Diagnosis: Perforated peptic ulcer (most likely a perforated duodenal ulcer) with generalized peritonitis

Why the erect CXR can be normal despite perforation

• Small or "sealed" perforation — omentum, adjacent liver, or an inflammatory adhesion plugs the defect quickly, limiting the amount of gas that escapes (self-limiting/contained perforation).
• Insufficient time upright — the patient needs to sit/stand for about 10 minutes before the film is taken to let air rise to the highest point (under the diaphragm); if imaged too soon, air may not have migrated there yet.
• Retroperitoneal perforation — a posterior duodenal ulcer can perforate into the retroperitoneum rather than the free peritoneal cavity, producing no pneumoperitoneum (may instead show retroperitoneal gas or a hazy right psoas outline).
• Very small volume of gas — a fresh, pinpoint perforation may release too little air (plain film can detect as little as 10-20 mL, but even this threshold can be missed).
• Technical/interpretive pitfalls — a gastric air bubble or Chilaiditi's syndrome (bowel interposed between liver and diaphragm) can also confuse interpretation, though these cause false positives rather than negatives.
• Patient too unwell to sit erect, resulting in a suboptimal or supine film.

Best next investigation: Contrast-enhanced CT of the abdomen and pelvis

Definitive management

• IV crystalloid resuscitation, correct electrolytes
• Nasogastric tube decompression
• Broad-spectrum IV antibiotics (covering gram-negative and anaerobic organisms) and IV proton pump inhibitor
• Analgesia (should not be withheld — adequate analgesia makes signs clearer, not more confusing)
• Urinary catheter and close monitoring; NPO

• Emergency laparotomy (or increasingly, laparoscopy) through an upper midline incision (or laparoscopic ports)
• Omental ("Graham") patch repair: the perforation is closed with interrupted sutures and a tongue of vascularized omentum is laid over/into the defect to seal it, followed by thorough peritoneal lavage and toileting to wash out spilled gastric contents and debris (Sabiston Textbook of Surgery, p. 2140-2141; Fischer's Mastery of Surgery, 8th ed.)
• For larger perforations or ulcers with friable/fibrotic edges that cannot be closed without tension, a larger omental patch or, rarely, a more definitive resection (partial gastrectomy) may be needed, especially if malignancy is suspected in a gastric ulcer (biopsy the ulcer edge)
• Laparoscopic repair is now supported by recent randomized-trial meta-analyses as at least equivalent to, and in some outcomes better than, open repair (shorter hospital stay, fewer wound complications), though with longer operative time — reasonable in hemodynamically stable patients when laparoscopic expertise is available.
• Selected, hemodynamically stable patients with a sealed perforation confirmed by CT/contrast study (no ongoing leak) may be managed non-operatively (Taylor's regimen: NG suction, IV antibiotics, PPI, close monitoring), but this is the exception, not the rule, and requires very close observation with a low threshold to operate if deterioration occurs.

• Continue IV PPI, then transition to oral once tolerating feeds
• Test for and eradicate Helicobacter pylori if present
• Stop NSAIDs/aspirin where possible; address other risk factors (smoking, steroids)
• Ongoing surveillance endoscopy for gastric ulcers to exclude malignancy after healing