Integrated Prediction of Extubation Outcome by the Spontaneous Breathing Trial in Newborn Infants

Prolonged endotracheal mechanical ventilation has significant adverse effects in newborn infants including subglottic stenosis, bacterial colonisation and bronchopulmonary dysplasia. Neonatal health professionals aim to limit the period of mechanical ventilation to the minimum possible duration. The decision to extubate is commonly a clinical one, based on current ventilator settings, blood gases analysis and the overall clinical condition of the infant. On the other hand, newborn infants commonly fail extubation and require reintubation which is a traumatic invasive procedure which could further prolong the total duration of invasive mechanical ventilation. Thus, accurate prediction of readiness for extubation outcome will probably incur a notable reduction in respiratory morbidity.

A number of studies have evaluated methods to predict extubation outcome. Of them, the spontaneous breathing trial (SBT) has been shown to hold the highest predictive value: it involves placing an infant from invasive mechanical pressure limited time cycled ventilation ventilation to endotracheal continuous positive end-expiratory pressure (ET-CPAP) for a period of 5 minutes during which saturation and heart rate are closely monitored. The infant would pass the test if there is no hypoxia or bradycardia during the test. A successful SBT has a positive predictive value of 93%, sensitivity of 97% and specificity of 73%. The combination of a successful SBT with increased variability in respiratory parameters during the SBT has been reported to further increase the sensitivity of the trial; alas the specificity remains below 75%.

It should be noted that at King's College Hospital the SBT forms part of routine clinical care as per clinical guideline and it is at the discretion of the attending physician to conduct the trial prior to extubation.

The work of breathing is undertaken by the respiratory muscles. Impaired respiratory muscle function is associated with a higher risk of respiratory muscle fatigue and respiratory failure and need for mechanical ventilation. Respiratory muscle function has been identified as an independent predictor of extubation outcome in mechanically ventilated infants. One simple way to quantify respiratory muscle function is the rate of relaxation of the inspiratory muscles which can be depicted by the rate of the decline of the airway pressure signal following a spontaneous breath. In concept, inefficient muscle function is manifested by a more gradual drop to baseline of the airway pressure over time. The principle on which the method is based is that when skeletal muscles contract against increased external loads, their relaxation slows as a result of slower calcium uptake by skeletal muscle fibres. In terms of physical properties, fatigued muscle fibres take longer to relax after contraction and this reduction in the relaxation rate can be quantified by an increase in the time constant of relaxation and a decrease in the maximal relaxation rate.

The rate of relaxation of the inspiratory muscles has been shown to accurately describe respiratory muscle function in non-neonatal populations. The information on the inspiratory muscle rate of relaxation is readily available in the ventilator graphics which, as part of routine clinical care, use a pressure port and a flow sensor to record airway flow, delivered volume and pressure over time. To the knowledge of the investigator, the potential value of this information in predicting extubation outcome in newborn infants has not been explored to date.