Author: John B. Downs, MD
The oxyhemoglobin dissociation curve is derived from laboratory data obtained by tonometry. O2 with a known partial pressure is bubbled through a blood sample at 37ºC until complete equilibrium is achieved. O2 saturation(s) is then measured and the curve is constructed for multiple values of PO2. Variation of temperature, pH, pCO2, 2.3 DPG, etc. will render different values of SO2 for any given PO2, leading to apparent “rightward or leftward” shift of the curve. It is important to note that O2 is the independent variable and the tonometry effect occurs only in the lung! In the periphery of the circulation, saturation is the independent variable and PO2 is the resultant, dependent variable. For example, a venous saturation of 70% will produce a PO2 of 40 mmHg at a pH of 7.36 and PCO2 of 45 mmHg. An increase in PvCO2 and decrease in pH will not change SvO2, but will increase PvO2.