What is a good fluid plan for anesthesia in an 80 kg patient undergoing a craniotomy?

Fluid Management for Craniotomy in an 80 kg Patient

Use 0.9% normal saline as your primary crystalloid, aiming for a mildly positive fluid balance of 1-2 liters by the end of the case, while maintaining strict euvolemia and avoiding both hypovolemia and fluid overload. 1

Primary Fluid Choice

• 0.9% normal saline is the crystalloid of choice for neurosurgical patients because it is the only commonly available truly isotonic crystalloid solution when measured by real osmolality (mosmol/kg⁻¹) rather than theoretical osmolality 1
• Hypotonic solutions including Ringer's lactate, Ringer's acetate, and gelatins must be avoided as they can increase brain water content and worsen cerebral edema 1
• Strongly avoid albumin and synthetic colloids (including hydroxyethyl starch) in neurosurgical patients, as they provide no benefit and may cause harm 1

Volume Strategy

Target approximately 80-160 mL total positive fluid balance by end of case (1-2 L for an 80 kg patient), calculated as follows: 1, 2

• Maintenance rate: 4-6 mL/kg/hour of 0.9% saline (320-480 mL/hour for 80 kg patient) 3
• Avoid excessive crystalloid administration beyond this range, as both restrictive "zero-balance" strategies increase acute kidney injury risk, while liberal strategies (>12 mL/kg/hour) increase complications 1, 3
• Replace blood loss milliliter-for-milliliter with additional crystalloid at 3:1 ratio or blood products as indicated 4

Hemodynamic Management

Maintain strict normotension with arterial line monitoring, measuring pressure at the level of the tragus (external auditory canal) to accurately reflect cerebral perfusion pressure when the head is elevated: 1

• Avoid hypotension at all costs - even brief episodes adversely affect neurological outcomes and worsen cerebral ischemia 1, 4
• Position patient with 20-30° head-up tilt to reduce intracranial pressure 1
• If hypotension occurs after correcting hypovolemia, use small boluses of alpha-agonists (phenylephrine or metaraminol) followed by infusion if needed 1, 5
• Manage hypertension with increased sedation first, then small boluses of labetalol 1

Specific Fluid Volumes for 80 kg Patient

Practical calculation for this case: 1, 2, 3

• Baseline maintenance: 320-480 mL/hour (4-6 mL/kg/hour)
• For 3-hour craniotomy: Approximately 960-1440 mL total maintenance
• Additional boluses: 250-500 mL for induction/positioning
• Blood loss replacement: 3:1 crystalloid ratio for estimated blood loss
• Total target: 1500-2000 mL by end of case (approximately 1-2 L positive balance)

Critical Monitoring Parameters

• Arterial blood pressure via arterial line at tragus level (mandatory for craniotomy) 1, 4
• Urine output: Maintain >0.5 mL/kg/hour (>40 mL/hour for 80 kg patient) 4
• Serum sodium: Check intraoperatively if case >3 hours; maintain normonatremia 1, 6
• Avoid PEEP >10 cmH₂O as higher levels may impair cerebral perfusion 1

Common Pitfalls to Avoid

• Never use Ringer's lactate or other "balanced" crystalloids in neurosurgery despite their benefits in other surgical populations - they are hypotonic and will worsen cerebral edema 1
• Do not pursue "zero-balance" or highly restrictive fluid strategies (<4 mL/kg/hour) as this increases acute kidney injury risk without neurological benefit 1, 3
• Avoid fluid overload (>12 mL/kg/hour or >3 L positive balance) which increases complications including cerebral edema 1, 3
• Do not use colloids (albumin, HES, gelatins) - strong evidence shows harm or no benefit in neurosurgical patients 1
• Correct hypovolemia before using vasopressors - hypotension in brain-injured patients should be assumed due to inadequate volume until proven otherwise 1

Special Considerations for Increased ICP

If clinical or radiological evidence of raised intracranial pressure exists: 1

• Mannitol 0.5 g/kg (40 grams for 80 kg patient) or hypertonic saline 2 mL/kg of 3% (160 mL for 80 kg patient) may be administered as bolus 1, 7
• These osmotic agents should be given in addition to, not as replacement for, maintenance crystalloid 1
• Continue 0.9% saline maintenance throughout to prevent dehydration 1