Arterial Blood Gas (ABGs) Analysis Ultimate Guide

Reading arterial blood gases is a skill you will use constantly on the floor, and most students find it intimidating the first few times. This guide breaks down what each ABG component tells you and walks you through interpreting results with the tic-tac-toe method.

What is arterial blood gas?

An arterial blood gas is a laboratory test that monitors a patient's acid-base balance. It shows how well the buffer system is compensating and measures the acidity (pH) and the levels of oxygen and carbon dioxide in arterial blood. Unlike samples drawn from a vein, an ABG sample is taken from an artery, commonly the radial or brachial artery.

Components of an ABG

There are six components of an arterial blood gas.

pH is the concentration of hydrogen ions and determines whether body fluids are acidic or alkaline. A pH below 7.35 indicates acidosis, and a pH above 7.45 indicates alkalosis. The normal range is 7.35 to 7.45.

PaCO2 (partial pressure of carbon dioxide) shows how well gas is exchanged between the alveoli and the outside air. When CO2 cannot escape because of alveolar damage, the excess combines with water to form carbonic acid and produces an acidotic state. Alveolar hypoventilation (as in COPD) raises PaCO2 and causes respiratory acidosis; alveolar hyperventilation lowers it and causes respiratory alkalosis. The normal range is 35 to 45 mmHg. This is the respiratory determinant.

PaO2 (partial pressure of oxygen) indicates how much oxygen is available to bind with hemoglobin. A low pH means less oxygen binds. The normal range is 75 to 100 mmHg.

SO2 (oxygen saturation) is the percentage of oxygen in the blood bound to hemoglobin. It can be calculated indirectly from PaO2 and pH, or measured directly by co-oximetry. The normal range is 94 to 100 percent.

HCO3 (bicarbonate) is an alkaline substance that makes up over half of the total buffer base in the blood. A deficit indicates metabolic acidosis; an excess indicates metabolic alkalosis.

BE (base excess) is checked alongside HCO3. A base excess below -2 is acidosis and above +2 is alkalosis. The normal range is -2 to +2 mmol/L.

Normal Values

To spot an acid-base imbalance, memorize these values so you can recognize what deviates from normal. Disorders are usually identified from blood pH. If the blood is basic, look at HCO3, since the kidneys regulate bicarbonate. If the blood is acidic, look at PaCO2, since the lungs regulate most of the acid.

Component	Normal Range
pH	7.35 to 7.45
PaCO2	35 to 45 mmHg (respiratory determinant)
PaO2	75 to 100 mmHg
HCO3	22 to 26 mEq/L (metabolic determinant)
Oxygen saturation (SO2)	94 to 100%
Base excess (BE)	-2 to +2 mmol/L

Interpreting ABG Imbalances

ABG interpretation detects respiratory or metabolic acidosis or alkalosis during acute illness. The easiest way to work through it is the tic-tac-toe method.

Goals of ABG analysis

There are three goals to accomplish with every ABG:

1. Determine whether the values indicate acidosis or alkalosis.
2. Determine whether the disorder is metabolic or respiratory.
3. Determine the compensation: fully compensated, partially compensated, or uncompensated.

Keep these three goals in mind, since the steps below build toward them.

Steps in ABG analysis using the tic-tac-toe method

There are eight steps to interpreting an ABG with the tic-tac-toe technique.

1. Memorize the normal values. Familiarize yourself with the normal ranges so abnormal results stand out: pH 7.35 to 7.45, PaCO2 35 to 45, HCO3 22 to 26. Note that PaCO2 is intentionally inverted for this method (a high value leans acidosis, a low value leans alkalosis), the opposite direction from pH and HCO3.

2. Create your tic-tac-toe grid. Draw a grid and label the top row ACIDOSIS, NORMAL, and ALKALOSIS. You will place pH, PaCO2, and HCO3 into one of those three columns based on their values.

3. Place pH under NORMAL, ACIDOSIS, or ALKALOSIS. The normal pH range is 7.35 to 7.45.

• A pH between 7.35 and 7.39 is normal but slightly acidic; place it under NORMAL.
• A pH between 7.41 and 7.45 is normal but slightly alkaline; place it under NORMAL.
• Any pH below 7.35 is acidosis; place it under ACIDOSIS.
• Any pH above 7.45 is alkalosis; place it under ALKALOSIS.

4. Place PaCO2 under NORMAL, ACIDOSIS, or ALKALOSIS. The normal range is 35 to 45.

• If PaCO2 is below 35, place it under ALKALOSIS.
• If PaCO2 is above 45, place it under ACIDOSIS.
• If PaCO2 is within range, place it under NORMAL.

5. Place HCO3 under NORMAL, ACIDOSIS, or ALKALOSIS. The normal range is 22 to 26.

• If HCO3 is below 22, place it under ACIDOSIS.
• If HCO3 is above 26, place it under ALKALOSIS.
• If HCO3 is within range, place it under NORMAL.

6. Solve goal #1: acidosis or alkalosis. Look at which column the pH landed in.

• If pH is under ACIDOSIS, it is acidosis.
• If pH is under ALKALOSIS, it is alkalosis.
• If pH is under NORMAL, decide whether it leans acidic or alkaline and interpret accordingly.

7. Solve goal #2: metabolic or respiratory. Check whether pH shares a column with PaCO2 or with HCO3.

• If pH is in the same column as PaCO2, it is respiratory.
• If pH is in the same column as HCO3, it is metabolic.
• If pH is under NORMAL, use the direction it leans to decide.

8. Solve goal #3: compensation.

• Fully compensated if pH is normal.
• Partially compensated if all three values are abnormal.
• Uncompensated if either PaCO2 or HCO3 is normal while the other is abnormal.

Application and examples

Practice Problem #1: pH = 7.26, PaCO2 = 32, HCO3 = 18. The pH of 7.26 is abnormal and acidic, so pH goes under ACIDOSIS. PaCO2 of 32 is abnormal and alkaline, so it goes under ALKALOSIS. HCO3 of 18 is abnormal and acidic, so it goes under ACIDOSIS. pH is under ACIDOSIS (goal #1: acidosis). pH shares a column with HCO3 (goal #2: metabolic). All three values are abnormal (goal #3: partially compensated). Answer: metabolic acidosis, partially compensated.

Practice Problem #2: pH = 7.44, PaCO2 = 30, HCO3 = 21. The pH of 7.44 is normal but leans alkaline, so place it under NORMAL pointing toward ALKALOSIS. PaCO2 of 30 is abnormal and alkaline (ALKALOSIS). HCO3 of 21 is abnormal and acidic (ACIDOSIS). pH leans alkaline (goal #1: alkalosis). pH leans toward the same side as PaCO2 (goal #2: respiratory). pH is normal (goal #3: fully compensated). Answer: respiratory alkalosis, fully compensated.

Practice Problem #3: pH = 7.1, PaCO2 = 40, HCO3 = 18. The pH of 7.1 is abnormal and acidic (ACIDOSIS). PaCO2 of 40 is normal (NORMAL). HCO3 of 18 is abnormal and acidic (ACIDOSIS). pH is acidosis (goal #1: acidosis). pH shares a column with HCO3 (goal #2: metabolic). pH and HCO3 are abnormal but PaCO2 is normal (goal #3: uncompensated). Answer: metabolic acidosis, uncompensated.

How to draw an ABG

Arterial blood is usually drawn from the radial or brachial artery.

• Tell the client about the procedure. There is no food or fluid restriction.
• Note whether the client takes anticoagulants or aspirin, since these affect results.
• Note whether the client is on oxygen therapy (flow rate and device) and record the current temperature.
• Using a heparinized needle and syringe, collect 1 to 5 mL of arterial blood from the radial or brachial artery.
• Place the syringe in an ice-water bag to slow the metabolic activity of the sample.
• Deliver the sample to the laboratory immediately.
• Apply pressure to the puncture site for 5 minutes or longer.

Acid-Base Balance and Imbalances

Acid-base imbalances develop when normal homeostatic mechanisms are overwhelmed or fail. Acidosis is blood that is too acidic (low pH), and because the body produces two types of acid there are two types: respiratory and metabolic acidosis. Alkalosis is blood that is too basic (high pH), and likewise comes in respiratory and metabolic forms.

When an imbalance occurs, the lungs and kidneys activate compensatory mechanisms to normalize pH. The kidneys compensate for respiratory imbalances, and the respiratory system compensates for metabolic ones. Compensation does not fix the root cause; if the underlying condition is not treated, these systems eventually fail.

Respiratory Acidosis

Respiratory acidosis occurs when breathing is inadequate (alveolar hypoventilation) and the lungs cannot excrete enough CO2, so respiratory acid builds up. The extra CO2 combines with water to form carbonic acid, a common occurrence in emphysema. The kidneys compensate slowly, often over 24 hours or more, by excreting more metabolic acid in the urine, which raises blood bicarbonate.

There are two forms. Acute respiratory acidosis comes on immediately and, left untreated, worsens progressively; it is a medical emergency. Chronic respiratory acidosis develops over time and often causes no symptoms because the body adapts, with the kidneys producing more bicarbonate to maintain balance. A new illness can tip chronic respiratory acidosis into acute.

Risk factors include hypoventilation (brain trauma, coma, hypothyroidism with myxedema), COPD, respiratory conditions such as pneumothorax, pneumonia, and status asthmaticus, and drug intake, particularly opioid overdose (morphine, tramadol, heroin, fentanyl) or magnesium sulfate.

Signs and symptoms include altered level of consciousness from encephalopathy or cerebral edema, confusion, stupor, drowsiness, and muscle jerks, disorientation and headache, coma in severe cases, tremors, and asterixis (the inability to hold a body part in position).

Management:

• Treat the underlying condition. Bronchodilators and corticosteroids reverse some airway obstruction, as in asthma and COPD. For obesity hypoventilation syndrome, significant weight loss may be needed to relieve compression of the lungs.
• Provide mechanical ventilation and supplemental oxygen to correct the low blood oxygen level.
• Manage hyperkalemia with Kayexalate. Acidosis drives potassium out of cells into plasma in exchange for hydrogen ions; Kayexalate binds potassium in the gut and increases fecal excretion.
• Maintain hydration with IV fluids and electrolytes as ordered.

Respiratory Alkalosis

Respiratory alkalosis results from hyperventilation, when the lungs excrete too much carbonic acid and pH rises. Because it happens quickly, the kidneys have no time to compensate. Neurological symptoms such as confusion, paresthesias, and increased cell membrane excitability appear when blood, CSF, and intracellular pH rise acutely.

Causes of the hyperventilation include panic and anxiety (the most common), fever, brainstem damage causing central neurogenic hyperventilation, metabolic acidosis, diabetic ketoacidosis, pregnancy (progesterone stimulates the respiratory center), and salicylate toxicity.

Signs and symptoms include numbness, a tingling sensation, palpitations, tetany, convulsions, and signs of hypokalemia and hypocalcemia (paresthesia, hyperreflexia, muscle spasm and twitching, a positive Chvostek's sign, and cardiac arrhythmias).

Management depends on the cause:

• For over-breathing from panic or anxiety, have the patient breathe into a paper bag to raise blood CO2, and use relaxation and diaphragmatic breathing exercises. Stay with the patient.
• Treat the underlying condition. An opioid pain reliever or anti-anxiety medication can reduce hyperventilation.
• Provide oxygen as indicated.
• After a massive aspirin ingestion, aggressive gut decontamination including gastric lavage is advisable.
• Correct hypokalemia and hypocalcemia.

Metabolic Acidosis

Metabolic acidosis occurs when bicarbonate falls and lactic acid builds up, as in diarrhea, ketosis, and kidney disorders. It has three main causes: increased acid production, loss of bicarbonate, and reduced renal excretion of acid.

Risk factors include diabetic ketoacidosis (acidic ketone bodies build up, mostly in type 1 diabetes), chronic renal failure, chronic hypoxia, obesity with insulin resistance, diarrhea (loss of bicarbonate stores), dehydration from prolonged vomiting, aspirin toxicity, and methanol poisoning.

Signs and symptoms include altered level of consciousness, confusion, disorientation, loss of appetite, coma, and jaundice.

Management:

• Sodium bicarbonate for severe metabolic acidosis (severe renal disease, uncontrolled diabetes, shock, cardiac arrest, severe lactic acidosis).
• Treat the underlying condition.
• Rehydration for diabetic ketoacidosis, which is the first priority.
• Dialysis for chronic renal failure, supplying bicarbonate during sessions.
• Diuretics and safety measures as appropriate.
• Kayexalate to manage the potassium shift that accompanies acidosis.

Metabolic Alkalosis

Metabolic alkalosis occurs when bicarbonate rises and blood pH climbs, as in excessive vomiting, dehydration, or endocrine disorders.

Risk factors include vomiting and nasogastric suction (both lose hydrochloric acid from gastric secretions, gaining a bicarbonate ion for every hydrogen ion lost), sodium bicarbonate overdose beyond the kidneys' capacity to excrete it, and hypokalemia (potassium shifts out of cells and hydrogen shifts in, raising pH).

Metabolic alkalosis may be silent, with complaints reflecting the underlying cause. When present, signs include numbness, vomiting, diarrhea, peripheral edema, fatigue, tingling, agitation, disorientation, seizures, and coma.

Management:

• Antiemetics in the case of vomiting.
• Ammonium chloride, a urinary acidifying agent converted to ammonia and hydrochloric acid by the liver, given IV for severe cases.
• Acetazolamide (Diamox), which is safe and effective for metabolic alkalosis following treatment of respiratory acidosis in COPD exacerbations.