What it is
This formula gives a first-guess internal diameter (ID, in mm) for a pediatric endotracheal tube from the child’s age. It is the Cole formula, carried into APLS/PALS teaching, used when there is no length-based tape to hand and time is short.
The formulae
For children roughly ≥ 1–2 years:
- Uncuffed ID (mm) = (age in years / 4) + 4
- Cuffed ID (mm) = (age in years / 4) + 3.5
Cuffed tubes are sized ~0.5 mm smaller because the cuff itself adds outer diameter and provides the seal that a snug uncuffed tube would otherwise need.
Modern guidelines (AHA/PALS, ERC) now favour cuffed tubes in most pediatric ages beyond the newborn, provided cuff pressure is monitored — they reduce reintubation for the wrong size and the need to exchange tubes, without raising airway-injury risk when used correctly.
When to use it
Emergency or elective pediatric intubation when a measured/length-based estimate (Broselow tape, which is generally more reliable) is unavailable. Always prepare the calculated size plus one half-size above and one below, because the formula is only an estimate.
Worked example
A 4-year-old needing intubation:
- Uncuffed = (4 / 4) + 4 = 5.0 mm ID.
- Cuffed = (4 / 4) + 3.5 = 4.5 mm ID.
- Have 4.0 and 5.0 (or 5.5) tubes immediately available, and confirm fit by a small audible leak around an uncuffed tube at ~20 cmH₂O (or appropriate cuff pressure for a cuffed tube).
Pitfalls and caveats
- Not for neonates or small infants. Below ~1 year the formula breaks down — use weight/length-based sizing (e.g. ~2.5 mm ID for a preterm <1 kg up to ~3.5 mm for a term newborn).
- It’s a starting estimate, not a guarantee — studies show the chosen tube is wrong by ≥0.5 mm in a meaningful minority of children, especially ages 2–5. Verify clinically and be ready to swap.
- Match tube depth too. A common depth guide is ID × 3 cm at the lips, or (age/2) + 12 cm — then confirm by auscultation, capnography and a chest film.
- Monitor cuff pressure (typically < 20–25 cmH₂O) to avoid mucosal ischaemia and post-extubation stridor.
Run it: ET Tube Size
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.