Endotracheal (ET) intubation and mechanical ventilation remains the standard of care for managing respiratory failure in very low birth weight (VLBW) infants. The adverse effects of prolonged endotracheal intubation include bacterial colonisation, sepsis, subglottic injury and bronchopulmonary dysplasia (BPD).1 The decision to extubate is usually based on clinical assessment. This is subjective and up to 40% of extubated infants require reinsertion of their ET tube.2 Reintubation is destabilising, traumatic and may prolong the duration of mechanical ventilation.3 Objective criteria to identify the earliest opportunity to successfully extubate VLBW infants are lacking.

We have previously described and evaluated respiratory drive in VLBW infants using a 3-minute spontaneous breathing trial (SBT) during ET continuous positive airway pressure (CPAP) before extubation.4 The test when applied once a clinical decision was made to extubate was highly sensitive and appeared to identify infants for whom extubation was most likely to fail (a high negative predictive value). The SBT was then incorporated into clinical practice at our hospital. The aim of our current study was to audit the effect of this change in policy on timing and success of extubation of VLBW infants.

METHODS

The study was conducted between 1 June 2005 and 30 June 2006 at The Royal Women’s Hospital, Melbourne and approved by the human research and ethics committees as an audit of practice. All ventilated infants with a birth weight <1250 g were eligible for inclusion. Infants with lethal or chromosomal malformations or who died before extubation were excluded.

Using the Dräger Babylog 8000plus (Lubeck, Germany), an SBT was performed when ventilated infants fulfilled entry criteria (set ventilator rate ⩽45 bpm, volume guarantee (VG) ⩽4.5 ml/kg or peak inspiratory pressures ⩽25 cm H₂O, inspired fractional oxygen ⩽40%). This was repeated daily, typically on ward rounds until either extubation criteria were met (“successful SBT”), unplanned extubation occurred or when the attending clinician made a decision to extubate (whichever was the sooner). Infants were ventilated and weaned using either assist control (AC) or synchronised intermittent mandatory ventilation (SIMV) ±VG modes. The ET CPAP pressure used was equivalent to the positive end expiratory pressure used in conventional ventilation (5–6 cm H₂O). A failed SBT was recorded if the infant had bradycardia (<100/min) for >15 seconds and/or the Spo₂ fell below 85% despite a 15% increase in fractional inspired oxygen (Fio₂) during the period of ET CPAP. At this point the SBT was discontinued and ventilation restarted. Infants who passed the SBT were extubated unless the attending clinicians disagreed. Non-compliance of the clinician with the results of the SBT was recorded.

The primary study outcome was reintubation within 72 hours of extubation. The indications for reintubation were specified a priori: (a) more than six episodes of apnoea requiring stimulation in 6 hours, or more than one significant episode of apnoea requiring bag and mask ventilation; (b) respiratory acidosis (Paco₂ >65 mm Hg (>8.5 kPa) and pH <7.25); (c) Fio₂ >0.60 to maintain Spo₂ in the target range (90–94%). BPD was defined as supplemental O₂ or CPAP requirement at 36 weeks’ corrected gestational age. Data were compared with a cohort of VLBW infants admitted to the same institution in 2002, before the SBT was part of clinical practice.

Power calculation and statistical analysis

During a 2-year period (2003–4), the rate of reintubation was 24%. Given the skewed nature of the duration of ventilation, the mean (SD) log transformed duration of ventilation for VLBW infants was 2.0 (0.7), corresponding to a geometric mean of 100 hours (102). We hypothesised a reduction of 50% in the duration of assisted ventilation compared with historical controls using the SBT to drive extubation (geometric mean of 50 hours, log transformed  = 1.7). Eighty-seven patients in each group were required to achieve 80% power and an α error of 0.05. Statistical analyses were performed using SPSS, version 11.5 (Cary, NJ, USA).

RESULTS

One hundred and eighty infants were studied (90 SBT and 90 controls). Table 1 demonstrates that the baseline variables were well matched between the two groups. However, fewer infants in the SBT cohort were intubated in the delivery room. During the latter period (SBT), VG ventilation was more commonly used compared with the control population (94% vs 26%) and there was a shift of weaning ventilation modes at the time of extubation from SIMV (93% in controls) to AC (81% in SBT cohort).

The mean (SD) number of SBTs applied to infants was 3 (2). Clinicians’ compliance with the SBT was excellent (97%). Six (7%) infants self-extubated after a failed SBT, three of whom were reintubated. The sensitivity of the SBT to predict successful extubation was 83%. The overall duration of mechanical ventilation was not significantly different (table 2). However, infants were extubated at significantly higher ventilator rates and mean airway pressure (P_aw) when the SBT was applied. Duration of CPAP was significantly higher in the SBT cohort, although there was no difference in the duration of all forms of ventilatory support or the incidence of BPD.

DISCUSSION

We showed previously that the SBT applied to infants judged by their clinician as being ready for extubation was accurate in predicting success or failure. This audit of a change in clinical practice was performed in order to evaluate the safety and effectiveness of the SBT to drive extubation of VLBW infants. Although using historical controls limits the number of conclusions that can be drawn from the study, there were two potentially harmful outcomes that we wished to exclude. First, the period of endotracheal intubation could be prolonged if clinicians waited for infants to pass the SBT when they might otherwise have decided to extubate earlier. Alternatively, by encouraging extubation sooner than the clinician might have decided, more infants might have required reintubation.

We have shown that invasive mechanical ventilatory support in VLBW infants can be safely discontinued using the SBT at significantly higher P_aw and set ventilator rates with similar reintubations compared with clinical decision-making. A clinical trial, randomising infants to have extubation guided by SBT versus clinical decision-making alone is therefore feasible. From our data, the median duration of ventilation for VLBW infants is <3 days and a very large sample size would be needed to demonstrate a significant reduction in duration of mechanical ventilation.

Limitations of our study design are highlighted by the differences seen in the two tables. While the patient demographics and indicators of illness severity are similar, the data demonstrate changes in the ventilator management of VLBW infants within one institution. Significantly fewer infants are being intubated in the delivery room and ventilator weaning modes have changed, with greater use of AC with VG. Extubating infants from a higher P_aw in the SBT group may be due changes in ventilator mode between the two time periods. The proportion of supported breaths during AC would be greater than infants received on SIMV. Our data suggest it is safe to wean and extubate from AC mode with a mean P_aw of 7.2 cm H₂O.

Our findings are consistent with reports in paediatric patients, where standardised protocols for weaning compared with clinical judgment confer no benefit on duration of ventilation and rates of extubation success.5 A single randomised study in newborn infants comparing defined weaning criteria based on minute volumes with clinical assessment found a shorter duration in weaning from randomisation but in an underpowered study, more extubations failed with the intervention.6

In conclusion a 3-minute SBT to determine readiness to extubate VLBW infants is safe and as effective as clinical decision-making alone. Whether this mode of weaning can reduce duration of all respiratory support, BPD and length of stay needs further evaluation in a prospective randomised controlled study.