Recent Severe CNS Altitude
Decompression Sickness
(DCS)
in U-2 Pilots
• Andrew Pilmanis, Ph.D.
Laboratory (Retired)
• USRA Seminar, March 10, 2010
Air Force Research
Publications
• JERSEY SL, BARIL RT, MCCARTY RD,
MILLHOUSE CM. Severe neurological
decompression sickness in a U-2 pilot. Aviat
Space Environ Med 2010; 81:64 – 8.
• 3 papers in preparation
• AsMA 2010 presentation
• UHMS 2010 presentation
Background
– USAF’s only remaining manned high altitude
platform
– U-2 has been flying since the 1950s
– Currently vital to US military operations
• U-2 Mission Profile
– High altitude (over 70,000 ft) reconnaissance
– Pressurized cockpit (3.8 psi differential)
– Pilots exposed to 29,000 to 30,000ft (4.3 psi)
– Exposure time: 8-12 hours
• Standard DCS Prevention Measure
– 1-hour resting prebreathe, 100% O2
– Full pressure suit (3.5 psi)
MAJOR VARIABLES FOR
ALTITUDE DCS
• Altitude
• Exposure time at altitude
• Preoxygenation (prebreathing)
• Exercise at altitude
DCS Countermeasures
•
•
•
•
•
Use of 100% oxygen
Preoxygenation
Limit time of exposure
Limit exercise at altitude
Cabin / pressure suit pressurization
% DCS
Cumulative Decompression Sickness Incidence vs
Exposure Time at 29,500-30,000 ft
(1 h Preoxygenation; N = 94)
100
90
80
70
60
50
40
30
20
10
0
0
60
120
Exposure Time, min
180
240
DCS Symptom Incidence (%) in Research Subjects
Exposed to Altitude Profiles Similar to These Operational
Scenarios
100
80
Standard U-2 High Flight
60
40
Previous Shuttle EVA
High Altitude Parachuting Above 30,000 Ft
20
0
Current Space Station EVA
AC-130H Long Duration Flight
Altitude Chamber Training
Decompression Sickness in High
Altitude Reconnaissance Pilots
•
•
•
(3) U-2 Pilots surveys completed:
Conclusion: DCS is more common than previously thought,
and there is mission impact
Results:
– 60%-80% of pilots report at least one case during career
– Of these 13% had neurological involvement
– 13-16% of these altered mission profile due to DCS
– Mission performance was affected by DCS in 34% of these
– Obstacle to reporting DCS: fear of grounding
• No
evidence of long-term medical problems from DCS
Motives for Reluctance to
Report Altitude DCS
•
•
•
•
•
•
•
•
Mission Protection
Career Protection
Inability to Detect and Diagnose
Perception of Unimportance
Treatment Inconvenience
Peer Pressure
Distrust of Medical Involvement
Concern of Impact on Operational Aspects
AFI 48-123 Attachment 6
– History of Decompression Sickness (DCS)
does not require a waiver. An episode of
DCS requires a minimum of 72 hours DNIF.
Consultation with AL/AOH (Hyperbaric
Medicine) and concurrence of
MAJCOM/SGPA is required before RTFS. In
cases of DCS with neurological
manifestations, a normal examination by a
neurologist is required before RTFS.
Previously
– Very few recorded cases of CNS DCS
– No recorded cases of permanent
neurological damage
Altitude DCS
versus
Diving DCS
1.
Decompression sickness starts from a
ground level tissue N2 saturated state
1.
Upward excursions from saturation diving
are rare
2.
Breathing gas usually high in O2 to prevent
hypoxia and promote denitorgenation
2.
Breathing gas mixtures usually high in inert
gas due to oxygen toxicity concerns
3.
Time of decompression exposure to altitude
is limited
3.
Time at surface pressure following
decompression is not limited
4.
Pre-mission denitrogenation reduces DCS
risk
4.
Not done
5.
DCS usually occurs during the mission
5.
DCS risk is usually after mission completion
6.
Symptoms usually mild and limited to joint
pain
6.
Neurological symptoms are common
7.
Recompression to ground level is
therapeutic and universal
7.
Therapeutic chamber recompression is time
limited and sometimes hazardous
8.
Individual susceptibility varies widely
8.
Less variability
9.
Tissue PN2 decreases to very low levels with
altitude exposure
9.
Tissue PN2 increases to very high levels with
hyperbaric exposure
10. Metabolic gases become progressively
important as altitude increases
11. Few documented chronic sequelae
10. Inert gases dominate
11.
Chronic bone necrosis and neurological
damage have been documented
DCS Symptom Descriptions in the
Database “Serious” Symptoms (6%)
Joint/Muscle Symptoms (72%)
Joint Pain (70%)
Muscle Pain (2%)
Skin Manifestations (19%)
Hot and/or cold sensation (1%)
Pins & Needles; tingling; prickling
(13%)
Pruritus (skin itch) (3%)
Skin Mottling (2%)
Other (3%)
Ataxia (incoordinate movements) (0.1%)
Blurred Vision (0.1%)
Cold Sweat (0.6%)
Cough (0.8%)
Dizziness (0.4%)
Dyspnea (Difficult or Labored Breathing) (0.9%)
Edema (0.1%)
Fatigue (inappropriate or sudden onset) (0.1%)
Headache (0.6%)
Hyperesthesia (increased sensitivity to stimulation)
(0.1%)
Light Headedness (0.3%)
Loss of consciousness (0.1%)
Muscular Weakness; including paralysis (0.3%)
Nausea (0.6%)
Numbness (0.5%)
Substernal Distress (Tightness in chest) (0.9%)
Tremor; shakes (0.1%)
AF U-2 DCS Working Group
August, 2009
•
TASKING
– Investigate recent severe decompression
incidents occurring in U-2 pilots
– Recommend, if possible, potential risk
mitigation strategies to decrease DCS
incidents
•
•
Report – Oct 2009
Brief pilots – Nov 2009
Neurological DCS (1991-2001)
• Nine
recorded DCS incidents of ALL types
• Two (2) confirmed case of chokes (1998)
• One (1) confirmed case of neurological DCS (1998)
• Two probable cases of neurological DCS (1991, 2000)
• Two possible cases of neurological or pulmonary DCS (1995, 1996)
Neurological DCS (20022009)
• 41 recorded DCS incidents of ALL types
• 16 confirmed cases neurological DCS (11 pilots)
– Three (3) neurological + pulmonary DCS
– Five (5) life and/or aircraft threatening
• Two possible cases of neurological DCS (2 pilots)
• Observations:
– Majority were men (one woman)
– No correlation to age, body habitus, or GelDex use
– No PFO detected among 6 pilots tested
– One pilot with detectable MRI lesions (most severe
case) of 7 tested
Recent CNS Manifestations
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
*Memory loss
Can’t fly airplane
Considered ejecting
Can’t recall landing
*confusion
*“foggy” thnking
Altered mental/visual perception
*Disorientation
Palpitation
Abrupt overwhelming sensation of
anxiety
**Difficulty concentrating
**Headaches
*malaise
Close to losing consciousness
**Personality changes
**Irritability
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
**Family problems
Vertigo
*Fatigue
*Dizziness
Tremor
Ataxia
*Numbness
Blurred vision
*Paresthesia
Hyperacusis
Photophobia
Rash
Nausea
Limb pain
Tinnitus
*Persisted
**Persisted long-term
Recent Symptoms for
Altitude DCS
• Late Onset of Neurological Symptoms
– 3 cases: sudden onset severe symptoms >4hrs in-flight
– 7 cases: delayed recognition of subtle symptoms after flight
• Recurrent Symptoms After Indicated Treatment
• Persistent Symptoms Despite Indicated Treatment
– Subtle neuropsychological symptoms persist for years
• Permanent Neurological Sequelae in 1 Severe Case
– Correlating lesions on MRI, clinical equivalent of stroke
• Symptoms similar to Traumatic Brain Injury (TBI)
Findings
• Cases are of a type and severity not documented
previously
• Appropriate treatment in the AOR
• Signs and symptoms recur with a temporal
relationship to the commercial flight home
• Symptoms persist for weeks, months or longer
Mission Duration & Frequency
Previous Generation
U-2 Pilots (1980’s –
1997)
Today’s U-2
Pilots
(1998 –
Present)
Percent
Change
Number of
Pilots
Available
49
37
24% fewer
pilots
Average
Annual Hours
per Pilot
172
329
91% more
hours
Average
Annual Sorties
per Pilot
42
92
121% more
sorties
7-10 years
3-5 years
They’re ALL
“1,000-Hour”
pilots now!
Time to
Achieve
“1,000-Hour”
Status
Factors
• Majority of cases occur at one
Detachment
– Increase in number of sorties per pilot
– Decrease in rest-cycle
– Increase in exposure time-greater than
9 hr sortie
– Increase in cockpit activity and stress
24
EXERCISE
• Exercise at altitude- Increased DCS
• Exercise during prebreathe- reduce DCS
• Post-flight exercise- no effect
Decompression Sickness and Mode of
Exercise
Factors
• Research data – DCS curves level off at 34 hours of exposure
• It now appears DCS occurs much longer
into the exposure
• More CNS DCS occurs with these long
exposures
• There are long-term sequelae
• New mind-set
Non-Factors
• No correlation to tail number
• No break/change in integration & prelaunch protocol
• Oxygen system and LOX checked out as
functioning normally and noncontaminated
– No mechanism to allow contaminate into O2
system while flying unless catastrophic failure
Non-Factors: Medical
• Treatment chamber in UAE: state-of-theart equipment with appropriate training
• No environmental exposures unique to
deployment locations (i.e., no endemic dz,
chem exposures, etc.)
• Dehydration not a factor
• No correlation to GelDex use
• No underlying medical issues or acute
illnesses
Bottom Line
•
•
•
•
Increased frequency of flights
Increased duration of flights
Increased physical activity
Reduced rest time
AF DCS Working Group ReportRisk Reduction Measures
• Exercise-enhanced prebreathe
• Routinely fly with FPS partially inflated
• Reduce length of sorties
– most of the severe CNS cases resulted from
9 to 11 hour missions
• Increase rest cycle
• Reduce mission operating altitude
PREOXYGENATION
• Zero-Prebreathe
• Resting Prebreathe
• Exercise-Enhanced Prebreathe
DCS at 30,000 ft with Mild Exercise,
4-hour Exposure (resting prebreathe)
Exercise-enhanced prebreathe
•
•
•
•
•
•
Described during WWII
Brooks studies confirmed
10 min at start of 60 min prebreathe
Heavy exercise at 75% VO2 max
Reduced DCS by 35-40%
Greatest reduction in CNS Sx
Prebreathe Profiles
Prebreathe Profile
0:10 Exer
0:10
0:50 Rest
0:05 Rest
0:60 Rest
0
10
20
30
Minutes of Prebreathe
40
50
60
IMPROVED DCS PROTECTION FOR
HIGH ALTITUDE MISSIONS
100
80
1:00 REST
77%
60
0:10 EXER
0:05 REST
64%
0:10 EXER
0:50 REST
% DCS
40
42%
20
0
1
2
3
PREBREATHE PROCEDURE
• Strenuous exercise during prebreathe resulted in greatly improved
denitrogenation and reduced DCS
• Recommend reevaluation of some operational prebreathe procedures
Symptom Type versus Prebreathe Profile
Number of Symptoms by Profile
20
15
10
5
0
Joint Pain
Joint Pain > 5
Paresthesia
Neuro/Respiratory
Symptom Type
60 min Rest
0:10 Exer, 0:05 Rest
10 min Exercise + 50 min Rest
TEST AND EVALUATION OF EXERCISEENHANCED PREOXYGENATION IN U-2
OPERATIONS
• One pilot had 2 cases of DCS during his
first 25 U-2 high flights using resting
prebreathe
• The next 36 high flights, using exerciseenhanced preoxygenation, were
completed with no reports of DCS.
• This statistically significant operational test
reinforced the laboratory studies.
EEP Procedures
Percent Max HR
Reach THR
by 5 min
point
Maintain
75%
EEP complete
Stop exercising
at 10 min point!
Warm up
@
30 – 40%
2
Cool down for
at least 5 min
with fan
Minutes
10
Total Denitrogenation Time =
1 hour prior to
traffic pattern departure
Reduce altitude
• Fly missions at a lower aircraft altitude
• Partially inflate pressure suit to reduce
physiological altitude
• Max suit pressure – 3.5 psi
– Need a pressure gage on suit
– Inflated suit can impact mission
– Inflate to what? Based on what?
USAF DCS Risk Prediction
Calculator (ADRAC)
Input Variables:
–Altitude
–Exposure time
–Preoxygenation time
–Exercise level
AFRL Altitude DCS Model
Validation (ADRAC)
– Operational application requires prospective
validation
– Many theoretical models – few previous
validations
– Two years of AFRL human trials for ADRAC
– The predictive ability and accuracy of
ADRAC model were validated by five
profiles
– ONLY PROSPECTIVELY VALIDATED
ALTITUDE MODEL
ADRAC
• Cannot calculate Exercise-enhanced
prebreathe
• AFRL data – 4 hr resting prebreathe equal
to 1 hr Exercise- enhanced prebreathe
Prebreathe Condition vs Cumulative DCS
Incidence (4.3 psia Exposures)
100
90
80
* Preliminary
60
50
40
30
20
10
Exposure Time, Min
4 h Supine Rest (N = 28*)
10 Exer + 50 Rest (N = 26)
240
210
180
150
120
90
60
30
0
0
% DCS
70
Table 1: Physiological altitude
(ft) vs %DCS risk
A
30,000
28,000
26,000
24,000
22,000
B
74
52
45
34
5
C
88
76
71
54
5
D
65
32
27
18
1
A Physiological altitude, i.e. the altitude the pilot’s body is exposed to inside
the suit
B Standard U-2 exposure profile: 60 min of resting prebreathe and mild
exercise at altitude
C Recent U-2 exposure profile: 60 min of resting prebreathe, more exercise at
altitude, longer exposure times at max altitude, more stress and fatigue; used
heavy exercise
D Recent U-2 exposure profile with addition of EEP (4 hr prebreathe)
Physiological altitude with suit
inflation
• Objective – reduce physiological altitude
below 24,000 ft
• Combination of flying lower and suit
inflation
• Suit inflation must be for Whole Mission,
not just when symptoms occur
• Suit inflation cannot interfere with mission
Table 2- Suggested suit inflation
pressures with aircraft altitude
•
•
•
•
•
Cabin altitude (ft)
29,000-30,000
27,000-29,000
25,000-27,000
25,000 or below
suit inflation (psi)
2.0
1.5
1.0
0.5
Medical
Recommendations
• Initial hyperbaric treatment for CNS DCS to use
full extensions at 60 FSW-Extended Table 6
• Return home from theater on supplemental
oxygen
• Establish protocol for post-DCS neuro eval
• Standing DCS Working Group for continuous
evaluation & refinement of practices
• Formation of expert panel to develop clinical
guidelines for central nervous system DCS
treatment
Aeromedical Management
•
•
•
•
•
•
•
Hyperbaric treatment
Return to flying
Define neurological symptoms
Long-term implications
Diagnostic procedures
Career implications
Theater requirements
U-2 changes Implemented
•
•
•
•
•
ALL high flights now require EEP
Suit inflation procedure approved
Aggressive hyperbaric treatment
Fly home on O2
Medical follow-up
Summary/Conclusions
• Increased Incidence of Neuro DCS
Among U-2 Pilots
• Patients Displaying Unprecedented
Symptoms
• Implications for Aeromedical
Management
• Implications for Treatment
• Implementation of new preventative
measures