What is the indication for using morphine (opioid analgesic) during a Hepatobiliary Iminodiacetic Acid (HIDA) scan?

Morphine Use in HIDA Scans for Diagnosis of Acute Cholecystitis

Morphine is indicated during HIDA scans when the gallbladder is not visualized within 60 minutes of radiotracer administration to differentiate between true cystic duct obstruction (acute cholecystitis) and other causes of non-visualization.

Mechanism and Rationale

Morphine works through two key mechanisms during HIDA scanning:

1. Increases pressure in the biliary system by causing contraction of the sphincter of Oddi 1
2. Forces bile reflux into the gallbladder if the cystic duct is patent 2

This pharmacological intervention helps distinguish between:

• True cystic duct obstruction (acute cholecystitis): Gallbladder remains non-visualized after morphine
• Functional or non-obstructive issues: Gallbladder visualizes after morphine administration

Protocol for Morphine Administration in HIDA Scans

Patient Preparation

• Patient should fast for at least 4-6 hours prior to the study 2
• Opiate and anticholinergic drugs should be withheld for at least 48 hours before testing 3

Standard Protocol

1. Inject HIDA radiotracer intravenously with patient supine
2. Obtain imaging up to 60 minutes to assess for gallbladder visualization
3. If gallbladder is not visualized by 60 minutes:
   • Administer 0.04 mg/kg morphine intravenously 2
   • Continue imaging for an additional 30 minutes

Interpretation

• Positive for acute cholecystitis: Gallbladder remains non-visualized after morphine administration (by 90 minutes total)
• Negative for acute cholecystitis: Gallbladder visualizes after morphine administration

Evidence Supporting Morphine Augmentation

The World Society of Emergency Surgery guidelines (2020) recognize HIDA scanning as having the highest sensitivity and specificity for diagnosing acute cholecystitis compared to other imaging modalities 3.

Morphine augmentation offers several advantages over traditional delayed imaging:

• Higher specificity: 95% for morphine-augmentation vs. 83% for delayed imaging 4
• Higher positive predictive value: 81% for morphine-augmentation vs. 45% for delayed imaging 4
• Improved accuracy: 95% for morphine-augmentation vs. 84% for delayed imaging 4
• Time savings: Eliminates need for delayed imaging up to 4 hours 5

Alternative Approaches

Some institutions use a modified protocol with morphine pretreatment:

• Administration of 2 mg morphine IV push at bedside before imaging 4
• This approach has shown comparable or superior results to post-treatment morphine protocols

Clinical Considerations and Precautions

• Contraindications: Morphine should not be used in pregnant patients as it may stimulate preterm labor 3
• Liver cirrhosis: Use with caution as morphine has a longer half-life and increased bioavailability in patients with liver disease 3
• Renal impairment: Morphine metabolites may accumulate, potentially causing neurotoxicity 3
• Drug interactions: Be aware of potential interactions with other medications that may affect interpretation

Diagnostic Algorithm for Suspected Acute Cholecystitis

1. Initial imaging: Ultrasound as first-line imaging modality 3
2. If ultrasound is equivocal: Proceed to HIDA scan
3. HIDA scan protocol:
   • If gallbladder visualizes within 60 minutes: No acute cholecystitis
   • If gallbladder does not visualize by 60 minutes: Administer morphine
   • If gallbladder visualizes after morphine: Functional disorder or partial obstruction
   • If gallbladder remains non-visualized after morphine: Acute cholecystitis

By following this evidence-based approach to morphine-augmented HIDA scanning, clinicians can accurately diagnose acute cholecystitis while minimizing unnecessary delays and improving diagnostic accuracy.