subject: Monitoring Of Respiration: Pulse Oximetry, Capnography, Oximetry [print this page] After a complete saturation of hemoglobin with oxygen is accompanied by a further increase Ra02 only a minor increase SaO2 by physically dissolved oxygen. Therefore, increasing the concentration of oxygen in the inspired gas or blown (F1O2) above the level that is sufficient for complete saturation of the hemoglobin capacity (Sa02 = 99-100%), is rarely justified.
Passing through the capillaries, arterial blood gives the tissues of the contained oxygen and is converted into the venous (PvO2 = 40 mm Hg. Art., SvO2 = 75%). Thus, in gas exchange involved only about 25% of the stock of oxygen in arterial blood, and oxygen saturation and desaturation of hemoglobin occur on the flat part of the dissociation curve.
Pathology of respiratory system leads to disruption of approx-sigenatsii blood in the lungs with the development of arterial hypoxemia, which quantifies the degree of pulse oximeter. Under these conditions, oxygen supply carried out in "emergency" mode, on the steep part of the curve, where a slight fall of PaO2 is sufficient for the separation of oxyhemoglobin required amount of oxygen. Emergency operation is to reduce stress and, consequently, the oxygen content in the tissues, as evidenced by low oxygen tension in venous blood.
Hemoglobin as a transport protein is called to solve two problems: up oxygen in the lungs and give it to the tissues. These tasks are opposite in nature, but are performed by the same agent, so the desire of hemoglobin binding to oxygen (hemoglobin affinity for oxygen) should be sufficient - to provide a blood oxygenation in the lungs, but not excessive - not to disrupt the impact of oxygen on the periphery . The normal position of the oxyhemoglobin dissociation curve corresponds precisely to the optimal hemoglobin readiness to implement both tasks. But under certain conditions, the balance between the desire to join the hemoglobin oxygen and willingness to pay is violated. Graphically this is expressed by the dissociation curve shift to the right or left.
If acidosis (respiratory or metabolic), hyperthermia and increased concentration of 2,3-diphosphoglycerate (2,3-DFG) in erythrocytes affinity of hemoglobin for oxygen is reduced and NO2 dissociation curve shifts to the right. In this case the saturation of hemoglobin with oxygen in the lungs deteriorate (decrease Sp02 at the same PaO2), but the separation of oxygen from oxyhemoglobin in the capillaries facilitated.
If gas exchange in the lungs was not affected, even dramatic shift the dissociation curve to the right accompanied by a very slight decrease in Sp02, as events occur in the lungs on the flat part of the curve. In the same tissue oxygen tension increases. With respect to oxygen homeostasis is generally a safe situation. Some experts even regarded-
melt, which is normal for the light acidosis contributes to the supply of oxygen to tissues.
A different picture is observed in the rough breathing disease, when blood is flowing from the lungs to low oxygen tension, corresponding to the steep part of the curve of dissociation of NO2. If this curve is shifted to the right, SpO2 can be much lower than in the normal position of the curve. This circumstance - an additional blow to the supply of oxygen to tissues and an important contribution to the development of hypoxia. Thus, when the initial arterial hypoxemia (low-level races "2) metabolic acidosis, hypercapnia and hyper-terminal can significantly reduce the oxygen saturation of hemoglobin (Sp02) and, consequently, the oxygen content in arterial blood.
Alkalosis (respiratory or metabolic), hypothermia and a decrease in concentration of 2,3-DFG increase the affinity of hemoglobin for oxygen, and dissociation curve shifts to the left N02. Under these conditions, hemoglobin avidly attaches itself to the oxygen in the lungs (SpO2 increases at the same PaO2) and reluctantly gave it to the tissues. It is believed that the shift of the dissociation curve to the left is always adversely affect tissue oxygenation, since a small increase in the content (but not the voltage) of oxygen in arterial blood does not pay for the subsequent reluctance to share oxyhemoglobin oxygen to tissues in the periphery. Perhaps, from the left of NO2 dissociation curve is not affected only babies. But that's another topic.
Inconsistency relationship between PaO2 and SpO2 may complicate interpretation of the data pulse oximetry: not always clear on what works hemoglobin dissociation curve at the moment.
About disgemoglobinah, paint and varnish, nail polish
Another factor that affects the pulse oximeter readings, - is the presence of additional blood hemoglobin fractions. Among them are disgemoglobiny (carboxy-and methemoglobin) and fetal hemoglobin.
The normal content of carboxyhemoglobin (dormice) in the blood is low (1-3%) and no effect on the SpO2. However, in cases of poisoning by carbon monoxide or in patients with recently received flame burns carboxyhemoglobin may be tens of percent of total hemoglobin. Dormice absorbs light in much the same as NO2, so instead of hemoglobin oxygen saturation pulse oximeter in these patients indicates the amount of interest and NOa concentrations of dormice. For example, if SaO2 = 65%, and dormice = 25%, pulse oximeter will display on the display value of SpO2, close to 90%.
