What acid-base abnormality would be expected in a patient with left-sided rib fractures and somnolence following a motor vehicle crash, as indicated by an arterial blood gas (ABG)?

Respiratory Acidosis (Answer: C)

In a somnolent patient with left-sided rib fractures following a motor vehicle crash, the expected acid-base abnormality is respiratory acidosis due to hypoventilation from pain-limited breathing and altered mental status. 1

Pathophysiologic Mechanism

The combination of rib fractures and somnolence creates a perfect storm for respiratory acidosis through multiple mechanisms:

• Pain-induced hypoventilation: Rib fractures cause severe chest wall pain that restricts respiratory excursion, leading to inadequate ventilation and CO2 retention 1
• Impaired respiratory mechanics: Chest wall injury directly limits the ability to generate adequate tidal volumes and clear secretions 1
• Altered mental status: Somnolence (whether from head injury, medications, or hypoxia) further depresses respiratory drive, compounding the hypoventilation 2
• Pulmonary complications: Rib fractures are associated with atelectasis, impaired clearance of secretions, and pneumonia—all of which worsen gas exchange 1

Expected ABG Findings

The arterial blood gas in this patient would show:

• pH < 7.35 (acidemia) 2, 3
• PaCO2 > 45 mmHg (hypercapnia) 2, 3
• Normal or slightly elevated bicarbonate initially (acute respiratory acidosis without renal compensation yet) 2, 4

Clinical Context and Risk Factors

This patient has multiple high-risk features that predict respiratory complications:

• Rib fractures themselves carry 10% overall mortality and are associated with pulmonary morbidity 1
• Somnolence indicates either CNS depression or severe hypoxia/hypercapnia, both of which worsen hypoventilation 2
• Acute trauma setting means no time for metabolic compensation to occur 2, 4

Why Not the Other Options

• Metabolic alkalosis (A): Would require vomiting, diuretic use, or volume contraction—none suggested by this scenario 4
• Metabolic acidosis (B): Could occur with severe shock/hypoperfusion, but the primary problem here is ventilatory failure, not tissue hypoperfusion 1, 4
• Respiratory alkalosis (D): Would require hyperventilation, which is impossible in a somnolent patient with painful rib fractures who cannot breathe adequately 2, 4

Critical Management Implications

Recognition of impending respiratory acidosis is essential because:

• Hypoventilation risks: Include progressive hypoxemia, hypercarbia, and eventual respiratory arrest 1, 2
• Aggressive pain control is mandatory: Regional anesthesia (thoracic epidural, paravertebral blocks, or newer techniques like erector spinae plane blocks) reduces pain, improves ventilation, and prevents progression to respiratory failure 1
• Close monitoring required: Somnolence with rib fractures warrants intensive monitoring for worsening respiratory acidosis and potential need for ventilatory support 1

The combination of mechanical chest wall injury and depressed consciousness creates acute respiratory acidosis through inadequate alveolar ventilation—the hallmark of Type II respiratory failure. 2, 3