Interpretation of the literature is complicated by use of various definitions of extubation success which include remaining extubated at 24 h (Belani et al. 1980; Wilson et al. 1998), 48 h (Dimitriou et al. 2002; Fox et al. 1993; Kavvadia et al. 2000; Smith et al. 1999; Szymankiewicz et al. 2005; Vento et al. 2004) or 72 h (Dimitriou et al. 1999; Fox et al. 1981; Kamlin et al. 2008, 2006; Veness-Meehan et al. 1990). In addition, although most studies define extubation failure as the need for reintubation (Wilson et al. 1998; Dimitriou et al. 2002; Fox et al. 1993; Kavvadia et al. 2000; Szymankiewicz et al. 2005; Vento et al. 2004; Fox et al. 1981; Kamlin et al. 2008, 2006; Shoults et al. 1979; Balsan et al. 1990; Gillespie et al. 2003), others include a requirement for nasal continuous positive airways pressure (Belani et al. 1980; Veness-Meehan et al. 1990). Approximately 30 % of infants fail extubation when extubation readiness is determined by clinical criteria (Sinha and Donn 2000). Successful weaning and extubation are dependent on the balance between the magnitude of the respiratory load, the effective respiratory drive and adequate respiratory muscle strength. Assessment of the individual components has yielded variable success, including measures of compliance (Dimitriou et al. 2002; Kavvadia et al. 2000; Smith et al. 1999; Szymankiewicz et al. 2005; Veness-Meehan et al. 1990; Balsan et al. 1990) and resistance (Fox et al. 1981; Veness-Meehan et al. 1990). Lung volume assessment was shown to be significantly associated with extubation failure (Dimitriou et al. 1996) but post rather than pre-extubation and measurement of functional residual capacity using a helium gas dilution technique requires considerable expertise. A more generalisable technique is computer-assisted analysis of the chest radiograph lung area. A low post extubation lung area was demonstrated to be predictive of extubation failure but, although had a 100 % positive predictive value, had only 57 % sensitivity (Dimitriou and Greenough 2000). Respiratory drive has been assessed by the ventilatory response to added dead space (tube breathing) but successful extubation was predicted with only 71 % sensitivity and 75 % specificity (Fox et al. 1993). Respiratory muscle strength has usually been determined by assessment of the maximum pressure generated during an occlusion (Pi_max). Although in some studies (Belani et al. 1980; Shoults et al. 1979) Pi_max has been found to differ significantly between those in whom extubation failed and succeeded, differences in one study (Dimitriou et al. 2002) disappeared once Pi_max was corrected for birthweight.

As a consequence, it seems likely that a better approach would be to determine the balance between the magnitude of the respiratory load, respiratory drive and respiratory muscle strength. This can be determined by the effectiveness of respiratory efforts, for example, the magnitude of minute volume. In one trial (Gillespie et al. 2003), infants randomised to extubation decided by a minute ventilation test rather than clinical judgement, were extubated significantly sooner (8 versus 36 h), but there were no significant differences in the extubation failure rate between the two groups. An alternative approach has been to measure the tension time index of the diaphragm (TTdi), which is calculated as the product of mean transdiaphragmatic pressure (Pdi) and maximal Pdi (Pdi_max) and the ratio of inspiratory time (T_I) to the total respiratory cycle time (T_TOT) that is TTdi = (Pdi/Pdi_max) × (T_I/T_TOT). Healthy human volunteers could not sustain a set breathing task for longer than 45 min when the TTdi was greater than 0.15 (Bellemare and Grassino 1982a, b). In addition, in patients with chronic obstructive airways disease, those failing weaning had a mean TTdi of 0.17 compared to 0.09 in those who were successfully weaned (Purro et al. 2000). In a paediatric intensive care population, a TTdi of greater than 0.15 was demonstrated to be 100 % sensitive and 100 % specific of extubation failure (Harikumar et al. 2006), retrospective analysis of neonatal data showed similar results (Currie et al. 2011). It is now important to prospectively investigate TTTdi and TTmus (a non invasive assessment of the force produced by the respiratory muscles) in a neonatal population.