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Prediction of extubation success using the tension time index in neonates

Presented at the Neonatal Society 2009 Summer Meeting (programme).

Currie A, Patel D, Rafferty G, Greenough A

School of Medicine, Division of Asthma, Allergy and Lung Biology, King's College London, London, UK

Background: Premature extubation leading to cardio-respiratory compromise necessitates reintubation and reinstitution of mechanical ventilation and increases mortality and morbidity, while prolonged ventilatory support exposes the child to increased risk of nosocomial infection and lung injury. Up to one third of neonates are unsuccessfully extubated. The duration of ventilation is reduced when objective criteria are used to define readiness for extubation(1). To date, however, neither univariate nor multivariate predictors have had a high degree of sensitivity or specificity with regard to predicting extubation outcome in neonates. The tension time index of the diaphragm (TTdi) measures the ability to generate a pressure and sustain a contraction of the diaphragm and, therefore, assesses the load and capacity of the muscle (2). A TTdi above 0.15 has been shown in adults (3) and children (4) to be an accurate predictor of extubation failure, but has not been investigated in neonates.

Aims: To determine whether the TTdi will provide an accurate predictor of the outcome of extubation and, in particular, whether a TTdi of greater than 0.15 predicted extubation failure.

Methods: TTdi was measured in 19 infants mechanically ventilated using the SLE 5000 ventilator, within six hours of extubation. A dual tipped catheter was used to measure gastric and oesophageal pressure and hence transdiaphragmatic pressure (Pdi). Airflow was recorded using a pneumotachograph inserted between the endotracheal tube and the ventilator circuit and inspiratory time (Ti) and total time for the breath (Ttot) recorded. Airway pressure measured from a side port on the pneumotachograph. Extubation failure was defined as reintubation within 48 hours.

Patients: Twelve male and seven female infants were included with a median birthweight of 1.6 kg (range 0.67 to 3.7kg) and birth gestation of 31 weeks (range 24 to 39 weeks). Parents gave informed written consent for their infants to take part in the studies.

Results: The median TTdi of infants that failed extubation (0.16, range 0.154-0.198) was higher than that of infants successfully extubated (0.06, range 0.017-0.124) (p = 0.001). Construction of receiver operator characteristic curves demonstrated the area under the curve for TTdi was one; a TTdi of >0.15 was 100% sensitive and 100% specific in predicting extubation failure

Conclusion: In ventilated neonates, pre-extubation TTdi is a sensitive and specific predictor of extubation failure.

1. Huon C, Moriette G, Mussat P, Parat S, Relier JP. Biol Neonate 1993; 63(2):75-9.
2. Bellemare F, Grassino A. J Appl Physiol 1982; 53(5):1190-1195.
3. Purro A, Appendini L, De Gaetano A, Gudjonsdottir M, Donner CF, Rossi A. Am J Respir Crit Care Med 2000; 161(4 Pt 1):1115-23.
4. Harikumar, G., Moxham, J., Greenough, A. & Rafferty, G. F. European Respiratory Journal 2006; 28: 492s

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