Gert Nilsson

Evaluation of a Laser Doppler Flowmeter for Measurement of Tissue Blood Flow

An instrument for measurement of tissue blood flow based on the laser Doppler principle was evaluated using a fluid model. A unique and linear relationship between flowmeter response and flux of red cells was demonstrated with red cell velocities and volume fractions within the normal physiological range of the microcirculatory network of the skin. Different degrees of oxygenation proved to influence the Doppler signal only to a minor extent. The study also shows that the Doppler signal is formed essentially by heterodyne mixing of light beams backscattered in static structures and moving red cells.

A New Instrument for Continuous Measurement of Tissue Blood Flow by Light Beating Spectroscopy

A new instrument for measurement of regional tissue blood flow based on the laser Doppler principle is reported. The theoretical background of light beating spectroscopy is discussed and a detection technique which makes possible the suppression of the adverse effects of laser-mode interference and wide-band beam amplitude noise is described. Instead of using a single square-law photodetector a differential detector technique is introduced that reduces common-mode noise to a negligible level, without influencing the blood flow related signal. The new instrument has proved to be highly stable and sensitive. Continuous recordings of tissue blood flow can be performed in the laboratory as well as at the bedside.

Laser Doppler perfusion imaging by dynamic light scattering

A laser Doppler perfusion imaging technique based on dynamic light scattering in tissue is reported. When a laser beam sequentially scans the tissue (maximal area approximately 12 cm*12 cm), moving blood cells generate Doppler components in the backscattered light. A fraction of this light is detected by a remote photodiode and converted into an electrical signal. In the signal processor, a signal proportional to the tissue perfusion at each measurement point is calculated and stored. When the scanning procedure is completed, the system generates a color-coded perfusion image on a monitor. A perfusion image is typically built up of data from 4096 measurement sites, recorded during a time period of 4 min. This image has a spatial resolution of about 2 mm. A theory for the system inherent amplification factor dependence on the distance between individual measurement points and detector is proposed and correction measures are presented. Performance results for the laser Doppler perfusion imager obtained with a flow simulator are presented. The advantages of the method are discussed.<<ETX>>

Signal processor for laser Doppler tissue flowmeters

Laser Doppler flowmeter for the assessment of tissue blood flow are generally equipped with a signal processor which generates the first moment of the unnormalised power spectral density as a continuous output signal. This signal is related to blood flow for low and moderate flow rates. At higher flow rates the interpretation of the output signal becomes ambiguous as a consequence of the multiple scattering in moving blood cells and the homodyne mixing of waves on the detector surface. The paper describes a new signal processor which takes these effects into account and establishes a linear relationship between the flowmeter output signal and blood flow for all flow rates. The performance of the signal processor was evaluated by an experimental fluid model which optically resembled the blood flow through the microvasculature. The transiently high blood flow in the initial phase of reactive hyperaemia, recorded from palmar skin, gave peak values which were almost double those recorded with the original signal processor of the flowmeter. In conjunction with these high peak values a high concentration of moving blood cells was recorded, indicating that the initially high flow rate is produced by an increased number of moving blood cells due to vasodulation rather than by a change in average velocity.

Evaluation of laser Doppler flowmetry in the assessment of intestinal blood flow in cat.

Laser Doppler flowmetry with a differential detector system has been used in the assessment of blood flow in the feline small intestine. Simultaneous mucosal and serosal laser Doppler flowmeter recordings were compared with total blood flow of a bowel segment measured by an optical drop-recorder unit in 6 cats. Blood flow through the muscularis layer was estimated using the 85Kr washout technique. A correlation coefficient of r = 0.96 (mucosal recordings = 90, serosal recordings = 80, p less than 0.001) was obtained between laser Doppler flowmeter output signals and total blood flow at different levels of vascular tone, regardless of whether the flowmeter recordings were made from the mucosal or the serosal side of the bowel wall. At intense vasodilation, the flowmeters showed a tendency to underestimate blood flow. The flowmeter signals were at variance with the muscularis blood flow but were clearly correlated to the calculated mucosal-submucosal blood flow. The uneven blood flow distribution in the intestinal wall did not affect the ability of the flowmeters to reflect total blood flow from either side of the bowel wall. A calibration curve could be constructed for approximate interpretation of the laser Doppler signals in absolute flow units. However, further experiments in humans and further development of the technique must be performed to elucidate clinical applications of the method.

Assessment of skin irritancy in man by laser Doppler flowmetry

It is desirable to use more objective methods than visual storing for the assessment of skin irritancy reactions. In the present study the blood flow in skin sites exposed to sodium lauryl sulfate (SLS) was recorded by a laser Doppler flowmeter. The irritant was applied to the volar forearm of healthy volunteers in concentrations ranginig from 0.001% to 5% under occlusion for 24 h. The test sites were scored visually and the blood flow was recorded at 3 different times: 26 h, 48 h and 72 h after the application of SLS. Approximately 950 recording were performed and a clear relationship was observed the applied doses of SLS, the recorded blood flow values and the corresponding scores. In a few cases a deviation from the general trend was observed, implying that the naked eye might be unreliable for the assessment of skin irritancy reactions.

The cutaneous vascular axon reflex in humans characterized by laser Doppler perfusion imaging.

1. Laser Doppler perfusion imaging was used to map the cutaneous vascular axon response induced by trains of electrical skin stimuli (1 ms, 2 Hz) on the dorsum of the hand, finger and foot in twenty‐four healthy subjects. Conduction anaesthesia was applied to nerves supplying the stimulated skin areas. Subtraction of images recorded before and after stimulation was used for data analysis of the intensity and area of the response. 2. The stimulation evoked a localized perfusion increase around the stimulating electrode which lasted approximately 30 min and increased in intensity and area with increasing stimulation strength to a maximum at 20 pulses and 20 mA. The intensity and area of the response was greater on the hand than on the foot. 3. Approximating the response area as a circle, the maximal perfusion increase in the hand extended 9 +/‐ 3 mm (mean +/‐ S.D.) outside the perimeter of the stimulating electrode. When stimulating within skin which had been subjected to surface anaesthesia, no response occurred, but when stimulating at the border of surface‐anaesthetized skin, the perfusion increase extended 2 +/‐ 1 mm (mean +/‐ S.D.) into anaesthetized skin. 4. The results show that the perfusion increase must have been due in part to impulse conduction to, and release of transmitters from, axon endings terminating in skin outside the contact area of the probe. It is concluded that the area of perfusion increase corresponds to the size of the receptive fields of afferent polymodal C fibres.

Sub-epidermal imaging using polarized light spectroscopy for assessment of skin microcirculation.

Background/aims: Many clinical conditions that affect the microcirculation of the skin are still diagnosed and followed up by observational methods alone in spite of the fact that non‐invasive, more user‐independent and objective methods are available today. Limited portability, high cost, lack of robustness and non‐specificity of findings are among the factors that have hampered the implementation of these methods in a clinical setting. The aim of this study is to present and evaluate a new, portable and easy‐to‐use imaging technology for investigation of the red blood cell (RBC) concentration in the skin microvasculature based on the method of polarization light spectroscopy using modified standard digital camera technology.

TRANSEPIDERMAL WATER LOSS IN NEWBORN INFANTS II. Relation to Activity and Body Temperature

Abstract. Using a method described in a previous article the transepidermal water loss (TEWL) was studied in 10 healthy newborn infants at rest and during activity. On the average TEWL was 37% higher during activity than during rest although no sweating was observed. In 9 infants placed in incubators with an ambient temperature slightly above the thermoneutral range measurements were made as the body temperature rose. TEWL was almost constant until a temperature of 37.1°C was reached whereupon the water loss suddenly increased as the infant started sweating.

Hyperbaric oxygen improves wound healing in normal and ischemic skin tissue.

The influence of hyperbaric oxygen on reepithelialization and on microvascular perfusion of wounds in normal and ischemic skin tissue was investigated by using a standardized model, in ears of hairless mice. Animals were treated within 2 hours of wound creation and then twice daily with 100% oxygen at 2 atmospheres of absolute pressure. Ischemia was induced by ligating two of the three major vessels of the ear 2.5 days before wound creation. Wound surface area was measured every third day after wound creation. In addition, microvascular blood flow before and during the wound healing process was measured by scanning the ear with a new laser Doppler perfusion imager. In normal tissue (n = 13), hyperbaric oxygen therapy significantly accelerated wound healing by 2 days (p < 0.01) as compared with controls (n = 16). In ischemic tissue (n = 16), treatment with hyperbaric oxygen reduced time for reepithelialization in control animals (n = 16) from 14.3 to 9.9 days (p < 0.001). Laser Doppler data showed no difference in tissue blood flow between treated and untreated animals. In comparison with normal tissue, wound healing in ischemic tissue was characterized by a reduced and less intense hyperemic response. These data suggest that hyperbaric oxygen therapy improves reepithelialization in normal and ischemic skin tissue. The beneficial effect is not associated with changes in microvascular perfusion and therefore is probably due to high arteriolar oxygen content and oxygen diffusion.