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The oxygen dissociation curve: quantifying the shift

Cardiac Perfusion Services, Life Systems, University Clinic of Tubingen, Tubingen, Germany, carole.hamilton{at}gmz.net

Barbara Steinlechner

Department of Cardiothoracic Anesthesia, University Clinic of Vienna, Vienna, Austria

Eva Gruber

Department of Cardiothoracic Anesthesia, University Clinic of Vienna, Vienna, Austria

Paul Simon

Department of Cardiothoracic Surgery, University Clinic of Vienna, Vienna, Austria

Gregor Wollenek

Department of Cardiothoracic Surgery, University Clinic of Vienna, Vienna, Austria

An oxyhaemoglobin dissociation curve (ODC) quantifies the most important function of red blood cells and that is the affinity for oxygen and its delivery to the tissues. Oxygen affinity for haemoglobin plays a critical role in the delivery of oxygen to the tissues and is changed by shifting to the left or right. A shift to the left implies an increased oxygen affinity and, hence, tighter binding due to the higher oxygen saturation in relation to the pO₂.On the other hand, a shift to the right corresponds to a decreased oxygen affinity and easier release of oxygen to the tissues. It is well known that the ODC shifts in response to changes in pH, pCO₂ and 2,3 diphosphoglycerate. However, how much the ODC shifts has never been quantified. Arterial and venous blood gases were taken during cardiopulmonary bypass and two indices were used to quantify the shift of the ODC; the p50 shift and the SO₂ difference. Arterial blood shifted to the right by 49-0.1 mmHg at a pH of 7.24 and shifted to the left by -3.59-0.05 mmHg at a pH of 7.51. The change in arterial saturation was minimal, rising by 0.8% and dropping by -5% and did not correlate to p50 shifting and changes in pH, but demonstrated changes dependent on the concentration of dyshaemoglobins. The venous blood exhibited a greater range of p50 shifting at each pH value. At a pH of 7.24, the p50 shifted to the right by 4.89-2 mmHg and at a pH of 7.51 the p50 shifted to the left by -4±1.8 mmHg. Unlike the arterial blood, the change in saturation correlated well to p50 shifting. It is shown here for the first time how much the curve shifts with changes in pH and how this may be used to evaluate treatment strategies.

Perfusion, Vol. 19, No. 3, 141-144 (2004)
DOI: 10.1191/0267659104pf734oa


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