Greenough et al., Airway pressure triggered ventilation for preterm neonates

J. Perinat. Med. 19(1991)471-476

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Airway pressure triggered ventilation for preterm neonates Anne Greenough, Michael F. Hird, and Vivien Chan

Department of Child Health, King's College Hospital, London, U.K.

1

Introduction

Patient triggered ventilation (PTV) has been available for neonatal use since 1986 [10]. Initially a conventional newborn ventilator (the SLE newborn) modified to be patient triggered by changes in abdominal expansion, sensed by a Graseby MR 10 respiration monitor, was used. Subsequently other triggering devices were incorporated [2, 3, 7]. A series of studies have suggested that PTV is frequently associated with acute improvements in blood gases when compared with conventional ventilation [2, 3, 7]. This change in gas exchange after the first hour of PTV is a useful predictor of the outcome on PTV, an improvement in oxygenation being significantly associated with PTV being successfully maintained until extubation [11]. Such studies have also demonstrated that PTV may be most successful in infants with mild respiratory distress and those who are being weaned from mechanical ventilation [3].

2 Methods Infants were entered into the study on the fifth day of life, as at this stage they were likely to be recovering from their respiratory distress. All the infants were being ventilated via oro-endotracheal tubes using Sechrist ventilators. The ventilator settings had been determined by the clinicians and chosen so as to keep the arterial blood gases within the clinically acceptable range prior to the study. On trial entry the infants were first studied for one hour on their conventional ventilator. An inspiratory time of 0.4 seconds was used in all infants regardless of ventilator rate. The infants were then transferred to the SLE 2000. For the first hour on the SLE 2000 no change was made in ventilator settings, but for the second hour the SLE 2000 was switched to patient triggered mode.

ation circuit. The patient circuit is supplied with More recently purpose-built patient triggered a constant fixed flow of fresh gas at five litres ventilators have appeared. One of these, the SLE per minute. The expiratory block of the SLE 2000 infant ventilator (SLE 2000) incorporates 2000 has two orifices, the front generates positive an airway pressure trigger. This form of trigger- end expiratory pressure (PEEP) and is supplied ing has been shown to be superior to earlier via the continuous positive airways pressure triggering systems as, when used with the pre- (CPAP)/PEEP regulator with the same oxygen vious type of SLE ventilator, had a shorter trig- concentration as the fresh gas supply. It generger delay and greater sensitivity than the Graseby ates an opposing flow to the fresh gas in the MR 10 respiratory monitor [8]. Another advan- exhalation tube and thus creates CPAP or PEEP. tage of the SLE 2000 is that it delivers a back- The rear orifice is used to generate the peak up rate if the infant fails to exceed the critical inspiratory pressure. An electronic module conchange in airway pressure. It is also claimed that, trols the rate and duration of the opening of a as the ventilator has a valveless pneumatic ex- solenoid valve. When the valve is open the drivhalation circuit, it has no inadvertent PEEP at ing gas enters the expiratory tube in opposition the fast rates which are likely to be triggered by to the fresh gas flow, this flow acts as a pneuvery small preterm infants [4]. The aim of this matic piston and creates a pressure wave at the study was to evaluate this ventilator and its use- endotracheal connector. The same gas mixture fulness in patient triggered mode amongst infants Brought from tothe oxygen blender is used to supply the you by | University of Arizona recovering from respiratory distress. patient with fresh gas and both the CPAP/PEEP Authenticated 1991 by Walter de Gruyter & Co. Berlin · New York

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472

Greenough et al., Airway pressure triggered ventilation for preterm neonates

and peak inspiratory pressure orifices, this avoids any possibility of dilution of the fresh gas mixture. The ventilator was tested in our laboratory using a lung model with a similar compliance and resistance to that of an infant suffering from RDS [5]. A series of rates from 30-120 bpm were used, peak inspiratory pressure 30 cmH2O, PEEP 3 cmH2O, inspiratory time 0.4 seconds and flow 5 I/minute. Inflating volume was calculated from the integrated flow signal measured by a pneumotachograph placed between the ventilator circuit and lung model and airway pressure changes were continuously monitored using a Vallidyne pressure transducer [5]. Throughout the rate sequence the inflating volume was maintained and no inadvertent PEEP was demonstrated even at rates of 150 bpm. In PTV mode the SLE 2000 is triggered by a change in airway pressure within the ventilator circuit of greater than or equal to 0.5 cmH2O. During PTV if the infant fails to trigger the ventilator there is a back-up rate which is set by the rate control when the ventilator is in conventional mode. When the mode switch is turned to PTV the last settings used in conventional mode become the back-up rate.

3 Patients Thirteen patients, all four days of age, were recruited into the study. Their median birthweight was 1640 g (range 838 — 3038) and gestational age 32 weeks (range 25 — 35). The infants were all ventilated for respiratory distress syndrome (RDS). Immediately prior to commencing the study their median ventilator rate was 30 bpm (range 10 — 60), peak inspiratory pressure 15 cmH2O (range 12 — 20) and inspired oxygen concentration 22% (range 21—40). 4 Results All 13 infants successfully completed the study, none requiring to be withdrawn because of low oxygen levels demonstrated by their continuous monitoring. The median trigger delay was found to be 80 msecs (range 40 — 100). The paO2 or paCO2 levels tended to improve on conventional ventilation using the SLE 2000 compared to the period on the conventional ventilator, but this did not reach statistical significance. The 95% confidence intervals of the differences of the means of these two periods being -4.6 to 3.7 mmHg (paO2) and -5.8 to 9.1 mmHg (paCO2) respectively.

During PTV simultaneous recordings were made a flow, volume, ventilator and oesophageal pressure changes, from this recording the trigger Table. Arterial blood bases during the three study delay was calculated [7, 8]. The trigger delay was periods calculated as the time lag from the start of in- median (range) spiration indicated by a negative deflection in paO2 paCO2 the oesophageal pressure trace and the onset of (mmHg) (mmHg) the ventilator breath. The trigger delay was expressed as the mean of the delays over the last 55 36 Conventional five minutes of the recordings. (47-92) (26-57) ventilator Throughout the study infants were monitored SLE ventilator: continuously using either a Searle intra-arterial Conventional 64 32 electrode or a transcutaneous oxygen electrode. ventilation (42-94) (25-46) Infants were withdrawn from the study if their continuous monitoring demonstrated the oxygen Patient 68 26 level to below 40 mmHg. Arterial blood gas triggered (45-104) (18-40) ventilation samples were obtained from the indwelling arterial line after each of the three one-hour peFollowing one hour of PTV on the SLE 2000 riods. Statistical analysis: Differences between the three there was a significant increase in paO2 period of ventilation were assessed for statistical (p < 0.01) and a significant reduction in paCO2 significance using the paired Wilcoxon rank sum (p < 0.01). The 95% confidence intervals of the test. The confidence intervals were calculated differences of the means of the period of conwith the appropriate p value from the standard ventional SLE ventilation and PTV SLE ventibeing 2.5 to 9.1 mmHg (paO2) and -2.77 error of the difference between the means ofBrought the lation to you by | University of Arizona to -6.76 mmHg (paCO2). three periods of ventilation. Authenticated Download Date | 6/10/15 7:56 AM

J. Perinat. Med. 19 (1991)

Greenough et al., Airway pressure triggered ventilation for preterm neonates

5 Discussion These results confirm that PTV acutely improves oxygenation compared to a similar period on conventional ventilation amongst infants recovering from respiratory distress [3]. An additional benefit was the significant reduction in paCO2 which had not previously been demonstrated in this group of infants. Infants were recruited into the study on the fifth day of age, as at this age it seemed likely they would be recovering from their respiratory distress and at this time PTV is most successful [3], The inspired oxygen concentration, ventilator rate and peak pressure at the start of the study support the hypothesis that recovery from respiratory distress was indeed taking place. There was a trend, but non-significant, for arterial blood gases to improve on changing from the Sechrist ventilator to the SLE 2000. For the Sechrist ventilator, as other conventional ventilators, the driving force for lung inflation is due to the build-up of pressure in the breathing circuit which results from obstructing gas exiting during the inspiratory phase while respiratory gas flow continues. While this occurs the gas in the breathing circuit is compressed and then discharged during the expiratory phase without contributing to the tidal gas exchange. This compressible volume is wasted tidal volume, its presence reduces the rate at which the peak inspiratory pressure is reached decreasing the time available to deliver tidal volume. The SLE 2000 has no occluding valve or thumb-like device to limit gas exit from the circuit during the inspiratory phase and thus behaves as if it has a low compressible volume [1]. We feel this difference in the performance of the two ventilators explains the improvement in blood gases on changing to the SLE 2000. Our laboratory study demonstrated that the SLE 2000 can be used at fast rates without inadvertent PEEP, as the inflating volume was maintained even up to rates of 150 bpm [5]. The SLE 2000 has a valveless pneumatic exhalation circuit and this prevents inadvertent PEEP at fast ventilator rates. We were able to demonstrate that the trigger delay of this new ventilator compared favourably with that of the airway pressure trigger and SLE

J. Perinat. Med. 19 (1991)

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Newborn ventilator [8] and also the other purpose-built patient triggered ventilator now commercially available, the Draeger 8000 [9]. It has been claimed that one form of airway pressure triggering was associated with a phase shift between inflation and inspiration [13]. This was not our previous experience [7, 8] and also did not occur with the new purpose-built ventilator used in the present study. We feel the explanation for this are differences in the sensitivity of the triggering devices and hence their trigger delay. In the present study the triggering system had a high sensitivity and hence short rigger delay, the median being 80 msecs. As the longest trigger delay experienced in the present study was only 100 msecs a phase shift between inspiration and inflation is clearly impossible. Inspiratory time was limited to 0.4 seconds throughout the study as during both conventional and PTV this has been shown to be advantageous. During weaning on conventional mode limitation of inspiratory time reduces the duration of ventilation [6]. During PTV tidal volume and minute volume are increased when compared to similar pressure settings on conventional ventilation when short inflation times are used [12]. The resultant improvement in alveolar ventilation partially explains the significant reduction in paCO2 after one hour of PTV compared to a similar period of conventional ventilation. The other explanation for the reduction in paCO2 following one hour of PTV is the greater number of breaths supported by positive pressure inflation during PTV. PaCO2 will be affected to a greater extent when the change to PTV is from conventional ventilation at a slow rate rather than at a higher rate. Thus it is not surprising that a significant reduction in paCO2 was seen in this study which only included infants who were weaning and not in previous investigations [2, 11] which were performed in infants with acute respiratory distress and thus likely to be on high rates during conventional ventilation. We conclude this form of airway pressure triggered PTV is a useful addition to ventilation of the newborn. Its use was associated with significant improvements in paCO2 and paO2 and thus would permit subsequent ventilation at a lower peak pressure, facilitating faster weaning. Brought to you by | University of Arizona Authenticated Download Date | 6/10/15 7:56 AM

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Greenough et al., Airway pressure triggered ventilation for preterm neonates

Abstract 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. Similtaneous 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 8383038). All were being ventilated for respiratory distress syndrome (RDS) and were 4 days of age. The median trigger delay was shown to be 80 msecs (range 40 — 100) and no phase shift was demonstrated between inspiration and inflation. The median paO2 following one hour on the conventional ventilator was 55 mmHg (range 47—92) and paCO2 was 36 mmHg (range 26 — 57). The arterial blood gases tended to improve, but not significantly, following one hour on the SLE ventilator in conventional mode, the median paO2 increased to 64 mmHg (range 42 — 94) and the paCO2 decreased to 32 mmHg (range 25—46). Following one hour of PTV both paO2 and paCO2 were significantly improved being a median of 68 mmHg (range 45 — 104), (p

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, ...
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