugs: A Review

Figures & data

Table 1 Overview of Characteristics for the Microvascular Imaging Modalities

	Operating Principle	Wavelength (s)	Frame Rate (fps)	Examples of Key Clinical Applications
Photoplethysmography	Tissue pulsatility by non-contact PPG	Near infrared and red	Up to 200	Heart rate variability; tissue oxygenation saturation mapping
Laser Doppler perfusion	Laser Doppler red blood cell	Red (633 nm), or near infrared (785 nm)	<1	Burn wound depth; endothelial function testing
Laser speckle contrast	Laser speckle contrast red blood cell	Near infrared (785 nm)	Up to 100	Perfusion-superficial nutritive microvessels
Nailfold capillaroscopy	Red blood cell light absorption, cold light epi-illumination	Green (for B&W imaging) White (for colour imaging)	Video (eg 25)	Connective tissue disease diagnostics
Thermography	Map of temperature distribution using thermal infrared	Typically 7–14 μm	Up to ∼60 k with windowing	Thermoregulatory perfusion in the periphery; inflammatory responses
Optical coherence tomography	Interferometry	Near infrared range (eg 800–1310 nm)	At least video rate	Microvascular structure visualization; histology; ophthalmology
Photoacoustic tomography	Reconstruction of photoacoustic source distribution	Near infrared or green	10s possible	Molecular imaging; melanoma detection; breast and brain imaging
Hyperspectral imaging	Spectroscopy	200–25,000 nm	100s possible	Tissue oxygenation mapping; shock; cancer
Tissue viability imaging	Proportional absorption of red and green light by skin	500–700 nm	Up to 50	Red blood cell concentration in superficial microvessels; studies of vasoconstriction

Figure 1 Schematic diagram of Photoplethysmography (PPG) waveform (Marker 1 is the first foot of the PPG waveform, marker 2 is the first peak of the waveform, marker 3 is The Valley, marker 4 is the second peak, and marker 5 is the second foot of the waveform; P1 and P2 indicate two positive waves respectively, V indicates the negative wave).

Figure 2 The comparison of PPG between hypertensive patients and normal young adults.

Figure 3 The comparison of PPG between hypertensive patients and patients with hypertension after the application of nitroglycerin. (A) PPG waveform examples of patients with hypertension. (B) PPG waveform examples of patients with hypertension after the application of nitroglycerin.

Figure 4 Schematic diagram of microcirculation. (A) Two Fluid Dynamics Diagrams: it seems that the vessel 1 had low resistance than vessel 2 because it had a greater cross-sectional area (electrical features); There were three components in vessel 1 (a, b and c). However, the fact is that the flow in vessel 1 drained more slowly than in vessel 2 because there was excessive friction in vessel 1, resulting in a greater resistance to flow. (B and C) The difference between parallel fluid pipeline and parallel circuit.

Figure 5 Diagram of the windkessel. (A) The elastic arteries and heart are similar with the windkessel and the pump, respectively. The change of air can somewhat represent the change of microvascular resistance during diastole and systole. (B) The components and structure of capillaries.

Figure 6 The blood vessel wall of Arteriovenous Anastomosis (AVA). The thick walls of AVAs are made up of circular and longitudinally organised smooth muscle fibres in cross-sections. The mother arterial segment’s vertical arrangement of spindle-shaped smooth muscle cells may resemble the acd valve, which regulates the opening of the AVA.

Figure 7 The structure diagram added at AVA. Mother and daughter vessels are in the spotlight. The construction of the fictitious AVA entrance serves as the link between the mother vessel and the daughter’s vessel. The muscle fibers are thickened and organised in the mother vessel’s direction to create a lip-like longitudinal structure.