A crucial tool that helps you recognize life-threatening conditions, arterial blood gas (ABG) analysis reveals your patient’s acid-base status and yields important information about pulmonary gas exchange and ventilatory control. Managing ABGs is an integral part of patient management, and every nurse must know how to interpret ABG results correctly.

The body’s balancing act
To function normally, the body must maintain acid-base balance. It achieves this balance through respiratory and metabolic mechanisms: Less than 3 minutes after chemoreceptors detect an abnormal partial pressure of arterial carbon dioxide (PaCO₂), respiratory mechanisms cause the lungs to either decrease or increase respiratory rate and depth. A rise or fall in pH triggers a metabolic response in which the kidneys restore a normal bicarbonate (HCO₃-) level—either by excreting HCO₃– as pH rises or retaining HCO₃– as pH declines. This response may take several hours to compensate for an imbalance.

Acid-base disorders can result from respiratory or metabolic imbalances. In an acidotic state (acidosis), blood pH is below 7.35. In an alkalotic state (alkalosis), pH exceeds 7.45. A pH below 6.8 or above 7.8 may lead to death unless corrected immediately.

Acidosis or alkalosis may be respiratory or metabolic:

• In respiratory acidosis, the respiratory system can’t eliminate enough CO₂ to maintain normal pH. CO₂ builds up in the bloodstream and blood pH drops.
• In metabolic acidosis, HCO₃– is lost from the extracellular fluid, metabolic acids accumulate, or both mechanisms occur. The plasma either gains acids or loses bases.
• In respiratory alkalosis, respiratory rate or depth increases, causing the lungs to eliminate CO₂, which in turn lowers PaCO₂ and raises pH.
• In metabolic alkalosis, hydro­gen ions   are lost, HCO₃– increases, or both mechanisms occur. Blood pH rises above 7.45.

Test your knowledge
Read the case studies below and answer the questions that follow to find out if you’re an ace at interpreting ABG values and managing abnormalities.

Sharon Lacosta, a 28-year-old woman weighing 110 lb (50 kg), suffered a left acetabular fracture, multiple rib fractures, and a small left frontal contusion during a motor vehicle accident. Earlier in the day, she underwent left acetabular repair.

She is now alert and oriented, but complains of extreme pain in her left hip. Her patient-controlled analgesia (PCA) pump is delivering a continuous infusion of morphine 3 mg/hour and a demand dose of morphine 2 mg every 5 minutes with no lockout. The physician also has ordered morphine 5 mg I.V. every 30 minutes as needed for breakthrough pain.

When Ms. Lacosta reports her pain as a 9 on a scale of 1 to 10, the nurse administers morphine 5 mg I.V. However, the patient continues to complain of extreme pain, so the nurse repeats a 5-mg I.V. dose of morphine 30 minutes later.

One hour later, Ms. Lacosta is difficult to arouse; her respirations are 6 to 8 breaths/minute and shallow. The physician orders ABG analysis, which reveals:

• pH: 7.23
• PaCO₂: 74 mm Hg
• PaO₂: 65 mm Hg
• HCO₃-: 24 mEq/L

1. Ms. Lacosta’s ABG values suggest which acid-base abnormality?

a. Metabolic acidosis
b.  Respiratory acidosis
c.  Metabolic alkalosis
d.  Respiratory alkalosis

Correct answer: b. Ms. Lacosta’s decreased pH, increased PaCO₂, and normal HCO₃– value indicate respiratory acidosis. A pH of 7.23 and PaO₂ of 65 mm Hg reflect an acute hypoventilatory respiratory disorder, which can arise abruptly and lead to respiratory arrest unless corrected. In light of her other ABG values, the normal HCO₃– value indicates that no metabolic compensation is occurring.

2. Based on the patient’s ABG values, which intervention should you implement?

a.  Call a code, because her ABG values indicate a life-threatening condition.
b.  Continue to monitor the patient, because her ABG values are within normal limits.
c.  Give medication as ordered to counteract her acid-base abnormality.
d.  Give naloxone hydrochloride 0.2 mg I.V. as ordered to reverse morphine’s hypoventilatory effects.

Correct answer: d. The patient needs naloxone
0.2 mg I.V. to reverse respiratory depression and the hypoventilatory effects of morphine. Other appropriate interventions include giving oxygen via face mask to correct low PaO₂ and, if necessary, providing airway support using a bag-valve mask. Increasing the patient’s respiratory depth and rate will boost CO₂ elimination and correct respiratory acidosis. Also, the PCA settings should be reevaluated to reduce the risk of morphine overdose.

Marc Jorgensen, age 36, is recovering from repair of a perforated duodenal ulcer. Two hours after surgery, nursing assessment reveals a respiratory rate of 30 breaths/minute; a pulse of 116 beats/minute; narrowed pulse pressure of 115/88 mm Hg; anxiety; and cool, clammy skin. The physician orders ABG analysis. Here are the results:

• pH: 7.49
• PaCO₂: 30 mm Hg
• PaO₂: 58 mm Hg
• HCO₃-: 26 mEq/L

1. What’s the correct interpretation of Mr. Jorgensen’s ABG values?

a.  Metabolic acidosis
b.  Respiratory acidosis
c.  Metabolic alkalosis
d.  Respiratory alkalosis

Correct answer: d. Increased pH, decreased PaCO₂, and normal HCO₃– indicate respiratory alkalosis, which may be associated with infection and peritonitis caused by the perforated duodenal ulcer.

2. Which interventions would you expect to implement based on these ABG values?

a.  Continue to monitor the patient; give medication to reduce anxiety.
b.  Obtain blood, urine, and sputum cultures and start broad-spectrum antibiotics to address suspected early sepsis.
c.  Send the patient for a ventilation-perfusion scan as ordered to rule out pulmonary embolism.
d.  Call a code because the ABG values indicate a life-threatening condition.

Correct answer: b. Besides collecting blood, urine, and sputum cultures and starting broad-spectrum antibiotics to treat infection and peritonitis, you should expect to give oxygen via nasal cannula or face mask and to obtain a complete blood count with differential. Mr. Jorgensen appears to be in the early stage of septic shock leading to oxygen hunger, increased respirations, and blowing off of CO₂. Although his anxiety might prompt you to give anxiolytics (if ordered), the healthcare team must first determine whether he has true agitation or oxygen hunger.

Aston Miller, age 27, is admitted with extreme thirst and an acetone breath odor. His Medic Alert bracelet states he has type 1 diabetes mellitus. He reports he has had a “stomach virus” for the last 3 days, with nausea and vomiting. The physician orders the following tests: blood glucose, serum potassium, anion gap, and ABG analysis. The laboratory report shows these results:

• blood glucose: 768 mg/dl
• serum potassium: 7.2 mEq/L
• anion gap: 18 mOsm/L
• pH: 6.91
• PaCO₂: 32 mm Hg
• PaO₂: 88 mm Hg
• HCO₃-: 7 mEq/L.

1. Which acid-base abnormality does Mr. Miller have?

a.  Metabolic acidosis
b.  Respiratory acidosis
c.  Metabolic alkalosis
d.  Respiratory alkalosis

Correct answer: a. The patient’s ABG values indicate metabolic acidosis, as noted by the low HCO₃– (metabolic component) and low pH. Like case study 2, this case underscores the importance of interpreting ABG values in conjunction with other test results. Based on Mr. Miller’s laboratory values, you should suspect he’s experiencing diabetic ketoacidosis (DKA), a life-threatening condition requiring immediate treatment. Low HCO₃– and the increased serum potassium level contribute to the markedly low pH. The elevated anion gap is another clue to metabolic acidosis: Normally, it’s below 12 mOsm/L; a gap above 15 mOsm/L reflects increased acid production caused by a metabolic process.

2. Based on the patient’s laboratory results, which interventions should you expect to implement?

a. Administer I.V. fluids and regular I.V. insulin bolus, followed by a continuous insulin drip; monitor blood glucose and serum potassium levels hourly.
b. Continue to monitor the patient; his assessment findings and laboratory values are normal.
c. Call a code, because his laboratory values indicate a life-threatening condition.
d. Administer two ampules of sodium bicarbonate I.V., kayexalate to decrease serum potassium, and subcutaneous insulin per sliding scale.

Correct answer: a. The patient needs I.V. fluid replacement with normal saline solution and a bolus of regular I.V. insulin followed by a continuous infusion, regulated by hourly monitoring of serum glucose and potassium levels. Close monitoring helps prevent hypoglycemia and hypokalemia—both of which can be life-threatening. Despite his blood pH of 6.91, you shouldn’t give sodium bicarbonate (to buffer acidosis) or kayexalate; as the patient is rehydrated and regular insulin causes serum glucose and potassium to move into cells, pH will return to a normal level and serum potassium level will decrease. Giving sodium bicarbonate and kayexalate would treat symptoms of DKA—not the underlying problem—and would lead to metabolic alkalosis and decreased serum potassium levels.

A tool for better patient care
Make sure you’re familiar with normal ABG values so you can quickly spot abnormal results. If your patient’s ABG values indicate an acid-base imbalance, always try to establish which system—respiratory or metabolic—is responsible. Identifying the problem early leads to improved patient care and can help reduce morbidity and mortality.

Selected references
Burns S. Indices of oxygenation. In: Wiegland D, Carlson K, eds. AACN Procedure Manual for Critical Care. 5th ed. St. Louis, Mo: Saunders; 2005:205-206.
Ellstrom K. The pulmonary system. In: Alspach J, ed. Core Curriculum for Critical Care Nursing. 6th ed. St. Louis, Mo: Saunders; 2006:45-69.
For a complete list of selected references, visit www.AmericanNurse Today.com.

Dawn Swiderski, RN, CCRN, is a Trauma Nurse Educator at Carolinas Medical Center in Charlotte, N.C. Diane Byrum, MSN, RN, CCRN, CCNS, FCCM, is a Critical Care Clinical Nurse Specialist at Presbyterian Hospital in Charlotte, N.C.