What do arterial blood gases tell you

For samples with high pa O 2 e. When analysis is expected to be delayed for more than 30 minutes, use of glass syringes and ice slurry is recommended.

Insufficient mixing can cause coagulation of the sample. It is recommended to mix the blood sample thoroughly by inverting the syringe 10 times and rolling it between the palms as shown in Figure 1. This prevents stacking such as coins or plates of red blood cells. Correct method of mixing of the arterial sample with the anticoagulant in two dimensions to prevent stacking of red blood cells. Modern blood gas syringes and capillary tubes are coated with various types of heparin to prevent coagulation in the sampler and inside the blood gas analyzer:.

Other anticoagulants, e. The use of liquid heparin as the anticoagulant causes a dilution of the sample, i. As a consequence, parameters such as p CO 2 and electrolytes are affected. Only 0. Dead space volume of a standard 5 mL syringe with 1 inch 22 gauge needle is 0.

If smaller sample volumes are obtained or more liquid heparin is left in the syringe, then the dilution effect will be even greater. The dilution effect also depends on the hematocrit value. Plasma electrolytes decrease linearly with the dilution of the plasma along with p CO 2 , cGlucose, and ctHb values.

Syringes for blood gas analysis can have a wide range of heparin amounts. In order to obtain a sufficient final concentration of heparin in the sample, blood volume recommended on the syringe must be drawn. If the user draws 2 mL of blood, then the resulting heparin concentration will be too low and the sample may coagulate.

If the user draws only 1 mL, then the resulting heparin concentration will be higher than that aimed for, which may lead to producing falsely low electrolyte results. Electrolytes bound to heparin cannot be measured by ion-selective electrodes, and the final effect will be measurement offalsely low values. The use of electrolyte-balanced heparin significantly reduces the binding effect and the resulting inaccuracy.

While making an interpretation of an ABG always check for the consistency of the report by the modified Henderson equation. The hydrogen ion is calculated by subtracting the two digits after the decimal point of pH from 80, e. The hydrogen can be calculated from Table 1. While making an interpretation of anABG, never comment on the ABG without obtaining a relevant clinical history of the patient, which gives a clue to the etiology of the given acid—base disorder.

For example, a patient with a history of hypotension, renal failure, uncontrolled diabetic status, of treatment with drugs such as metformin is likely to have metabolic acidosis; a patient, with a history of diuretic use, bicarbonate administration, high-nasogastric aspirate, and vomiting, is likely to have metabolic alkalosis.

Respiratory acidosis would occur in COPD, muscular weakness, postoperative cases, and opioid overdose, and respiratory alkalosis is likely to occur in sepsis, hepatic coma, and pregnancy. The oxygenation status of the patient is judged by the paO 2 ;however, never comment on the oxygenation status without knowing the corresponding FiO 2.

Calculate the expected paO 2 generally five times the FiO 2. Based on the expected paO 2 classify as mild, moderate, and severe hypoxia. Then look at paCO 2 which is a respiratory acid, whether it is increased, i.

When the pH and paCO 2 change in the same direction which normally should not , the primary problem is metabolic; when pH and paCO 2 move in opposite directions and paCO 2 is normal, then the primary problem is respiratory. After the primary disorder is established as respiratory, then the following points will help us to approach further with regard to the respiratory disorder.

Hypoxemic respiratory failure can be associated with normal 10—15 mmHg or increasedalveolar arterial oxygen gradient. Figure 2 shows the alogrithim for approach in a patient with hypoxemic respiratory failure. Central nervous system—Respiratory center depression due to causes such as drug overdose, primary alveolar hypoventilation, and myxedema. Respiratory muscles—Hypophosphatemia, muscle fatigue, myasthenia gravis, and polymyositis.

The compensatory response depends upon the proper functioning of the organ system involved in the response lungs or kidneys and on the severity of acid—base disturbance. Acute compensation occurs within 6—24 h and chronic within 1—4 days.

Respiratory compensation occurs faster than metabolic compensation. In clinical practice, it is rare to see complete compensation. However, in chronic respiratory alkalosis, the pH may actually completely return to normalcy in some cases.

Chronic : [HCO 3 - ] increase by 3. In patients with metabolic acidosis, an excess of acid or loss of base is present. For more than 40 years, the AG theory has been used by clinicians to exploit the concept of electroneutrality and has evolved as a major tool for evaluating the acid—base disorder. Because electroneutrality must be maintained, the difference reflects the unmeasured ions. Normally, this difference or the gap is filled by the weak acids A - principally albumin, and to a lesser extent phosphates, sulfates, and lactates.

When the AG is greater than that produced by the albumin and phosphate, other anions e. Figure 3 shows the alogrithm for the approach to patients with normal AG acidosis.

The primary problem with AG is its reliance on the use of the normal range produced by the albumin and to a lesser extent phosphate, the level of which may be grossly abnormal in critically ill patients.

Because these anions are not strong anions, their charges will be altered by changes in pH. Collecting blood from an artery is more painful than collecting it from a vein because the arteries are deeper and are protected by nerves.

There is little chance of a problem from having a blood sample taken from an artery. Though problems are rare, be careful with the arm or leg that had the blood draw. Do not lift or carry objects for about 24 hours after you have had blood drawn from an artery. An arterial blood gas ABG test measures the acidity pH and levels of oxygen and carbon dioxide in the blood.

The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab and depend upon the elevation above sea level. Your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors.

This means that a value that falls outside the normal values listed here may still be normal for you or your lab. Greater than 80 mm Hg greater than The normal values for children may differ from the adult values listed here. The concentration of oxygen being breathed, called the fraction of inhaled oxygen FiO2 , is also usually reported. This is only useful if you are receiving oxygen therapy from a tank or are on a ventilator.

Many conditions can change blood gas levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.

Reasons you may not be able to have the test or why the results may not be helpful include the following:. Russo MD - Internal Medicine. Author: Healthwise Staff.

Medical Review: E. This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use and Privacy Policy.

Learn how we develop our content. To learn more about Healthwise, visit Healthwise. 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. Topic Contents What is it? Why is this test done?

How do you prepare for the test? How is the test done? How does having an arterial blood gases ABG test feel? What happens after the test? Where can you learn more? Top of the page. What is it?

Your doctor will tell you if there is anything you need to do to prepare for the test. You may feel light-headed, faint, dizzy, or nauseated while the blood is being taken from your artery. Most people feel a brief, sharp pain as the needle to collect the blood sample enters the artery.

If you get a local anesthetic, you may feel nothing at all from the needle puncture. Or you may feel a brief sting or pinch as the needle goes through the skin. You may feel more pain if the person drawing your blood has a hard time finding your artery, if your artery is narrowed, or if you are very sensitive to pain.