What are the formulas for estimating endotracheal tube (ETT) size in pediatric patients of different age groups, including neonates to 1 year, 1 to 6 years, and more than 6 years?

Endotracheal Tube Size Formulas for Pediatric Patients

For pediatric ETT sizing, use age-based formulas with cuffed tubes preferred: infants <1 year use 3.0 mm ID, children 1-2 years use 3.5 mm ID, and children >2 years use the formula ID (mm) = (age/4) + 3.5 for cuffed tubes or ID (mm) = (age/4) + 4 for uncuffed tubes. 1

Neonates to 1 Year

Cuffed Tubes (Preferred)

• For infants ≥3.5 kg and <1 year of age: use 3.0 mm ID cuffed ETT 1, 2
• This represents the standard recommendation across major resuscitation guidelines 1

Uncuffed Tubes (Alternative)

• For infants up to 1 year: use 3.5 mm ID uncuffed ETT 1, 3
• This sizing applies when cuffed tubes are unavailable or not preferred 1

Children 1 to 6 Years

Ages 1-2 Years

Cuffed tubes:

• Use 3.5 mm ID cuffed ETT for all children 1-2 years of age 1, 2

Uncuffed tubes:

• Use 4.0 mm ID uncuffed ETT for children 1-2 years 1

Ages 2-6 Years (and Beyond)

Cuffed tubes (preferred):

• Formula: ID (mm) = (age in years ÷ 4) + 3.5 1, 2
• This formula, known as the modified Khine formula, is recommended by the American Heart Association 1
• Example: For a 4-year-old child: (4 ÷ 4) + 3.5 = 4.5 mm ID cuffed tube 1

Uncuffed tubes (alternative):

• Formula: ID (mm) = (age in years ÷ 4) + 4 1, 3
• This is the traditional Cole formula for uncuffed tubes 4
• Example: For a 4-year-old child: (4 ÷ 4) + 4 = 5.0 mm ID uncuffed tube 1

Children Over 6 Years

The same formulas continue to apply beyond age 6:

• Cuffed: ID (mm) = (age ÷ 4) + 3.5 1
• Uncuffed: ID (mm) = (age ÷ 4) + 4 1
• Example: For an 8-year-old: Cuffed = 5.5 mm, Uncuffed = 6.0 mm 1

Critical Implementation Points

Tube Preparation

• Always have tubes 0.5 mm smaller AND 0.5 mm larger than calculated size immediately available 1
• If the tube meets resistance during insertion, use the tube 0.5 mm smaller 1
• If excessive air leak occurs with deflated cuff, consider reintubation with 0.5 mm smaller tube when patient is stable 1

Cuff Management (When Using Cuffed Tubes)

• Monitor and maintain cuff pressure <20-25 cm H₂O 1, 3
• Cuffed tubes are particularly preferable in poor lung compliance, high airway resistance, or large glottic air leak 1, 2
• Cuffed tubes reduce aspiration risk 2, 3

Length-Based Tape Alternative

• Length-based resuscitation tapes are more accurate than age-based formulas for children up to approximately 35 kg 1, 3
• This method is particularly useful for children with short stature 1

Depth of Insertion

• Primary formula: Depth at lip (cm) = weight (kg) + 6 2, 3
• Alternative formula: Depth (cm) = 3 × internal diameter of tube 2
• For oral intubation in children >2 years: Depth (cm) = (age ÷ 2) + 12 2

Common Pitfalls to Avoid

• Do not rely solely on formulas: Age-based formulas are only 53.5% accurate in some populations 5, which is why having multiple sizes ready is essential 1
• Avoid oversizing: A tube that is too large risks airway trauma, post-intubation croup, and subglottic stenosis 4
• Avoid undersizing: A tube that is too small increases risk of inadequate ventilation, air leakage, and aspiration 4
• Do not replace a functional tube unnecessarily: Tube replacement carries significant risk and should only be done by experienced personnel in appropriate settings 1