[Affinity of oxygen for hemoglobin--its significance under physiological and pathological conditions]

Abstract

Hemoglobin as a vehicle for oxygen carries roughly 65 times the volume of oxygen that would otherwise be transported by simple solution in plasma. Conformational shifts of the molecule induce a cooperative oxygen-hemoglobin affinity. This property is reflected in the sigmoidal shape of the oxygen-hemoglobin dissociation curve. The affinity of hemoglobin is affected by temperature, hydrogen ions, carbon dioxide, and intraerythrocytic 2,3-DPG, with all these factors mutually influencing each other. Physiologic conditions associated with shifts in hemoglobin-oxygen affinity are oxygen uptake in the lung, oxygen delivery in the capillaries, and particularly oxygen delivery in working muscle, diaplacental oxygen transfer, and the regulation of erythropoesis. Hemoglobin-oxygen affinity attains pathological significance for oxygen supply during respiratory or metabolic alkalosis when the hemodynamic and tissue responses of the individual are limited: the increased affinity can critically lower capillary oxygen tension. Methemoglobin and carbon monoxide shift the oxygen dissociation curve to the left, so that intoxication with both substances reduces both total oxygen capacity and oxygen delivery of the remaining hemoglobin able to bind oxygen. This effect of methemoglobin and carbon monoxide must be considered in intensive care of intoxicated victims. Transfusions of large volumes of stored red cells, whose hemoglobin shows high affinity, can force the capillary oxygen tension down, especially in patients with impaired cardiac performance. The lowered oxygen affinity of patients with chronic renal disease and anemia must be preserved by avoiding an increase in the acidotic plasma pH. In the neonate, hemoglobin possesses a high affinity for oxygen physiologically; the hemodynamic reserve of the neonate is limited. Therefore, the hemoglobin content plays a crucial role in oxygen transport capacity during the initial months of extrauterine life. Consequently, red cell transfusion must be started much earlier in neonatal surgery than in adults. The red cells must be fresh, or at best "rejuvenated". Normally, oxygen affinity is not relevant for oxygen supply, but the position of the oxygen-hemoglobin dissociation curve may be a critical factor in the situations described above, particularly when blood flow is additionally restricted.