Detection and quantification of small right-to-left shunts by the phase-plane method

In six anesthetized dogs 70 indicator-dilution curves and phase-plane plots were recorded. Indocyanine green was injected into the superior vena cava and sampled through a dichromatic cuvette densitometer from the femoral artery. Dye-dilution curves were recorded from dogs with normal cardiovascular systems and repeated after right-to-left shunts of different magnitude were surgically produced. The percent of blood shunting was calculated according to standard indicator-dilution equations and also by means of oximetric determination of blood samples. They were compared with shunt estimates based on phase-plane loop measurements. Good correlation was found with standard indicator-dilution procedures (r = 0.890) but not with oximetry (r = 0.466). The phase plane shows its utility in the detection and evaluation of small right-to-left shunts. These shunts affect the initial portion of the phase plane and the alterations appear to be more evident than in the corresponding concentration-vs.-time curves.