Thursday, September 23, 2010

3.3d Oxygen and CO2 in blood/Measurements/Effects

Physician's Notebooks 3  - - See Homepage

(Continues Chapter 3. About oxygen in blood and effects. Also CO2 and effect on brain)
Update 25 November 2018
Oxygen Saturation of Arterial Blood (SaO2)

 Recall that hemoglobin (Hgb) is the red pigment inside the red blood cells that, in the lungs, combines with oxygen O2 and, as Hgb-O2, goes in the blood inside each freshly oxygenated RBC to the tissues where it releases O2 to cells in exchange for CO2, carbon dioxide. The combining of O2 with Hgb in the lungs is efficient. Nearly every molecule of Hgb is filled at its bond site with O2 (97%-100% saturation by oxygen, or SaO2). Testing SaO2 by Pulse Oximeter is a simple, painless finger pinch that checks for low O2 in breathed air at high altitude or with poor lung air-exchange; and it also checks the efficiency of air-exchange in your lungs as affected by lung disease or by heart failure or by shunting of low-O2 venous blood from right to left side of heart. The SaO2 test is not affected by anemia, ie, you can be severely anemic with low RBC yet have SaO2 99%. The oximeter also measures the heart's pulse rate, and the latest one may measure the hemoglobin concentration in capillary blood.

The SaO2 while at rest breathing sea level air should stay above 95%. If one gets in a jet and goes up to 35,000 feet, the SaO2 may fall to 90% with no problem. But if SaO2 goes much below 90%, it means critical lack of oxygen (hypoxia) is developing, and, if below 75%, damage from lack of oxygen to brain may be imminent. Low SaO2 requires oxygen inhalation.

Oxygen Physically Dissolved in your Blood Plasma: the SaO2 measures how much O2 is in chemical combination with Hgb in RBC in artery blood but it does not measure the physically dissolved O2 in blood plasma. Normally, this is insignificant – 0.3% of the total O2 in blood circulation while breathing air at sea level. But in acute myocardial infarction (AMI, a coronary artery blockage heart attack), brain stroke, or in an underwater diver ascending too quickly and having nitrogen bubbles in blood, the damage may be prevented by prompt breathing 100% oxygen or even better by hyperbaric (at pressure of several atmospheres) oxygen in sealed chamber. In AMI and stroke, the good effect is because pure O2 under high pressure breathing will increase the oxygen dissolved physically in the tissues as molecular gas, even when the SaO2 is 100%, and this difference gives better heart and brain tissue oxygenation and may make the difference between life and death for heart or brain cell already affected by the low oxygen of poor circulation.

Color Appearance of Whole Blood depends on ratio of the O2 to the CO2 connected to hemoglobin in the red blood cells. When the heart’s right side chamber pumps high-CO2 venous blood into lungs, the blood exchanges its CO2 acquired in tissue for newly breathed-in O2 and becomes bright red. Then the bright red blood is returned to left heart, pumped into aorta, and distributed via arteries to tissues where it gives up the O2 for CO2 to the cells of your body tissues, and becomes dark, as seen in blood returning in veins to right side of heart. When such vein blood, freshly drawn by needle syringe, is put into a glass tube, it should appear dark red. But occasionally the vein blood appears almost black due to severe disease affecting heart or lungs causing low O2 in veins. Because the tissue extraction of the already low O2 depletes oxygenation of vein blood more than usual, it leads to blood more black than red and a dusky skin appearance in fair-skin person. Blood color changes relating to oxygenation also are the cause of the blue-black tinged skin of methemoglobin poisoning (seen with overdose of APC tablets against fever) and the cherry red color about a patient’s lips, of carbon monoxide poisoning. The late President JFK's wife Jackie Kennedy's father was nicknamed Black Jack Bouvier because he had the methemoglobinemia condition as genetic trait.

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