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
- Paco2: 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
- Paco2: 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
- Paco2: 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
- Paco2: 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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Ayers P, Warrington L. Diagnosis and treatment of simple acid-base disorders. Nutr Clin Pract. 2008;23(2):122-127.
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Rhoades R, Pflanzer R. Human Physiology. 4th ed. Fort Worth, TX: Saunders College Publishing; 2003.
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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.
A 45 year-old-female suffering from bronchial asthma was brought to emergency in a critical state with extreme difficulty in breathing.
The blood gas analysis revealed the following
What is your Interpretation?
Low PO2 and PCO2 excess signify Primary respiratory problem
Thus, the patient is suffering from Acute respiratory acidosis.
A 4 day old girl neonate became lethargic and uninterested in breast-feeding. Physical examination revealed tachypnea (rapid breathing) with a normal heart beat and breath sounds. Initial blood chemistry values included normal glucose, sodium, potassium, chloride, and bicarbonate (HCO3-) levels.
Blood gas values revealed a pH of 7.53, partial pressure of oxygen (PO2) was normal (103 mm Hg) but PCO2 was 27 mmHg.
What is the probable diagnosis?
The baby is suffering from Respiratory Alkalosis
Tachypnea in term infants may be due to brain injuries and metabolic diseases that irritate the respiratory center. The increased respiratory rate removes carbon dioxide from the lung alveoli and lowers blood CO2, forcing a shift in the indicated equilibrium towards left
CO2 + H2O çè H2CO3 çè H+ + HCO3-
Carbonic acid (H2CO3) can be ignored because negligible amounts are present at physiological pH, leaving the equilibrium
The leftward shift to replenish exhaled CO2 decreases the hydrogen ion (H+) concentration and increases the pH to produce alkalosis. This respiratory alkalosis is best treated by diminishing the respiratory rate to elevate the blood [CO2], to force the above equilibrium to the right, elevate the [H+], and decrease the pH.
A new-born with tachypnea and cyanosis (bluish color) is found to have a blood pH of 7.1. Serum bicarbonate is measured as 12 mM while pCO2 is 40 mm Hg.
What is the probable diagnosis?
Low p H and low bicarbonate indicate metabolic acidosis. Since p CO2 is normal it can not be compensatory respiratory acidosis (If the baby had respiratory acidosis, the PCO2 would have been elevated).This is a hypoxia related metabolic acidosis. Hyperventilation is as a compensation to metabolic acidosis.
Thiscondition can be treated by administration of oxygen to improve tissue perfusion and decrease metabolic acidosis.
A 60-year-old man was brought to hospital in a very serious condition. The patient complained of constant vomiting containing several hundred mL of dark brown fluid from the previous two days plus several episodes of melaena. Past history of alcoholism, cirrhosis, portal hypertension and a previous episode of bleeding varices was there.
Arterial Blood Gases revealed-
Na+ 131 mmol/l., Cl– 85 mmol/l. K+ 4.2 mmol/l., “total CO2” 5.1, glucose 52mg/dl, urea 38.6mg/dl, creatinine1.24mg/dl, lactate 20.3 mmol/l Hb 6.2 G%, and WBC- 18 x103/mm3
The patient is severely ill with circulatory failure and GI bleeding on a background of known cirrhosis with portal hypertension.
The very low pH indicates a severe acidosis. The combination of a low pCO2 and low bicarbonate indicates either a metabolic acidosis or a compensatory respiratory alkalosis (or both). As this patient has a severe acidosis, so the most probable diagnosis is metabolic acidosis. The anion gap is 31 indicating the presence of a high anion gap disorder. The lactate level of 20.3mmol/l is extremely high and this confirms the diagnosis of a severe lactic acidosis. Hb is very low consistent with the history of bleeding and hypovolemia. Urea and creatinine are elevated (renal failure) but at these levels there would not be retention of anions sufficient to result in a renal acidosis. Hence,
Lactic acidosis can be suspected. The respiratory efforts may be due to the distress or as a consequence of a metabolic acidosis (ie compensatory).
A 56- year -old man who smoked heavily for many years developed worsening cough with purulent sputum and was admitted to the hospital because of difficulty in breathing. He was drowsy and cyanosed. His arterial blood gas analysis was as follows;
What is the likely diagnosis?
The patient is suffering from Respiratory acidosis. Difficulty in breathing, cough and purulent sputum signify the underlying lung pathology. Low p H and raised pCO2 indicate respiratory acidosis. Slightly high HCO3- may be due to compensation as a result of increased reabsorption from the kidney. The low pO2 is due to associated hypoxia. The treatment is based on the treating the primary cause.O2 and mechanical ventilation are often needed.
A 5-year old girl displayed increased appetite, increased urinary frequency, and thirst. Her physician suspected new onset diabetes mellitus and confirmed that she had elevated urine glucose and ketones.
Blood gas analysis revealed
The patient is suffering from Diabetic ketoacidosis
In the presence of insulin deficiency, a shift to fatty acid oxidation produces the ketones that cause metabolic acidosis. The pH and bicarbonate are low, and there is frequently some respiratory compensation (hyperventilation with deep breaths) to lower the PCO2. A low pH with high PCO2 would have represented respiratory acidosis which is not there in the given case.
A 19-year-old boy was brought to the emergency department with loss of consciousness. Apparently the patient was a homeless found on the street.
Arterial blood gases revealed-
The blood level of methanol was 0.4 mg/dl.
The patient is suffering from metabolic acidosis as evident from the low p H and low bicarbonate levels. Low p CO2 and high p O2 signify that the patient is in a state of respiratory compensation. Blood methanol level is high, so it might be the case of Methanol poisoning producing metabolic acidosis.
A 66-year-old man had a postoperative cardiac arrest. Past history of hypertension treated with an ACE inhibitor was there. There was no past history of Ischemic heart disease. Following reversal and extubation, myocardial ischemia was noticed on ECG. He was transferred to ICU for overnight monitoring. On arrival in ICU, BP was 90/50, pulse 80/min, respiratory rate was 16/min and S pO2 99%. During handover to ICU staff, he developed ventricular fibrillation which reverted to sinus rhythm with a single 200J counter shock. Soon after, blood gases were obtained from a radial arterial puncture:
Arterial Blood Gases
Biochemistry Results (all in mmol/l): Na+ 138, K+ 4.7, Cl– 103, urea 6.4, creatinine 0.07
What is the probable diagnosis?
1) p H– low , Acidosis is present.
2) p CO2- high, hypoventilation(The residual depressant effect of the Anesthetic agents is considered the most likely cause)
3) Bicarbonate– near normal
4) pO 2- high- This is because the patient is breathing a high inspired oxygen concentration. If the patient had been breathing room air (FIO2 = 0.21), then a depression of alveolar pO2 must have occurred. Most ill patients in hospital breathe supplemental oxygen so it is common for the pO2 to be elevated on blood gas results.
5) An acidemia with the pattern of elevated pCO2 and normal HCO3 is consistent with an acute respiratory acidosis.
6) Anion gap– The anion gap is about 11 which is normal so no evidence of a high anion gap acidosis.
Diagnosis- Acute respiratory acidosis
Cause- Resuscitation from postoperative ventricular fibrillation
A 72-year-old male with diabetes mellitus is evaluated in the emergency room because of lethargy, disorientation, and long, deep breaths (Kussmaul respiration). Initial chemistries on venous blood demonstrate high glucose level of 380 mg/dl (normal up to 120 mg/dl) and pH of 7.3. Bicarbonate 15mM and PCO2 30 mmHg, What is the probable diagnosis ?
The man is acidotic as defined by pH lower than normal 7.4. His hyperventilation with Kussmaul respiration can be interpreted as compensation by lungs to blow off CO2 to lower PCO2, to increase [HCO3-]/[CO2] ratio, and to raise pH. Thus the patient has metabolic acidosis due to underlying Diabetic ketoacidosis.
A 24 year female with broken ankle was brought to emergency with acute pain.
Blood gas analysis revealed the following
What is the probable diagnosis?
pH:- 7.55 – indicates Alkalosis
PCO2: 27 -low, it is a Primary respiratory disturbance
PCO2 Deficit = 40-27 = 13
It is Respiratory alkalosis due to pain related hyperventilation.