To investigate the impact of synovial fluid volume on oxygen tension (Po,) and other metabolic correlates, 24 specimens of synovial fluid from the knees of 22 patients were analyzed for volume, number of leukocytes (WBC), pH, PO,, Pco,, glucose, protein, and complement (CH50) levels. Concurrent arterial blood samples were obtained in 21 instances. Synovial fluid Po, values varied inversely with volumes of synovial fluid (r = -0.54, P < 0.01), but when patients with rheumatoid arthritis were excluded, the correlation was more significant (r = -0.76, P c 0.001). When synovial fluid Po, dropped below 45 mm Hg, intraarticular acidosis resulted. The decrease in pH (r = 0.93, P < 0.001), the lowering of glucose values (r = 0.89, P < 0.001), and the rise in PCO, (r = -0.79, P < 0.01) can be explained by a shift toward anaerobic metabolism coupled with the impaired elimination of its products. Systemic acidosis and hypoxia were not found. Intraarticular hypoxia most likely represents circulatory imbalance at the level of the synovial membrane, although an inverse relationship of synovial fluid PO, and WBC was also noted. Complement and protein levels had no correlation with volume, pH, or respiratory gas tensions of synovial fluids. Our data support the importance of the effective blood Address reprint requests to George Ho, Jr., MD, Veterans Administration Medical Center, Davis Park, Providence, Rhode Island 02908.Submitted for publication July 8, 1980; accepted in revised form November 25, 1980 flow to the joint in maintaining homeostasis. The volume of synovial effusion and the compliance of the joint c a p sule appear to be important determinants of the articular blood supply.The study of synovial fluid respiratory gases began when easily reproducible measurements of pH, Po,, Pco2, and HCO, by blood gas analyzers became available. Several studies examined the diagnostic specificity of synovial fluid respiratory gases and their metabolic and histologic correlates (1-3). Most have shown that progressive hypoxia was associated with increased glucose consumption leading to lactate accumulation, bicarbonate consumption, acidosis, and hypercapnia. The lowest PO, and pH levels with the highest Pco2 and lactate levels were usually found in synovial fluids from adults with rheumatoid arthritis who had the most advanced histopathologic changes (2). The precise mechanism of intraarticular hypoxia is unknown, but a "circulatory-metabolic" imbalance was proposed and investigated by Goetzl et a1 in 1971 (4). The present study was undertaken to confirm the profound metabolic consequences of intraarticular hypoxia and to examine the influence of synovial fluid volume on respiratory gases in joint effusions of various causes. MATERIALS AND METHODSTwenty-four specimens of synovial fluid from 23 knee joints of 22 patients were obtained during diagnostic or therapeutic arthrocentesis. All patients were supine with the knee in maximal extension. The initial 2-3 ml were aspirated into a heparinized syringe and the remainder was remove...