Assessment of validity and predictability of the FiO2-SpO2 transfer-function in preterm infants.

In this paper an investigation of the gain, delay, and time-constant parameters of the transfer function describing the relation between fraction of inspired oxygen (FiO2) and oxygen saturation in the blood (SpO2) in preterm infants is presented. The parameters were estimated following FiO2 adjustments and goodness of fit was used to assess the validity of the model when using an assumed first-order transfer function. For responses identified to be first-order, the estimated parameters were then clustered to identify areas where they tended to be concentrated. Each group described an operating region of the transfer function; thus, predicting the right operating region could potentially assist a range-based robust inspired oxygen controller to provide more optimal control by adapting itself to different clusters. Accordingly, the samples were assigned labels based on their cluster associations and 14 features available at the time of each adjustment were used as inputs to an artificial neural network to classify the clustered samples. The validity study suggested that 37% of the adjustments were followed by first-order responses. Prediction studies on the first-order responses indicated that the clusters could be predicted with an average accuracy of 64% when the parameters were divided into two groups.

Lower back-up rates improve ventilator triggering during assist-control ventilation: a randomized crossover trial.

OBJECTIVE: The objective of this study is to compare the effects of back-up ventilation rates (BURs) on triggered inflations and patient cardiorespiratory stability during assist-control/volume guarantee ventilation (AC/VG). STUDY DESIGN: This study is a randomized crossover trial conducted in a neonatal unit in an Australian tertiary NICU. In all, 26 stable preterm infants on AC/VG ventilation were studied at BUR settings of 30, 40 and 50 min(-1). Inflation rate, triggering and cardiorespiratory measures of patient stability were compared during 20 min epochs with 10 min washout periods. RESULT: The 26 infants studied were median (inter-quartile range) gestational age 27 (26, 30) weeks, birth weight 0.84 (0.75, 1.14) kg and FiO(2) 0.24 (0.21, 0.31) and age 6 (4, 19) days. At BURs of 30, 40 and 50, the proportions of inflations, which were triggered, were mean (s.d.) 85% (11), 75% (19) and 61% (25); P<0.01 for all comparisons. Total delivered inflation rates were 56 (8), 58 (9) and 62 (8) min(-1), respectively. Cardiorespiratory parameters did not vary between the settings. CONCLUSION: Using a lower BUR allows greater triggering of ventilator inflations. Cardiorespiratory parameters including CO(2) levels were stable at all rates.

Endotracheal DNase for atelectasis in ventilated neonates.

Management of atelectasis and lung collapse in ventilated neonates remains a common challenge in the neonatal intensive care unit. Recombinant human DNase (rhDNase) is an established treatment of atelectasis in cystic fibrosis and its use is also reported in the management of asthma, respiratory syncitial virus bronchiolitis and bronchiectasis to liquefy sputum and aid its clearance from the lungs. We report the use of rhDNase in a subgroup of ventilated neonates with severe end-stage respiratory failure and atelectasis. Three of the four patients showed clinical improvement. A previously undiagnosed lung anomaly was subsequently identified in the fourth patient. Future randomized studies could examine any potential benefits of this emerging therapy.

A practical guide to neonatal volume guarantee ventilation.

A recent systematic review and meta-analysis shows that volume-targeted ventilation (VTV) compared with pressure-limited ventilation (PLV) reduce death and bronchopulmonary dysplasia, pneumothorax, hypocarbia and severe cranial ultrasound abnormalities. In this paper, we present published research and our experience with volume guarantee (VG) ventilation, a VTV mode available on the Dräger Babylog 8000plus and VN500 ventilators. The VG algorithm measures the expired tidal volume (V(T)) for each inflation and adjusts the peak inflating pressure for the next inflation to deliver a V(T) set by the clinician. The advantage of controlling expired V(T) is that this is less influenced by endotracheal tube leak than inspired V(T). VG ventilation can be used with an endotracheal tube leak up to ∼50%. Initial set V(T) for infants with respiratory distress syndrome should be 4.0 to 5.0 ml kg(-1). The set V(T) should be adjusted to maintain normocapnoea. Setting the peak inflating pressure limit well above the working pressure is important to enable the ventilator to deliver the set V(T), and to avoid frequent alarms. This paper provides a practical guide on how to use VG ventilation.

High-frequency ventilation with the Dräger Babylog 8000plus: measuring the delivered frequency.

BACKGROUND: Ventilator frequency is one of the determinants of tidal volume delivery during high-frequency ventilation. Clinicians increasingly use data on ventilator displays to inform their decisions. AIM: To measure the frequencies delivered by the Dräger Babylog 8000plus ventilator when used in high-frequency mode. METHODS: Ventilator waveforms using a test lung were recorded at the full range of settings 5-20 Hz using Spectra software at 1000 Hz. The changes in frequency produced by a 1-Hz change in set frequency were calculated. Actual and displayed frequencies were compared. RESULTS: For settings up to 12 Hz, median (range) difference between set and delivered frequencies was 0 (-0.4 to +0.1) Hz. Above 12 Hz, delivered frequency varied by -0.3 (-1.9 to +0.3) Hz. For 1-Hz changes in frequency settings, in the range 5-12 Hz, 1-Hz changes produced a change in delivered frequency of 1.0 (0.6-1.4) Hz. Above 12 Hz, the corresponding changes were 0.7 (0-2.9) Hz. The ventilator displays the set frequency during operation rather than the delivered frequency. CONCLUSION: At 12 Hz and below, the differences between set and delivered frequencies were relatively small compared with those at 13 Hz and higher. Above 13 Hz, the difference between set and delivered frequencies was up to 2.9 Hz. Some frequency setting changes did not result in a change in delivered frequency.

Assist control volume guarantee ventilation during surfactant administration.

OBJECTIVE: To measure changes in ventilator parameters in preterm infants receiving surfactant during assist control volume guarantee (AC/VG) ventilation. METHODS: 22 preterm infants (up to 32 weeks' gestation) receiving surfactant for respiratory distress syndrome were enrolled in a prospective study of ventilator parameters during AC/VG ventilation at a tertiary neonatal intensive care unit. Ventilator pressures, flow and tidal volume waveforms were recorded from the Dräger Babylog 8000 plus in real time, and compared to pre-surfactant measurements. RESULTS: Following surfactant administration, 21 of 22 babies experienced completely obstructed endotracheal gas flow. Peak inflation pressure (PIP) increased by a median (IQR) of 8 (4-10) cm H2O, and took 30-60 min to return to baseline. Inspired oxygen concentration was reduced from a median (IQR) of 39% (26%-44%) to 26% (21%-30%) in the first 5 min. The set maximum PIP (Pmax) limited the delivered PIP such that most babies received tidal volumes less than the target value (V(Ttarget)) immediately following surfactant delivery. Four infants, in a subgroup of 11 infants where Pmax was set to less than 10 cm H2O above baseline PIP, were still receiving <90% of V(Ttarget) 20 min post surfactant. CONCLUSIONS: When giving surfactant during AC/VG ventilation, complete obstruction is common. PIPs increased and remain elevated for 30-60 min. The Pmax setting may restrict tidal volume delivery.

Volume-guarantee ventilation: pressure may decrease during obstructed flow.

BACKGROUND: Two unexpected observations were made during ventilation with the Dräger Babylog 8000+ in volume-guarantee mode: (a) during complete obstruction to gas flow down the endotracheal tube (ETT), positive inspiratory pressure (PIP) was reduced to half way between the maximum inflating pressure and the positive end expiratory pressure (PEEP) even though the set expired tidal volume had not been achieved; (b) an external Dräger waveform monitor may stop displaying real-time waveforms when a tube-obstructed alarm is activated. OBJECTIVE: To investigate these phenomena using a test lung. METHOD: A 50 ml Dräger test lung was attached to the ventilation circuit of a Dräger Babylog 8000+. Partial obstruction to ETT flow was induced by compressing the tubing leading to the test lung, and complete obstruction was achieved by clamping. Recordings were made from the digital output of the ventilator at 125 Hz. RESULTS: When the ETT flow was completely obstructed during VG ventilation, a constant PIP was set midway between the set maximum and PEEP. This did not happen during partial obstruction. The external waveform monitor display froze when ETT flow was completely obstructed. CONCLUSIONS: During complete ETT obstruction, the PIP is set to a pressure midway between maximum PIP and PEEP even if this is less than the PIP used before the obstruction. Further research is needed to evaluate whether this reduction in PIP is associated with prolongation of precipitating events.

Analysis of MRI images of a silicone elastomer under different axial loading conditions.

Silicone elastomers have been used as implants in orthopaedics for replacements of small joints such as the metacarpophalangeal joint. A series of experiments was conducted an axially tensioned silicone elastomer tendon spacer samples to determine whether there is a relationship between stress in the material and image intensity. With increasing load, reduced image intensities were observed using spin echo (SE), gradient echo (GE) and magnetic resonance imaging (MRI) snapshot techniques. MR attenuation was accentuated in the GE experiment. The T1 and T2 snapshot images were dim and showed a low signal-to-noise ratio, so analysis was limited. Changes in magnetic susceptibility are suggested as the mechanism causing reduced image intensity, due to the more pronounced attenuation in the GE image. An alternative mechanism is the alteration of crystallinity during loading, whereby molecular realignment modifies nuclear relaxation. The changes in spin lattice (T1) and spin spin (T2) relaxation times, which would support this, were not, however, satisfactorily demonstrated.