Abstract

It has been known since 1851 that atmospheric oxygen is taken up by the human epidermis. The contribution to total respiration is negligible. Until now the significance for the local oxygen supply of the skin has remained unknown. With a newly developed sensor, the oxygen fluxoptode, it has become possible to make local measurements of the transcutaneous oxygen flux (tcJ(O2)). In this study the sensor was calibrated so that absolute values of tcJ(O2) could be reported. At rest, tcJ(O2) was determined on normal, humidified skin on the volar forearm of 20 volunteers of different age groups. In order to evaluate the contribution of the blood flow to the oxygen supply of the skin, tcJ(O2) was recorded at the end of a 5 min suprasystolic occlusion of the forearm. At normal skin surface partial oxygen pressure (163 +/- 9 Torr), tcJ(O2) was 0.53 +/- 0.27 ml O2 min(-1) x m(-2). A 5 min interruption of blood flow resulted in an increase of 9.5 +/- 6.3 % in tcJ(O2). The value of tcJ(O2) was unaffected by the age of the subject. Published data on the oxygen diffusion properties of skin and simulations of intracutaneous profiles of oxygen partial pressure indicated that under these conditions, the upper skin layers to a depth of of 0.25-0.40 mm are almost exclusively supplied by external oxygen, whereas the oxygen transport of the blood has a minor influence. As a consequence, a malfunction in capillary oxygen transport cannot be the initiator of the development of superficial skin defects such as those observed in chronic venous incompetence and peripheral arterial occlusive disease.

The skin is the only organ besides the lungs that is directly exposed to atmospheric oxygen. Apart from the stratum corneum, oxygen is consumed in all layers of the epidermis and dermis. The oxygen demand is partially satisfied by the blood: the dermis exhibits a vasculature that is arranged in two tiers that are parallel to the skin surface. The superficial plexus between the papillary and the upper reticular dermis deep plexus in the lower reticular dermis are connected by perpendicularly orientated communicating vessels. Arcades of capillaries loop upwards into the papillae from the subpapillary plexus (Braverman, 1989). exposed directly to the atmosphere. As early as 1851, Gerlach was able to show that human skin takes up oxygen from the atmosphere.

Local relative measurements of the changes in cutaneous oxygen uptake from the atmosphere, the so-called transcutaneous oxygen flux (tcJO2), have become possible with the development of an oxygen fluxoptode (Holst, 1994; Holst et al. 1995). Measurements of tcJO 2 on the humidified skin of the volar forearm at normal skin temperature (33 °C) during artificially induced variations in blood perfusion have indicated the functional relevance of the external oxygen supply (Stücker et al. 2000a). The induction of hyperaemia in moist skin with a combination of nonivamide and nicoboxil resulted in a distinct decrease of tcJO2 to 70 % of the resting values. These experiments clearly demonstrated that the oxygen supply of the corium is a balance between oxygen transport by the blood and uptake from the atmosphere. If the oxygen supply from the blood increases, a lower tcJO2 suffices to cover the oxygen demand of the skin. Stopping capillary oxygen transport was compensated by an increase of tcJO2 of only 9 %. This indicates that under normal conditions a substantial part of the upper skin is supplied by direct oxygen uptake from the atmosphere. Until now it has not been possible to determine the thickness of the layer (T) that characterises the contribution of tcJO2 to the total skin oxygen supply.

In a theoretical analysis, Fitzgerald (1957) estimated a mean T of 48 µm, with a range of 34–84 µm, which would cover the main part or the whole of the epidermis. His calculations were based on data for the diffusion coefficient measured on the anterior abdominal wall of the frog following removal of the skin, because there were no comparable measurements on mammals. In fact, the true values for the oxygen permeability of skin tissue are an order of magnitude greater, whilst the oxygen consumption under normal conditions is about four times lower (actual data: 1470–2110 ml O2m-3 min-1; ; Fitzgerald used 7800 ml O2 (ml tissue)-1 min-1). The approximate partial pressure of oxygen (PO2) of capillary blood, 95 Torr (1Torr = 0.1333 kPa), was taken as the minimum Po2 of the skin. This is higher than the minimum value of 51Torr that was measured in the skin in vivo using needle electrodes (Evans & Naylor, 1966a; Roszinski & Schmeller, 1995). A greater penetration depth T of the external oxygen is calculated using the latter values. Furthermore, Fitzgerald had to use data for the absorption of oxygen through the skin surface, which had a wide range of 0.4–2.9 ml O2m-2 min-1. This was due to different measuring locations and temperatures, large measuring areas and poor sensitivity of the measuring devices (for example, changes in the oxygen absorption caused by increased or decreased blood flow could not be detected).

In 1987, Baumgärtl et al. measured the intracutaneous profile of PO2 directly with needle electrodes. The skin surface of the lower limb was covered by a film of water, which resulted in a reduced skin surface PO2 of 78 Torr. Furthermore, the needle puncture probably produced a local hyperaemia and increased the oxygen supply by the blood. Under these conditions, with reduced skin surface PO2 and hyperaemia, the PO2 profile had a distinct minimum at a depth of about 100 μm, roughly at the level of the capillary loops (Fig. 1). These invasive measurements demonstrated a penetration depth of atmospheric oxygen into the skin, double that of Fitzpatrick’s estimated values.
According to Fick’s law of diffusion:

where JO2 is the oxygen flux, K the conductivity and πPO2/πx the pressure gradient. These measurements show, therefore, that in the upper 100 µm at least, there can only be a diffusion of oxygen from the skin surface to that depth instead of from the blood to the skin surface.

In this study, we have examined the importance of the cutaneous uptake of external oxygen for the skin supply by quantifying tcJO2 at a normal atmospheric PO2 using a new, highly sensitive, non-invasive measuring device, which only covers small homogeneous skin areas rather than the large, heterogeneous skin areas with varying skin thicknesses and different densities of skin adnexes of earlier studies. In contrast to these earlier studies, it has been possible on the one hand to quantify absolute values (rather than relative changes in oxygen) by using a newly developed calibration system and, on the other, to measure at a normal instead of reduced skin surface PO2 (Stücker et al. 2000a).

Figure 1. Oxygen partial pressure (PO2 ) measured by a needle electrode inserted perpendicularly into the skin

The depth z of the electrode is given in µm (skin surface at 0 µm). The skin surface was covered by a water film, resulting in a reduced skin surface PO2 (ssPO2) of 78 Torr. The PO2 profile has a distinct minimum at a depth of approximately 100 µm, roughly at the level of the dermo-epidermal junction (according to Baumgärtl et al.1987). The needle puncture probably resulted in a local hyperemia. Under more physiological conditions, it is expected that the minimum would occur at a greater depth.

Methods

Measurement of tcJ02
Measurements were carried out using an oxygen fluxoptode developed by our group (Holst, 1994; Holst et al. 1995). This device consists of three different layers in a sandwich arrangement (Fig. 2). The fluxoptode is applied to the skin surface and represents an artificial barrier to the external atmosphere. The upper polymer layer serves as a diffusion barrier with a defined oxygen conductivity, whilst the oxygen-sensing silicon layer (with a negligible oxygen conductivity) allows measurement of the skin surface PO2 (ssPO2). ssPO2 is lower than the external atmospheric value as a result of the decreased oxygen flux (JO2) through the diffusion barrier. By increasing the external PO2, ssPO2 can be varied until a stable value close to the normal atmospheric pressure is reached (Fig. 3C). At a given oxygen permeability P, the pressure difference (DPO2 ) between ssPO2 and the external PO2 is proportional to tcJO2:

tcJO2 = DPO2P. (1)

It is thus possible to obtain an absolute value for tcJO2 if the oxygen permeability P of the diffusion barrier is known.

Oxygen fluxoptode
In order to obtain constant diffusion properties, the oxygen fluxoptodes were produced in our laboratory according to a standardised protocol. A commercially available polymer membrane (PFA 6510, Nowofol, Siegsdorf, Germany) with athickness d = 55 µm was used as the diffusion barrier. The value of P of this layer is given by:

P = K/d = aD/d, (2)

where K is the oxygen conductivity, a is the oxygen solubility and D is the oxygen diffusion constant.

ssPO2 was measured optically using the oxygen indicator RuBiPy (Tris (2,2-bipyridyl) ruthenium (II) chloride hexahydrate; Strem Chemicals, Kehl, Germany) adsorbed onto silica gel particles, which were embedded in a silicone layer with a high oxygen conductivity (Wacker Chemie, Burghausen, Germany). A further, blackened silicone layer in direct contact with the skin served as optical insulation to suppress fluorescence artefacts from the skin (Fig. 2). The measuring principle is based on the reduction of the fluorescence lifetime (quenching) of the indicator by oxygen. If the indicator is excited harmonically, the phase shift (DF) between excitation and fluorescence intensity is a measure of the oxygen concentration at the indicator molecules. The calibration curve is non-linear and is described by eqn (3) (Holst, 1994):