Hypoxia Following Rapid Decompression to 18,288 m (60,000 ft) Attributable to Alveolar Hypoventilation Desmond M Connolly PhD QinetiQ Aircrew Systems Senior Medical Officer Timothy J D Oyly BSc Amanda S McGown MSc Vivienne M Lee PhD Tuesday May 14th 2013 Aerospace Medical Association 84th Annual Scientific Meeting, Chicago, IL QinetiQ Proprietary 1
Declaration 84th Annual AsMA Scientific Meeting Dr Desmond M Connolly I have the following financial relationships to disclose: Consultant for: Stockholder in: Employee of: QinetiQ plc QinetiQ plc QinetiQ plc I will not discuss off-label use or investigational use in my presentation 2
Publication Lung volumes, pulmonary ventilation, and hypoxia following rapid decompression to 60,000 ft (18,288 m) Aviation Space and Environmental Medicine 2013; 84(6): 551-559 dmconnolly@qinetiq.com 3
Rapid decompression risk of hypoxic incapacitation PAO2 (mmhg) 60 Useful consciousness determined by the time taken to replace alveolar nitrogen with oxygen 40 30 mmhg Fall in PAO2 post-decompression 140 mmhg.s loss of consciousness 20-40 -20 0 20 40 60 Time (s) 4
Rapid decompression risk of hypoxic incapacitation PAO2 (mmhg) 60 At final altitudes above 40,000 ft, require immediate delivery of 100% oxygen with positive pressure breathing (PPB) 40 30 mmhg 20-40 -20 0 20 40 60 Time (s) 5
Determinants of Hypoxia Severity Post-Rapid Decompression Initial pressure altitude (i.e. initial absolute respiratory tract pressure) Final pressure altitude (i.e. final absolute respiratory tract pressure) Time course of pressure change (inc PPB) Initial breathing gas FIO2 (hence PAO2 prior to decompression) Final breathing gas FIO2 (PAO2 achievable after decompression) Time course of change in FIO2 and, hence, purging of dead space Rate of elimination of alveolar nitrogen (lung volumes and ventilation) 6
Determinants of Hypoxia Severity Post-Rapid Decompression Initial pressure altitude 22,500 ft Final pressure altitude 60,000 ft (+72 mmhg PPB) Time course of pressure change 3 s (90%); PPB @ 38,000 ft Initial breathing gas FIO2 56% oxygen Final breathing gas FIO2 100% oxygen (from 25,000 ft) Time course of change in FIO2 First/second breath Rate of elimination of alveolar nitrogen Lung volumes unknown Steady pressure breathing cycle N = 6 subjects who undertook two RDs each (main and standby regulators) 7
All RDs were completed safely but one subject experienced significant hypoxia during the minute at final altitude Diminished peripheral and colour vision with blurring, few seconds Pronounced dizziness, light-headedness and paresthesiae throughout 8
All RDs were completed safely but one subject experienced significant hypoxia during the minute at final altitude? 9
So was Subject B genuinely hypoxic? Respired gases? (1. O2) PETO2 seemed ok! 10
So was Subject B genuinely hypoxic? Respired gases? (2. CO2) 11
So was Subject B genuinely hypoxic? Respired gases? (3. N2) 10 mmhg 5 mmhg 12
Accurate data on subjects lung volumes and pulmonary ventilation were obtained VC > 7L! TLC > 10L! 13
Pulmonary ventilation main regulator 8 breaths of mean TV < 250 ml Physiological dead space ~145 ml Alveolar ventilation with oxygen only ~1.2L by 30 s 14
Summary - Subject B Severe hypoxia explained by delayed elimination of nitrogen, due to: 1.Poor alveolar ventilation - frequent but very shallow breaths 2. Unusually large lung volumes - poor alveolar ventilation with oxygen: - in first 10 s ~110 ml - in first 20 s ~310 ml - in first 30 s ~1.16 L 3.False estimates of end-tidal oxygen and carbon dioxide - poor alveolar gas mixing due to very low tidal volumes - but nitrogen must be from alveolar region indicating slow elimination 15
Comparison between subjects Subject SpO2 Lung volumes / ventilation B 53% sustained C 62% sustained E 64% transient Low RR @ 8 min-1 but deep breaths, MV 12.3 L Average lungs (TLC 6.85L; VC 5.2L) Effective alveolar ventilation with oxygen, but slow F 68% transient Large lungs (TLC >9L; VC ~7L); RR 10 min-1, MV 15.0L Reasonably effective ventilation of big lungs A/D Average lungs, RR 7-10 min-1, large MV, large breath volumes Slow nitrogen elimination (high RR but shallow breaths) Very large lungs, ineffectively ventilated for first 30 s Slow nitrogen elimination, lowest minute volume Average lungs (VC 5.7L), RR 11 min-1, MV 7 L.min-1 NB Tidal volume during PPB tends to be higher than normal 16
Conclusions Lung volumes and pulmonary ventilation are critical, idiosyncratic determinants of nitrogen elimination (and hence severity of hypoxia) following rapid decompression to high altitudes Following rapid decompression to pressure breathing altitudes, an initial two deep clearing breaths are recommended before adopting a steady breathing cycle Partial pressure assemblies can provide reliable protection to an altitude ceiling of 18,288 m (60,000 ft), even when pulmonary ventilation of large lungs is sub-optimal, but protection at 60,000 ft (and above) warrants careful training in appropriate breathing techniques 17
Work sponsored by UK Ministry of Defence With very grateful thanks to my co-authors, our experimental subjects and our many QinetiQ colleagues who worked on the programme 18
Respiratory pressures, flows and gas compositions were monitored continuously Mass spectrometry Pressures: Mask cavity (MCP) Mask tube (MTP) Counterpressure garments (CCPG) Regulator inlet Flows: Expiratory Inspiratory (regulator outlet) However, wished to estimate minute volume at final altitude based on breath-by-breath respired volumes calibrating, zeroing, and integrating inspiratory flow rate data (with reference to changes in respired gas composition to provide accurate timing of inspiration and expiration) while also compensating for the phase delay in measurement of respired gas composition (with reference to corresponding MCP/CCPG pressure swings). 19
Monitoring of respiratory status during Subject B decompression 0s Subject (thumb s up) 1s Switch to 100% (visual) 3s MCP increasing (display) 3s Garments inflating (visual) 8s 60,000 ft (PCO) 10s MCP 69 mmhg (PLO) 12 s 100% O2 in mask (MSO) 15s ET O2 rising (MSO) 19s Garment pressures (PLO) 26s ET O2 42 mmhg (MSO) 30s 30 seconds (PCO) 34s Sats 62% (MM)? 35s ET O2 53 mmhg (MSO) 40s ET CO2 26 mmhg (MSO) 45s Sats 53% (MM)??? 46s ET O2 55 mmhg (MSO) 51s Sats rising (MM) 60 s Recompression 20