Submitted on November 16, 2007
Accepted on March 25, 2008
PO2 Measurements in the Microcirculation using Phosphorescence Quenching Microscopy at High Magnification
Aleksander S Golub1 and Roland Nathan Pittman1*
1 Physiology, Virginia Commonwealth University, Medical College of Virginia, Richmond, Virginia, United States
* To whom correspondence should be addressed. E-mail: pittman{at}vcu.edu.
In phosphorescence quenching microscopy (PQM) the multiple excitation of a reference volume produces the integration of oxygen consumption artifacts caused by individual flashes. We analyzed the performance of two types of PQM instruments in order to explain reported data on PO2 in the microcirculation. The combination of a large excitation area (LEA) and high flash rate produces a large oxygen photo-consumption artifact manifested differently in stationary and flowing fluids. A LEA instrument strongly depresses PO2 in a motionless tissue, but less in flowing blood, creating an apparent transmural PO2 drop in arterioles. The proposed model explains the mechanisms responsible for producing apparent transmural and longitudinal PO2 gradients in arterioles, a PO2 rise in venules, a hypothetical high respiration rate in the arteriolar wall and mesenteric tissue, a low PO2 in lymphatic microvessels, and both a low and uniform tissue PO2. This alternative explanation for reported paradoxical results of PO2 distribution in the microcirculation obviates the need to revise the dominant role of capillaries in oxygen transport to tissue. Finding a way to eliminate the photo-consumption artifact is crucial for accurate microscopic oxygen measurements in microvascular networks and tissue. The PQM technique which employs a small excitation area (SEA), together with a low flash rate, was specially designed to avoid accumulated oxygen photo-consumption in flowing blood and lymph. The related scanning SEA instrument provides artifact-free PO2 measurements in stationary tissue and motionless fluids. Thus, the SEA technique significantly improves the accuracy of microscopic PO2 measurements in the microcirculation using the PQM.
