Little is known about the microvascular perfusion of the skin postnatally. Skin microvascular parameters can be assessed noninvasively with orthogonal polarization spectral imaging (OPS), a technique where, through the use of special optics, a virtual light source is created at a depth of 1 mm within the tissue. The light is absorbed by the Hb, yielding an image of the illuminated Hb-carrying structures in negative contrast. In nine term (weight 2100 -4470 g) and 28 preterm infants (weight 550 -2070 g; gestational age 24 -33 wk) red blood cell velocity and vessel diameter and density were determined off-line with the CapImage program in vessels video-recorded by OPS near the axilla on d 1 and 5 of life. Blood pressure, heart rate, hematocrit, and body and incubator temperature were noted. Vessel diameter ranged from 6 to 24 m, vessel density from 219 to 340 cm/cm 2 with no change between d 1 or 5 and no difference between term and preterm infants. Red blood cell velocity increased in preterm infants from d 1 [median 528 m/s, 95% confidence interval (CI) 486 -564 m/s] to d 5 (median 570 m/s; 95% CI 548 -662 m/s; p ϭ 0.001) and correlated with the decrease in median hematocrit from 44% (CI 40%-60%) to 39% (CI 37%-43%) with r 2 ϭ Ϫ0.37 with a 95% CI Ϫ0.59 to Ϫ0.11, p ϭ 0.006. Hematocrit correlates with red blood cell velocity in the microvessels of the skin. The new technology can be used to assess quantitative changes in the microvessels and thus allows noninvasive assessment of tissue perfusion in term and preterm infants. At birth, the skin is richly supplied by a dense subepidermal plexus that shows relatively little regional variation. Even the middle and deep dermis are richly endowed with vasculature. The mature pattern of capillary loops and of the subpapillary venous plexus is not present at birth. With exception of the palms, soles, and nail beds, the skin at birth has almost no papillary loops, but demonstrates a disorderly capillary network. By the end of the first week of life, the capillary network assumes a more orderly pattern. Papillary loops begin to appear as small superficial dilatations or buds in the second week and cooling of the skin appears to encourage maturation. The skin architecture in newborns is notably different from those of an adult. Whereas the latter regularly shows loops of capillaries running orthogonal to the surface of the skin, the neonate has a more horizontal structure, which is readily seen through the very thin upper layers (1-4).Cardiac output normalized to mass is much higher in the newborn than in the adult. Because of the high resting cardiac output of neonates there is limited reserve to further augment blood flow under stress. Perfusion pressure is maintained by redistributing marginal cardiac output and oxygen supply to brain, heart, and adrenal gland. Under no stress conditions the skin has high blood flow in relation to its oxygen requirement. Assessment of skin perfusion is therefore of great interest, but there is only very scant data about the microcirculator...

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