Do I need an arterial blood gas (ABG) for a patient with a rib fracture (fx)?

Arterial Blood Gas Testing in Patients with Rib Fractures

Arterial blood gas (ABG) testing is not routinely required for all patients with rib fractures but should be performed if the patient has oxygen saturation below 94% on room air, signs of respiratory distress, or risk factors for hypercapnic respiratory failure.

When to Obtain ABGs in Rib Fracture Patients

ABG testing should be performed in the following clinical scenarios:

Indications for ABG in Rib Fracture Patients:

• Oxygen saturation (SpO2) below 94% on room air or requiring oxygen to maintain target saturation 1
• Signs of respiratory distress (tachypnea >30 breaths/min, accessory muscle use)
• Deteriorating oxygen saturation (fall of ≥3%) 1
• Critically ill patients with rib fractures 1
• Patients requiring oxygen therapy to maintain target saturation ranges 1
• Patients with risk factors for hypercapnic respiratory failure:
  • Severe or moderate COPD
  • Chest wall or spinal disease
  • Neuromuscular disease
  • Severe obesity
  • Cystic fibrosis or bronchiectasis 1

Clinical Factors That Increase Need for ABG:

• Multiple rib fractures (≥3 ribs) 1, 2
• Flail chest segment 1
• Displaced rib fractures 1
• Elderly patients (>65 years) 1
• Decreased vital capacity (<30% of predicted) 2
• Presence of pulmonary contusion 1

Importance of ABG Testing When Indicated

ABG testing provides critical information that pulse oximetry alone cannot:

• Detects hypercapnia (elevated PaCO2) which may not be evident with pulse oximetry 1
• Identifies respiratory acidosis (pH <7.35 with PaCO2 >45 mmHg) 3
• Helps guide oxygen therapy and ventilatory support decisions 1
• Identifies patients who may need escalation of care (NIV or invasive ventilation) 1

Limitations of Pulse Oximetry

The BTS guidelines emphasize that "the presence of a normal SpO2 does not negate the need for blood gas measurements especially if the patient is on supplemental oxygen therapy" 1. Pulse oximetry:

• Cannot detect abnormal pH or PCO2 levels
• May appear normal despite significant acid-base disturbances
• Will not identify hypercapnic respiratory failure 1

ABG Sampling Technique

When obtaining ABGs:

• Use local anesthesia for all ABG specimens except in emergencies 1
• Perform Allen's test before radial ABG to ensure dual blood supply to the hand 1, 3
• For most patients who are not critically ill or in shock, arterial or arterialised earlobe blood gases may be used to measure pH and PCO2 1

Alternative Approaches

If arterial sampling is difficult or not feasible:

• Capillary blood gases (CBG) can be used for re-measuring PCO2 and pH during oxygen titration 1
• Cutaneous capnography can be used for re-measuring PCO2 alone 1
• For community-based assessments, a combination of CBGs and oximetry could be used as an alternative 1

Management Based on ABG Results

• If pH <7.35 and PCO2 >6.0 kPa (45 mmHg): Consider non-invasive ventilation or invasive ventilation 1
• If oxygen saturation falls below target range (94-98% for most patients, 88-92% for those at risk of hypercapnic failure): Titrate oxygen therapy and repeat ABGs after 30-60 minutes 1
• If respiratory acidosis develops during oxygen therapy: Seek immediate senior review 1

Conclusion

ABG testing is an important diagnostic tool in selected patients with rib fractures who demonstrate respiratory compromise or are at risk for respiratory failure. While not needed for all rib fracture patients, ABGs provide crucial information about ventilation and acid-base status that guides management decisions and helps predict outcomes.