Mixed venous P02 (P902) Medical Assignment Help

This is the partial pressure of oxygen in pulmonary arterial blood that has been thoroughly mixed during its passage through the heart. If Pa02 remains constant, the PV<>2 will fall if more oxygen has to be extracted from each unit volume of blood arriving at the tissues. A fall in PV<>2 therefore indicates that either oxygen delivery has fallen or that tissue oxygen requirements have increased without a compensatory rise in cardiac output. If PV<>2 falls, the effect of a given degree of pulmonary shunting on arterial oxygenation will be exacerbated. Thus, worsening arterial hypoxaemia does not necessarily indicate a deterioration in pulmonary function but may instead reflect a fall in cardiac output and/or a rise in oxygen consumption.
The Pv02 is also influenced by the position of the oxyhaemoglobin dissociation curve, a factor not incorporated in the concept of oxygen flux. Thus, if the arteriovenous oxygen content difference remains constant, a shift of the curve to the right, which occurs with acidosis, hypercarbia, pyrexia and a rise in red cell 2,3- diphosphoglycerate (2,3-DPG) levels, may cause the Pv02 to rise. If the Pv02 remains unchanged, more oxygen will be unloaded at tissue level. A shift of the curve to the left, on the other hand, will cause a fall in the Pv02′ It might be argued, then, that under certain circumstances an acidosis may be beneficial in terms of tissue oxygenation, provided that it is not severe enough to interfere with cardiac function. It is probable though, that shifts of the dissociation curve are of little clinical significance.

The carbon dioxide dissociation curve.

The carbon dioxide dissociation curve.

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