What is the recommended sedation regimen using atropine (anticholinergic) and ketamine (anesthetic) prior to a lumbar puncture (lumbar tap)?

Atropine + Ketamine for Lumbar Puncture Sedation

For pediatric lumbar puncture sedation, use ketamine 4 mg/kg IM with atropine 0.01 mg/kg, which provides superior sedation with faster onset (3 minutes) and shorter discharge time (85 minutes) compared to alternative regimens. 1

Recommended Dosing Regimen

Intramuscular Administration (Preferred for Lumbar Puncture)

• Ketamine: 4 mg/kg IM 1, 2
• Atropine: 0.01 mg/kg IM (minimum 0.1 mg, maximum 0.5 mg) 1
• This combination demonstrated significantly faster onset of sedation (3 minutes vs 18 minutes, P<0.01) and shorter time to discharge (85 minutes vs 113 minutes, P=0.01) in a double-blinded randomized controlled trial of children aged 6 months to 6 years undergoing lumbar puncture 1

Intravenous Administration (Alternative)

• Ketamine: 1-1.5 mg/kg IV 2, 3
• Atropine: 0.01 mg/kg IV 1
• IV route provides more rapid onset (30-96 seconds) but requires IV access 3
• At 1.5 mg/kg IV, only 5.5% of patients required additional ketamine compared to 54% at 1 mg/kg 2, 3

Clinical Efficacy and Outcomes

Sedation Quality

• All patients achieved adequate sedation for lumbar puncture procedures with the ketamine/atropine combination 1
• Mean behavioral distress scores were significantly lower (2.7 vs 9.8, P<0.003) compared to alternative sedation regimens 1
• 100% of patients were cooperative during the procedure 1

Timing Considerations

• IM onset: 3-5 minutes (average 4 minutes) 2, 3
• IV onset: 30-96 seconds 3
• Duration of sedation: 82 minutes (IM) 1
• Recovery time: 85 minutes (IM), 84 minutes (IV) 1, 3

Role of Atropine

Antisialagogue Effect

• Atropine significantly reduces hypersalivation during ketamine sedation (mean secretion score 16.5 vs 27.0, P<0.05) 4
• However, this reduction does not provide substantial clinical benefit, as only 9.7% of patients without atropine required simple airway interventions (suction or repositioning) 4
• No patients in either group required advanced airway management 4

Cardiovascular Effects

• Atropine causes a modest increase in heart rate (approximately 11 bpm increase) 5, 4
• This may theoretically counteract ketamine's sympathomimetic effects, though ketamine itself increases heart rate and blood pressure 1, 3

Current Evidence on Necessity

• Recent evidence suggests atropine may not be clinically necessary for all patients, as hypersalivation-related complications are rare and easily managed 4
• However, the established regimen of ketamine 4 mg/kg + atropine 0.01 mg/kg IM remains the most studied and validated approach specifically for lumbar puncture 1

Critical Safety Considerations

Intracranial Pressure Effects

• Ketamine increases cerebrospinal fluid opening pressure by approximately 4-7 cm H₂O 6, 7
• Mean opening pressure with ketamine alone: 26.5 cm H₂O vs 17.3 cm H₂O without ketamine (P=0.002) 6
• This elevation is clinically significant but has not been associated with adverse outcomes in children undergoing lumbar puncture 7
• Consider adding midazolam 0.09 mg/kg to blunt this rise (reduces opening pressure to 22.0 cm H₂O, P=0.013) 6

Respiratory Safety

• Ketamine demonstrates superior respiratory safety compared to opioid/benzodiazepine combinations 8
• Oxygen saturation should remain >93% on room air; all patients maintained SpO₂ >95% in clinical trials 3, 5
• Transient desaturation occurs in only 6-8% of patients and responds to simple interventions 8, 9
• Avoid midazolam doses >0.3 mg/kg, as 50% of patients developed desaturation at this threshold 9

Monitoring Requirements

• Continuous pulse oximetry, heart rate, and blood pressure monitoring are mandatory 3
• Capnography should be used when available 1
• End-tidal CO₂ typically remains <47 mmHg 1

Common Adverse Effects

Expected Side Effects

• Emesis: 7-9% of patients 3, 8
• Nausea: 4-5% of patients 3
• Ataxia: 7-8% of patients during recovery 3
• Emergence reactions: 8.7-13.6% (hallucinations, agitation) 9
  • More common without midazolam pretreatment (4 patients in ketamine-only group vs 0 with midazolam) 6
  • Can be minimized by co-administration of midazolam 0.05-0.1 mg/kg 1, 6

Serious Adverse Events

• Laryngospasm: 0.9-1.4% (very rare) 8
• No episodes of clinically significant respiratory depression or cardiovascular instability in large pediatric series 1

Contraindications

Avoid ketamine in patients with: 1, 3

• Ischemic heart disease
• Cerebrovascular disease
• Uncontrolled hypertension
• Active psychosis
• Severe hepatic dysfunction
• Elevated intracranial or intraocular pressure (relative contraindication)

Practical Algorithm

1. Verify patient eligibility: Age 6 months to 6 years, no contraindications 1
2. Establish monitoring: Continuous pulse oximetry, heart rate, blood pressure 3
3. Administer sedation:
   • Standard regimen: Ketamine 4 mg/kg IM + atropine 0.01 mg/kg IM 1
   • If IV access available: Ketamine 1.5 mg/kg IV + atropine 0.01 mg/kg IV 2
   • If concerned about ICP or emergence reactions: Add midazolam 0.05-0.09 mg/kg 6
4. Wait for onset: 3-5 minutes (IM) or 1-2 minutes (IV) 2, 3
5. Perform lumbar puncture when patient achieves dissociative state 1
6. Monitor recovery: Average 85 minutes; watch for emergence reactions 1
7. Discharge criteria: Patient ambulatory, vital signs stable, no ongoing adverse effects 3

Key Clinical Pearls

• The IM route is preferred for lumbar puncture as it avoids the need for IV access in already distressed children 1
• Adding midazolam reduces both ICP elevation and emergence reactions but may prolong recovery time 6
• Atropine's antisialagogue benefit is modest; the primary rationale for its inclusion is the established safety and efficacy data of the combination regimen 4
• Younger children may require higher doses; be prepared to administer additional ketamine 1-2 mg/kg if initial sedation is inadequate 9