Acute severe asthma: performance of ventilator at simulated altitude

Objective Exacerbation of asthma can be seen during air transport. Severe patients, not responding to conventional therapy, require ventilator support. We evaluated the performance of two transport ventilators, built with turbine technology, the T-birdVSO2 and the LTV-1000, for use during aeromedical evacuation of acute severe asthma. We have assessed the ability of both the ventilators to deliver to an acute severe asthma model a tidal volume (Vt) set at different simulated altitudes, by changing the ambient air pressure. Methods The simulated cabin altitudes were 1500, 2500, and 3000 m (decompression chamber). Vt was set at 700 and 400 ml in an acute severe asthma lung model. Comparisons of the preset with the actual measured values were accomplished using a t-test. Results Comparisons between the actual delivered Vt and set Vt showed a significant difference starting at 1500 m for both the ventilators. The T-birdVSO2 showed a decrease in the volume delivered, with a negative variation of more than 10% compared with the Vt set. The LTV-1000 showed mostly an increase in the volume delivered. The delivered Vt remained within 10% of the set Vt. Conclusion The accuracy of Vt delivery was superior with the LTV-1000 than with the T-birdVSO2, but the higher delivered Vt of the LTV-1000 are likely to be more harmful than lower delivered Vt of the T-birdVSO2.


Introduction
Flying is accompanied by stress and inhalation of cool, often dry, hypoxic air. By itself, this could represent a risk for asthmatic patients [1,2]. Moreover, aeromedical evacuations are becoming more frequent [3]. More than 400 aeromedical evacuations were carried out by the French Air Force between 2000 and 2009 in favor of 529 patients (the medical team usually consists of one general practitioner experienced in air medical transport and two nurses, reinforced with an intensivist in 52% of the flights; usual transport time 5 ± 2 h). Exacerbation of asthma can be seen during air transport. Severe patients, not responding to conventional therapy, require ventilator support [4]. In the most severe cases, often referred to as near-fatal asthma, the institution of mechanical ventilation may be required with a transport ventilator [5]. Ventilation of acute severe asthma is a special challenge, and there is definitely a need for lack of variations between delivered and set tidal volume (Vt) on the ventilators that are being used [5].
Evaluations of ground transport ventilators have been carried out by other groups, including up to 15 ventilators [6]. At sea level, few ventilators met all the targets when ventilating an acute severe asthma lungs model, mostly because of the large difference between the set and actual Vt [6].
With increasing altitude, changes in barometric pressure are associated with alterations in gas density, temperature, and humidity. This requires a considerable amount of flexibility in terms of ventilator options, without the alteration of Vt across a wide range of hypobaric conditions [7][8][9][10][11]. Unfortunately, mechanical ventilators can suffer in their performance from variations in the environmental pressure [12][13][14][15][16][17][18]. Criteria exist, however, which can help the user focus on the equipment and potential problems unique to the ventilation of acute severe asthma during air transport.
We evaluated the performance of two transport ventilators, built with turbine technology, the T-birdVSO2 (Bird products, Palm springs, California, USA) and the LTV-1000 (Pulmonetic Systems Inc., Colton, California, USA), for use during aeromedical evacuation of acute severe asthma (both ventilators are routinely used in the French military hospitals). We have assessed the ability of both the ventilators to deliver, to an acute severe asthma model, a Vt set at different simulated altitudes by changing the ambient air pressure using a decompression chamber.

Methods
The experiment was performed in the French laboratory of aviation and space medicine of the Air Force, at Bretigny.

Cabin altitude simulation
We used a decompression chamber to mimic the hypobaric environment of an aircraft cabin. The aircraft cabin altitude during an aerial evacuation is usually less than 2500 m (International Civil Aviation Organization), and practically the condition of pressurization of most commercial flights is 1500 m. In some military flights the cabin altitude is about 3000 m (this particular pressurization can be used by the French Air Force). These three hypobaric conditions (1500, 2500, 3000 m; 4000, 6670, 8000 ft) were simulated.

Ventilation parameters
One ventilator of each type was randomly chosen from the warehouse. Both the ventilators had current manufacturer certification, and were checked by the maintenance service of medical materials of the French Air Force. Ventilators were tested with various parameters. The inspired oxygen content was 90%. The Vt were set at 700 and 400 ml (corresponding to standard Vt for adults, male, and female, respectively). Respiratory rate was 12 breaths/min. The ratio inspiratory time/ expiratory time was 1/2. The positive end expiratory pressures (PEEPs) were set at 0 cm H 2 O. The cabin altitude was manually input in T-birdVSO2 to execute the manufacturer's altimetric correction (with recalibration of gas delivery).

Lung simulation
The lung model was obtained from the VT1 Bioteck (Winooski, Vermont, USA). Resistance and compliance were 3.9 cm H 2 O/l/min and 50 ml/cm H 2 O, respectively, to mimic an acute severe asthma.

Measurement of volume delivered
We measured the actual volume delivered with a dedicated instrument of the physiological laboratory of the Air Force: Fleisch pneumotachograph. It was connected between the ventilator circuit and the acute severe asthma lung model. Pressure drop across the pneumotachograph was measured by a differential pressure transducer (Enertec, Dorval, Québec, Canada). The spirometer was checked at each altitude using a calibration syringe. Signals for flow, volumes, and pressure were collected and recorded for analysis using a Hewlett Packard computer (Palo Alto, California, USA). The protocol included three measurements for each Vt set at sea level, 1500, 2500, and 3000 m.

Ethical aspect
Three of the seven operators had to be inside the hypobaric chamber. The local human research committee reviewed the project. As long as the exposure of the operators inside the hypobaric chamber was limited to 3000 m, ethical approval was given.

Statistical analysis
The protocol included three measurements for each Vt and cabin altitude. The mean and standard deviation were calculated. Comparisons between the set and actual measured values were accomplished using a t-test. A significant difference was defined by a P value of less than 0.05.

Results
The standard deviation for the three measurements obtained at each altitude was consistently less than 10 ml. The respiratory rate delivered was 12 breaths/min in all cases. Figure 1 summarizes the data with the T-bird-VSO2. With altitude, the ventilator showed a decrease in volume delivered. Comparisons between the actual delivered Vt and set Vt showed a significant difference starting at 1500 m for a Vt set at 700 ml, at 2500 m for a Vt set at 400 ml, with a negative variation of more than 10% compared with the Vt set at 3000 and 2500 m, respectively. Comparisons between the actual delivered Vt and Vt internally measured by the ventilator showed a significant difference at 3000 m for a Vt set at 400 ml, with a positive variation of 7% compared with the set Vt.   the actual delivered Vt and set Vt showed a significant difference at 1500 m for a Vt set at 700 ml and at 2500 m for a Vt set at 400 ml. The delivered Vt remained within 10% of the set Vt. Comparisons between the actual delivered Vt and Vt internally measured by the ventilator showed a significant difference starting at 2000 m for a Vt set at 700 ml, at 1500 m for a Vt set at 400 ml, with a negative variation of more than 10% compared with the Vt set at 2500 and 3000 m (for both the Vt set).

Discussion
The major finding of this investigation is that the ventilators performed differently when exposed to ventilation of an acute severe asthma in a hypobaric environment. The accuracy of Vt delivery was superior with the LTV-1000 than with the T-birdVSO2. The T-birdVSO2 did not compensate well for altitude and progressively delivered lower volumes as the barometric pressure decreased, whereas the LTV-1000 showed a moderate increase in the volume delivered for the acute severe asthma lung model, with increasing altitude, but maintained the delivered volume within 10% of the set Vt up to 3000 m (the American Society for Testing and Materials standard for volume delivery is ± 10%) [19].
The T-birdVSO2 performed well up to 1500 m. This study suggests that performance is reduced regarding Vt at 2500 m, with a negative variation of more than 10% compared with the Vt set. On the one hand, patients could suffer from a lack of Vt accuracy, and in the right context this may affect patient care [5]. On the other, the ventilator itself clearly displayed a good evaluation of the Vt delivered, as far as the Vt internally measured by the T-birdVSO2 was accurate. Hence, the physician involved in aeromedical evacuation can easily correct the ventilation.
With altitude, the LTV-1000 showed an increase in the volume delivered for the acute severe asthma lung model, maintaining the delivered volume within 10% of the set Vt up to 3000 m. Performance of the ventilator remained almost the same despite the decrease in barometric pressure. Although we concluded that the T-birdVSO2 is less accurate, it consistently had a lower Vt than the set Vt, whereas the LTV-1000 had higher delivered Vt. Given what we know from the acute severe asthma ventilation, the higher delivered Vt of the LTV-1000 is likely to be more harmful than the lower delivered Vt of the T-birdVSO2 [5]. Moreover, the ventilator itself measured and displayed an evaluation of the Vt delivered that was not accurate at 2500 and 3000 m (variation of more than 10% starting at 2500 m).
Unfortunately, benchmarking other studies is difficult because of the lack of studies and test procedure standardization. For example, Thomas and Brimacombe studied the Oxylog 1000 (Dräger Medical, Lübeck, Germany) with normal and noncompliant lung models, at altitude ranges of 2000 and 9100 m [14]. At a simulated altitude of 2000 m, the delivered volume was 28% greater than the Vt set, and moreover the respiratory rate set was not respected.
More recently, Flynn and Singh evaluated the Oxylog 1000, 2000, and 3000 (Dräger Medical) [17]. They used models for asthma, normal lung, and acute respiratory distress syndrome. The simulated altitudes were 1800 and 3400 m. With regard to the asthma lung, the test lung was the Demonstration Thorax TE020790 (Dräger Medical  A decompression chamber was used to mimic the hypobaric environment at the altitude ranges of 1500, 3000, and 5600 m. The LTV-1000 was delivering an increasing Vt with altitude. At 1500 and 3000 m, the increase in the Vt delivered was 5-12%. At 5600 m, the LTV-1000 delivered a Vt 30-37% greater than set Vt. The Impact 754 (Impact Instrumentation Inc.) compensated the volume output to deliver the set Vt regardless of the change in environmental pressure. With decreasing barometric pressure, the delivered Vt remained within 5% of the set Vt.
Moreover, there is definitely a place for international consensus regarding the testing of ventilators in hypobaric environment. Medical evacuations of patients with significant pulmonary impairments are frequent. Thus, ventilation should be tested with three models: normal lung, acute respiratory distress syndrome, and asthma. With regard to the altitudes studied, the international rules can serve as a basis. The aircraft cabin altitude during an aeromedical evacuation is less than 2500 m (International Civil Aviation Organization), and practically the condition of most commercial flights is 1500 m. In some military flights, the cabin altitude is about 3000 m. These three hypobaric conditions (1500, 2500, 3000 m) should be used in the subsequent studies for standardization purposes. Concomitant to the development of such experimentation is the need to have an international consensus.

Conclusion
The LTV-1000 met the trial targets in all the settings, whereas the T-birdVSO2 did not compensate well for altitude and progressively delivered lower volumes as the barometric pressure decreased. Such variations between the delivered and set Vt suggest a lack of efficacy of the altimetric correction in hypobaric conditions in some devices. The LTV-1000 showed a moderate increase in the volume delivered for the acute severe asthma lung model with increasing altitude, but maintained the delivered volume within 10% of the set Vt up to 3000 m. The accuracy of Vt delivery was superior with the LTV-1000 than with the T-birdVSO2, but the higher delivered Vt of the LTV-1000 is likely to be more harmful than the lower delivered Vt of the T-birdVSO2. Vt is a critical metric in the care of acute severe asthma during air transport. It is crucial for the clinicians to be well versed in the performance characteristics of the ventilator that they are using. Moreover, it is important to have expert personnel capability to know and maximize the performance envelope of the devices for the benefit of the patient.