Acceleration Atelectasis: New risks from an old friend

Transcription

1 Acceleration Atelectasis: New risks from an old friend Ross Pollock Alec Stevenson SAFE Europe 2018

2 What is atelectasis? Partial collapse of the lung resulting from alveoli (small sack like structures where gas exchange occurs) becoming gasless and collapsing shut No Atelectasis Difficult to re-open without large change in pressure across the lung chest wall interface Often occurs clinically in anesthetised patients Atelectasis

3 Signs, Symptoms and Risks Signs and symptoms associated with the development of acceleration atelectasis: Coughing Chest Pain Breathing difficulty and shortness of breath Lowered O 2 content of the blood (hypoxaemia) Risks associated with acceleration atelectasis: Respiratory symptoms can be distracting or, at worst, debilitating Lowered O 2 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 3

4 Protection against Head-to-Foot (+Gz ) Acceleration 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 Normal (+1Gz) Under +Gz without protection Under +Gz with protection 4

5 Acceleration Atelectasis To protect against the effects of altitude the O 2 concentration of the gas supplied to pilots is increased O 2 concentrations of >60% are thought to cause atelectasis Gz and anti-g trouser compresses the lung Airways become unstable and close O 2 uptake by blood reduces alveoli volume and they collapse Once Gz is offloaded collapsed alveoli remain shut 5

6 Re-emergence of Acceleration Atelectasis? 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 exposure 2 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 performed 3 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 6 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

7 Possible causes of increased incidence 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 Figure 1. Typical OBOGS O 2 delivery schedule limits 7

8 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 O 2 Phase 1: To investigate the effects of +Gz duration on acceleration atelectasis Acceleration (Gz) FiO 2 (%) 1 0 Phase 2: To investigate the effects of inspired O 2 concentration on acceleration atelectasis Acceleration (Gz) FiO 2 (%) 1 0 Phase 3: To investigate the effects of cumulative exposure to +Gz and different O 2 concentrations on acceleration atelectasis Acceleration (Gz) FiO 2 (%) 8

9 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 Made Forced Inspiratory Vital Capacity (FIVC) Regional FIVC Symptomology Predicted Effect on Atelectasis 9

10 Example FIVC and Symptoms Post +5Gz breathing air (21% O 2 ) Post +5Gz breathing 94% O 2 10

11 Results: Effect of Gz duration on lung volume * * * * * Data are mean ± SE * Significantly Different from baseline 11

12 Results: Effect of O 2 concentration on lung volume Phase 2 Phase 1 Data are mean ± SE 12 12

13 Symptomology: Effect of +Gz Duration and Inspired O 2 Concentration 13 94% O 2

14 Phase 3 Results Symptoms and FIVC Mean ± SE * Significantly different from 60 % Significantly different from 95 % baseline Significantly different from 60 % baseline 14

15 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 15

16 Where do we go from here? We have found evidence of atelectasis during cumulative exposure to +Gz acceleration despite breathing O 2 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? Flight trials to investigate the incidence of atelectasis when breathing high O 2 concentrations Limited ability to control O 2 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 Typical air combat manoeuvre Gz profile in the F-18 taken from: Newman DG & Callister R (1999); Av Spa Enviro Med. 70(4);

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