• Mechanism: Binds to 30S ribosomal subunit -> Inhibits protein synthesis.
• Spectrum: Gram Negatives (E. coli, Pseudomonas, Klebsiella). Synergistic against Staph (Endocarditis).
• Post-Antibiotic Effect: Bacteria continue to die even after drug levels drop (allows once daily dosing).

1. Sepsis of Unknown Origin (Gram Negative cover).
2. Pyelonephritis / UTI (Severe).
3. Endocarditis (Synergy with Penicillin).
4. Neutropenic Sepsis (Tazocin + Gentamicin).

Toxicity

1. Ototoxicity: Accumulates in inner ear hair cells. Causes deafness (high frequency first) and Vertigo (vestibular damage). Irreversible.
2. Nephrotoxicity: Acute Tubular Necrosis (ATN). Reversible.

Contraindications

• Myasthenia Gravis (Worsens weakness).
• Severe CKD (Relative - needs dose adjustment).
• Pregnancy (Ototoxic to fetus).

Most hospitals use a ONCE DAILY regimen based on weight.

┌─────────────────────────────────────────────────────────────────────────────┐
│                    GENTAMICIN PRESCRIBING PROTOCOL                          │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│   STEP 1: CALCULATE DOSE                                                    │
│   • Dose: **5-7 mg/kg** (Once Daily).                                       │
│   • Weight: Use IDEAL Body Weight if obese (Gentamicin distributes in       │
│     water, not fat).                                                        │
│   • Max typical dose often capped (e.g., 480mg or 560mg).                   │
│   └─────────────────────────────────────────────────────────────────────┘   │
│                          ↓                                                  │
│   STEP 2: ADMINISTER                                                        │
│   • Give as slow IV infusion (over 30-60 mins).                             │
│   • Record precise time.                                                    │
│   └─────────────────────────────────────────────────────────────────────┘   │
│                          ↓                                                  │
│   STEP 3: MONITORING (The "Trough" or "Pre-Dose")                           │
│   • **Regimen A (Traditional Multiple Daily)**:                             │
│     - Measure Peak (1hr post) and Trough (pre-dose).                        │
│     - Rarely used now except Endocarditis.                                  │
│                                                                             │
│   • **Regimen B (Once Daily / Hartford)**:                                  │
│     - Take blood sample **6 to 14 hours AFTER** the start of infusion.      │
│     - Plot level on the Nomogram Graph.                                     │
│     - The Graph tells you dosing interval:                                  │
│       * Zone A (Low): Give next dose at 24 hrs.                             │
│       * Zone B (Med): Give next dose at 36 hrs.                             │
│       * Zone C (High): Give next dose at 48 hrs.                            │
│   └─────────────────────────────────────────────────────────────────────┘   │
│                          ↓                                                  │
│   STEP 4: DURATION                                                          │
│   • Review daily. Stop usually after 3 days (switch to oral).               │
│   • If treating >3 days, toxicity risk skyrockets.                          │
│   └─────────────────────────────────────────────────────────────────────┘   │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

• Different rule applies!
• For synergy (e.g., Strep/Enterococcus endocarditis), we need LOW dose (1mg/kg TDS).
• Target: Peak 3-5 mg/L, Trough <1 mg/L.

Scenario: 70kg Male. Normal renal function.

• Dose: 7mg/kg x 70kg = 490mg.
• Round to nearest vial size (usually 80mg vials) -> 480mg.
• Give 480mg IV in 100ml Normal Saline over 60 mins.

Scenario: Obese Male (120kg). Height 1.8m.

• Don't use 120kg! (Risk of overdose).
• Calculate Ideal Body Weight (IBW) ~75kg.
• Add 40% of excess weight for Adjusted Body Weight.
• Practice Point: Most guidelines say use IBW or Adjusted calc.

Acute Kidney Injury (AKI)

• Gentamicin CAUSES AKI.
• If Creatinine rises while on Gentamicin -> STOP/WITHHOLD.
• Do not rely on nomogram if renal function is rapidly changing.

Fluid Balance

• Dehydration increases toxicity. Ensure patient is hydrated.

Exam-Focused Points

1. Ideal Body Weight: Used because adipose tissue contains little water (Gent is water soluble).
2. Ototoxicity: If patient complains of ringing ears or dizziness, STOP immediately. It does not come back.
3. Myasthenia: The "Killer" contraindication. Can precipitate respiratory crisis.
4. Trough Level: The most important level to prevent toxicity. Must be low (usually <1 mg/L).
5. Peak Level: Determines efficacy.

Common Exam Scenarios

• Patient on Gentamicin develops ringing in ears. (Stop Gentamicin - Ototoxicity).
• "Prescribe Gentamicin for this 100kg lady". (Calculate IBW first).
• Nurse asks when to take the level? (For Hartford: 6-14 hours post dose. For Traditional: Just before next dose).

Why this strong antibiotic?

"We are using Gentamicin because it is very effective at killing the specific bacteria causing your severe infection. It works faster than other antibiotics."

Why do you need so many blood tests?

"This drug is cleared by your kidneys. If it builds up in the blood, it can damage hearing or kidneys. We measure the level to ensure we are giving enough to kill the bacteria, but not enough to cause harm."

Key Guidelines

Evidence-Based Recommendations