INTRODUCTION. Some manufacturers of reduced oxygen (O2) breathing devices claim a comparable hypobaric hypoxia (HH) training experience by providing F(sub I) O2 < 0.209 at or near sea level pressure to match the ambient O2 partial pressure (iso-pO2) of the target altitude. METHODS. Literature from investigators and manufacturers indicate that these devices may not properly account for the 47 mmHg of water vapor partial pressure that reduces the inspired partial pressure of O2 (P(sub I) O2). Nor do they account for the complex reality of alveolar gas composition as defined by the Alveolar Gas Equation. In essence, by providing iso-pO2 conditions for normobaric hypoxia (NH) as for HH exposures the devices ignore P(sub A)O2 and P(sub A)CO2 as more direct agents to induce signs and symptoms of hypoxia during acute training exposures. RESULTS. There is not a sufficient integrated physiological understanding of the determinants of P(sub A)O2 and P(sub A)CO2 under acute NH and HH given the same hypoxic pO2 to claim a device that provides isohypoxia. Isohypoxia is defined as the same distribution of hypoxia signs and symptoms under any circumstances of equivalent hypoxic dose, and hypoxic pO2 is an incomplete hypoxic dose. Some devices that claim an equivalent HH experience under NH conditions significantly overestimate the HH condition, especially when simulating altitudes above 10,000 feet (3,048 m). CONCLUSIONS. At best, the claim should be that the devices provide an approximate HH experience since they only duplicate the ambient pO2 at sea level as at altitude (iso-pO2 machines). An approach to reduce the overestimation is to at least provide machines that create the same P(sub I)O2 (iso-P(sub I)O2 machines) conditions at sea level as at the target altitude, a simple software upgrade

Conkin, J.

Wessel, J. H., III

NASA Technical Reports Server

1 
 
 
APPROXIMATE SIMULATION OF ACUTE HYPOBARIC HYPOXIA WITH 
NORMOBARIC HYPOXIA 
 
J. Conkin1, J.H. Wessel, III2. Universities Space Research Association1, 3600 Bay Area 
Boulevard, Houston, TX 77058-3696, Wyle Integrated Science and Engineering Group2, 
1290 Hercules Drive, Suite 120, Houston, TX 77058-2769. 
 
INTRODUCTION.  Some manufacturers of reduced oxygen (O2) breathing devices 
claim a comparable hypobaric hypoxia (HH) training experience by providing FIO2 < 
0.209 at or near sea level pressure to match the ambient O2 partial pressure (iso-pO2) 
of the target altitude.  METHODS.  Literature from investigators and manufacturers 
indicate that these devices may not properly account for the 47 mmHg of water vapor 
partial pressure that reduces the inspired partial pressure of O2 (PIO2).  Nor do they 
account for the complex reality of alveolar gas composition as defined by the Alveolar 
Gas Equation.  In essence, by providing iso-pO2 conditions for normobaric hypoxia (NH) 
as for HH exposures the devices ignore PAO2 and PACO2 as more direct agents to 
induce signs and symptoms of hypoxia during acute training exposures.  RESULTS.  
There is not a sufficient integrated physiological understanding of the determinants of 
PAO2 and PACO2 under acute NH and HH given the same hypoxic pO2 to claim a device 
that provides isohypoxia.  Isohypoxia is defined as the same distribution of hypoxia 
signs and symptoms under any circumstances of equivalent hypoxic dose, and hypoxic 
pO2 is an incomplete hypoxic dose. Some devices that claim an equivalent HH 
experience under NH conditions significantly overestimate the HH condition, especially 
when simulating altitudes above 10,000 feet (3,048 m).  CONCLUSIONS.  At best, the 
claim should be that the devices provide an approximate HH experience since they only 
duplicate the ambient pO2 at sea level as at altitude (iso-pO2 machines).  An approach 
to reduce the overestimation is to at least provide machines that create the same PIO2 
(iso-PIO2 machines) conditions at sea level as at the target altitude, a simple software 
upgrade.    
 
Learning Objectives:   
 
1. Applying basic principles of respiratory physiology to the design of reduced 
oxygen breathing devices. 
2. Working toward a better understanding of hypoxia.   
https://ntrs.nasa.gov/search.jsp?R=20110015909 2019-08-30T17:25:50+00:00Z
ABSTRACT 
RESULTS 
INTRODUCTION 
  
METHODS 
REFERENCES 
CONCLUSIONS  
   
APPROXIMATE SIMULATION OF ACUTE HYPOBARIC HYPOXIA WITH NORMOBARIC HYPOXIA 
J Conkin1 and JH. Wessel2, III.  Universities Space Research Association1, Wyle Integrated Science & Engineering Group2, Houston, Texas, USA 77058. 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Reduced O2 breathing devices create a normobaric hypoxic (NH) exposure by providing an FIO2 < 0.209, 
breathed either through a mask or within a "hypoxia tent".   
  
The Some manufacturers claim an equivalent acute hypobaric hypoxic (HH) experience but under NH 
conditions.  This eliminates the need for an expensive hypobaric chamber and the risk of decompression 
sickness associated with hypobaric exposure, creating .  So a cost-effective hypoxia training niche is 
created with these devices, if they these devices deliver what they  as promised. 
 
INTRODUCTION.  Some manufacturers of reduced oxygen (O2) breathing devices claim a comparable 
hypobaric hypoxia (HH) training experience by providing FIO2 < 0.209 at or near sea level pressure to match 
the ambient O2 partial pressure (iso-pO2) of the target altitude.  METHODS.  Literature from investigators and 
manufacturers indicate that these devices may not properly account for the 47 mmHg of water vapor partial 
pressure that reduces the inspired partial pressure of O2 (PIO2).  Nor do they account for the complex reality of 
alveolar gas composition as defined by the Alveolar Gas Equation.  In essence, by providing iso-pO2 conditions 
for normobaric hypoxia (NH) as for HH exposures the devices ignore PAO2 and PACO2 as more direct agents to 
induce signs and symptoms of hypoxia during acute training exposures.  RESULTS.  There is not a sufficient 
integrated physiological understanding of the determinants of PAO2 and PACO2 under acute NH and HH given 
the same hypoxic pO2 to claim a device that provides isohypoxia.  Isohypoxia is defined as the same 
distribution of hypoxia signs and symptoms under any circumstances of equivalent hypoxic dose, and hypoxic 
pO2 is an incomplete hypoxic dose. Some devices that claim an equivalent HH experience under NH 
conditions significantly overestimate the HH condition, especially when simulating altitudes above 10,000 feet 
(3,048 m).  CONCLUSIONS.  At best, the claim should be that the devices provide an approximate HH 
experience since they only duplicate the ambient pO2 at sea level as at altitude (iso-pO2 machines).  An 
approach to reduce the overestimation is to at least provide machines that create the same PIO2 (iso-PIO2 
machines) conditions at sea level as at the target altitude, a simple software upgrade. 
1. Artino Jr AR, Folga RV, Vacchiano C.  Normobaric hypoxia training: the effects of breathing-gas 
flow rate on symptoms.  Aviat Space Environ Med 2009; 80:547-52. 
2. Conkin J.  pH2O and simulated hypobaric hypoxia.  Aviat Space Environ Med (in press). 
3. Conkin J, Wessel JH, III. Critique of the equivalent air altitude model. Aviat Space Environ Med 
2008; 79:975-82. 
4. Fenn WO, Rahn H, Otis AB. A theoretical study of the composition of the alveolar air at altitude. 
Amer J Physiol 1946; 146:637-53. 
5. Haab P, et al.  Ventilation-perfusion relationships during high-altitude adaptation. J Appl Physiol 
1969; 26:77-81. 
6. Loeppky JA, et al.  Ventilation during simulated altitude, normobaric hypoxia, and normoxic 
hypobaria.  Respir Physiol 1997; 107:231-9.              
7. Moniaga NC, Griswold CA.  Loss of consciousness and seizure during normobaric hypoxia 
training.  Aviat Space Environ Med 2009; 80:485-8. 
8. Self DA, et al.  Physiological equivalence of normobaric and hypobaric exposures of humans to 
25,000 feet (7620 m).  Aviat Space Environ Med 2011; 82:97-103. 
9. U.S. Standard Atmosphere – 1976.  US Committee on Extension to the Standard Atmosphere.  
NOAA, Washington D.C. (NOAA-S/T 76-15672: Superintendent of Documents, U.S. Government 
Printing Office (Stock No. 003-017-00323-0), 1976. 
10. West JB.  Safe upper limits for oxygen enrichment of room air at high altitude.  High Alt Med 
and Biol 2001; 2:47-51. 
We reviewed Literature was reviewed to understand the operations of three reduced O2 breathing devices: 
ROBD (1), PROTE (8), and GO2Altitude (http://www.hypoxic-training.com). 
  
The devices seem to duplicate the ambient partial pressure of O2 (iso-pO2) at sea level as exists at the 
target altitude, or something else besides PIO2 (7). 
  
The method to convert feet altitude to ambient pressure was never specified, a necessary detail to 
understand the operation of these devices.  But Through analysis, it appears that Eq. 1 is used. 
  
Eq. 1 defines a “Standard Atmosphere - 1976” where distance in kilometers is converted to the equivalent 
ambient pressure as mmHg.   
  
PBhypo (mmHg) =  760 ∗ [288.15 / (288.15 – 6.5 ∗ altitude (km))]-5.25588   Eq. 1 
  
Eq. 2 is an alternative to Eq. 1 (10).   
  
PBhypo (mmHg) = exp[6.63268 – 0.1112 ∗ altitude (km) – 0.00149 ∗ altitude2 (km)]  Eq. 2 
 
0 5000 10000 15000 20000 25000 30000
altitude (feet above MSL)
200
256
312
368
424
480
536
592
648
704
760
am
bi
en
t p
re
ss
ur
e 
(m
m
Hg
)
West
Standard Atmosphere
Fig. 1:  The relationship between 
pressure and altitude as feet above 
mean sea level (MSL) diverges given 
two equations.   
 
Eq. 3 converts the y-axis in Fig. 1 to FIO2 under normobaric pressure (FIO2normo) that represents the 
pO2 while breathing air at these pressures for the iso-pO2 machines (Fig. 2). 
  
FIO2normo = PBhypo ∗ FIO2hypo / PBnormo        Eq. 3 
  
where PBnormo is most often 760 mmHg, but could be different if the training is done at a location 
other than sea level, FIO2hypo is most often 0.209 but could be different if you are breathing an O2 
mixture that is not “air”, and PBhypo comes from either Eq. 1 or 2 that computes the ambient 
pressure for a particular altitude. 
 
0 5000 10000 15000 20000 25000 30000
altitude (feet above MSL)
5
7
9
11
13
15
17
19
21
23
25
pe
rc
en
ta
ge
 O
2 
(F
IO
2)
West
Standard Atmosphere
Fig. 2:  FIO2 needed at 760 
mmHg to provide the equivalent 
pO2 you would breathe while 
breathing air at the given 
altitude.   
 
Eq. 4 computes the FIO2 under normobaric conditions (FIO2normo) to provide for iso-PIO2.  
  
FIO2normo = (PBhypo – 47) ∗ FIO2hypo / (PBnormo – 47)     Eq. 4 
  
This simple improvement would provide a device that delivers a simulation of HH while under 
NH conditions accounting for 47 mmHg of water vapor pressure, as seen in Fig. 3. 
 
0 5000 10000 15000 20000 25000 30000
altitude (feet above MSL)
5
7
9
11
13
15
17
19
21
23
25
pe
rc
en
ta
ge
 O
2 
(F
IO
2)
Standard ATM iso-pO2
Standard ATM iso-PIO2
Fig. 3:  FIO2 needed at 760 mmHg to 
provide the equivalent pO2 (Standard 
ATM iso-pO2 curve) or the equivalent 
PIO2 (Standard ATM iso-PIO2 curve) 
you would breathe while breathing air 
at the given altitude.   
 
The difficulty in using the upper curve in Fig. 3 is that when you provide a FIO2 at sea level to 
match the pO2 at the target altitude you create a PIO2 that is greater than the PIO2 at the target 
altitude, a consequence of ignoring pH2O.   
  
It is best to provide a FIO2 at sea level as defined by the lower curve in Fig. 3 that at least 
produces the equivalent PIO2 at sea level as at the target altitude, a consequence of not 
ignoring  which would account for pH2O. 
 
Example: 9.0% FIO2 at 1 ATA on the display of an iso-pO2 machine would indicate that you are 
at about 21,500 feet altitude with a pO2 of 68.5 mmHg (Eq. 3).  But PIO2 at 1 ATA is 64.1 mmHg, 
equivalent to breathing air at 19,700 feet, so an iso-pO2 machine overestimates the simulated 
altitude by 1,800 feet.  This is a consequence of ignoring the contribution of pH2O.  
  
 
Even accounting for pH2O is not sufficient to account for PAO2 and PACO2 as more direct agents to 
induce signs and symptoms of hypoxia during acute training exposures (2).  
 
Fenn et al., as early as 1946, provided the theoretical foundation on why alveolar gas composition 
would never be identical under NH and HH conditions given the same hypoxic PIO2, a 
consequence captured in the derivation of the Alveolar Gas Equation. 
 
Fig. 4 is an example of how NH and HH given the same PIO2 of 57.3 mmHg (22,000 ft) would not 
produce identical PAO2 and PACO2.   
Fig. 4.   Application of the Alveolar Gas 
Equation to demonstrate the inability to 
accurately reproduce HH under NH 
conditions given an example of an acute 
hypobaric training exposure to 22,000 ft.  
PIO2 is 57.3 mmHg in both conditions, the 
intercept for all respiratory quotient (RQ) 
diagonals when PACO2 is zero.  Diagonals 
for HH are solid lines and those for NH (8% 
FIO2) are dashed lines, all from the Alveolar 
Gas Equation (Eq. 5).  
  
 
PAO2 = PIO2 – PACO2 ∗ [FIO2 + ((1 – FIO2) / RQ)].  Eq. 5  
  
  
Point 1:  NH trainee dons mask at 760 mmHg with FIO2 of 8% and RQ 
of 0.8, and PAO2 quickly drops to 13 mmHg – a stimulus to 
hyperventilate. 
  
Point 2:  HH trainee rapidly ascends on air to 22,000 feet with same 
RQ of 0.8, and PAO2 quickly drops to about 14 mmHg – a slightly less 
stimulus to hyperventilate than NH. 
  
Point 3:  Hyperventilation of rarified air in this example acutely places 
HH trainee on the 1.1 RQ diagonal given the conditions in Table 1, 
column 2. 
 
parameter HH Example 
PIO2 57 mmHg 
NH Example 
PIO2 57 mmHg    
PB (mmHg) 321 760 
FIO2 0.21 0.08 
VE (lBTPS/min) 14.3 16.5  
VA (lBTPS/min) 10.4 12.0  
VCO2(mlSTPD/min) 289 389  
VO2 (mlSTPD/min) 262 278  
RQ 1.1 1.4  
VA/VCO2 0.036 0.031  
PAO2 mmHg 35 37  
PACO2 mmHg 24 28  
PAN2 (mmHg) 215 648  
TABLE 1.  Reasonable Response to Acute HH and NH Exposures 
Point 4:  Hyperventilation of dense air in this example acutely places 
NH trainee on the 1.4 RQ diagonal given the conditions in Table 1, 
column 3. 
 
Loeppky et al. 1997 (and others) shows a greater increase in the rate 
and depth of breathing in NH relative to HH.  In the above example 
the increase in minute ventilation (VE) and alveolar ventilation (VA) in 
NH relative to HH results is a greater PAO2 and PACO2 as a result of 
increased VCO2 from breathing the relatively dense gas during NH. 
It follows from Loeppky and our example in Fig. 4 that physiological 
responses would be different after peripheral and central 
chemoreceptor responses are integrated within the central nervous 
system. 
 
Even if the Alveolar Gas Equation was used in reduced O2 breathing 
devices one must account for the complex time-dependent role that 
PAN2 has in modifying PAO2 and PACO2 under a particular hypoxic 
PIO2. 
 
An accurate application of the Alveolar Gas Equation requires that 
the inspired N2 volume be equal to the expired N2 volume: 
 
VN2 = VI ∗ FIN2 – VE ∗ FEN2 = 0.      Eq. 6 
       
Eq. 6 is applicable in maneuvers such as breath holding, voluntary 
hyperventilation, or exercise.  But Eq. 6 is invalid to greater or lesser 
degree when ambient pressure changes or FIO2 ≠ 0.209, or some 
combination of both until a new PAN2 equilibrium is established.   
 
NH necessarily requires N2 molecules to move from the lungs into 
the tissues while HH requires N2 molecules to move from the tissues 
into the lungs, each moving under different concentration gradients 
and possibly different time constants until a new dynamic equilibrium 
is achieved during a chronic NH or HH exposure. 
 
The transient movement of N2 changes PAN2 at constant PB, so 
changes the O2-CO2 point between NH and HH until the differences 
eventually become small and constant as each O2-CO2 point 
migrates onto its appropriate RQ diagonal near 0.8.   
 
parameter HH NH acutetimechronic 
fv   ? 
VT   ? 
VE   ? 
VA   ? 
VCO2   ? 
VO2    ? 
RQ   ? 
VA/VCO2   ? 
PAO2   ? 
PACO2   ? 
FEN2/FIN2   ? 
VD/VT ? ? ? 
Q ? ? ? 
VA/Q  (5) ? ? 
pHCSF ? ? ? 
% AMS   ? 
TABLE 2.  Unresolved Issues Given Same Hypoxic PIO2 
There is not an adequate integrated physiological understanding of the 
determinants of PAO2 and PACO2 under NH and HH given the same hypoxic 
pO2 to claim a device that provides TRUE isohypoxia.   
 
Isohypoxia is defined as the same distribution of hypoxia signs and symptoms 
under any circumstances of equivalent hypoxic dose, and hypoxic pO2 or 
even PIO2 are incomplete doses (3). 
 
Both time-dependent PAO2 and PACO2 should be considered in a calculation 
of hypoxic dose. 
 
We hypothesize that the integrated hypoxic dose over the same exposure 
time is less in NH than HH for the same hypoxic PIO2.  
 
Some devices that claim an equivalent HH experience under NH conditions 
significantly overestimate the HH condition, especially when simulating 
altitudes above 10,000 feet (3,048 m). 
 
At best, the claim should be that the devices provide an approximate HH 
experience since they only duplicate the ambient pO2 at sea level as at 
altitude (iso-pO2 machines).   
 
A first step n approach to reduce the overestimation is to at least provide 
machines that create the same PIO2 (iso-PIO2 machines) conditions at sea 
level as at the target altitude, a simple software upgrade from Eq. 3 to Eq. 4.    


Approximate Simulation of Acute Hypobaric Hypoxia with Normobaric Hypoxia

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