ABGs, or arterial blood gases, are a set of blood test results that tell nurses and other clinicians how well a patient’s lungs are working and whether the blood’s acid-base balance is normal. The test measures five key values from a sample of arterial blood: pH, partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2), bicarbonate (HCO3), and oxygen saturation (SaO2). For nursing students and practicing nurses, understanding ABGs is essential because these results guide critical decisions about ventilation, oxygenation, and fluid management.
The Five ABG Values and What They Mean
Each component of an ABG result tells you something different about what’s happening in a patient’s body.
pH reflects the overall acid-base balance of the blood. The normal range is 7.35 to 7.45. A pH below 7.35 means the blood is too acidic (acidosis), and a pH above 7.45 means it’s too alkaline (alkalosis). Even small shifts outside this range can affect organ function and become life-threatening if left uncorrected.
PaCO2 measures carbon dioxide dissolved in the blood, with a normal range of 35 to 45 mmHg. Carbon dioxide is an acidic waste product your body creates constantly. The lungs control it: when you breathe faster, you blow off more CO2 and the blood becomes less acidic. When breathing slows or becomes shallow, CO2 builds up and the blood turns more acidic. This makes PaCO2 the respiratory component of the ABG.
HCO3 (bicarbonate) is the metabolic component, with a normal range of 22 to 26 mEq/L. The kidneys regulate bicarbonate levels by either retaining or excreting it. Because bicarbonate acts as a buffer against acid, a drop in HCO3 signals metabolic acidosis, while a rise points to metabolic alkalosis.
PaO2 measures the pressure of oxygen dissolved in the blood, showing how effectively the lungs transfer oxygen from inhaled air into the bloodstream. SaO2 (oxygen saturation) shows the percentage of red blood cells currently carrying oxygen. Together, these two values paint a picture of how well a patient is being oxygenated.
How to Interpret ABG Results Step by Step
Nurses commonly use a systematic approach to read ABGs. The process works in three steps: check the pH, identify the primary cause, then look for compensation.
Start with the pH. If it’s below 7.35, the patient is in acidosis. If it’s above 7.45, alkalosis. Next, determine whether the problem is respiratory or metabolic. Look at PaCO2 first. If it’s abnormal and moving in the direction that would explain the pH change, the problem is respiratory. If PaCO2 is normal but HCO3 is abnormal, the problem is metabolic. For example, a pH of 7.30 with a PaCO2 of 55 mmHg points to respiratory acidosis, because the elevated CO2 is making the blood acidic.
The third step is checking whether the body is compensating. The lungs and kidneys work as a team. When one system causes an imbalance, the other tries to correct it. In acute respiratory acidosis, bicarbonate will still be in the normal range because the kidneys haven’t had time to respond. In chronic respiratory acidosis, you’ll see an elevated bicarbonate level, meaning the kidneys have started retaining bicarbonate to buffer the extra acid. If the pH has returned to normal range despite abnormal PaCO2 and HCO3 values, compensation is complete. If the pH is still outside normal range but the opposing system is clearly responding, compensation is partial.
The Four Acid-Base Imbalances
Respiratory Acidosis
This happens when breathing becomes too slow or too shallow, allowing CO2 to accumulate. The ABG shows a pH below 7.35 and a PaCO2 above 45 mmHg. Common causes include COPD, opioid use or overdose, sedation, neuromuscular diseases like Guillain-Barré syndrome, and conditions that weaken the muscles of breathing. Nursing priorities focus on improving ventilation. That might mean repositioning the patient, administering prescribed bronchodilators, or preparing for mechanical ventilation if the patient is becoming lethargic or confused. For opioid-related cases, reversal agents can quickly restore normal breathing. One important caution: in chronic respiratory acidosis, correcting CO2 levels too rapidly can cause seizures, so gradual correction is the goal.
Metabolic Acidosis
Here the pH drops below 7.35 because of a bicarbonate deficit (HCO3 below 22 mEq/L) rather than a CO2 problem. The most common triggers are uncontrolled diabetes (where the body produces acidic ketone bodies for energy), severe diarrhea (which flushes out bicarbonate), lactic acid buildup from poor tissue oxygenation, and kidney failure. Patients often present with rapid, deep breathing as the lungs try to compensate by blowing off CO2. Other signs include a fast heart rate, confusion, nausea, fatigue, and sometimes a sweet or fruity smell on the breath (particularly with diabetic ketoacidosis). Nursing care revolves around treating the underlying cause, closely monitoring blood sugar when diabetes is involved, and watching for worsening mental status.
Respiratory Alkalosis
The opposite of respiratory acidosis: the patient is breathing too fast or too deeply, blowing off excess CO2. The ABG shows a pH above 7.45 and a PaCO2 below 35 mmHg. Anxiety, pain, and fear are among the most common triggers, but respiratory alkalosis also occurs with head injuries, fever, pneumonia, pulmonary embolism, asthma exacerbations, and even high altitude. In mechanically ventilated patients, the ventilator settings themselves can cause it. Nursing interventions center on identifying and addressing whatever is driving the hyperventilation, whether that’s managing pain, reducing anxiety, or adjusting ventilator settings.
Metabolic Alkalosis
This occurs when bicarbonate levels climb above 26 mEq/L, pushing the pH above 7.45. The two classic causes in a hospital setting are prolonged vomiting (or gastric suctioning, which removes stomach acid) and overuse of diuretics, which cause the kidneys to excrete too much acid. Treatment typically involves replacing fluids and electrolytes, particularly potassium, magnesium, and chloride. If a diuretic is the culprit, the dose may need to be reduced.
How an ABG Sample Is Collected
Unlike routine blood draws that use a vein, ABGs require arterial blood, most commonly from the radial artery at the wrist. The procedure is more uncomfortable than a standard blood draw because arteries sit deeper and have more nerve endings around them.
Before puncturing the radial artery, a modified Allen test is performed to confirm that the hand has adequate backup blood supply through the ulnar artery. The patient makes a fist (or the nurse closes the hand tightly), and the nurse compresses both arteries at the wrist. When the patient opens the hand, the palm should appear pale. The nurse then releases pressure only on the ulnar artery. If color returns to the hand within 5 to 15 seconds, the test is positive, meaning it’s safe to proceed. If the hand stays pale, the ulnar artery isn’t supplying enough blood, and a different site should be used.
During the draw, the needle is inserted at about a 45-degree angle. The syringe is pre-treated with a small amount of heparin to prevent clotting. Once enough blood is collected, the nurse removes the needle and applies firm pressure to the site for at least five minutes, and longer if the patient is on blood thinners or has high blood pressure. The sample must be placed on ice and transported to the lab quickly. Any air bubbles in the syringe need to be expelled, because air exposure alters the oxygen and CO2 readings. It’s also important that the patient is breathing calmly during the draw, since clenching, breath-holding, or crying can change the results.
Complications of Arterial Puncture
Serious complications are rare when proper technique is followed. The most common issue is bruising or soreness at the puncture site. Less common risks include a hematoma (a pocket of blood under the skin), excessive bleeding, the need for multiple puncture attempts to locate the artery, and, very rarely, infection. Patients are generally advised to avoid lifting heavy objects for 24 hours after the draw.
Why ABGs Matter in Nursing Practice
ABGs give nurses real-time information that pulse oximetry and basic lab work can’t provide. A pulse oximeter tells you oxygen saturation, but it says nothing about CO2 levels or acid-base status. A patient can have a normal oxygen saturation and still be in serious trouble from CO2 retention or metabolic acidosis. ABG results help nurses recognize deterioration early, especially in patients on mechanical ventilation, those with chronic lung disease, anyone receiving opioids or heavy sedation, and critically ill patients whose organ function is shifting rapidly.
Being able to read and act on ABG results is one of the skills that separates routine patient monitoring from the kind of clinical thinking that catches problems before they escalate. For nursing students, practicing the three-step interpretation method on sample ABGs builds the pattern recognition that eventually becomes second nature in clinical settings.