Diclofenac Sodium

Introduction

From an anesthetic standpoint, Diclofenac Sodium is not a primary anesthetic agent but a critical component of multimodal analgesia. As a potent non-steroidal anti-inflammatory drug (NSAID), its primary role in the operating room and post-anesthesia care unit (PACU) is to provide opioid-sparing analgesia. By targeting the inflammatory component of pain, it enhances patient comfort, reduces opioid-related side effects like post-operative nausea and vomiting (PONV) and respiratory depression, and facilitates a faster, smoother recovery. Its use requires a careful risk-benefit analysis by the anesthesia team.

Structure and Classification

Diclofenac is a phenylacetic acid derivative. It belongs to the class of non-selective NSAIDs, meaning it inhibits both cyclooxygenase (COX) isoforms, COX-1 and COX-2. This non-selectivity is the source of both its therapeutic benefits (via COX-2) and its significant adverse effects (via COX-1).

Mechanism of Action (MOA)

The perioperative analgesic effect of diclofenac stems from its inhibition of the cyclooxygenase enzymes.

  • COX-2 Inhibition (Therapeutic Effect): COX-2 is an inducible enzyme upregulated at sites of tissue injury and inflammation. By inhibiting COX-2, diclofenac blocks the conversion of arachidonic acid to prostaglandins (specifically PGE2), which are key mediators of pain, inflammation, and fever. This provides potent analgesia and reduces postoperative swelling.
  • COX-1 Inhibition (Adverse Effects): COX-1 is a constitutive enzyme responsible for maintaining normal physiological functions, including protecting the gastric mucosa and supporting platelet aggregation. Inhibition of COX-1 is responsible for the well-known NSAID toxicities: gastrointestinal ulceration and impaired platelet function.

Pharmacokinetics in the Perioperative Setting

  • Onset of Action: The intravenous (IV) formulation has a rapid onset, providing analgesia within 15-30 minutes, making it suitable for intraoperative or immediate post-op administration.
  • Distribution: It is highly protein-bound (>99%), which can lead to drug interactions by displacing other highly-bound medications.
  • Metabolism and Excretion: Diclofenac undergoes extensive hepatic metabolism, with metabolites excreted renally. While the active drug has a short half-life (1-2 hours), its effect on platelet function lasts for the lifespan of the platelet (7-10 days) due to irreversible COX-1 inhibition.

Pharmacodynamics: The Anesthesiologist's View

  • Analgesic Profile: Provides effective analgesia for somatic pain, particularly following orthopedic, abdominal, and gynecological surgery. It is less effective for pure visceral or neuropathic pain.
  • Opioid-Sparing Effect: This is its most valuable pharmacodynamic contribution. By reducing opioid requirements by 20-40%, it directly decreases the incidence of PONV, sedation, pruritus, and ileus, thereby accelerating recovery milestones and PACU discharge.
  • Anti-inflammatory Effect: Reduces local edema and inflammatory pain, which is especially beneficial in procedures involving significant tissue trauma (e.g., joint replacement, tonsillectomy).
  • Antiplatelet Effect: The inhibition of thromboxane A2 synthesis impairs platelet aggregation, increasing the risk of perioperative bleeding.

Perioperative Uses

The use of diclofenac is always within a balanced analgesic plan.

  1. As Part of Multimodal Analgesia: Administered alongside regional anesthesia (nerve blocks, epidurals), acetaminophen, and low-dose opioids.
  2. Intraoperative Administration: A single IV dose is often given after surgical incision or toward the end of the case to preempt postoperative pain.
  3. Post-operative Pain Management: Used regularly in the PACU and on the surgical ward. The IV form is crucial for patients who cannot take oral medications. Patients can then be transitioned to oral or rectal (suppository) forms.
  4. Ambulatory Surgery: Highly valuable in day-case procedures, as effective analgesia with minimal nausea and sedation is essential for timely discharge.

Dosage and Administration

  • IV/IM Dose: A common adult dose is 75 mg once or twice daily, administered over at least 15-30 minutes to minimize renal and cardiovascular risks.
  • Oral/Rectal Dose: Typically 50 mg three times daily.
  • Key Anesthetic Consideration: The maximum daily dose should be strictly adhered to (usually 150 mg/day) to avoid toxicity.

Contraindications: Anesthesiologist's Red Flags

The decision to use diclofenac involves screening for these critical contraindications:

  • High Bleeding Risk Surgery: Absolutely contraindicated in procedures where even minor bleeding can be catastrophic (e.g., neurosurgery, intracranial surgery, prostatectomy/TURP, major plastic surgery).
  • Renal Impairment: Contraindicated in patients with moderate to severe pre-existing renal failure due to the high risk of perioperative acute kidney injury (AKI).
  • Active Peptic Ulcer Disease or GI Bleed: The risk of gastrointestinal hemorrhation is significantly increased.
  • Severe Heart Failure: NSAIDs can cause sodium and water retention, exacerbating heart failure.
  • Aspirin-Exacerbated Respiratory Disease (AERD): Can precipitate severe, potentially fatal bronchospasm in susceptible asthmatic patients.
  • Pregnancy (3rd Trimester): Contraindicated due to the risk of premature closure of the fetal ductus arteriosus.

Adverse Effects Requiring Perioperative Vigilance

  • Renal Toxicity: Perioperative AKI is the most feared complication. It is caused by inhibiting prostaglandins that maintain renal blood flow during states of hypovolemia or hypotension. Vigilant monitoring of urine output and renal function is paramount.
  • Increased Surgical Bleeding: The antiplatelet effect can lead to increased intraoperative blood loss and postoperative hematoma formation.
  • Gastrointestinal Effects: Can cause gastritis, ulceration, and bleeding, a risk heightened by the stress of surgery.
  • Cardiovascular Risk: NSAIDs carry a "black box warning" for increased risk of myocardial infarction and stroke. Caution is advised in patients with established cardiovascular disease.

Special Considerations

  • Elderly Patients: Are at a much higher risk for renal, GI, and cardiovascular side effects. A "start low, go slow" approach is necessary, often with a reduced dose.
  • Patients on Anticoagulants: The combination of diclofenac with warfarin, DOACs, or heparin dramatically increases bleeding risk and is generally avoided.
  • Pediatrics: Diclofenac is used in children for analgesia (e.g., post-tonsillectomy), but dosing is weight-based and the same contraindications, particularly regarding renal function and asthma, apply.

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