QINETIQ PROPRIETARY Acceleration Atelectasis: New risks ... · QINETIQ PROPRIETARY 3 QINETIQ PROPRIETARY • Signs and symptoms associated with the development of acceleration atelectasis:

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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

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Accele

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• 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

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QINETIQ PROPRIETARY Acceleration Atelectasis: New risks ... · QINETIQ PROPRIETARY 3 QINETIQ PROPRIETARY • Signs and symptoms associated with the development of acceleration atelectasis:

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