Arterial Blood Gas (ABG) Interpretation Guide
Introduction
Arterial blood gas (ABG) analysis is essential in assessing and managing patients with respiratory and metabolic disorders. It provides information on the acid-base balance, oxygenation status, and ventilation. Key components of an ABG report include:
- pH: Indicates the acidity or alkalinity of the blood (normal range: 7.35-7.45).
- PaCO2: Partial pressure of carbon dioxide (normal range: 4.7-6.0 kPa or 35-45 mmHg).
- HCO3: Bicarbonate concentration (normal range: 22-26 mmol/L).
- PaO2: Partial pressure of oxygen (normal range: 10-13 kPa or 75-100 mmHg).
- SaO2: Oxygen saturation (normal range: 94-100%).
- Base Excess (BE): Indicates the amount of excess or insufficient bicarbonate in the system (normal range: -2 to +2 mmol/L).
Steps to Interpret ABGs
- Assess the pH to determine if the patient is acidotic (<7.35) or alkalotic (>7.45).
- Evaluate the PaCO2 to determine if the respiratory system is contributing to the acid-base imbalance.
- Examine the HCO3 to see if the metabolic (renal) system is contributing to the acid-base imbalance.
- Look for compensation:
- If pH is normal but PaCO2 and HCO3 are abnormal, this suggests complete compensation.
- If pH is abnormal, look for partial compensation.
Hypoxaemia
Definition
Hypoxaemia refers to low levels of oxygen in the blood, typically defined as PaO2 < 10 kPa (75 mmHg).
Causes
- V/Q mismatch (e.g., pulmonary embolism)
- Shunt (e.g., congenital heart disease)
- Hypoventilation (e.g., CNS depression)
- Diffusion impairment (e.g., pulmonary fibrosis)
- Reduced FiO2 (e.g., high altitude)
ABG Findings
- PaO2: < 10 kPa (75 mmHg)
- Other parameters depend on the underlying cause
Type 1 vs Type 2 Respiratory Failure
Type 1 Respiratory Failure
Characterised by hypoxaemia with normal or low PaCO2.
Causes
- Pneumonia
- Pulmonary embolism
- Acute respiratory distress syndrome (ARDS)
- Asthma
ABG Findings
- PaO2: < 10 kPa (75 mmHg)
- PaCO2: Normal or low
Type 2 Respiratory Failure
Characterised by hypoxaemia and hypercapnia.
Causes
- COPD
- Severe asthma
- Drug overdose
- Neuromuscular disorders
ABG Findings
- PaO2: < 10 kPa (75 mmHg)
- PaCO2: > 6.0 kPa (45 mmHg)
Mixed Acidosis and Alkalosis
Definition
Occurs when there are simultaneous disorders causing both acidosis and alkalosis.
Causes
- Chronic renal failure with vomiting
- Cardiac arrest with vomiting
- Severe sepsis with vomiting
ABG Findings
- Variable pH
- PaCO2 and HCO3 may both be abnormal in different directions
Respiratory Acidosis
Causes
- COPD
- Respiratory muscle weakness (e.g., myasthenia gravis, Guillain-Barré syndrome)
- CNS depression (e.g., sedatives, opiates)
- Severe pneumonia or asthma
ABG Findings
- pH: < 7.35
- PaCO2: > 6.0 kPa (45 mmHg)
- HCO3: may be increased if compensation is occurring
- PaO2: variable
Respiratory Alkalosis
Causes
- Hyperventilation (e.g., anxiety, pain, hypoxia)
- Central nervous system disorders (e.g., stroke, infection)
- Severe anaemia
- Salicylate overdose
ABG Findings
Metabolic Acidosis
Causes
- Lactic acidosis (e.g., sepsis, shock)
- Ketoacidosis (e.g., diabetic ketoacidosis)
- Renal failure
- Diarrhoea (loss of bicarbonate)
ABG Findings
- pH: < 7.35
- HCO3: < 22 mmol/L
- PaCO2: may be decreased if compensation is occurring
- PaO2: variable
Metabolic Alkalosis
Causes
- Vomiting or nasogastric suction
- Diuretic use
- Excessive bicarbonate intake
- Hypokalaemia
ABG Findings
- pH: > 7.45
- HCO3: > 26 mmol/L
- PaCO2: may be increased if compensation is occurring
- PaO2: variable
Mixed Respiratory and Metabolic Acidosis
Causes
- Cardiac arrest
- Severe sepsis
- Multi-organ failure
ABG Findings
- pH: < 7.35
- PaCO2: > 6.0 kPa (45 mmHg)
- HCO3: < 22 mmol/L
- PaO2: variable
ABG Interpretation Quiz
Scenario 1
Question: A 28-year-old female with anxiety presents with the following ABG results:
- pH 7.50 (normal range: 7.35-7.45)
- PaCO2 3.2 kPa (normal range: 4.7-6.0 kPa)
- HCO3 22 mmol/L (normal range: 22-26 mmol/L)
- PaO2 14 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has respiratory alkalosis.
Explanation: The pH is elevated (7.50), indicating alkalosis. The PaCO2 is decreased (3.2 kPa), suggesting a respiratory origin. The HCO3 is within the normal range (22 mmol/L), indicating that there has not been enough time for metabolic compensation. The underlying cause is likely hyperventilation due to anxiety.
Scenario 2
Question: A 65-year-old male with COPD presents with the following ABG results:
- pH 7.35 (normal range: 7.35-7.45)
- PaCO2 7.0 kPa (normal range: 4.7-6.0 kPa)
- HCO3 28 mmol/L (normal range: 22-26 mmol/L)
- PaO2 9 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has type 2 respiratory failure.
Explanation: The pH is at the lower end of the normal range (7.35), the PaCO2 is elevated (7.0 kPa), and the HCO3 is elevated (28 mmol/L), indicating partial metabolic compensation. The PaO2 is decreased (9 kPa), indicating hypoxaemia. Type 2 respiratory failure is characterized by both hypoxaemia and hypercapnia and is often caused by conditions such as COPD, severe asthma, or neuromuscular disorders.
Scenario 3
Question: A 60-year-old female with chronic kidney disease and recent vomiting presents with the following ABG results:
- pH 7.38 (normal range: 7.35-7.45)
- PaCO2 5.2 kPa (normal range: 4.7-6.0 kPa)
- HCO3 20 mmol/L (normal range: 22-26 mmol/L)
- PaO2 12 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has mixed acidosis and alkalosis.
Explanation: The pH is normal (7.38), but the HCO3 is decreased (20 mmol/L), suggesting metabolic acidosis. The PaCO2 is slightly increased (5.2 kPa), indicating a respiratory component that could be compensatory. The normal pH with these mixed changes suggests both metabolic acidosis (likely from chronic kidney disease) and metabolic alkalosis (likely from vomiting).
Scenario 4
Question: A 55-year-old male with diabetic ketoacidosis presents with the following ABG results:
- pH 7.25 (normal range: 7.35-7.45)
- PaCO2 3.5 kPa (normal range: 4.7-6.0 kPa)
- HCO3 14 mmol/L (normal range: 22-26 mmol/L)
- PaO2 12 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has metabolic acidosis.
Explanation: The pH is decreased (7.25), indicating acidosis. The HCO3 is also decreased (14 mmol/L), suggesting a metabolic origin. The PaCO2 is decreased (3.5 kPa), indicating respiratory compensation. The underlying cause is likely diabetic ketoacidosis, given the patient's history of diabetes.
Scenario 5
Question: A 75-year-old male with a history of COPD presents with the following ABG results:
- pH 7.30 (normal range: 7.35-7.45)
- PaCO2 8.0 kPa (normal range: 4.7-6.0 kPa)
- HCO3 28 mmol/L (normal range: 22-26 mmol/L)
- PaO2 8 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has respiratory acidosis.
Explanation: The pH is decreased (7.30), indicating acidosis. The PaCO2 is elevated (8.0 kPa), suggesting a respiratory origin. The HCO3 is elevated (28 mmol/L), indicating partial metabolic compensation. The PaO2 is decreased (8 kPa), indicating hypoxaemia. The underlying cause is likely COPD, leading to inadequate ventilation and CO2 retention.
Scenario 6
Question: A 55-year-old male presents with shortness of breath. ABG results:
- pH 7.40 (normal range: 7.35-7.45)
- PaCO2 5.0 kPa (normal range: 4.7-6.0 kPa)
- HCO3 24 mmol/L (normal range: 22-26 mmol/L)
- PaO2 8 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has hypoxaemia.
Explanation: The pH, PaCO2, and HCO3 are within normal ranges, indicating a balanced acid-base status. However, the PaO2 is decreased (8 kPa), indicating hypoxaemia. The underlying cause could be a variety of conditions affecting oxygenation, such as pneumonia, pulmonary embolism, or heart failure.
Scenario 7
Question: A 70-year-old female with a history of heart failure presents with the following ABG results:
- pH 7.45 (normal range: 7.35-7.45)
- PaCO2 4.6 kPa (normal range: 4.7-6.0 kPa)
- HCO3 24 mmol/L (normal range: 22-26 mmol/L)
- PaO2 7.5 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has type 1 respiratory failure.
Explanation: The pH, PaCO2, and HCO3 are within normal ranges, indicating a balanced acid-base status. However, the PaO2 is decreased (7.5 kPa), indicating hypoxaemia. Type 1 respiratory failure is characterized by hypoxaemia without hypercapnia and is often caused by conditions such as heart failure, pneumonia, or pulmonary embolism.
Scenario 8
Question: A 70-year-old male with sepsis and chronic renal failure presents with the following ABG results:
- pH 7.10 (normal range: 7.35-7.45)
- PaCO2 6.5 kPa (normal range: 4.7-6.0 kPa)
- HCO3 12 mmol/L (normal range: 22-26 mmol/L)
- PaO2 10 kPa (normal range: 10-13 kPa)
What is the diagnosis and underlying cause?
Answer: The patient has mixed respiratory and metabolic acidosis.
Explanation: The pH is significantly decreased (7.10), indicating severe acidosis. The PaCO2 is elevated (6.5 kPa), suggesting a respiratory origin, while the HCO3 is markedly decreased (12 mmol/L), indicating a metabolic origin. The PaO2 is within the normal range (10 kPa). The underlying causes are likely sepsis and chronic renal failure, leading to both respiratory and metabolic acidosis.
References
Adrogué, HJ & Madias, NE 2007, 'Arterial blood gas analysis in the assessment of suspected acid-base disorders', New England Journal of Medicine, vol. 356, no. 9, pp. 917-929.
Malley, WJ 2018, Clinical Blood Gases: Assessment & Intervention, 2nd edn, Elsevier Health Sciences, St. Louis, MO.
Cairo, JM 2010, Pilbeam's Mechanical Ventilation: Physiological and Clinical Applications, 5th edn, Mosby, St. Louis, MO.
West, JB 2012, Respiratory Physiology: The Essentials, 9th edn, Lippincott Williams & Wilkins, Philadelphia, PA.
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