closed-circuit helium dilution

closed-cir·cuit he·li·um di·lu·tion

(klōzd-sĭr'kŭt hē'lē-ŭm di-lū'shŭn) A gas dilution technique for measuring functional residual capacity (FRC); the patient rebreathes helium from a spirometer while oxygen is added and carbon dioxide is removed to maintain a constant system volume.
Synonym(s): helium dilution method.