When should an Arterial Blood Gas (ABG) be ordered for a patient with Shortness of Breath (SOB)?

From the Guidelines

Patients with shortness of breath (SOB) should have an arterial blood gas (ABG) ordered in specific clinical scenarios, including critically ill patients, unexpected or inappropriate fall in SpO2, deteriorating oxygen saturation, and those with risk factors for hypercapnic respiratory failure, as recommended by the BTS guideline for oxygen use in adults 1. When evaluating a patient with SOB, it is essential to consider the clinical context and potential underlying conditions that may require ABG testing. The BTS guideline provides specific recommendations for ordering ABGs in various situations, including:

• Critically ill patients
• Unexpected or inappropriate fall in SpO2 below 94% in patients breathing air or oxygen
• Deteriorating oxygen saturation (fall of ≥3%) or increasing breathlessness in a patient with previously stable chronic hypoxaemia
• Most previously stable patients who deteriorate clinically and require increased fraction of inspired oxygen (FiO2) to maintain a constant oxygen saturation
• Any patient with risk factors for hypercapnic respiratory failure who develops acute breathlessness, deteriorating oxygen saturation, drowsiness or other features of carbon dioxide retention
• Patients with breathlessness who are thought to be at risk of metabolic conditions such as diabetic ketoacidosis or metabolic acidosis due to renal failure These recommendations are based on grade D evidence, indicating that the guideline development group considered the recommendations to be good practice points, despite the lack of high-quality evidence 1. In clinical practice, ABGs provide crucial information about oxygenation, ventilation, acid-base status, and can help distinguish between different types of respiratory failure. While pulse oximetry offers continuous, non-invasive monitoring of oxygen saturation, it cannot assess ventilation or acid-base status, making ABGs necessary when comprehensive respiratory assessment is needed. The requirement for an increased concentration of oxygen is an indication for urgent clinical reassessment of the patient and repeat blood gas measurements in most instances, as stated in the BTS guideline 1.

From the Research

Guideline Recommendations for Ordering ABG on a Patient with SOB

• The decision to order an arterial blood gas (ABG) on a patient with shortness of breath (SOB) depends on various factors, including the severity of symptoms, underlying medical conditions, and clinical settings 2.
• ABG analysis is a diagnostic tool that evaluates the partial pressures of gas in blood and acid-base content, providing valuable information on respiratory, circulatory, and metabolic disorders 2.
• In emergency medicine, ABG is frequently ordered for acute conditions, but it may also be necessary in other clinical settings, such as in patients with severe sepsis, septic shock, hypovolemic shock, diabetic ketoacidosis, or chronic respiratory failure 2.
• However, serial arterial blood gas measurements may not be useful in all cases, such as in post-extubation patients, where clinical deterioration can be detected through other means, like pulse oximetry or increasing drowsiness 3.
• In resource-limited settings, pulse oximetry and pulmonary ultrasound may be useful alternatives to ABG and chest radiography for diagnosing acute respiratory distress syndrome (ARDS) 4, 5.
• The use of pulse oximetry-based indices, such as the SpO2/FiO2 ratio, has been validated for the diagnosis and risk stratification of patients with ARDS, but its accuracy may be reduced in certain conditions, like poor perfusion states or darker skin pigmentation 4, 5.
• It is essential to consider the potential risks and benefits of oxygen therapy and to avoid excessive oxygen exposure, which can worsen clinical outcomes in patients with ARDS 6.