Methods of gas-balance control to be used with a portable hyperbaric chamber in the treatment of high altitude illness

Transcription

1 Journal of Wilderness Medicine 1, (1990) Methods of gas-balance control to be used with a portable hyperbaric chamber in the treatment of high altitude illness R.I. GAMOW 1 *, G.D. GEER2, J.F. KASIC3 and R.M. SMITR 4 JDepartmentofChemicalEngineering, University ofcolorado, Campus Box 424, Boulder, Colorado 80309, USA 2Department ofemergency Medicine, Hennepin County Medical Center, Minneapolis, Minnesota, USA 3Department ofkinesiology and 4 DepartmentofGeology and CIRES, University ofcolorado, Boulder, Colorado 80309, USA A portable fabric hyperbaric chamber (Gamow Bag TM ) has been designed for on site treatment ofaltitude illness. Thechamberincludes a manual pumping system, weighs 6.57 kg (14.5 pounds), and can be carried in a backpack. A laboratory study was undertaken to document safety, subject comfort and gas retention characteristics of the chamber. The chamber is designed to be used in one of three different modes: pump mode, bladder mode, and oxygen-scrubber mode. In the pump mode, 521 min-i offresh air is continuously supplied via an air pump. Using this technique, ten healthy volunteers were placed in the chamber for 90 min at pressures up to 16.0 kpa (120 mmhg) above ambient pressure. Ambient CO 2 and O 2 levels were found to be less than 1% and more than 20%, respectively. Testing the bladder mode, six subjects exhaled via a nasal mask into a latex bladder, not allowing CO 2 to accumulate in the chamber. Using this mode, the pumping rate can be decreased by a factor of ten, a great advantage when the pumper is working at altitudes above 5792 m (19000 ft). In the oxygen-scrubber mode, O 2 was continuously added via a pressure regulator. CO 2 accumulation was eliminated with a high efficiency CO 2 scrubber made of LiOH. One pound of scrubber and 150 lof pressurized O 2 can keep an average person in an atmosphere of 21 % O 2 with less than 1% CO 2 for 6 h. In this mode, all pumping, except the initial pressurization of the bag, is eliminated. Key words: altitude illness, hyperbaric chamber, Gamow Bag, HAPE, HACE, HBO, decompression chamber Introduction Righ altitude illness affects persons who ascend rapidly to altitudes above 2500 m. Acute mountain sickness (AMS) is the most frequent manifestation, characterized by headache, nausea, vomiting and lassitude [1,2). Symptoms of AMS typically begin 6-8 h after rapid ascent [3,4). AMS is rarely serious and usually self limited, but may lead to high altitude pulmonary edema (RAPE) or high altitude cerebral edema (RACE) [2). RAPE is uncommon below 3000 m, although fatal cases have been reported at as low as 2500 m [2,5). If untreated, it may progress to coma and death. RACE starts with a severe headache and progresses to ataxia, irritability, hallucinations, lethargy and eventually coma and death. RAPE and RACE may affect young, healthy individuals in an unpredictable manner [6). The complex sequence of physiological changes that leads to altitude illness is not fully understood, although it is generally agreed that a low environmental partial pressure of oxygen is the initial inciting event [1,2). *To whom correspondence should be addressed /90 $ Chapman and Hall Ltd.

2 166 Gamowet al. Despite various pharmacological trials [1], the only definitive treatment for AMS, HAPE, or HACE is descent to a lower altitude [1-6]. Dramatic improvement has been noted after a descent of only m [2,4,6]. Oxygen has been reported to be effective treatment for AMS when used at a rate of 1-21 min-i [1]. HAPE also responds to oxygen, but flows of min-i are required [1-4]. Oxygen is not definitive for the treatment of HACE [2]. Although HAPE responds to oxygen, recovery is more rapid with descent, which should not be delayed [7]. Hyperbaric therapy has been used successfully in the treatment of mountain sickness, but little data appear in the literature [1]. Rapid improvements have been reported to occur after 30 min at a simulated altitude of 1600 m (5200 ft) inside a large steel hyperbaric chamber at Pheriche, Nepal, 4000 m (14000 ft) [8]. Altitude illness often occurs in settings which preclude rapid descent. Climbers may be stranded in isolated regions with rough terrain and poor weather, making ground or air evacuation impossible. Oxygen is usually in short supply. This is the situation in which a lightweight portable hyperbaric chamber would be efficacious in treating altitude illness. The purpose of this study was to test such a chamber under controlled laboratory conditions. Material and methods The portable hyperbaric chamber used in our studies is manufactured and marketed by Du Pont as the 'Gamow Bag' (Fig. 1). The chamber itself weighs 3.8 kg (8.5 Ib) and, for field use, comes with a 1.8 kg (4 Ib) bellows-type foot pump. When electricity is available, inflation and pressurization can be obtained using an oil-free diaphragm compressor that delivers air at 4.25 X 10-2 m 3 min- 1 (1.5 ft3 min-i). The volume inside the standard Gamow Bag is 0.5 m 3 (17 ft3). The chamber used in the 'pump mode' experiments was slightly larger, 0.6 m 3 (22 ft3). The inside pressure of the Gamow Bag is controlled by two 13.7 kpa (2 psi) pressure relief valves. The chambers are equipped with a pressure gauge so that the internal pressure can be monitored continuously. In experiments using the pump mode, ten healthy university student volunteers, six males and four females, were used as subjects. Each was questioned regarding history of ear or sinus problems or surgery, spontaneous pneumothorax or other pulmonary disease, and current upper respiratory infection. Subjects with any positive responses were excluded from the study. Additionally, any subject who reported a history of claustrophobia was excluded. Each subject had an otoscopic exam to ensure external ear canal patency prior to compression; any subject with an occluded canal was excluded. Subjects were instructed on the proper procedure to clear their ears during pressure changes. Prior to entering the chamber, each subject was weighed. The V02 and VC02 for each subject were measured by collecting exhaled air for one minute while each subject stood at rest. A tight fitting face mask was used for gas collection. Oxygen concentration was analyzed using an Applied Electrochemistry analyzer, model S3-A (Applied Electrochemistry, Sunnyvale, CA). The CO 2 concentration was determined using a Beckman medical gas analyzer, model LB2. Each subject was fitted with a transmitted pulse monitor (CIC Unique). The chamber was inflated with the Thomas electric pump and the Sevylor foot pump simultaneously at a rate of approximately 3501 min-i (the maximum rate for the

3 Methods ofgas-balance control for portable hyperbaric chamber 167 Fig 1. Photograph of the Gamow Bag fully inflated. combined pumps). It required 2 min to inflate the chamber to full volume with no increase in pressure. Inflation was continued with a pause after each 2.7 kpa (20 mmhg) pressure increase to allow the subject to clear his or her ears. When the subject verbally communicated clear ears, inflation continued with pauses until the pressure release valve opened at 13.7 kpa (2 psi). Time from chamber entry until full pressure was reached was approximately 3 min. At this time, manual pumping was stopped while the electric pump continued to deliver 52 I min-i, equivalent to foot pumps per minute. Because 52 I min- 1 is a higher flow rate than a single pressure relief valve will handle, the chamber pressure rose to 16.0 kpa (120 mmhg) and remained constant throughout the experiment. A 0.5 I gas sample was removed from the head of the chamber every 5 min. The samples were analyzed for oxygen and carbon dioxide. Heart rate was continuously monitored and recorded every 5 min. Continuous verbal and visual communication were maintained with the subjects throughout the trials. Each subject remained in the chamber for 90 min. The air supply was then discontinued and the pressure released over a period of one minute, with pauses as necessary for ear clearing. The zipper was opened and the subject exited the chamber. All tests were done in our laboratory at the University of Colorado at Boulder (1600 m (5300 ft» with the exception of the bladder field tests, which were conducted with the help of the Colorado Altitude Research Institute in Keystone, Colorado (2700 m (9000 ft».

4 168 Gamowetal. In experiments using the 'bladder mode', the procedure is identical to the 'pump mode' except that the patient breathes into an oral cup that allows exhalation into a large 100 I latex bladder and inhalation of air from within the chamber. A 1 in flexible hose carries the exhalent into the latex bladder. The accumulated exhalent can be vented directly to the outside via another 1 in flexible hose that is connected directly to the 13.8 kpa (103 mmhg) exhaust valve. In experiments using the oxygen-carbon dioxide scrubbing mode, the Gamow Bag is equipped with COz scrubbing pads and a high quality pressure regulator. The scrubber (Du Pont) consists of a series of one foot square pads that have been impregnated with LiOH. Each pad lasts approximately 20 min. The pads remove both COz and accumulated moisture. A Matheson model 8-2 pressure regulator, full scale range 0 to PA (3 psi), was used to maintain chamber pressure and replace the spent oxygen. Although the Matheson is an ideal pressure regulator for the laboratory experiment, in field tests we used a light 0.39 kg pressure regulator produced by Circle Seal Controls (Anaheim, California). The oxygen bottle contains 136 I when pressurized to 12.0 kpa (1750 psi). This will supply enough Oz for a person at rest for 6 h. For field use, the Oz bottles canbe filled to 21.0 kpa (3000 psi), significantly extending theduration ofsupply. COz and Oz concentrations were determined with a Hewlett Packard Patient Gas Monitor, model 78386A. Results Pumping mode All ten subjects completed the experiment. Each reported noticing the pressure changes during both compression and decompression, but none complained of pain or difficulty in ear clearing. The subjects were comfortable inside the chamber without complaints of claustrophobia. All subjects were able to shift from a supine to prone position while inside the chamber; one subject was able to turn so that her head was at the foot end of the chamber. Table 1 shows the Oz and COz concentrations for each subject from min in the chamber. These values represent steady state concentrations and remained constant Table 1. Measured steady state Oz and COz concentrations inside the chamber, calculated Va, and VC02' and mean heart rate for each subject. Subject Sex Weight Steady state Voz V eoz Mean (kg) %Oz %COz heart rate 1 M ± 4 2 F ± 6 3 M ± 5 4 M ± 6 5 F ± 4 6 M ± 5 7 F ± 5 8 M ± 5 9 F ± 4 10 M ± 3

5 Methods ofgas-balance control for portable hyperbaric chamber 169 after 60 min. Average heart rates are also listed. Rate remained approximately constant throughout the experiments, varying less than 6 beats per min, without significant acceleration or deceleration. The V02 and VC02 calculated from the measured expired air of each subject before entering the chamber are also given. The following equations were used to calculate V02 and VC02 V 02 = VC02 = (0.209S-f o2 exhaled) (Volume exhaled) (f co2 exhaled) (Volume exhaled) Figure 2 compares the measured O 2 and CO 2 inside the chamber averaged for the ten subjects with the concentrations calculated from theory. The theoretical calculation is based on a mole balance over the chamber. The predicted O 2 and CO 2 concentrations are: 2' J %02.hoty %02 N , J IM (INn) C02.,. G time (min) C02.1heory Fig 2. Comparison of measured 0z and COz inside the chamber with concentrations calculated from theoretical equations.

6 170 fc = Po V 02 (e -PFt-1) PF P V c =PoVe02(1_ e -PFt) Je02 PF P V c where: t0 2 = fractional concentration of oxygen inside the chamber te02 = fractional concentration of CO 2 inside the chamber P = ambient barometric pressure in mmhg Po = barometric pressure where V02 and VCO2 were measured Pe = total barometric pressure inside the chamber in mmhg F = volume flow rate of fresh air into the chamber in I min- t V = total chamber volume in liters V 02 = volume of O 2 used by subject in I min- t t = time in minutes as t approaches 00, the concentrations reach a steady state, where: Gamowet al. (1) (2) (3) The time to reach steady state is dependent on the parameters in equations 1 and 2. For the experimental conditions used, steady state is reached in approximately 60 min. These equations assume that air in the bag is well mixed. Random samples were taken from the foot of the bag during the experiments and compared with samples from the head. There was never more than 0.1 % discrepancy, indicating that the air was quite well mixed. Table 2 compares the measured f 02 and f co2 with that calculated using equations 3 and 4. A paired test was used to compare the predicted with the measured values. It (4) Table 2. Measured steady state O 2 and CO 2 concentration inside the chamber compared with the theoretically predicted concentrations. Subject Measured Predicted (Measured - predicted) %02 %C02 %02 %C02 02 C Mean Standard Deviation

7 Methods ofgas-balance control for portable hyperbaric chamber 171 could not be shown that there was a significant difference between the measured and predicted values (p < 0.05). Bladder mode This mode of the Gamow Bag TM is that which mountain climbing teams working above 5792 m (19000 ft) will probably find most useful, since it is lightweight, self-regulating and easy to use. This method utilizes a face mask and a latex bladder which traps the patient's exhalent. The weight of the added system is less than 1 kg. The advantage of this system is that the pumping rate can be decreased to pumps per min. The patient can be left with no maintenance for the time it takes to fj.j.i the bladder, with the pumping/ bladder emptying procedure performed at min intervals. Alternatively, pumping can be calculated to approximately match the patient's breathing rate (2-3 pumps per min), never allowing the bladder to fill completely. Operation anywhere between these two extremes is possible to suit the needs and convenience of those maintaining the chamber from the outside. The Gamow Bag TM is operated at 13.7 kpa (2 psi) above ambient pressure and is fitted with two relief valves to prevent over-pressurization. To one of these valves we have attached tubing, which leads to an air-tight bladder, which in turn is attached to the exhaust port of a face mask worn by the patient (Fig.3). As the patient breathes, he or she consumes ambient air from the chamber through the intake ports of the mask. The Fig. 3. RIG is demonstrating the use of the bladder. The expanding 100 I latex bag is shown at the lower left.

8 172 Gamowetal. air is exhaled through the exhaust port into the bladder, where it is contained separate from any inhalent. As the bladder fills, it can be emptied using the foot pump on the outside of the chamber. As the foot pump is operated, the pressure in the bag increases. At 13.7 kpa (2 psi), the relief valve opens and dumps out the exhausted air from the bladder (Fig. 4). Pumping continues until the bladder is empty or nearly empty, and the process is repeated. We have found that the 100 I bladder fills in approximately min, thus placing an upper limit on time available with no maintenance. This time can be calculated as follows [9]: Vbladder = minutes to fill bladder (Vou/min) (5) where: Vbladder = volume of bladder YOU! = volume of air exhaled by patient -.- ( YOU!) = volume of exhalent produced per min mm In the lower-limit case, where one averages the pumping such that there is zero time with no maintenance (except for that between pumps), we have found the pumping rate to be approximately 2 pumps per min to hold inflation of the bladder in a steady-state. This is calculated as follows: (:;!)(i) = pumps per min (6) where: C = capacity of foot pump in liters per pump. If the bladder and face-mask system were completely leak-proof, one would find no mixing of gases between inhalent and exhalent. Thus, the patient would always be consuming an ambient air mixture. Due to the fact that one consumes a greater volume of air than one exhausts, the volume which is metabolized (d V) must be replaced. (7) where: Yin = volume of air inhaled by patient you! = volume exhausted from bladder d V = volume metabolized by patient = Vo zc 1- RQ) RQ = v coz = respiratory quotient v oz Due to the configuration of this system, the volume (d V) is automatically regulated, as it is the volume which must fill the bag to regain 13.7 kpa (2 psi) before bladder exhaust

9 rj) 'hi So.Q, 18" I /IO>'CL.../; ]1'-r :J --_.-/ I _.. _---...'! -1 :::».. I WAY VALVe..s l0,4j XAL.JI... [JO \5) ii 0 l::l S' ;::l 'C'... C... 0 l::l... (:j Fig. 4. Schematics ofthe entirebladder system. (1)oral cup oxygen mask (2) 100 liter latex bag (3)wall of latex bag (4) one-inch diameter air exhaust flange (5) one-inch flexible medical tubing (6) one-inch stiff tubing (7) three fourths by one inch double barbed hose connector (8) two way valve (9) six inch by one inch medical tubing (10) eighteen by one inch stiff tubing...:i W

10 174 Gamowet al. can begin. Any other loss of volume, such as that caused by a leak in the chamber, is controlled in the same way. If the breathing system is leak-proof and there is no gas mixing, then the volume of air which must be pumped in from the outside is limited to that which is necessary to regain pressure and deflate the bladder. This will equal the volume of air inhaled by the patient plus any volume which may escape due to a leak in the chamber. where: V p = volume pumped in V L = volume leaked out Yin will vary from patient to patient and from situation to situation. However, we have found the average pumping rate to be 2-3 pumps per min. This represents a dramatic improvement over the pumps per min necessary to vent the chamber with no gas control system. The theoretical calculations quoted above were verified when we field-tested the bladder mode. For the majority of subjects, the oxygen concentration stayed well above 18% and carbon dioxide levels below 2% (Fig. 5). The amount of pumping ranged from pumps per min (Fig. 6) and appeared to be linearly related to subject weight (Fig. 7). (8) a Z «c::: I- 12 Z w U oxygen Z 0 u CJ) «<..:l a Cl. /! ao carbon dioxide 100 TIME (min) Fig 5. Gas concentrations versus time in tests of the bladder system.

11 Methods ofgas-balance control for portable hyperbaric chamber 175 SUBJECT Fig 6. Pumping rate for various subjects using the bladder system O or--..., , or--... " WEIGHT (b) Fig 7. Pumping rate as a function of subject weight using the bladder system. Oxygen scrubber model In this mode, all pumping is eliminated exceptthat needed for the original pressurization. Oxygen levels remain at 21 % and CO 2 levels below 1% via an oxygen pressure regulator apparatus and a high efficiency CO 2 scrubber, which are both incorporated into the Gamow Bag TM (Fig. 8). The theoretical calculations and data presented below were first reported elsewhere [10].

12 176 Gamowetal. Fig. 8. A fully inflated chamber with an internal frame is shown from the inside. In the center of the head of the tent a Circle Seal regulator is seen with an oxygen bottle attached. The white material on the right of the chamber is the LiOH scrubber. When the Gamow Bag TM is used in this mode, it is important to maintain a negligible leak, both to maintain pressure and to avoid excess concentration of oxygen in the bag. Leak rates were calculated as follows: Using the ideal gas law approximation, the amount of air pumped into or leaked out of the chamber versus the gauge pressure on the bag is given by: where: d V = volume of air (at ambient pressure) pumped in or leaked out P = pressure on gauge V= volume of bag (476/) PAM = ambient pressure = 85.3 kpa (640 mmhg) in Boulder, CO = kpa (760 mmhg) at sea level This equation gives a result of 5.58 X 10-3 I Pa- I ( mmhg- 1 ) in Boulder, and 4.69 X 10-3 I PA-I (0.626 I mmhg-i) at sea level. Leakage was measured directly in mmhg per unit time. Combining these measured values with equation (1) gives the leak rate in l/min: dv dp _ dv dp dt - dt (10) (9)

13 Methods ofgas-balance control for portable hyperbaric chamber The value obtained for the Gamow Bag under study was: (0.744) 1/mm Hg 3g) = 0.221/min 177 (11) After the leak test, the second phase of testing measured the kinetics of the CO 2 absorption portion of the system. Figure 9 shows percentage CO 2 versus time for two different input rates of CO 2 gas. Ten Du Pont LiOH CO 2 scrubbing pads were used. CO 2 was supplied from a gas cylinder bottle via a flow regulator, and no human subject was in the chamber. Figure 10 shows percentage CO 2 as a function of time with a human subject as the producer of CO 2, The plot shows CO 2 buildup when no LiOH pads were used. The LiOH pads successfully prevented CO 2 buildup in the chamber. On average, the usable lifetime per pad is 20 min. The third phase in the testing process measured the oxygen consumption of a human subject as a function of time. These measurements were taken with and without LiOH pads, but with no other regulation of gases. Figure 11 shows the dramatic decrease of oxygen inside the chamber when no supplementation was available. The results indicate that with or without LiOH pads, O 2 concentration reaches dangerous levels within approximately two hours. The final phase of testing combined a human subject, pressurized chamber, LiOH pads, and 02 supplementation system. The chamber was inflated with a foot pump to 2 psi gauge. The O 2 regulator was set to maintain the chamber at that pressure. With a completely leak-proof chamber, the only loss of pressure in the system is due to O o/j:1:j2. 3 CO2 51,m 2 CO2 31.m time (min) Fig 9. CO 2 from a gas cylinder was bled into the chamber via a flow regulator set to deliver 0.3 I min- 1 in one case and 0.5 I min- 1 in the second case. Ten UOH pads were suspended in the chamber. These data demonstrate the kinetics of CO 2 absorption by the UOH pads.

14 178 Gamowet al. 40 :l "0 ;( 30 0 "5 c: 0.c 2.0 (.ij u 14 pacs II o ::::acs Q time (min) Fig 10. A human subject (R.I.G.) was placed in the chamber; CO 2 concentration was measured with no LiOR pads in one case, and 14 LiOR pads in the second case. c: Cl) Cl > )( o cj. II no pads 6 pads time (min) Fig 11. A human subject (J.F.K.) was placed in the chamber with no LiOR pads in the first test, and six pads in the second test.

15 Methods ofgas-balance control for portable hyperbaric chamber 179 II II II II 20 II II II II 15 O/.,0 10 II 02 CO time (min) Fig 12. A human subject (H.M.S.) was in the chamber for 6 h. CO 2 was removed by means of 22 LiOH pads; 02 was replaced by a pressure regulator attached to an O 2 gas cylinder. The pressure in the bag fell from 98 mmhg to 40 mmhg above ambient pressure during the course of the experiment, because of chamber leakage and because chamber air was bled out in order to measure O 2 concentration. consumption by the subject; thus, the O 2 regulator allows replacement of exactly that which has been used. Twenty-two LiOH pads were placed in the chamber, since this was determined to be more than sufficient for a six hour test, the lifetime of a 136 I O 2 bottle. In Fig. 12, the CO 2 and O 2 gas concentrations are shown as functions of time, and both curves are essentially flat over the six hour duration of this experiment. We have shown that a leak-rate of 0.22 I min- 1 can be considered essentially air-tight. The LiOH pads successfully control CO 2 concentration, and the O 2 bottle/regulator component successfully replaces O 2 used by the subject, while at the same time maintaining chamber pressure. Conclusion The Gamow Bag TM, a portable hyperbaric chamber, exactly duplicates descent. Physicians working in the field in Alaska, Nepal and Tibet continue to document its efficacy. Since the bag must be airtight in order to hold pressure, there must exist a way of maintaining safe gas balances for the patient inside. We have discussed three different methods of gas-balance control. Each method has advantages in given situations. The major variations between the methods are in weight and effort of maintenance. The foot pump method adds no extra weight to the system, but requires the pump to be operated times per min by someone outside the chamber. While the lack of extra weight is an advantage to climbing teams, the effort required to maintain this pumping rate can be very taxing at high altitudes. The bladder system adds less than two pounds to the weight of the basic bag, while reducing the effort of maintenance by 60-80%. It will

16 180 Gamowet al. probably prove to be the most useful option for small climbing teams. Where weight is less of a consideration, the full closed-circuit rebreathing system requires no maintenance at all for periods of approximately six hours. Permanent medical facilities and the base camps of large expeditions will probably be suitable locations for this system. Acknowledgements We acknowledge the Candle Foundation, Los Angeles, California, The University of Colorado Graduate School- CRCW, and Du Pont de Nemours and Company (Inc.) for their generous financial support. Du Pont also donated the LiOH scrubbers. We thank the University of Colorado Human Performance Laboratory, and Professors Art Dickenson and Bill Byrnes of the University of Colorado Department of Kinesiology for providing laboratory space and valuable technical assistance and encouragement. We also thank the Colorado Altitude Research Institute and the Snake River Medical Clinic for allowing experiments to be done at facilities in Keystone, Colorado. The chambers used in these studies were donated by Du Pont Co. and Hyperbaric Mountain Technologies, Inc. References 1. Hackett, P.H. and Roach, RC. Medical therapy of altitude illness. Ann Emerg Med 1987; 16, Houston, C.S. Altitude illness. Emerg C/in North Am 1984; 2, Foulke, G.E. Altitude related illness. Am J Emerg Med 1985; 3, Hackett, P.H. Acute mountain sickness - The clinical approach. Adv Cardio/1987; 27, Dickinson, J.G. Severe acute mountain sickness. Postgrad Med 1979; 55, Schoene, RB. High altitude pulmonary edema: pathophysiology and clinical review. Ann Emerg Med 1987; 16, Hackett, P.H., Rennie D. Acute mountain sickness. Sem Resp Med 1983; 5, Houston, C.S. Going Higher. Boston: Little, Brown & Co., 1987: Smith, H.M., Kasic, J.F. and Gamow, RI. A gas balance control system for use with a portable hyperbaric chamber. 11th Annual International Conference of IEEE Engineering and Biology Society, Kasic, J.F., Smith, H.M. and Gamow, RI. A self contained life support system designed for use with a portable hyperbaric chamber. Biomaterials Sciences Instrumentation 25. Ames, Iowa: Iowa State University, 1989.