The Effects on the Pulmonary Shunt Value of Using Measured versus Calculated Hemoglobin Oxygen Saturation and of Correcting for the Presence of Carboxyhemoglobin and Methemoglobin

In a clinical study we evaluated the effects on the pulmonary shunt value (Qs/Qt) of using measured versus calculated values for hemoglobin oxygen saturation (HbO2) and of correcting for the presence of carboxyhemoglobin (HbCO) and methemoglobin (HbMet). We compared the Qs/Qt determined by four methods: Method 1 used pulmonary capillary HbO2 = 1.0 and calculated arterial and mixed venous HbO2; Method 2 used pulmonary capillary HbO2 = 1.0 and measured arterial and mixed venous HbO2; Method 3 used pulmonary capillary HbO2 = 1.0 – HbCO and measured arterial and mixed venous HbO2; Method 4 used pulmonary capillary HbO2 = 1.0 – HbCO - HbMet and measured arterial and mixed venous HbO2. The Qs/Qt determined by Method 4 was significantly lower (P < 0.001) than that of any other method. The HbCO and HbMet values in our study population were similar to published normal values. The calculated arterial HbO2 was significantly greater (P < 0.001) than the measured HbO2. There was no statistically significant difference between the measured and calculated mixed venous HbO2, although there was a large difference between the calculated and measured values in many individual cases. For correct determination of the Qs/Qt, measured values for arterial and mixed venous HbO₂ must be used, and the pulmonary capillary HbO₂ must be corrected for HbCO and HbMet (Method 4). (Hess D, Elser RC, Agarwal NN. The Effects on the Pulmonary Shunt Value of Using Measured versus Calculated Hemoglobin Oxygen Saturation and of Correcting for the Presence of Carboxyhemoglobin and Methemoglobin. Respir Care 1984;29:1001-1005.