Objective The tension-time index of the diaphragm (TTdi) is a composite assessment of the load on and the capacity of the diaphragm. TTmus is a non-invasive tension-time index of the respiratory muscles. Our aim was to determine whether TTdi or TTmus predicted extubation outcome and performed better than respiratory muscle strength (Pimax, Pdimax), respiratory drive (P_0.1) and work of breathing (transdiaphragmatic pressure-time product (PTPdi)) or routinely available clinical data.

Design Prospective study.

Setting Tertiary neonatal intensive care unit.

Patients Sixty infants, median gestation age 35 (range 23–42) weeks and postnatal age of 55 (range 1–115) days.

Interventions Airway occlusions were performed to measure Pimax, Pdimax and P_0.1. TTdi and PTPdi were derived from measurements of transdiaphragmatic pressure. TTmus was derived from airway pressure measurements. Measurements were made within 6 h of extubation.

Main outcome measures Extubation failure defined as reintubation within 48 h of extubation.

Results Twelve infants failed extubation. The infants who failed extubation were significantly more immature (medians 25 vs 37 weeks) and of greater postnatal age (23 vs 5 days) and had higher TTdi (0.15 vs 0.04) and TTmus (0.17 vs 0.08). TTdi and TTmus were only significantly better predictors than the peak inflation pressure immediately prior to extubation and did not perform significantly better than gestational age or birth weight.

Conclusions Assessment of TTdi and TTmus cannot be recommended for use in routine clinical practice.