Reassessment of the assumed A-V oxygen content difference in the shunt calculation

A study was undertaken in 15 patients to compare measured and assumed arteriovenous oxygen (A-V O2) content differences and their effects on resultant shunt calculations. All patients were on volume ventilators and demonstrated a stable cardiovascular state. Simultaneous measurements of the O2 content of a pulmonary artery (PA) and of a superior vena cava (SVC) sample were compared. A mean A-V 02 content difference of 3.5 plus or minus 0.8 volumes percent was obtained from the PA and 2.6 plus or minus 1.1 volumes percent from the SVC. The resultant shunt calculations derived from measured A-V 02 content differences were compared with the calculation based on an assumed A-V O2 content difference of 5 volumes percent. A method for extrapolating a "true" A-V 02 content difference from an SVC blood sample was obtained. The extrapolated value resulted in a more representative "true shunt" calculation in 13 of the 15 patients.