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QINETIQ PROPRIETARY
QINETIQ PROPRIETARY
QINETIQ PROPRIETARY
QINETIQ PROPRIETARY
Ross Pollock
Alec Stevenson
SAFE Europe 2018
Acceleration Atelectasis:
New risks from an old friend
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What is atelectasis?
• Partial collapse of the lung resulting from alveoli (small sack like structures where gas exchange occurs) becoming gasless and collapsing shut
• Difficult to re-open without large change in pressure across the lung – chest wall interface
• Often occurs clinically in anesthetised patients
No Atelectasis
Atelectasis
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• Signs and symptoms associated with the development of acceleration atelectasis:
– Coughing
– Chest Pain
– Breathing difficulty and shortness of breath
– Lowered O2 content of the blood (hypoxaemia)
• Risks associated with acceleration atelectasis:
– Respiratory symptoms can be distracting or, at worst, debilitating
– Lowered O2 content of the blood could increase susceptibility to hypoxia (e.g. altitude, Gz exposure)
– Subsequent increased risk of loss of consciousness
– Atelectasis symptoms could be confused with other hazards (e.g. hypoxia) leading to inappropriate mitigation actions
being taken
Signs, Symptoms and Risks
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• High performance aircraft can pull high G forces that can be sustained for many seconds
– Typhoon aircraft = +9 Gz
– Protection by increasing blood pressure to overcome the hydrostatic forces
– Achieved by compressing the lower body
– Moves blood centrally, reduces volume of vessels (↓volume = ↑ pressure ), diaphragm and chest contents pushed up (reduced distance between heart and head)
• However, G Protection (Anti-G Trousers) compresses the lung
Protection against Head-to-Foot (+Gz ) Acceleration
Normal (+1Gz)Under +Gz without protection Under +Gz with protection
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• To protect against the effects of altitude the O2 concentration of the gas supplied to pilots is increased
• O2 concentrations of >60% are thought to cause atelectasis
Acceleration Atelectasis
Gz and anti-G trouser compresses
the lung
Airways become unstable and close
O2 uptake by blood reduces alveoli
volume and they collapse
Once Gz is offloaded collapsed
alveoli remain shut
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• Approximately 30 % of Typhoon pilots, surveyed in 2011, reported symptoms suggestive of acceleration atelectasis (coughing and chest pain)1
– The majority were reported 5 – 10 mins post +Gz exposure but some remained post sortie
• A survey of Hawk T2 aircrew revealed the majority had experienced coughing and chest tightness post +Gz exposure2
– Symptoms lasted for between 5 mins post exposure and 2-3 hours post sortie
• Symptoms suggestive of acceleration atelectasis (coughing and chest pain) reported by F-22 pilots for up to 4 hours after high Gz sorties performed3
Re-emergence of Acceleration Atelectasis?
1. Wilkinson (2011). Typhoon Pilot Medical Questionnaire (Unpublished); 2. Monberg (2013). Av Spa Enviro Med. 84:247; 3. Flottmann (2013). Av Spa Enviro Med. 84:428
• “Physiological Incidents” have resulted in the grounding of aircraft in military air forces outside of the UK– Encompasses a wide range of causes/effects
– Acceleration atelectasis could be a contributory factor
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• On board oxygen generation systems (OBOGS) designed to adhere to the limits set out in Figure 1
– Do they adhere to these limits?
– Are these limits still valid?
• Aircraft capabilities and anti-G systems have improved since limits were set
– Longer durations of +Gz can be sustained at higher altitudes
– Higher Gz levels can be experienced without the need to perform the anti-G straining manoeuvre
Possible causes of increased incidence
Figure 1. Typical OBOGS O2 delivery schedule limits
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Aim and Protocol
• Overarching aim to improve our understanding of acceleration atelectasis when wearing modern anti-G trousers and breathing gas mixtures containing high levels of O2
1
3
5
Accele
ration (
Gz)
0
20
40
60
80
100
FiO
2 (%
)
1
3
5A
ccele
ration (
Gz)
0
20
40
60
80
100
FiO
2 (%
)
1
3
5
Accele
ration (
Gz)
0
20
40
6080
100
FiO
2 (%
)
• Phase 1: To investigate the effects of +Gz duration on acceleration atelectasis
• Phase 2: To investigate the effects of inspired O2 concentration on acceleration atelectasis
• Phase 3: To investigate the effects of cumulative exposure to +Gz and different O2 concentrations on acceleration atelectasis
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Experimental Protocol
• 14 subjects completed all phases of testing (2 to 3 females in each phase)
• All runs to 5 Gz (at 1.0 G.s-1) with subjects wearing Typhoon AEA
– Positive pressure breathing for Gz protection was not utilised
• During all Gz exposures subjects maintained clear vision using muscle tension only
– i.e. the breathing component of the anti-G straining manoeuvre (AGSM) was not performed
Measurements MadePredicted Effect
on Atelectasis
Forced Inspiratory Vital
Capacity (FIVC)↓
Regional FIVC ↓
Symptomology ↑
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Example FIVC and Symptoms
Post +5Gz breathing air (21% O2) Post +5Gz breathing 94% O2
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Results: Effect of Gz duration on lung volume
***
**
Data are mean ± SE
* Significantly Different from baseline
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Results: Effect of O2 concentration on lung volume
12
Phase 1Phase 2
Data are mean ± SE
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Symptomology: Effect of +Gz Duration and Inspired O2 Concentration
94% O2
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Phase 3 Results – Symptoms and FIVC
Mean ± SE
* Significantly different from 60 %†Significantly different from 95 % baseline
‡ Significantly different from 60 % baseline
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Operational Significance of Findings
• The majority of individuals will develop acceleration atelectasis when exposed to >90 s of moderate +Gz breathing high oxygen concentrations– Reducing the length of exposure moderates this but less than 30 s required to prevent it
– Reducing the % of oxygen inspired markedly reduces the extent of atelectasis
– 60% still produces atelectasis in susceptible individuals
– <45% required to prevent it
• Cumulative exposure can lead to atelectasis– Exposures that would otherwise not be expected to result in atelectasis (e.g. 30 s breathing 60%) can do if they are
repeated a number of times
• On average the performance of two deep breaths can reverse acceleration atelectasis– A greater number may be required for those individuals more prone to developing atelectasis
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Where do we go from here?
• We have found evidence of atelectasis during cumulative exposure to +Gz acceleration despite breathing O2
concentrations that are thought to reverse it
– Are operational flight profiles causing atelectasis in fast jet aircrew?
– Could we minimise the effect of cumulative exposure by having aircrew taking deep breaths after exposure to high Gz?
Typical air combat manoeuvre Gz profile in the F-18 taken from: Newman DG & Callister R (1999); Av Spa Enviro Med. 70(4); 310
• Flight trials to investigate the incidence of atelectasis when breathing high O2
concentrations
– Limited ability to control O2 concentration
• Operational relevant profiles could be investigated on centrifuges with simulator capabilities (e.g. the new RAF centrifuge being built at RAF College Cranwell)
– Would allow safe study of techniques to
reverse atelectasis or prevent