The oxyhemoglobin dissociation curve has received scant attention in clinical practice. The circumstances associated with a shift of the dissociation curve (abnormal temperature, PaCO2, pH, and 2,3-diphosphoglycerate levels) are almost always present in critically ill patients to varying degrees and in varying combinations. In addition, there may be other hemoglobin ligands such as carboxyhemoglobin or methemoglobin which, if present in significant concentrations, could effectively lower CaO2 and also lead to errors in oxygen transport calculations.
Shifts of the oxyhemoglobin dissociation curve mainly affect the middle portion, leaving the upper and lower portions relatively unaffected. The concept of consumable or available oxygen has been proposed based on clinical work, notably that of Bryan.Brown,.et a[ (.1973.) which strongly suggests that oxygen bound to hemoglobin below a tension of 2.7 kPa (20 mmHg) is unavailable for tissue oxygen consumption_because of its inability to diffuse into mitochondria at this partial pressure. It has generally been believed that a shift of the curve to the right is beneficial in that if PV02 remains constant there will be a reduction in 5^02 and therefore an automatic increase in Vo2 due to reduced hemoglobin affinity for oxygen at the mixed venous saturation. This has been referred to as improved 'off-loading' of oxygen. This is a purely teleological point of view and ignores the multiplicity of factors which affect tissue oxygen availability in critically ill patients.
If Do2 is reduced but the tissues maintain the ability to extract oxygen, Vo2 can be maintained at a normal or even high value by an increase in OER which produces a reduction in Si^02. This is an unsatisfactory mechanism as it may limit oxygen availability towards the venous side of the capillary bed in some tissues, notably the liver. This type of oxygen transport pattern is seen most commonly in poorly resuscitated trauma victims or in cardiogenic shock following acute myocardial infarction.
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