When to Evaluate for Free Air in a Vomiting Patient
Evaluate for free air immediately when a vomiting patient develops sudden-onset abdominal pain with peritoneal signs, or when clinical suspicion for perforation exists based on physical examination findings of localized or generalized peritonitis. 1, 2
Clinical Triggers for Free Air Evaluation
High-Risk Presentations Requiring Immediate Imaging
Sudden onset of severe abdominal pain in conjunction with vomiting is the hallmark presentation of gastroduodenal perforation and mandates evaluation for free air 1, 2
Peritoneal signs on examination (guarding, rigidity, rebound tenderness) should prompt immediate imaging, though these findings may be present in only two-thirds of perforated peptic ulcer patients 1, 2
Progressive symptoms despite initial management, particularly when accompanied by hemodynamic changes, fever, or signs of sepsis 1
Elderly patients with vomiting and abdominal pain warrant lower threshold for imaging, as 30-50% of bowel perforations in this population will not show free air on plain radiographs 3
Laboratory Findings That Should Lower Your Threshold
- Leukocytosis, metabolic acidosis, or elevated serum amylase in a vomiting patient with abdominal pain are associated with perforation and should trigger imaging evaluation 1, 2
Imaging Algorithm
First-Line Imaging Strategy
CT scan is the preferred initial imaging modality when promptly available, with 93-96% sensitivity and 93-100% specificity for detecting free air and characterizing perforation (Strong recommendation, 1C) 1, 4, 5
Chest/abdominal X-ray series should be performed as initial assessment only when CT is not immediately available (Strong recommendation, 1C) 1, 2
For plain radiographs, have the patient stand or lie in left lateral decubitus position for 10-15 minutes before imaging to maximize detection of small amounts of free air 3
Critical Imaging Pitfalls
Plain X-rays detect free air in only 30-85% of perforations, making negative films unreliable for excluding perforation 1, 2, 3
Up to 12% of patients with perforation have normal CT scans, requiring additional diagnostic measures when clinical suspicion remains high 1, 2
When initial CT is negative but suspicion persists, administer water-soluble contrast orally or via nasogastric tube and perform triple-contrast CT to improve sensitivity (Weak recommendation, 2D) 1, 2
CT Findings Indicating Perforation
Pneumoperitoneum (free intraperitoneal air) is the primary finding 1, 4
Distant free air with air-fluid levels is pathognomonic for perforation requiring surgical exploration 4
Unexplained intraperitoneal fluid, bowel wall thickening, mesenteric fat streaking, or extraluminal contrast are additional suspicious findings 1, 2
When NOT to Delay Imaging
Do not wait for "classic" peritonitis before imaging—physical examination may be equivocal, particularly in patients with contained or sealed perforations 1, 2
Patients on steroids may have blunted peritoneal signs despite significant pathology 1
Clear history and examination findings of peritonitis with free air on any imaging modality is sufficient to justify surgical exploration without further testing 1
Special Considerations
Point-of-care ultrasound by trained operators can detect free intraperitoneal air under the abdominal fascia, though its role requires further validation 1, 2
In vomiting patients with progressive symptoms, hemodynamic instability, or worsening laboratory parameters, proceed directly to CT rather than starting with plain films 1
Contained perforations may present with minimal peritonitis but still require identification—CT is superior for detecting these cases 1