Crossover study of proportional assist versus assist control ventilation.

OBJECTIVE: To test the hypothesis that in very prematurely born infants remaining ventilated beyond the first week, proportional assist ventilation (PAV) compared with assist control ventilation (ACV) would be associated with reduced work of breathing, increased respiratory muscle strength and less ventilator-infant asynchrony which would be associated with improved oxygenation. DESIGN: Randomised crossover study. SETTING: Tertiary neonatal unit. PATIENTS: 12 infants with a median gestational age of 25 (range 24-26) weeks were studied at a median of 43 (range 8-86) days. INTERVENTIONS: Infants were studied for 1 h each on PAV and ACV in random order. MAIN OUTCOME MEASURES: At the end of each hour, the work of breathing (assessed by measuring the diaphragmatic pressure time product), thoracoabdominal asynchrony and respiratory muscle strength (maximal inspiratory pressure, maximal expiratory pressure (Pemax) and maximal transdiaphragmatic pressure (Pdimax)) were assessed. Blood gas analysis was performed and the oxygenation index (OI) calculated. RESULTS: After 1 h on PAV compared with 1 h on ACV, the median OI (5.55 (range 5-11) vs 10.10 (range 7-16), p=0.002) and PTP levels were lower (217 (range 59-556) cm H2O.s/min vs 309 (range 55-544) cm H2O.s/min, p=0.005), while Pdimax (44.26 (range 21-66) cm H2O vs 37.9 (range 19-45) cm H2O, p=0.002) and Pemax (25.6 (range 6.5-42) cm H2O vs 15.9 (range 3-35) cm H2O levels p=0.010) were higher. CONCLUSIONS: These results suggest that PAV compared with ACV may have physiological advantages for prematurely born infants who remain ventilated after the first week after birth.

Randomised trial of volume-targeted ventilation versus pressure-limited ventilation in acute respiratory failure in prematurely born infants.

BACKGROUND: During volume-targeted ventilation (VTV), a constant volume is delivered with each ventilator inflation. OBJECTIVES: To determine whether VTV compared to pressure-limited ventilation (PLV) reduced the time to reach weaning criteria in prematurely born infants with acute respiratory distress, and if any difference was explained by better respiratory muscle strength and/or a lower work of breathing (WOB). METHODS: Infants of <34 weeks of gestational age ventilated for <24 h in the first week after birth were randomised to receive either VTV or PLV. The primary outcome was the time to achieve pre-specified weaning criteria. Respiratory muscle strength was assessed by the measurement of the maximum inflation and expiratory pressures, and the WOB assessed by the transdiaphragmatic pressure time product. Other outcomes reported are the duration of ventilation, occurrence of patent ductus arteriosus, pneumothorax, intraventricular haemorrhage, periventricular leukomalacia and episodes of hypocarbia. RESULTS: Forty infants, median gestational age 27 (range 23-33) weeks, were recruited. The time taken to achieve weaning criteria was similar in the two groups [median 14 h (VTV) vs. 23 h (PLV)]. There were no significant differences between the groups with regard to respiratory muscle strength, WOB or other outcomes, except that fewer of the VTV compared to the PLV group had episodes of hypocarbia (8 vs. 19; p < 0.001). CONCLUSION: In prematurely born infants with acute respiratory failure, use of VTV did not reduce the time to reach weaning criteria, but was associated with a reduction in episodes of hypocarbia.

Prediction of extubation outcome in infants using the tension time index.

OBJECTIVE: Approximately one-third of ventilated infants fail extubation. The objective of this study was to determine whether assessment of the load relative to the capacity of respiratory muscles by measurement of the tension time index (TTI) successfully predicted extubation outcome in infants. DESIGN: Prospective study. SETTING: Tertiary neonatal intensive care unit. PATIENTS: Twenty ventilated infants, with a median gestational age of 31 (range 24-39) weeks, were studied at a median postnatal age of 2.5 (range 1-37) days. INTERVENTIONS: The diaphragm tension time index (TTdi) was derived from measurements of transdiaphragmatic pressure using a dual-pressure transducer tipped catheter. The respiratory muscle tension time index (TTmus) was derived from non-invasive airway pressure measurements. Measurements were made within the 6 h prior to extubation. MAIN OUTCOME MEASURES: Extubation failure was defined as the need for reintubation within 48 h of extubation. RESULTS: Five infants failed extubation; their median TTdi (p=0.001) and TTmus (p=0.001) were significantly higher than those of the successfully extubated infants. A TTdi of >0.15 and a TTmus of >0.18 were 100% sensitive and 100% specific in predicting extubation failure CONCLUSION: In ventilated infants, invasive and non-invasive measurements of the TTI could provide an accurate prediction of extubation outcome.

Work of breathing and volume targeted ventilation in respiratory distress.

OBJECTIVE: To determine the level of volume targeting (VT) associated with the lowest work of breathing (WOB) for prematurely born infants being ventilated with acute respiratory distress. DESIGN: Prospective study. SETTING: Tertiary neonatal intensive care unit. PATIENTS: 18 infants, median gestational age 29 (range 25-34) weeks, being ventilated for acute respiratory distress. INTERVENTIONS: Infants were studied first without VT (baseline). Volume targeted levels of 4 ml/kg, 5 ml/kg and 6 ml/kg were then delivered in random order. After each VT level, the infants were returned to baseline. Each step was maintained for 20 minutes. MAIN OUTCOME MEASURE: The transdiaphragmatic pressure time product (PTPdi) as an estimate of the WOB. RESULTS: The mean PTPdi was higher at a VT level of 4 ml/kg (median 154 cm H(2)O·s/min) compared to baseline (median 112 cm H(2)O·s/min) (p<0.001) and a VT level of 6 ml/kg (median 89 cm H(2)O·s/min) (p<0.001). CONCLUSION: A low level of VT increased the WOB in infants with acute respiratory distress syndrome. The authors' results suggest that, during acute respiratory distress, a VT level of at least 5 ml/kg rather than a lower level might avoid an increased WOB. The most appropriate level of VT needs to be determined in a randomised controlled trial with long-term outcomes.

In vitro assessment of proportional assist ventilation.

OBJECTIVE: During proportional assist ventilation (PAV) the timing and frequency of inflations are controlled by the patient and the patient's work of breathing may be relieved by elastic and/or resistive unloading. It is important and the authors' objective to determine whether ventilators delivering PAV function well in situations mimicking neonatal respiratory conditions. DESIGN: In vitro laboratory study. SETTING: Tertiary neonatal ICU. INTERVENTIONS: Dynamic lung models were developed which mimicked respiratory distress syndrome, bronchopulmonary dysplasia and meconium aspiration syndrome to assess the performance of the Stephanie neonatal ventilator. MAIN OUTCOME MEASURES: The effects of elastic and resistive unloading on inflation pressures and airway pressure wave forms and whether increasing unloading was matched by an 'inspiratory' load reduction. RESULTS: During unloading, delivered pressures were between 1 and 4 cm H(2)O above those expected. Oscillations appeared in the airway pressure wave form when the elastic unloading was greater than 0.5 cm H(2)O/ml with a low resistance model and 1.5 cm H(2)O/ml with a high resistance model and when the resistive unloading was greater than 100 cm H(2)O/l/s. There was a time lag in the delivery of airway pressure of at least 60 ms, but increasing unloading was matched by an inspiratory load reduction. CONCLUSIONS: During PAV, unloading does reduce inspiratory load, but there are wave form abnormalities and a time lag in delivery of the inflation pressure. The impact of these problems needs careful evaluation in the clinical setting.

Work of breathing and different levels of volume-targeted ventilation.

OBJECTIVES: The objectives of this study were to determine the impact of different volume-targeted levels on the work of breathing and to investigate whether a level that reduced the work of breathing below that experienced during ventilatory support without volume targeting could be determined. METHODS: The transdiaphragmatic pressure-time product, as an estimate of the work of breathing, was measured for 20 infants (median gestational age: 28 weeks) who were being weaned from respiratory support by using patient-triggered ventilation (either assist-control ventilation or synchronous intermittent mandatory ventilation). The transdiaphragmatic pressure-time product was measured first without volume targeting (baseline) and then at volume-targeted levels of 4, 5, and 6 mL/kg, delivered in random order. After each volume-targeted level, the infants were returned to baseline. Each step was maintained for 20 minutes. RESULTS: The mean transdiaphragmatic pressure-time product was higher with volume targeting at 4 mL/kg in comparison with baseline, regardless of the patient-triggered mode. The transdiaphragmatic pressure-time product was higher at a volume-targeted level of 4 mL/kg in comparison with 5 mL/kg and at 5 mL/kg in comparison with 6 mL/kg. The mean work of breathing was below that at baseline only at a volume-targeted level of 6 mL/kg. CONCLUSIONS: Low volume-targeted levels increase the work of breathing during volume-targeted ventilation. Our results suggest that, during weaning, a volume-targeted level of 6 mL/kg, rather than a lower level, could be used to avoid an increase in the work of breathing.