When it comes to acids and bases, the difference between life and death is balance. The body’s acid-base balance depends on some delicately balanced chemical reactions. The hydrogen ion (H+) affects pH, and pH regulation influences the speed of cellular reactions, cell function, cell permeability, and the very integrity of cell structure.
When an imbalance develops, you can detect it quickly by knowing how to assess your patient and interpret arterial blood gas (ABG) values. And you can restore the balance by targeting your interventions to the specific acid-base disorder you find.
Basics of acid-base balance
Before assessing a patient’s acid-base balance, you need to understand how the H+ affects acids, bases, and pH.
- An acid is a substance that can donate H+ to a base. Examples include hydrochloric acid, nitric acid, ammonium ion, lactic acid, acetic acid, and carbonic acid (H2CO3).
- A base is a substance that can accept or bind H+. Examples include ammonia, lactate, acetate, and bicarbonate (HCO3-).
- pH reflects the overall H+ concentration in body fluids. The higher the number of H+ in the blood, the lower the pH; and the lower the number of H+, the higher the pH.
A solution containing more base than acid has fewer H+ and a higher pH. A solution containing more acid than base has more H+ and a lower pH. The pH of water (H2O), 7.4, is considered neutral.
The pH of blood is slightly alkaline and has a normal range of 7.35 to 7.45. For normal enzyme and cell function and normal metabolism, the blood’s pH must remain in this narrow range. If the blood is acidic, the force of cardiac contractions diminishes. If the blood is alkaline, neuromuscular function becomes impaired. A blood pH below 6.8 or above 7.8 is usually fatal.
pH also reflects the balance between the percentage of H+ and the percentage of HCO3-. Generally, pH is maintained at a ratio of 20 parts HCO3– to 1 part H2CO3. (See Fast facts on acid-base balance by clicking the PDF icon above.)
Regulating acid-base balance
Three regulating systems maintain the body’s pH: chemical buffers, the respiratory system, and the renal system.
Chemical buffers, substances that combine with excess acids or bases, act immediately to maintain pH and are the body’s most efficient pH-balancing force. These buffers appear in blood, intracellular fluid, and extracellular fluid. The main chemical buffers are bicarbonate, phosphate, and protein.
The second line of defense against acid-base imbalances is the respiratory system. The lungs regulate carbon dioxide (CO2) in the blood, which combines with H2O to form H2CO3. Chemoreceptors in the brain sense pH changes and vary the rate and depth of respirations to regulate CO2 levels. Faster, deeper breathing eliminates CO2 from the lungs, and less H2CO3 is formed, so pH rises. Alternatively, slower, shallower breathing reduces CO2 excretion, so pH falls.
The partial pressure of arterial CO2 (Paco2) level reflects the level of CO2 in the blood. Normal Paco2 is 35 to 45 mm Hg. A higher CO2 level indicates hypoventilation from shallow breathing. A lower Paco2 level indicates hyperventilation. The respiratory system, which can handle twice as many acids and bases as the buffer systems, responds in minutes, but compensation is temporary. Long-term adjustments require the renal system.
The renal system maintains acid-base balance by absorbing or excreting acids and bases. Also, the kidneys can produce HCO3– to replenish lost supplies. The normal HCO3– level is 22 to 26 mEq/L. When blood is acidic, the kidneys reabsorb HCO3– and excrete H+. When blood is alkaline, the kidneys excrete HCO3– and retain H+. Unlike the lungs, the kidneys may take 24 hours before starting to restore normal pH.
Compensating for imbalances
The two disorders of acid-base balance are acidosis and alkalosis. In acidosis, the blood has too much acid (or too little base). In alkalosis, the blood has too much base (or too little acid). The cause of these acid-base disorders is either respiratory or metabolic. If the respiratory system is responsible, you’ll detect it by reviewing Paco2 or serum CO2 levels. If the metabolic system is responsible, you’ll detect it by reviewing serum HCO3– levels.
To regain acid-base balance, the lungs may respond to a metabolic disorder, and the kidneys may respond to a respiratory disorder. If pH remains abnormal, the respiratory or metabolic response is called partial compensation. If the pH returns to normal, the response is called complete compensation. Keep in mind that the respiratory or renal system will never overcompensate. A compensatory mechanism won’t make an acidotic patient alkalotic or an alkalotic patient acidotic.
Understanding acidosis and alkalosis
Caused by hypoventilation, respiratory acidosis develops when the lungs don’t adequately eliminate CO2. The hypoventilation may result from diseases that severely affect the lungs, diseases of the nerves and muscles of the chest that impair the mechanics of breathing, or drugs that slow a patient’s respirations. Respiratory acidosis causes a pH below 7.35 and a Paco2 above 45 mm Hg. HCO3– is normal. (See Causes of acid-base imbalances at a glance by clicking the PDF icon above.)
Caused by hyperventilation, respiratory alkalosis develops when the lungs eliminate too much CO2. The most common cause of hyperventilation is anxiety. Respiratory alkalosis causes a pH above 7.45 and a Paco2 below 35 mm Hg. HCO3– is normal.
Metabolic acidosis may result from:
- ingestion of an acidic substance or a substance that can be metabolized to an acid
- production of excess acid
- an inability of the kidneys to excrete normal amounts of acid
- a loss of base.
Metabolic acidosis causes a HCO3– below 22 mEq/L and a pH below 7.35. Paco2 is normal.
Metabolic alkalosis may result from:
- loss of stomach acid
- an excess loss of sodium or potassium
- a renal loss of H+
- a gain of base.
Metabolic alkalosis causes a HCO3– above 26 mEq/L and a pH above 7.45. Paco2 is normal.
ABG analysis in four steps
ABG analysis is a diagnostic test that helps you assess the effectiveness of your patient’s ventilation and acid-base balance. The results also help you monitor your patient’s response to treatment. ABG analysis provides several test results, but only three are essential for evaluating acid-base balance: pH, Paco2, and HCO3-. Memorize these normal values for adults:
- pH: 7.35 to 7.45
- Paco2: 35 to 45 mm Hg
- HCO3-: 22 to 26 mEq/L.
Remember, the key to interpreting ABG values at the bedside is consistency. Follow these four simple steps every time:
- Step 1. List the results for the three essential values: pH, Paco2, and HCO3-.
- Step 2. Compare them with normal values. If a result indicates excessive acid, write an
A next to it. If a result indicates excessive base, write a
B next to it. And if a result indicates a normal balance, write an N next to it. The pH will tell you whether the patient has acidosis or alkalosis.
- Step 3. If you’ve written the same letter for two or three results, circle them. If you circle pH and Paco2, your patient has a respiratory disorder. If you circle pH and HCO3-, your patient has a metabolic disorder. If you circle all three results, your patient has a combined respiratory and metabolic acid-base disturbance. (See Interpreting arterial blood gas values by clicking the PDF icon above.)
- Step 4. To check for compensation, look at the result you didn’t circle. If it has moved from the normal value in the opposite direction of those circled, compensation is occurring. If the value remains in the normal range, no compensation has occurred. Once compensation is complete, the pH will return to normal.
Keep in mind that several factors can make ABG results inaccurate:
- using improper technique to draw the arterial blood sample
- drawing venous blood instead of arterial blood
- drawing an ABG sample within 20 minutes of a procedure, such as suctioning or administering respiratory treatment
- allowing air bubbles in the sample
- delaying transport of the sample to the lab.
ABG values provide important information about your patient’s condition. But never underestimate the importance of your clinical assessment and judgment. As a nurse, you are the most important advocate for your patients because you are constantly at the bedside, monitoring, assessing, intervening, and reevaluating.
Your role begins with identifying patients at risk for acid-base disturbances, including those who have or are at risk for:
- significant electrolyte imbalances
- net gain or loss of acids
- net gain or loss of bases
- ventilation abnormalities
- abnormal kidney function.
Assess patients carefully to identify early clues of acid-base disturbances. Consider what your patient’s vital signs are telling you. Count your patient’s respirations for a full minute. What are the rate and the depth? Are they clues to an impending or underlying respiratory or metabolic problem? What is your patient’s level of consciousness? Confusion can be an early sign of an acid-base disturbance. Correlate your patient’s fluid balance and creatinine levels with kidney function. Always correlate your assessment findings with your patient’s diagnosis. Do they match? Or is some clue pointing in a different direction? Be sure to double-check the implications and adverse effects of all drugs you administer.
Treating acid-base imbalances
Treatment for metabolic acidosis focuses on correcting the underlying cause. For a diabetic patient, treatment consists of controlling blood glucose and insulin levels. In a case of poisoning, treatment focuses on eliminating the toxin from the blood. Correcting the underlying cause of sepsis may include antibiotic therapy, fluid administration, and surgery. You may also treat the acidosis directly. If it’s mild, administering I.V. fluid may correct the problem. If acidosis is severe, you may give bicarbonate I.V., as prescribed.
Treatment for metabolic alkalosis also focuses on the underlying cause. Frequently, an electrolyte imbalance causes this disorder, so treatment consists of replacing fluid, sodium, and potassium.
The treatment goal for respiratory acidosis is to improve ventilation. Expect to administer drugs such as bronchodilators to improve breathing and, in severe cases, to use mechanical ventilation. Maintain good pulmonary hygiene.
Usually, the only treatment goal for respiratory alkalosis is to slow the breathing rate. If anxiety is the cause, encourage the patient to slow his or her breathing. Some patients may need an anxiolytic. If pain is causing rapid, shallow breathing, provide pain relief. Breathing into a paper bag allows a patient to rebreathe CO2, raising the level of CO2 in the blood.
Practice makes perfect
Use the case histories below to test your acid-base knowledge with some examples. Read each history and try to determine the cause of the signs and symptoms. Then, read the interpretation section to see how well you did. (See Beyond pH, Paco2, and HCO3– by clicking the PDF icon above.)
Case history 1
Mary Barker, 34, comes to the emergency department (ED) with acute shortness of breath and pain on her right side. She smokes one pack of cigarettes a day and recently started taking birth control pills. Her blood pressure is 140/80 mm Hg; her pulse is 110 beats/minute; and her respiratory rate is 44 breaths/minute. Her ABG values are as follows:
- pH: 7.50
- Paco2: 29 mm Hg
- Partial pressure of arterial oxygen (Pao2): 64 mm Hg
- HCO3-: 24 mm Hg
- Oxygen saturation (SaO2): 86%.
Interpretation: These ABG values reveal respiratory alkalosis without compensation. The patient’s pH and Paco2 are alkalotic, and her HCO3– is normal, indicating no compensation. You would administer oxygen (O2) therapy, as ordered, to increase SaO2 to more than 95%; encourage the patient to breathe slowly and regularly to decrease CO2 loss; administer an analgesic, as ordered, to ease pain; and support her emotionally to decrease anxiety. Based on the clues, the probable underlying cause is pulmonary embolism.
Case history 2
John Stewart, 22, is brought to the ED for an overdose of a tricyclic antidepressant. He’s unconscious and has a respiratory rate of 5 to 8 breaths/minute. His ABG values are as follows:
- pH: 7.25
- Paco2: 61 mm Hg
- Pao2: 76 mm Hg
- HCO3-: 26 mm Hg
- SaO2: 89%.
Interpretation: These ABG values reveal respiratory acidosis without compensation. The patient’s pH and Paco2 are acidotic, and his HCO3– is normal, indicating no compensation. You would administer O2, as ordered. The patient may be intubated to protect his airway and placed on a mechanical ventilator. You would also treat the underlying cause by performing gastric lavage and administering activated charcoal. This patient’s condition may progress to metabolic acidosis. If so, you would give sodium bicarbonate to reverse the acidosis.
Case history 3
Steve Burr, 38, has type 1 diabetes. He hasn’t been feeling well for the last 3 days and hasn’t eaten or injected his insulin. He’s confused and lethargic. His respiratory rate is 32 breaths/minute, and his breath has a fruity odor. His serum glucose level is 620 mg/dL. While receiving 40% O2, his ABG values are:
- pH: 7.15
- Paco2: 30 mm Hg
- Pao2: 130 mm Hg
- HCO3-: 10 mm Hg
- SaO2: 94%.
Interpretation: These ABG values reveal metabolic acidosis with partial respiratory compensation. The patient’s pH and HCO3– indicate acidosis. His Paco2 is lower than normal, reflecting the lungs’ attempt to compensate. Because pH is abnormal, you know compensation isn’t complete.
ABG values only
Try interpreting this set of ABG values without a clinical scenario:
- pH: 7.49
- Paco2: 40 mm Hg
- Pao2: 85 mm Hg
- HCO3-: 29 mm Hg
- SaO2: 90%
Interpretation: These values reveal uncompensated metabolic alkalosis. The pH and HCO3– indicate alkalosis. Paco2 is normal, indicating no compensation.
Now, interpret these values:
- pH: 7.25
- Paco2: 56 mm Hg
- Pao2: 80 mm Hg
- HCO3-: 15 mm Hg
- SaO2 : 93%
Interpretation: These values reveal mixed acidosis. The pH, HCO3-, and Paco2 all indicate acidosis.
Back in balance
How did you do? Whether you aced this practice quiz or not, remember that integrating your ABG interpretation skills into your patient assessments takes practice. By becoming more adept at identifying specific acid-base disorders, you can ensure that patients receive the appropriate nursing interventions and get back in balance as quickly as possible.
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Michelle Fournier is Founder and CEO of A Choice Above in Denver, Colorado, and a healthcare consultant for ja thomas & Associates in Smyrna, Georgia. The planners and author of this CNE activity have disclosed no relevant financial relationships with any commercial companies pertaining to this activity.
Please fix the PaCO2 in the case study. This is an excellent article. I have emailed this before. If you can get to a PDF the errors are fixed. In a digital copy you have two PaCO2s. I know one is PaO2. I cannot get to a PDF anymore. I use it for my nursing students. It is really a great article even though it is older. Some things do not change! Thanks for the article.
Thank you for bringing this error to our attention! We were able to correct the changes this morning, which you will see in the web copy above. Additionally, our tech team was able to fix the issue with the PDF so you should now be able to download the article which was in the journal through the link at the top of the article.
Thank you again,
Lydia Kim, Digital Content Editor
I learned a lot from you
Repeatedly in the case scenarios, interpretation is difficult because different values for PaCO2 are reported. E.g. pH: 7.15
Paco2: 30 mm Hg
Paco2: 130 mm Hg
HCO3-: 10 mm Hg
Thank you for bringing this error to our attention! We were able to correct the changes this morning, which you will see in the web copy above.
Thank you again,
Lydia Kim, Digital Content Editor