little reminder about the :
[font=verdana, geneva, helvetica]oxygen transport and the oxyhemoglobin disassociation curve
[font=verdana, geneva, helvetica]lung diseases can effect any or all factors needed for oxygen transport.
[font=verdana, geneva, helvetica]the transport of [color=#236eb5]oxygen throughout the body is done by the blood stream and the red blood cells. factors that affect this transport include the available oxygen to the lungs, the ability of oxygen to get passed the [color=#236eb5]alveolar/capillary membrane, and the oxygen carrying capacity of the blood.
[font=verdana, geneva, helvetica]even after blood has received its load of oxygen, there are more factors that affect blood's willingness to release its cargo such as temperature and ph.
these two factors effect what is known as the oxyhemoglobin disassociation curve.[font=verdana, geneva, helvetica]
lung diseases can affect any or all factors needed for oxygen transport. we treat a diminished ability to get oxygen into the alveoli and then into the capillaries by increasing the concentration of oxygen available to the patient. we can easily do that with [color=#236eb5]supplemental oxygen delivered by nasal cannula or mask, but this doesn't treat the cause of the disease. what it does is buy time for other treatments to work.
once oxygen has made its way past the lung it's up to the blood stream and red blood cells to deliver oxygen to all parts of the body. oxygen is carried two ways; bound to hemoglobin within the red blood cells and dissolved in the blood's plasma. the pao2 measured by an
arterial blood gas (abg) looks at the dissolved portion of oxygen in the blood
. this is a very small part but does give the clinician an idea of how much oxygen is making its way passed the alveolar/capillary membrane. the bulk of oxygen carried is bound to hemoglobin. the oxygen saturation or o2 sat. is a measurement of the amount of oxygen bound to hemoglobin.
the amount of hemoglobin available to oxygen, hemoglobin's ability to bind with oxygen, and then hemoglobin's ability to release its store of oxygen to the tissues are all important to oxygen transport. a low red blood cells count (anemia) reduces the amount of hemoglobin available. high oxygen saturations matter little since very little oxygen may be available to the body.
this can be caused by bleeding, reduced red blood cell production, or other factors. other gasses binding with hemoglobin such as carbon monoxide, which has a 240 times greater affinity to hemoglobin than oxygen, can reduce oxygen delivered to the body but still show as high oxygen saturation.
the [color=#236eb5]oxyhemoglobin disassociation curve
describes how oxygen interacts with hemoglobin. the curve itself is based on a comparison of pao2 and oxygen saturation. as pao2 drops, so does oxygen saturation, but the relationship is not linear, the curve is more "s" shaped. oxygen saturations will stay in the 90 and above percent range until the pao2 drops to about 60mmhg. below that the curve drops drastically along with oxygen saturations. variations in ph, carbon dioxide, 2,3-dpg, and temperature can cause the curve to shift to the right or left. a shift to the right means it will take higher pao2s to maintain a high oxygen saturation, where a shift to the left will have high saturations even with normally low oxygen levels.
this helps explain why people drowning in cold water can be saved with little or no brain damage even when they've been submerged to long periods of time. hypothermia and an acidotic blood chemistry shift the curve to the left maintaining high oxygen levels in the blood. hypothermia also lowers the body's metabolic rate so it uses less oxygen. the flip side of the curve is that shifts to the left means hemoglobin has an increased affinity for oxygen and releases less of it to the body. a shift to the right, even with lower oxygen saturations, means hemoglobin releases its supply of oxygen more easily.
factors and how the curve is affected:
- variation of the hydrogen ion concentration. this changes the blood's ph. a decrease in ph shifts the standard curve to the right, while an increase shifts it to the left. this is known as the bohr effect.
- effects of carbon dioxide. carbon dioxide affects the curve in two ways: first, it influences intracellular ph (the bohr effect), and second, co2 accumulation causes carbamino compounds to be generated through chemical interactions. low levels of carbamino compounds have the effect of shifting the curve to the right, while higher levels cause a leftward shift.
- effects of 2,3-dpg. 2,3-diphosphoglycerate, or 2,3-dpg, is an organophosphate, which are created in erythrocytes during glycolysis. the production of 2,3-dpg is likely an important adaptive mechanism, because the production increases for several conditions in the presence of diminished peripheral tissue o2 availability, such as hypoxemia, chronic lung disease, anemia, and congestive heart failure, among others. high levels of 2,3-dpg shift the curve to the right, while low levels of 2,3-dpg cause a leftward shift, seen in states such as septic shock and hypophosphatemia.
- temperature. temperature does not have so dramatic effect as the previous factors, but hyperthermia causes a rightward shift, while hypothermia causes a leftward shift.
- carbon monoxide. hemoglobin binds with carbon monoxide 240 times more readily than with oxygen, and therefore the presence of carbon monoxide can interfere with the hemoglobin's acquisition of oxygen. in addition to lowering the potential for hemoglobin to bind to oxygen, carbon monoxide also has the effect of shifting the curve to the left. with an increased level of carbon monoxide, a person can suffer from severe hypoxemia while maintaining a normal po2.
- effects of methemoglobinemia (a form of abnormal hemoglobin). methemoglobinemia causes a leftward shift in the curve.
- fetal hemoglobin. fetal hemoglobin (hbf) is structurally different from normal hemoglobin (hb). the fetal dissociation curve is shifted to the left relative to the curve for the normal adult. typically, fetal arterial oxygen pressures are low, and hence the leftward shift enhances the placental uptake of oxygen.
[font=verdana, geneva, helvetica]http://asthma.about.com/gi/dynamic/o...so/dissoc.html
when you have less that 88% saturation, you can see how steeply the percent of oxygen in the body drops.
[font=impact]what do the readings indicate?
[font=impact][color=#660066]as a guide:
- [font=impact][color=#660066]95% - 99% - normal
- [font=impact][color=#660066]91% - 94% - mild hypoxia
- [font=impact][color=#660066]86% - 91% - moderate hypoxia
- [font=impact][color=#660066]85% and lower - severe hypoxia
supplemental o2 is indicated for < 88% saturation level.
oximetry should always be measured: at rest and with activity (often drops with activity).
medicare guidelines for oxygen
payment is standard most insurance companies use for oxygen payment.
for patients that don't qualify, a doctor can still write a rx for oxygen and patient pay for rental of an e tank--costs less than $75.00/month.
if a person does not meet the following guidelines, medicare may not pay for home oxygen therapy:
hope this helps!
- arterial oxygen pressure is less than or equal to 55 mm hg (millimeters of mercury, a measure of pressure).
- arterial oxygen saturation is less than or equal to 88%.
- arterial oxygen pressure is between 56 mm hg and 59 mm hg or oxygen saturation is 89% and the person has:
- evidence of right-side heart failure due to breathing problems (cor pulmonale).
- heart failure.
- an increased number of red blood cells (erythrocytosis).
- arterial oxygen saturation is greater than 88% when the person is resting but becomes less than or equal to 88% when the person is exercising or sleeping.