As prevsiouly discussed, the blood oxygen content is directly proportional to the haemoglobin concentration and, consequently, anaemia (and polycythaemia) has largely predictable effects on the ODC. In simple terms, a reduction of haemoglobin concentration to half of the normal value is accompanied by a similar reduction in arterial oxygen content with no (or very little) change in % saturation or PaO2 ( figure 2 ). Despite reduced oxygen carriage by the blood, tissue oxygen consumption is likely to show little change and is maintained by several compensatory factors, including higher cardiac output and greater extraction of oxygen by the tissues. Consequently, the blood “reserve” of oxygen is diminished and the venous oxygen content, saturation and partial pressure are all less than normal.
5 g ? dL ?1 compared to normal haemoglobin concentration of 15 g matchocean? dL ?1 . When oxygen content is plotted against PO2 the curve in anaemia is scaled down by fifty%, reflecting the halving of oxygen carrying capacity (dissolved oxygen is ignored); when SaO2 is plotted the anaemic and normal curves are superimposed.
A large number of genetically determined abnormal haemoglobins have been described, one of the more familiar being HbS which is found in patients with sickle cell disease. In individuals with a haemoglobinopathy, the abnormal molecules comprise a variable proportion of the total haemoglobin and consequently, the effects are similarly variablepared to the normal adult HbA, the abnormal haemoglobin molecules are associated with shifts of the ODC which can be either to the right (low oxygen affinity haemoglobins) or to the left (high oxygen affinity haemoglobins). The position of the ODC can be quantified by the P50, which is measured in vitro as the partial pressure of oxygen at a saturation of 50%. The P50 of normal adult blood is approximately 26 mmHg; low affinity haemoglobins are characterised by higher P50 and high-affinity haemoglobins by a lower than normal P50. Such abnormal haemoglobins can have major consequences for tissue delivery of oxygen but their effects are mitigated by various compensatory mechanisms, one of which is the haemoglobin concentration. High-affinity molecules, by definition, release oxygen less readily than normal and, because tissue hypoxia is a stimulus to haemoglobin production, affected individuals often have polycythaemia. By contrast those with low affinity haemoglobins are usually anaemic.
CO competes reversibly which have oxygen for binding internet sites into haemoglobin molecule but haemoglobin provides an even greater affinity (about two hundred-times) toward former and a large proportion of one’s binding sites might possibly be filled of the CO, even at the lowest partial tension of CO. On top of that, the current presence of carboxyhaemoglobin leads to a shift of your ODC left, after that decreasing cells fresh air delivery.
The usual indices of oxygenation are potentially misleading in CO poisoning; in particular, PaO2 is likely to remain normal and saturation may also appear normal when measured as SpO2 since most pulse oximeters (which utilise only two wavelengths of light) measure carboxyhaemoglobin together with oxyhaemoglobin. Distinction is possible using a specific CO pulse oximeter, which utilises several wavelengths, but such devices are not currently widely available . In suspected CO poisoning, carboxyhaemoglobin should be measured on a blood sample by multiwavelength spectrophotometry, as incorporated in modern blood gas analysers. The binding of CO to haemoglobin is reversible and can be reduced by increasing the inspired (and consequently the arterial) PO2. Patients with CO poisoning are therefore treated with the highest possible inspired oxygen concentration; sometimes hyperbaric oxygen is employed with the rationale that the higher the PaO2, the more CO molecules will be displaced from haemoglobin and that the increase in the very small amount of dissolved oxygen at very high PaO2, may help to sustain life .