A new, sensitive, qualitative technic is described for detecting small left-to-right and right-to-left circulatory shunts. Advantages over other sensitive shunt detection devices are: (1) the use of a readily available, nonradioactive, nonexplosive, nontoxic gas, namely Freon 22; (2) the ability to sample through standard cardiac catheters as small as 5F; (3) an immediate answer due to recording of the gas concentration in blood; and (4) extremely high sensitivity allowing the detection of the smallest left-to-right and right-to-left shunts.