Oxygen transport from systemic arteries to capillaries - Studies using thephosphorescence quenching technique

Oxygen transport from systemic arteries to capillaries is a very complex en tity. Main determinants are arterial oxygen content (CaO2) as a function of pulmonary gas exchange and oxygen carrying capacity (hematocrit) as well a s macro- and microvascular perfusion as a function of cardiac output, blood pressure and vascular resistance. The phosphorescence quenching technique, an optical method, allows in consc ious animals for the first time investigation of oxygen transport down to t he smallest blood vessels in a microcirculatory system as well as simultane ous analysis of resulting (interstitial) tissue oxygenation. Results, which are highly relevant also for clinicians exhibit significantly lower oxygen tension levels in microvessels and tissue (approximately 40-70% and 30% of paO(2) respectively), which is indicative of a precapillary "loss" of oxyg en and questions the function of capillaries as sole "tissue oxygen donors" . Under physiological conditions, tissue oxygenation is maintained by autor egulation independently of systemic arterial pO(2), so that this parameter does not provide direct information about microvascular oxygen transport an d tissue oxygenation. During general anaesthesia or in hemorrhagic shock, there is an increased d iscrepancy between systemic arterial and microcirculatory oxygen tension le vels. Quality of peripheral oxygen transport and tissue oxygenation may not be judged accurately by parameters such as p (v) over barO(2) or base exce ss, which are frequently used in clinical routine for the assessment of con ditions of critically ill patients. Simultaneous monitoring of intravascular and interstitial pO(2) in differen t peripheral and central microvascular systems would be very useful in thes e patients. However, this is not yet possible and may be accomplished only by indirect procedures such as the gastral mucosa pH analysis.