AIM To determine the optimum resting tension(ORT) for in vitro human pulmonary artery(PA) ring preparations.METHODS Pulmonary arteries were dissected from disease free sections of the resected lung in the operating theatre and tissue samples were directly sent to the laboratory in Krebs-Henseleit solution(Krebs).The pulmonary arteries were then cut into 2 mm long rings.PA rings were mounted in 25 m L organ baths or 8 m L myograph chambers containing Krebs compound(37 ℃,bubbled with 21% O_2:5% CO_2) to measure changes in isometric tension.The resting tension was set at 1-gram force(gf) with vessels being left static to equilibrate for duration of one hour.Baseline contractile reactions to 40 mmol/L KCl were obtained from a resting tension of 1 gf.Contractile reactions to 40 mmol/L KCl were then obtained from stepwise increases in resting tension(1.2,1.4,1.6,1.8 and 2.0 gf).RESULTS Twenty PA rings of internal diameter between 2-4 mmwere prepared from 4 patients.In human PA rings incrementing the tension during rest stance by 0.6 gf,up to 1.6 gf significantly augmented the 40 mmol/L KCl stimulated tension.Further enhancement of active tension by 0.4 gf,up to 2.0 gf mitigate the 40 mmol/L KCl stimulated reaction.Both Myograph and the organ bath demonstrated identical conclusions,supporting that the radial optimal resting tension for human PA ring was 1.61 g.CONCLUSION The radial optimal resting tension in our experiment is 1.61 gf(15.78 m N) for human PA rings. AIM To determine the optimum resting tension (ORT) for in vitro human pulmonary artery (PA) ring preparations. METHODS Pulmonary arteries were dissected from disease free sections of the resected lung in the operating theater and tissue samples were directly sent to the laboratory in Krebs The Henseleit solution (Krebs). The pulmonary arteries were then cut into 2 mm long rings. PA rings were mounted in 25 mL of organ baths or 8 mL of myograph chambers containing Krebs compound (bubbled with 21% O 2: 5% CO_2) to measure changes in isometric tension. The resting tension was was at set-1-gram force (gf) with vessels being left static to equilibrate for duration of one hour.Baseline contractile reactions to 40 mmol / L KCl were obtained from a resting tension of 1 gf. Contracttile reactions to 40 mmol / L KCl were then obtained from stepwise increases in resting tension (1.2, 1.4, 1.6, 1.8 and 2.0 gf) .RESULTS Twenty PA rings of internal diameter between 2-4 mmwere prepared from 4 patients .In human PA rings increm enting the tension during rest stance by 0.6 gf, up to 1.6 gf significantly augmented the 40 mmol / L KCl stimulated tension. Thermal enhancement of active tension by 0.4 gf, up to 2.0 gf mitigate the 40 mmol / L KCl stimulated reaction. and the organ bath demonstrated identical conclusions, supporting that the radial optimal resting tension for human PA ring was 1.61 g. CONCLUSION The radial optimal resting tension in our experiment was 1.61 gf (15.78 mN) for human PA rings.