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Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated. Top of the page. Test Overview An arterial blood gas ABG test measures the acidity pH and the levels of oxygen and carbon dioxide in the blood from an artery. This measures the pressure of oxygen dissolved in the blood and how well oxygen is able to move from the airspace of the lungs into the blood.
Partial pressure of carbon dioxide PaCO2. This measures the pressure of carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body.
The pH of blood is usually between 7. A pH of less than 7. So blood is slightly basic. Bicarbonate HCO3. Bicarbonate is a chemical buffer that keeps the pH of blood from becoming too acidic or too basic. O2 content measures the amount of oxygen in the blood. Oxygen saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen O2. See how well treatment for lung diseases is working. Find out if you need extra oxygen or help with breathing mechanical ventilation.
Find out if you are receiving the right amount of oxygen when you are using oxygen in the hospital. Measure the acid-base level in the blood of people who have heart failure , kidney failure , uncontrolled diabetes , sleep disorders, severe infections, or after a drug overdose.
How To Prepare Tell your doctor if you: Take a blood thinner , or if you have had bleeding problems. Are taking any medicines. Are allergic to any medicines, such as those used to numb the skin anesthetics. How It Is Done A sample of blood from an artery is usually taken from the inside of the wrist radial artery , but it can also be taken from an artery in the groin femoral artery or on the inside of the arm above the elbow crease brachial artery. The health professional taking a sample of your blood will: Clean the needle site with alcohol.
You may be given an injection of local anesthetic to numb that area. Put the needle into the artery. More than one needle stick may be needed. Allow the blood to fill the syringe. Be sure to breathe normally while your blood is being collected. Put a gauze pad or cotton ball over the needle site as the needle is removed.
Put a bandage over the puncture site and apply firm pressure for 5 to 10 minutes possibly longer if you take blood-thinning medicine or have bleeding problems. The ABG parameters generated during analysis will be defined here and their relationship to pulmonary gas exchange and acid-base balance discussed. The article begins with consideration of some relevant aspects of respiratory physiology. Oxygen is fundamental to life. The cells of all human tissues derive the energy they require to survive and function from the continuous aerobic metabolism of dietary-derived nutrients carbohydrates, fats, etc.
This aerobic metabolism requires a constant supply of oxygen and results in a continuous production of carbon dioxide, a waste product that must be eliminated from the body. The delivery of oxygen to, and removal of carbon dioxide from tissue cells is a major function of blood that occurs in tissues where the smallest blood vessels microcapillaries surround each tissue cell.
Blood returning in veins to the heart from this tissue microvasculature i. Pulmonary gas exchange, central to the process of respiration, accounts for the vital role that the lungs play in the delivery of oxygen to tissue cells and the elimination of carbon dioxide from the body.
The dual objective of respiration is transfer of oxygen present in inspired air to venous blood blood oxygenation and elimination of carbon dioxide from venous blood to the environment in expired air. The site of gaseous exchange between blood and air within the lungs is the alveolar-capillary membrane, which comprises two elements: the alveolar membrane that lines the microscopic, bubble-like, cul-de-sacs of lung structure called alveoli; and the endothelium wall of blood microcapillaries.
The amount of gas in any system is defined by the pressure it exerts, traditionally measured as height in millimeters mm of a column of mercury Hg. For example, the pressure of atmospheric air i. This means that at sea level, the gases contained in the air we breathe have a combined pressure sufficient to support a column of mercury mm high. In a mixture of gases, as air is, the total pressure is simply the sum of the partial pressures represented by the symbol p of each gas.
To convert values in the traditional unit mmHg to the equivalent SI unit kPa value, simply multiply by 0. It is important to note that these are a measure only of the amount of gas that is dissolved in arterial blood or venous blood , not the total amounts. For example, most of the oxygen in blood is bound to the protein hemoglobin. This protein-bound oxygen is not included in the p O 2 measurement see text.
Maintenance of two measured ABG parameters, p O 2 a and p CO 2 a within normal limits implies effective pulmonary gas exchange, which is dependent on:.
Ventilation is continuously regulated, principally by respiratory centers located in the brain stem. These respond to the amount of CO 2 in arterial blood i. By their neural connection to the chest musculature involved in breathing, these respiratory centers increase the rate and depth of breathing, and thereby increase alveolar ventilation, if p CO 2 is rising and reduce alveolar ventilation if p CO 2 a is falling.
By this means sufficient CO 2 is eliminated in expired air to maintain p CO 2 a within narrow normal limits.
Inappropriately reduced ventilation hypoventilation leads to inadequate pulmonary gas exchange, evident on blood gas analysis as increased p CO 2 and decreased p O 2 a. Hyperventilation overbreathing is always associated with decreased p CO 2 a but not necessarily, as might be supposed, increased p O 2 a.
Partial pressure of oxygen in arterial blood p O 2 a is not the only parameter measured during ABG that reflects blood oxygenation; the other is oxygen saturation s O 2. To understand the difference and relationship between p O 2 a and s O 2 a , we must examine how oxygen is transported in blood.
Each erythrocyte contains million hemoglobin molecules and each hemoglobin molecule can bind a maximum of four oxygen molecules. The product of the reversible binding of oxygen by hemoglobin is called oxyhemoglobin; the term deoxyhemoglobin is used to describe hemoglobin that has no oxygen bound to it. The oxygen delivery function of hemoglobin, i. In the deoxy state hemoglobin has low affinity for oxygen and in the oxy state it has high affinity for oxygen.
A number of environmental factors in blood determine the hemoglobin state deoxy or oxy and thereby the relative affinity for oxygen. The most significant of these is the p O 2. Hemoglobin present in blood with relatively high p O 2 has much greater affinity for oxygen than hemoglobin present in blood with relatively low p O 2. The oxygen dissociation curve ODC describes this relationship graphically Fig.
The percentage of total hemoglobin saturated with oxygen i. By contrast in the milieu of the tissues where p O 2 is much lower, hemoglobin affinity for oxygen is also much lower and oxygen is released from hemoglobin to the tissues. Relationship between the amount of oxygen dissolved in blood PO 2 and the amount of oxygen carried by hemoglobin SO 2. If p O 2 a is reduced, then less oxygen can be carried by hemoglobin i.
Examination of the oxygen dissociation curve Fig. The delivery of oxygen to tissues becomes increasingly compromised as p O 2 a falls below this level. All biochemical reactions are sensitive to change in pH, so that optimum survival and function of cells require that blood pH is maintained within the narrow range of 7.
Even mild excursion outside the normal range has multiple deleterious effects, and a pH of less than 6. The maintenance of normal blood pH is a complex synergy of action involving the chemical buffers present in blood principally bicarbonate , red blood cells and the function of three organs: the kidneys, lungs and brain stem.
The following discussion assumes an outline understanding of some basic concepts: pH, acids, bases and buffers see BOXES for a reminder.
Pure water has a pH of 7 neutral, i. From this equation: pH 7. An acid is a substance that dissociates in solution to release hydrogen ions. A base accepts hydrogen ions. By contrast, carbonic acid is a weak acid ; it dissociates much less easily, yielding fewer hydrogen ions and therefore higher pH than hydrochloric acid. A chemical buffer is a compound in solution the conjugate base of a weak acid that resists change in the pH of the solution when acid is added, by "mopping up" hydrogen ions.
Partial pressure of oxygen, or PaO2. This measurement shows the pressure of oxygen in your blood. Healthy adults will generally have a resting Pa02 greater than 80 millimeters of mercury mmHg.
Acid-base balance pH. When carbon dioxide builds up in your blood, it dissolves and creates an acid.
If your blood acid level is out of balance, it can mean your body isn't able to get rid carbon dioxide efficiently. This may happen because your lungs aren't working well or your kidneys can't get rid of the acid. A normal result is between 7. The test is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand.
Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. How far you live above sea level can affect your results.
Your blood oxygen goes down as your elevation goes up. Smoking, inhaling secondhand smoke, having a fever, or breathing quickly can also affect the test results. You don't need to do anything to prepare for the test. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. Search Encyclopedia. Why do I need this test? Here are some of the more common reasons an ABG is ordered: You're often short of breath. You're unusually tired.
You feel confused or disoriented.
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