Randomised controlled trial of two methods of weaning from high frequency positive pressure ventilation

Ultrashort inspiratory times homogenize ventilation distribution in an inhomogeneous two-compartment model of the neonatal lung

BACKGROUND

Inhomogeneous lung ventilation and pulmonary air leaks are common adverse effects of mechanical ventilation in preterm infants suffering from respiratory distress syndrome. We hypothesized that shortening of inspiratory times can improve the homogeneity of lung aeration. We assumed that ultrashort inspiratory times lead to a full build-up of pressure in regions with low compliance while maintaining incomplete filling of regions with high compliance.

METHODS

We connected a two-compartment model of the lung with different compliances of the two compartments to a neonatal ventilator. Pressures and flow rates were measured separately for each compartment at inspiratory times ranging from 0.1 to 0.8 s and various combinations of tidal volumes.

RESULTS

An inspiratory time of 0.8 s resulted in near total pressure equalization between the airway pressure and the two compartments, as 97% of the maximum applied peak pressure was reached in the low compliance compartment and 95% in the high compliance compartment. The distribution of the tidal volume was proportional to the compartment compliance. Ultrashort inspiratory times lowered the peak pressure and tidal volume in the high compliance compartment but maintained higher pressure and volume in the low compliance compartment. An inspiratory time of 0.2 s resulted in a peak pressure of 80% of peak airway pressure in the low compliance compartment and in 61% of peak airway pressure in the high compliance compartment (p < .001).

CONCLUSION

Ventilation with ultrashort inspiratory times may improve homogeneity of air distribution in inhomogeneous lungs of ventilated preterm or term infants.

Predictors of extubation readiness in preterm infants: a systematic review and meta-analysis

CONTEXT

A variety of extubation readiness tests have already been incorporated into clinical practice in preterm infants.

OBJECTIVE

To identify predictor tests of successful extubation and determine their accuracy compared with clinical judgement alone.

METHODS

MEDLINE, Embase, PubMed, Cochrane Library and Web of Science were searched between 1984 and June 2016. Studies evaluating predictors of extubation success during a period free of mechanical inflations in infants less than 37 weeks' gestation were included. Risk of bias was assessed using the Quality Assessment of Diagnostic Accuracy Studies-2 tool. After identifying and describing all predictor tests, pooled sensitivity and specificity estimates for the different test categories were generated using a bivariate random-effects model.

RESULTS

Thirty-five studies were included, showing wide heterogeneities in population characteristics, methodologies and definitions of extubation success. Assessments ranged from a few seconds to 24 hours, provided 0-6 cmH₂O positive end-expiratory pressure and measured several clinical and/or physiological parameters. Thirty-one predictor tests were identified, showing good sensitivities but low and variable specificities. Given the high variation in test definitions across studies, pooling could only be performed on a subset. The commonly performed spontaneous breathing trials had pooled sensitivity of 95% (95% CI 87% to 99%) and specificity of 62% (95% CI 38% to 82%), while composite tests offered the best performance characteristics.

CONCLUSIONS

There is a lack of strong evidence to support the use of extubation readiness tests in preterm infants. Although spontaneous breathing trials are attractive assessment tools, higher quality studies are needed for determining the optimal strategies for improving their accuracy.

Long versus short inspiratory times in neonates receiving mechanical ventilation

BACKGROUND

When intermittent positive pressure ventilation (IPPV) was introduced in newborn infants with hypoxic respiratory failure from hyaline membrane disease (HMD), mortality was high and air leaks problematic. This barotrauma was caused by the high peak inspiratory pressures (PIP) required to oxygenate stiff lungs. The primary determinants of mean airway pressure (and thus oxygenation) on a conventional ventilator are the inspiratory time (IT), PIP, positive end expiratory pressure and gas flow rates. In the 1970s uncontrolled studies on a small number of infants demonstrated a benefit in reducing barotrauma using a long IT and slow rates. This strategy was subsequently widely adopted. Current neonatal ventilators have been designed to minimise lung injury but rates of bronchopulmonary dysplasia (BPD) remain high. It is therefore important that the inspiratory time causing least harm is used.

OBJECTIVES

To determine in mechanically ventilated newborn infants whether the use of a long rather than a short IT reduces the rates of death, air leak and BPD.

SEARCH STRATEGY

The standard search strategy of the Cochrane Neonatal Review Group (CNRG) was used. Searches of electronic and other databases were performed. These included MEDLINE (1966 - April 2004) and the Cochrane Central Register of Controlled Trials (CENTRAL, The Cochrane Library, Issue 4, 2003). In order to detect trials that may not have been published, the abstracts of the Society for Pediatric Research, and the European Society for Pediatric Research were searched from 1998 - 2003.

SELECTION CRITERIA

All randomised and quasi-randomised controlled trials enrolling mechanically ventilated infants with or without respiratory pathology evaluating the use of long versus short IT (including randomised crossover studies with outcomes restricted to differences in oxygenation).

DATA COLLECTION AND ANALYSIS

The standard method of the Cochrane Collaboration and its Neonatal Review Group were used. Two authors independently assessed eligibility, and the methodological quality of each trial, and extracted the data. The data were analysed using relative risk (RR) and risk difference (RD) and their 95% confidence intervals. A fixed effect model was used for meta-analyses.

MAIN RESULTS

In five studies, recruiting a total of 694 infants, a long IT was associated with a significant increase in air leak [typical RR 1.56 (1.25, 1.94), RD 0.13 (0.07, 0.20), NNT 8 (5, 14)]. There was no significant difference in the incidence of BPD. Long IT was associated with an increase in mortality before hospital discharge that reached borderline statistical significance [typical RR 1.26 (1.00, 1.59), RD 0.07 (0.00, 0.13)].

REVIEWERS' CONCLUSIONS

Caution should be exercised in applying these results to modern neonatal intensive care, because the studies included in this review were conducted prior to the introduction of antenatal steroids, post natal surfactant and the use of synchronised modes of ventilatory support. Most of the participants had single pathology (HMD) and no studies examined the effects of IT on newborns ventilated for other reasons such as meconium aspiration and congenital heart disease (lungs with normal compliance). However, the increased rates of air leaks and deaths using long ITs are clinically important; thus, infants with poorly compliant lungs should be ventilated with a short IT.

Bronchopulmonary dysplasia in infants with respiratory distress syndrome in a developing country: a prospective single centre-based study

The aim of this prospective study was to determine the incidence of bronchopulmonary dysplasia (BPD) in and the outcome of neonates ventilated for respiratory distress syndrome (RDS). The study was conducted in a developing country prior to the use of surfactant replacement therapy and the results are compared to published reports from the developed world. BPD was defined as oxygen dependency beyond day 28 of life. The incidence of BPD over a 9-month-period was 8.2% of all neonates requiring ventilation (n = 169) and 41% (n = 38) of neonates ventilated for RDS (n = 92). Of those neonates who developed BPD, 26% were still being ventilated on day 28. Of the infants, 21 (55%) developed type 1 BPD and 17 (45%) type 2 BPD. There was no statistical difference in the severity of lung disease on any of the study days between type 1 and type 2 BPD although neonates with type 2 BPD required assisted ventilation and supplemental oxygen for a longer period: 30 versus 12 days and 95 versus 49 days, respectively. Of those neonates who developed BPD, 8 (21%) died prior to discharge from hospital and a further 5 infants (17%) died subsequent to discharge. Of the latter five, three died from treatable causes (gastroenteritis n = 2, pneumonia n = 1). Of the 25 (83%) children seen at follow up, 68% were developing normally, 20% were classified as having suspect development and 12% had developed cerebral palsy at corrected postnatal ages of 12-24 months. None of the results differed significantly from those of neonates being ventilated in the developed world, except for the causes of post-discharge deaths.

CONCLUSION

Health services providing ventilation for neonates in the developing world will have to take the needs of children with BPD into account when planning a neonatal service which should include among others a widely available and easily accessible primary health care system.

Synchronous intermittent mandatory ventilation modes compared with patient triggered ventilation during weaning

The efficacy of combining rate and pressure reduction during weaning by synchronous intermittent mandatory ventilation (SIMV) were compared with weaning by patient triggered ventilation (PTV) (pressure reduction alone) in two randomised trials. Regardless of ventilation mode, pressure was reduced to the same level according to the size of the infant. In the first trial, the SIMV rate was also reduced progressively to a minimum of 20 breaths/minute, and in the second to five breaths/minute. Forty premature infants aged 15 days of age or less were randomly allocated into each trial. No significant differences were found in the first trial between ventilation modes in either the duration of weaning or the number of infants in whom weaning failed. In the second trial, the duration of weaning was shorter by PTV than by SIMV (median 24 hours, range 7-432 v 50 hours, range 12-500; p < 0.05); weaning failed in two infants in the PTV group and in five in the SIMV group. It is concluded that weaning by a combination of pressure and rate reduction, such as can be achieved during SIMV, offers no significant advantage over pressure reduction alone.

Inspiratory and expiratory times for infants ventilator-dependent beyond the first week

The aim of this study was to determine optimum inspiratory and expiratory times to be used for ventilation of infants older than one week of age. Each infant was studied at a rate of 30 breaths/min (inspiratory times (TI) of 1.0, 0.67 and 0.5 s with expiratory times (TE) of 1.0, 1.33 and 1.5 s, respectively) and at a rate of 60 breaths/min (TI 0.5, 0.33 and 0.25 s and TE 0.5, 0.67 and 0.75 s, respectively). Arterial blood-gases were examined after 20 min on each setting. Fifteen infants with a median gestational age of 27 weeks were studied at a median postnatal age of 9 days and 10 infants with a median gestational age of 27 weeks at a median postnatal age of 24 days. All infants had type I chronic lung disease. Oxygenation did not consistently improve as TI was prolonged, elevating mean airway pressure but, particularly in older infants, was better at TI > or = 0.5 s compared with TI < 0.5 s. In both groups, carbon dioxide elimination was better at 60 than at 30 breaths/min. Thus we suggest that in infants fully ventilator-dependent beyond the first week of life, an inspiratory and expiratory time of 0.5 s should be used as the first choice.

Comparison of weaning by patient triggered ventilation or synchronous intermittent mandatory ventilation in preterm infants

Forty preterm infants were entered into a randomized controlled trial to compare the efficacy and duration of weaning by patient triggered ventilation (PTV) to that of synchronous intermittent mandatory ventilation (SIMV). Infants were randomized during recovery from respiratory distress once ventilator rate had been reduced to 40 breaths per minute; weaning during PTV was by reduction in ventilator pressure only, whereas infants randomized to SIMV were weaned by reduction in rate only. Weaning failed in 12 infants, 6 from each group, the 12 infants were more immature than those in whom weaning succeeded (p < 0.01). Overall, the duration of weaning did not differ significantly between the PTV and SIMV groups.

Randomised trial of methods of extubation in acute and chronic respiratory distress

Sixty infants (median gestational age 29 weeks) with acute and 60 infants (median gestational age 25 weeks) with chronic respiratory distress were randomised to be extubated either directly into a headbox or onto 3 cm H2O nasal continuous positive airway pressure (CPAP). Our aim was to test the hypothesis that extubation onto nasal CPAP rather than directly into a headbox was more likely to be associated with successful extubation in infants with acute rather than chronic respiratory distress. Overall the failure rate of extubation was approximately 33%, with no significant difference between the infants with acute and chronic respiratory distress. There was no significant difference in the failure rate of extubation among infants randomised to receive nasal CPAP or headbox oxygen in either the acute or chronic respiratory distress groups.

Randomised controlled trial of weaning by patient triggered ventilation or conventional ventilation

A group of preterm infants (n = 40) were entered into a randomised controlled trial to compare the duration and efficacy of weaning by patient triggered ventilation (PTV) or conventional ventilation. Once recovery from respiratory distress had begun, enabling the ventilator rate to be reduced to 40 breaths/min, infants were randomised to either regime. Infants randomised to PTV were weaned by reduction in ventilator pressure only, whereas infants randomised to conventional ventilation were weaned by reduction in ventilator rate only. Only one infant required re-ventilation within 24 h of extubation; this infant had been weaned by conventional ventilation. Three infants, all of less than 28 weeks gestation, did not tolerate weaning by PTV and were subsequently weaned conventionally. The duration of weaning was analysed according to the original randomisation allocation and was significantly shorter in the PTV group, being a median of 30 h (mean 39, range 3-186) compared to a median of 61 h (mean 65, range 15-262) in the conventional group, P < 0.02. We conclude PTV is the more advantageous form of weaning in preterm infants of greater than 27 weeks gestational age.

Neonatal respiratory support and lung function abnormalities at follow-up

We have investigated if respiratory distress syndrome (RDS) treated by an increased inspired oxygen concentration, rather than mechanical ventilation, was associated with impaired lung function at follow-up and/or an increase in respiratory symptoms. Thoracic gas volume (TGV) and airways resistance (RAW) were measured in eight pre-term infants (median gestational age 29 weeks) at 6 and 12 months of age. The infants had suffered from RDS but had not required mechanical ventilation. Their results were compared to 16 other infants, matched for gestational age; eight who had required ventilation in the neonatal period and eight who had had no RDS. In all three groups the occurrence of respiratory symptoms was recorded. The lung function of the infants requiring oxygen in the neonatal period was similar to those who had not suffered from RDS, but their airways resistance was significantly lower at 6 but not 12 months than that of infants ventilated in the neonatal period (P less than 0.05). There was no significant difference in recurrent respiratory symptoms between the three groups although a greater proportion of the infants ventilated in the neonatal period were symptomatic in the first 6 months of life. These results suggest that oxygen therapy alone does not result in an impairment of lung function which is independent of the effect of prematurity.

Airway pressure triggered ventilation for preterm neonates

The usefulness of airway pressure triggered ventilation for the preterm newborn has been assessed using a new patient triggered valveless ventilator, the SLE 2000 infant ventilator (SLE 2000). This ventilator performs well at fast rates with no inadvertent positive end expiratory pressure (PEEP) even at rates of 150 breaths per minute (bpm). The ventilator is triggered by a change in airway pressure equal to or exceeding 0.5 cmH2O. If the infant fails to achieve the change in airway pressure which will trigger the ventilator the infant is ventilated at the back-up rate which is predetermined in conventional mode prior to commencing PTV. Infants were ventilated for one hour on a conventional neonatal ventilator, then for one hour on the SLE 2000 in conventional mode without changing the ventilator settings and finally for one hour on the SLE 2000 in patient triggered mode. Arterial blood gases were checked at the end of each hour. During patient triggered ventilation (PTV) the peak pressure, inspiratory time and inspired oxygen concentration were the same as those used during conventional mode. Simultaneous recordings were made of flow, volume, ventilator and oesophageal pressure change, from this recording the trigger delay during PTV was calculated. The trigger delay, being the time lag from the start of spontaneous inspiration, indicated by the negative deflection in the oesophageal pressure trace, and the onset of the ventilator breath. Thirteen infants were included in the study, median gestational age 32 weeks (range 25-35) and birthweight 1640 g (range 838-3038). All were being ventilated for respiratory distress syndrome (RDS) and were 4 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)