Nursing Pocket Cards


Lippincott Nursing Pocket Card

Arterial Blood Gas (ABG) Analysis

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What is an Arterial Blood Gas?

The arterial blood gas (ABG) measures the acid-base balance (pH) and oxygenation of an arterial blood sample. An ABG can be used to assess respiratory compromise, status peri- or post-cardiopulmonary arrest, and medical conditions that cause metabolic abnormalities (such as sepsis, diabetic ketoacidosis, renal failure, toxic substance ingestion, drug overdose, trauma or burns). An ABG can also be used to evaluate the effectiveness of oxygen therapy, ventilatory support, fluid and electrolyte replacement, and during perioperative care.

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Normal Values and Clinical Significance

VALUE NORMAL RANGE CLINICAL SIGNIFICANCE
pH 7.35-7.45 The pH tells you if your patient is acidotic or alkalotic. It is a measurement of the acid content or hydrogen ions [H+] in the blood. Low pH indicates a higher concentration of hydrogen ions (acidosis) while a high pH indicates a lower concentration of hydrogen ions (alkalosis).
PaCO2 35-45 mm Hg The PaCO2 level is the respiratory component of the ABG. It is a measurement of carbon dioxide (CO2) in the blood and is affected by CO2 removal in the lungs. A higher PaCO2 level indicates acidosis while a lower PaCO2 level indicates alkalosis. 
HCO3- 22-26 mEq/L The HCO3- level is the metabolic component of the ABG. It is a measurement of the bicarbonate content of the blood and is affected by renal production of bicarbonate. A lower HCO3- level indicates acidosis while a higher HCO3- level indicates alkalosis.
PaO2 80-100 mm Hg The PaO2 level is a measurement of the amount of oxygen dissolved in the blood. A PaO2 level less than 60% results in tissue hypoxia.
SaO2 95-100% SaO2, or oxygen saturation, refers to the number of hemoglobin binding sites that have oxygen attached to them. How easily oxygen attaches to hemoglobin can be affected by body temperature, pH, 2,3-diphosphoglycerate levels, and CO2 levels.

Six Steps for ABG Analysis

ABG ANALYSIS
STEPS CLINICAL SIGNIFICANCE
Step 1: Analyze the pH
pH < 7.35 = acidosis
pH > 7.45 = alkalosis
Determine if the pH is within the normal range, or reflects acidosis or alkalosis.
Step 2: Analyze the PaCO2
PaCO2 > 45 = acidosis
PaCO2 < 35 = alkalosis
Carbon dioxide is produced in the tissues of the body and eliminated in the lungs. Changes in the PaCO2 level reflect lung function. 
Step 3: Analyze the HCO3-
HCO3- < 22 = acidosis
HCO3- > 26 = alkalosis
Bicarbonate is produced by the kidneys. Changes in the HCO3- level reflect metabolic function of the kidneys.
Step 4: Match the PaCO2 or HCO3- with pH  

If pH < 7.35 and PaCO2 > 45 and HCO3- level is normal, the patient has respiratory acidosis.

Causes of respiratory acidosis include hypoventilation, respiratory infection, severe airflow obstruction as in COPD or asthma, neuromuscular disorders, massive pulmonary edema, pneumothorax, central nervous depression, spinal cord injury, and chest wall injury.

If pH < 7.35 and HCO3-  < 22 and PaCO2 level is normal, the patient has metabolic acidosis.

Causes of metabolic acidosis include renal failure, DKA, lactic acidosis, sepsis, shock, diarrhea, drugs, and toxins such as ethylene glycol and methanol.

If pH > 7.45 and PaCO2 < 35 and the HCO3- level is normal, the patient has respiratory alkalosis.

Causes of respiratory alkalosis include hyperventilation, pain, anxiety, early stages of pneumonia or pulmonary embolism, hypoxia, brainstem injury, severe anemia, and excessive mechanical ventilation.

If pH is > 7.45 and HCO3- > 26 and the PaCO2 level is normal, the patient has metabolic alkalosis.

Causes of metabolic alkalosis include diuretics, corticosteroids, excessive vomiting, dehydration, Cushing syndrome, liver failure, and hypokalemia.

Step 5: Assess for compensation by determining whether the PaCO2 or the HCO3- go in the opposite direction of the pH.
 
When a patient has an acid-base imbalance, the respiratory and metabolic systems try to correct the imbalances the other system has produced. 
If pH 7.35-7.40, PaCO2 > 45, and HCO3- > 26, the patient has compensated respiratory acidosis. To compensate for respiratory acidosis, the kidneys excrete more hydrogen ions and elevate serum HCO3-, in an effort to normalize the pH.
If pH 7.35-7.40, PaCO2 <35, and
HCO3- <22, the patient has compensated metabolic acidosis.
 
To compensate for metabolic acidosis, the patient's respiratory center is stimulated and the patient hyperventilates to blow off more CO2, raising the pH. 
If pH 7.40-7.45, PaCO2 <35, and
HCO3- < 22, the patient has compensated respiratory alkalosis.
To compensate for respiratory alkalosis, the metabolic system is activated to retain hydrogen ions and lower serum HCO3-, in an effort to raise the pH.
If pH 7.40-7.45, PaCO2 > 45, and
HCO3- > 26, the patient has compensated metabolic alkalosis.
To compensate for metabolic alkalosis, the patient’s respiratory center is suppressed; decreased rate and depth of respiration causes CO2 to be retained, lowering the pH.
Step 6: Analyze the PaO2 and SaO2              
If PaO2 < 80 mm Hg or SaO2 < 95%, the patient has hypoxemia.
 
Causes of hypoxemia include COPD, ARDS, certain medications, high altitudes, interstitial lung disease, pneumothorax, pulmonary embolism, pulmonary edema, pulmonary fibrosis, anemia, heart disease, and sleep apnea.

References:

Lian, J. (2013). Using ABGs to optimize mechanical ventilation. Nursing2013, 43(6), 46-52.

Lian, J. (2010). Interpreting and using the arterial blood gas. Nursing2010 Critical Care, 5(3), 26-36.

Woodruff, D. (2007). Six Steps to ABG Analysis. Nursing2007 Critical Care, 2(2), 48-52. 
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