Using the ABG to Identify Acidosis

deciphering diagnostics

Using the ABG to identify acidosis

By Amanda Perkins, MSN, RN

Acidosis is an acid-base imbalance that’s a low serum pH and corresponding respi- characterized by an abnormal accumula- ratory acidosis. tion of acid in the blood. It can also be Respiratory acidosis can develop quickly caused by a loss of alkali in the blood, over a short period of time or slowly over a leading to an acid-base mismatch in which prolonged amount of time. The speed with there’s more acid than base. Regardless of which respiratory acidosis develops will the cause, acidosis will lead to a serum pH depend on the underlying condition causing level below 7.35. An acidic environment it. Respiratory acidosis may be caused by the can be very dangerous because it alters following: cellular function, which in turn affects all • chronic obstructive pulmonary disease body systems. Additionally, acidosis can (COPD) alter a patient’s oxygenation by making it • pneumonia more difﬁ cult for hemoglobin to bind with • airway obstruction available oxygen. • atelectasis Acidosis is either respiratory or metabolic • asthma in nature. In this article, you’ll learn about • respiratory failure both disorders, including recognition, • any disorder/condition resulting in pain causes, and nursing care. with breathing • drug overdose Respiratory • head injury As the name implies, respiratory acidosis is • anesthesia. caused by problems with the respiratory The signs and symptoms associated with system. In order to understand respiratory respiratory acidosis are as follows: acidosis, you’ll need to have a generalized • restlessness understanding of anatomy and physiology • headache as it relates to the respiratory system, • light-headedness which takes oxygen from the air and trans- • decreased level of consciousness (LOC), ports it to the blood. Once oxygen is in the such as confusion, lethargy, or coma blood, it’s transported throughout the • dysrhythmias body. In addition to taking in oxygen, the • slow or difﬁ cult breathing lungs also remove carbon dioxide from the • tremor/muscular twitching blood by exhaling it into the environment. • BP changes When the lungs aren’t able to remove car- • cyanosis bon dioxide effectively, the carbon dioxide • drowsiness that remains in the body will form an acid. • muscle weakness. This acid accumulates in the blood, causing Metabolic consider this Unlike respiratory acidosis in which the cause is a problem with the respiratory A 75-year-old male patient is admitted to your ED with altered mental sta- system, metabolic acidosis can be caused tus, headache, cyanosis, and slowed breathing. The patient’s family by a variety of problems in the body, such reports that he developed a cold approximately 1 week ago. This patient as tissue hypoperfusion (the tissues don’t has a diagnosis of COPD, asthma, and hypertension. An ABG test is receive enough oxygen) or a lack of oxygen obtained with the following results: pH, 7.30; PaCO , 49; PaO , 85%; HCO , 2 2 3 to the kidneys. When the tissues in the 24; and SaO2, 88%. The diagnostic results indicate respiratory acidosis. body don’t receive enough oxygen,

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Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved. anaerobic metabolism takes over and pro- acid-base imbalances. With this test, a duces lactic acid, creating an acidic envi- blood sample from an artery instead of a ronment within the body. When the kid- vein is analyzed. Arterial blood is used neys don’t receive enough oxygen, they because it provides the most accurate lose their ability to excrete acid in the form picture of what’s going on with the pa- of hydrogen ions, leading to the excess acid tient. There are fi ve components to the remaining in the body and causing an ABG: pH, Paco2, Pao2, HCO3, Sao2 (see acidic environment. Metabolic acidosis can Normal ABG values). also occur as the result of excess loss of bi- pH is the measurement of the blood’s carbonate from the gastrointestinal (GI) acidity or alkalinity. Basically, the pH is a tract. Patients may lose bicarbonate from measurement of the concentration of the GI tract when they have excessive diar- hydrogen ions in the blood. Increased rhea or vomiting, or need to be suctioned hydrogen ions cause an acidic environ- for a prolonged period of time. ment, whereas decreased hydrogen ions As with respiratory acidosis, metabolic create an alkaline environment. Slight acidosis can develop quickly or over a more shifts in the pH can be very dangerous for prolonged time period. The speed with a patient. which metabolic acidosis develops is depen- Paco2 is the measurement of the partial dent on the underlying condition. Metabolic pressure of carbon dioxide dissolved in arte- acidosis can be caused by the following: rial blood. The rate and depth of respirations • ketoacidosis (diabetes, starvation) alter Paco2 levels. Hypoventilation causes • alcoholism increased Paco2 levels and resulting respira- • renal failure tory acidosis. • diarrhea Pao2 is the measurement of the partial • oliguric renal disease pressure of oxygen dissolved in arterial • poor perfusion (shock) blood. Pao2 levels that fall below 60 mm Hg • diuretic use cause anaerobic metabolism with result- • lactic acidosis ing lactic acid production and metabolic • isoniazid, salicylate, or iron poisoning acidosis. • methanol ingestion HCO3 is the measurement of the concen- • pancreatic fi stula tration of bicarbonate ions. Bicarbonate • hypermetabolic state (burns, infection). levels in the blood provide a window into The signs and symptoms associated with how the kidneys are handling metabolic metabolic acidosis include: acid. Low bicarbonate levels are indicative • tachypnea of metabolic acidosis. • decreased LOC, such as confusion, Sao2 is the measurement of the lethargy, or coma percentage of hemoglobin saturated with • abdominal pain oxygen. Factors that can alter Sao2 levels • dysrhythmias include temperature, pH, and Paco2. • increased rate and depth of respirations When analyzing ABG test results (compensatory hyperventilation) for acid-base imbalances, you should • fl ushed skin do the following: • decreased BP • muscle twitching consider this • Kussmaul respirations • nausea A 22-year-old female patient with diabetes is admitted to your ED • vomiting with lethargy, headache, abdominal pain, and tachypnea. Her mother, • diarrhea. who’s present at the time of admission, reports that the symptoms began yesterday. Upon admission, the patient’s blood glucose level How to use the ABG is 500. An ABG test is obtained with the following results: pH, 7.31; PaCO2, 45; PaO2, 85%; HCO3, 20; and SaO2, 90%. The diagnostic An arterial blood gas (ABG) test is the results indicate metabolic acidosis. lab test used to diagnose and evaluate www.NursingMadeIncrediblyEasy.com March/April 2016 Nursing made Incredibly Easy! 7

when respiratory acidosis has occurred Normal ABG values and the kidneys may not be functioning When using an ABG test to determine acid- correctly when metabolic acidosis has base imbalances, you must know the normal occurred. values for the five components: The body compensates for respiratory • pH: 7.35 to 7.45 acidosis by increasing HCO ; it compen- • 3 PaCO2: 35 to 45 mm Hg • sates for metabolic acidosis by hyperven- PaO2: 80 to 100 mm Hg • tilating and decreasing Paco2. It may help HCO3: 22 to 26 mEq/L • to remember that CO levels are con- SaO2: 95% to 100%. 2 trolled by the respiratory system and bicarbonate levels are controlled by the 1. Review the pH to determine if it’s renal system. Basically, when one system high, low, or normal. When pH levels are becomes acidic, the other will compensate high, hydrogen ion concentration is low; by trying to become alkalotic. if pH levels are low, hydrogen ion concentration is high. A high pH and low Management hydrogen ion concentration corresponds Respiratory acidosis is treated by to an alkaline environment, whereas a correcting or managing the underlying low pH and high hydrogen ion concen- respiratory cause. When addressed tration corresponds to an acidic environ- quickly, the treatment of acute condi- ment. This step won’t identify respiratory tions will usually lead to no long-term versus metabolic disturbances, but it will effects. Left untreated, patients with re- provide information about acidity versus spiratory acidosis may become comatose alkalinity. or die.

2. Look at the Paco2 and Sao2 values Corticosteroids, bronchodilators, to determine the patient’s oxygenation noninvasive positive-pressure ventila- status. tion, mechanical ventilation, and oxy-

3. Examine the Paco2 and HCO3 levels gen administration are all potential to determine if the problem is respira- treatment options for a patient with tory or metabolic in nature. If the prob- respiratory acidosis.

lem is respiratory, the Paco2 will be Corticosteroids and bronchodilators elevated and the HCO3 will be normal. may be used to decrease airway inﬂ am- If the problem is metabolic, the Paco2 mation and resistance in patients with will be normal and the HCO3 will be breathing difﬁ culties. When administer- decreased. ing these medications, it’s important to Keep in mind that with respiratory educate your patient regarding the rea-

imbalances, the pH and Paco2 will be son for their use and potential adverse abnormal in opposite directions. With reactions.

metabolic imbalances, the pH and HCO3 Noninvasive positive-pressure ventila- will be abnormal in the same direction. tion may be utilized to assist patients with breathing. This treatment involves The body tries to compensate the use of a continuous positive airway It’s important to be aware that the body pressure or bilevel positive airway pres- will attempt to compensate for acid-base sure machine. imbalances when they occur. The body In some cases, you may need to assist attempts to compensate to normalize the your patient with ventilation. If the under- serum pH and return itself to a state of lying cause is a chronic breathing condition, homeostasis. The lungs will try to com- the patient may need to be placed on a pensate for metabolic problems and the ventilator. kidneys will try to compensate for respi- Low oxygen levels may need to be ratory problems. This makes sense be- treated with oxygen administration. cause the lungs aren’t functioning well When administering oxygen, you’ll want

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Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved. to make sure that you’re aware of your potential complication is alkalosis, so patient’s underlying respiratory condition. patient monitoring is essential. For example, if your patient has COPD Interpretation skills needed and is a CO2 retainer, you’ll want to administer oxygen cautiously and moni- When caring for patients with respi- tor your patient closely to ensure that you ratory or metabolic acidosis, one of aren’t administering oxygen at a level the most important aspects is the that’s too high. In addition to these inter- ability to interpret ventions, you’ll want to educate patients the ABG test. An regarding the importance of smoking inability to do so Respiratory acidosis = cheat cessation because smoking can increase may delay treat- sheet pH ↓, HCO normal, PaCO ↑ your patient’s risk for respiratory ment and nega- 3 2 problems. tively affect patient Metabolic acidosis = As with respiratory acidosis, the treat- outcomes. Being pH ↓, HCO ↓, PaCO normal ment for metabolic acidosis focuses on knowledgeable 3 2 correcting the underlying cause. about the causes of Treatments that may be used for a both respiratory patient with metabolic acidosis include and metabolic acidosis is also an impor- detoxification for drug and/or alcohol tant factor to keep patients safe and pro- poisoning, insulin for diabetic ketoacido- mote positive outcomes. Patients who are sis (DKA), I.V. fluids, and sodium bicar- at risk for acidosis should be monitored bonate. closely and treated promptly if signs and If drug and/or alcohol poisoning is symptoms develop. causing the acidosis, detoxifi cation will Stay tuned next issue where we’ll take a look bring about a reversal. Always make sure at respiratory and metabolic alkalosis. ■ to follow facility protocols for drug and/ or alcohol detoxifi cation. REFERENCES With DKA, the body becomes acidic as Emergency Nurses Association. Trauma Nursing Core Course: Provider Manual. 7th ed. Des Plaines, IL: the result of an inability to produce Emergency Nurses Association; 2014. enough insulin. The administration of Jones MB. Basic interpretation of metabolic acidosis. Crit insulin will reverse the acidotic state. Care Nurse. 2010;30(5):63-69. When administering insulin, always Potter P, Perry A, Stockert P, Hall A. Essentials for Nursing Practice. 8th ed. St. Louis, MO: Mosby; 2015. monitor for and be aware of the signs and Pruitt B. Interpreting ABGs: an inside look at your patient’s symptoms of hypoglycemia. status. Nursing. 2010;40(7):31-35. When administering I.V. fl uids, ensure Rogers KM, McCutcheon K. Four steps to interpreting that you’re administering the correct fl u- arterial blood gases. J Perioper Pract. 2015;25(3):46-52. Rogers KM, McCutcheon K. Understanding arterial blood ids because incorrect I.V. fl uid adminis- gases. J Perioper Pract. 2013;23(9):191-197. tration can be harmful for your patient. Terry CL, Weaver A. Critical Care Nursing Demystifi ed: Additionally, you should monitor Hard Stuff Made Easy. New York, NY: McGraw Hill; 2011. any patient receiving fl uids for fl uid Williams LS, Hopper PD. Understanding Medical-Surgical overload. Nursing. 5th ed. Philadelphia, PA: F.A. Davis Company; The administration of sodium bicar- 2015. bonate can correct pH and avoid compli- cations associated with acidosis. It’s Amanda Perkins is an Assistant Professor of Nursing at Vermont important to remember that bicarbonate Tech in Randolph Center, Vt. won’t fi x the underlying cause, but it will help reduce acid in the blood. Bicarbonate is typically administered to maintain a The author has disclosed no financial relationships related to this serum pH above 7.2. In many cases, bicar- article. bonate will only be administered when the serum pH is 7.0 or below. When administering sodium bicarbonate, a DOI-10.1097 /01.NME.0000479872.01931.cc www.NursingMadeIncrediblyEasy.com March/April 2016 Nursing made Incredibly Easy! 9