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The pressure in either the radial (n = 88) or proximal brachial artery (n = 82) was compared with aortic pressure before and after cardiopulmonary bypass (CPB) in patients receiving coronary artery bypass grafts. Radial artery pressures were measured via 20-G 5-cm long catheters, brachial artery pressures via 20-G 12.7-cm catheters, and aortic pressures were measured via a luer port in the aortic perfusion cannula. Transducers were connected via 122-cm long tubing. For the various systems, mean natural frequencies were 16.1 to 17.7 Hz and damping coefficients were 0.16 to 0.27. Before CPB the brachial systolic, diastolic, and mean pressures were 108.2 +/- 5.2%, 100.9 +/- 2.8%, and 99.6 +/- 2.3% of aortic; respective radial pressures were 113.9 +/- 9.6%, 99.5 +/- 2.8%, and 98.4 +/- 2.8% of aortic. Immediately after CPB the brachial pressures were 99.5 +/- 7.5%, 98.9 +/- 3.5%, and 97.4 +/- 2.9% of aortic, whereas respective radial pressures were 92.1 +/- 14.6%, 94.7 +/- 5.6%, and 90.8 +/- 7.4%. All brachial and radial as a per cent of aortic pressure medians were significantly different, and except for prebypass diastolic and mean, the variance for brachial pressures was significantly less than that for pressures in the radial artery. The prebypass brachial correlation (r) with aortic for systolic, diastolic, and mean were 0.90, 0.98, and 0.98; respective radial correlations with aortic were 0.78, 0.97, and 0.95. Postbypass brachial systolic, diastolic, and mean correlations were 0.91, 0.97, and 0.98; radial were 0.50, 0.93, and 0.83. Brachial artery pressures were more accurate and reliable than radial artery pressures.