The Diluter-Demand Oxygen System
Used During the International Himalayan Expedition to Mount Everest
F. DUANE BLUME AND NELLO PACE
IN 1969 the White Mountain Research Station, University of California Berkeley, was asked by Barry Hagen to assist with plans for oxygen-breathing equipment to be used in support of an attempt to scale the Southwest Face of Everest in 1971. The use of supplementary oxygen as an aid in ascents of major peaks has had a long history, dating back to the original suggestion in 1878 of the French physiologist, Paul Bert, that by the respiration of super-oxygenated air the summit of Mount Everest would no longer be theoretically inaccessible to man.1
Oxygen was first employed on Everest by George Finch and Geoffrey Bruce during the 1922 British expedition.2 Porters carried oxygen cylinders to a camp at 25,000 feet on the northwest ridge, where Finch and Bruce spent two nights before their summit attempt. On the third day, breathing oxygen through a mask, they reached an altitude of 27,300 feet before turning back. Finch and Bruce also left behind the first of now many hundreds of empty oxygen cylinders which adorn the high slopes of the Goddess Mother of the World.
The most widely used oxygen unit in recent years was the one designed by Thomas Hornbein for the American Mount Everest Expedition in 19633, which comprises an ingeniously designed face mask arranged so that a variable quantity of oxygen, manually regulated, is delivered to the oro-nasal region of the mask with each breath. The major disadvantage of the system is the limitation imposed by manual selection of oxygen flow rates (1 to 4 liters per minute) consistent with the degree of physical activity. Generally, the flow is selected prior to a given period of climbing and remains at that level until the need for additional oxygen is felt subjectively. Owing to the fact that climbing of this type involves irregular patterns of activity, the selected flow rate is apt to be wasteful at the lower respiratory rates and potentially deficient at the very high rates.
Preliminary consideration of the problem indicated two possible approaches for improvement of oxygen delivery under the severe constraints of Himalayan mountaineering. One was the utilization of the closed-circuit principle tested by Bourdillon during the British Expedition to Mount Everest in 19534 and used so successfully by Lunar Astronauts in the vacuum of outer space. The other was the application of the diluter-demand regulator, developed and used extensively for high-altitude flying during World War II.
While the closed-circuit apparatus permits the ultimate in efficient utilization of a given supply of oxygen, it has several disadvantages. The system is bulky by nature and is complex mechanically. It requires the use of a carbon dioxide absorbant which must be renewed, thus doubling the logistics problem. Finally, should the oxygen supply fail while the unit is in use, the oxygen concentration in the circulating gas can actually fall below that in the ambient air without the awareness of the wearer.
Accordingly, our development effort was directed toward exploration of the diluter-demand regulator principle. As used in aviation, these regulators are designed so that as the individual inspires he simultaneously draws ambient air into his mask through an orifice in the regulator, and pure oxygen from a tank through a demand valve in the regulator. The size of the ambient air orifice is made directly proportional to the barometric pressure by use of a passive aneroid valve, so that as altitude increases the ambient air orifice is automatically made smaller and the individual inspires a greater proportion of oxygen. The diluter-demand system thus accomplishes two things: maintenance of a near-sea-level partial pressure of oxygen in the inspired air regardless of altitude, and considerable economy of bottled oxygen.
A major feature to be included in the design of a diluter-demand oxygen regulator for Himalayan mountaineering is the level of oxygen partial pressure to be delivered to the user. The regulators designed for use by aviators insure that the inspired oxygen partial pressure (PIo2) does not fall below that at an altitude of 8,000 feet; namely, 115 to 120 torr. However, the Himalayan mountaineer is acclimatized to high altitude and does not require the same degree of oxygen supplementation. It is well established5 that individuals can acclimatize successfully to altitudes up to 17,000 or 18,000 feet where the PIo2 is about 80 torr, and the traditional march-in by Everest expeditions from Kathmandu to a Base-Camp site between 17,000 and 18,000 feet below the Khumbu icefall is designed to help attain such acclimatization. Therefore, the assumption we made was that the diluter-demand oxygen regulator for high climbing should be designed to provide a PIo2 of approximately 80 torr, regardless of the level of physical activity involved and regardless of the altitude reached above 18,000 feet. Thus, the climbers would always be working at an equivalent physiological altitude of 18,000 feet, taking full advantage of their natural high-altitude acclimatization yet making efficient use of their available oxygen supply.
In 1970, Dr. F. Duane Blume was appointed Oxygen Officer of the International Himalayan Expedition; Mount Everest, Southwest Face, Direttissima, under the leadership of Norman Dyhrenfurth, and development of a diluter-demand oxygen system was pressed vigorously. At the suggestion of Captain Walter L. Goldenrath, MSC, USN, Director of the Aerospace Crew Equipment Department, U. S. Naval Air Development Center, Johnsville, Pennsylvania, the authors contacted the Robertshaw Controls Company, Anaheim, California, whose personnel had developed a miniaturized diluter-demand regulator which is in wide operational use in civilian and military aviation.
The Robertshaw Controls Company, in particular Mr. Robert Hamilton, was most cooperative and undertook the redesign of their regulator to meet our specifications. The modifications ultimately resulted in a regulator which omitted the emergency full-flow oxygen feature of the original and which substituted 4 click-stop ambient-air orifices for the aneroid valve of the original because the aneroid valve lacked sufficient sensitivity. The orifices, numbered 1 to 4, were to be selected by the climber so as to provide a minimum PIo2 of 80 torr at any elevation up to the summit of Everest. Each of the 4 settings corresponded roughly to 2000-foot increments in altitude between 22,000 feet and 30,000 feet.
Development Testing of Diluter-Demand Regulator. At each stage of modification, the regulator was tested physiologically in a low-pressure chamber. The tests consisted in having one of us (F.D.B.) prepared with ECG electrodes for measuring heart rate, and wearing a standard A-14 face mask which was connected to the diluter-demand regulator under test. The regulator in turn was connected to an oxygen gas supply line at 50 psi. Incorporated into the mask was a miniaturized oxygen electrode supplied by Dr. Boyd W. Coon of this laboratory, which served to sense PIo2 continuously during the test. The subject was seated on a Quinton bicycle ergometer in the chamber. When the desired test altitude was reached in the chamber, heart rate and PIo2 were recorded with the subject at rest, during a standard 800 kpm exercise for 5 minutes, and then during recovery from the exercise. Chamber air Po2 was monitored by means of a Thermo-Lab Instruments Company Thermox Probe. Performance of the regulator was evaluated on the basis of change in heart rate and PIo2, and on the basis of the sensations and behavioral characteristics of the subject.
A typical test is shown in Table 1. In this instance, an early version of the regulator was found to deliver an excessively high PIo2 and was returned to the Robertshaw Company for further modification. It may be pointed out that physiological testing of the regulator posed a special problem because, inasmuch as it was designed to provide a PIo2 equivalent to 17,000 or 18,000 feet, a subject acclimatized to sea level would be at a severe disadvantage, particularly during exercise. However, Dr. Blume is regularly the resident scientist at the White Mountain Research Station and had acquired a substantial degree of high-altitude acclimatization before he served as the subject for these tests. Thus, it was feasible to examine the performance of the regulator in the low-pressure chamber at equivalent altitudes up to 30,000 feet before the expedition went into the field.
Heart Rate (beats/min)
Table 1. Low-Pressure Chamber Performance Test at Click-Stop Setting 2 of a Development Model of the Diluter-Demand Regulator at an Altitude of 25.200 ft., a Barometric Pressure of 280 torr, and a Chamber Po2 of 58 torr.
The operating characteristics of the final version of the regulator are shown in Figure 1. The lower diagonal line for each of the 4 click-stop settings gives the PIo2 as a function of the ambient barometric pressure when the individual’s inspiratory minute volume is 20 Hters/min, which represents mild activity. The upper diagonal line gives the PIo2 when the inspiratory minute volume is 80 liters/min, which is typical of moderately severe exercise. The dashed horizontal lines at PIo2 values of 90 torr and 75 torr represent the ambient oxygen partial pressures at altitudes of 15,000 feet and 19,000 feet, respectively. It may be seen that the regulator not only effectively keeps the PIo2 at the equivalent of 17,000 or 18,000 feet, but that it has the additional feature of providing a higher PIo2 at increased ventilatory rates. Thus, during periods of exertion the climber actually receives more oxygen than he does at rest.
As discussed below, the oxygen cylinders used on the expedition contained 1200 liters of oxygen at STP when they were fully charged at 3000 psi. Figure 2 shows the time a cylinder lasts as a function of the inspiratory minute volume, at each of the 4 click-stop settings of the diluter-demand regulator. Even taking the worst case, at setting 4 on the regulator, it may be seen that one cylinder would be expected to last from 20 hours if the individual were at rest, to 4.2 hours if he were very active with a continuous inspiratory minute volume of 80 liters/ min. These duration times correspond to constant oxygen flow rates of 1 liter/min and 4.8 liters/min, respectively. In practice the oxygen flow rate, and hence the cylinder duration time, would be strictly proportional to the integrated inspiratory minute volume of the individual. Thus, considerable oxygen economy is achieved automatically.
Total Oxygen System. The complete oxygen-breathing system used during the 1971 Everest expedition is shown in Figure 3. It comprises the oxygen cylinder, a pressure reducer and pressure gauge, a length of high-pressure plastic tubing, the diluter-demand regulator, and a length of flexible corrugated tubing integral with the face-mask, and altogether weighs 18.5 lb. An auxiliary port on the pressure reducer accommodates another length of plastic tubing which supplies a light plastic sleeping mask when desired.
The steel oxygen cylinders used on the expedition were purchased from Walter Kidde & Company, Inc., Los Angeles, California (P/N 271773). The tank weighs 16 lb when filled with 1200 liters STP oxygen at 3000 psi. It is 5.4 inches in diameter and 21.9 inches in overall length, including the hand-valve. Each climber’s back pack was fitted with three pockets for carrying oxygen cylinders in the upright position. The two outer pockets were used for portage of cylinders between camps, but only one cylinder was carried in the center pocket to maintain proper balance during climbing.
The pressure reducer is a specially modified Robertshaw On-Otf Reducer (P/N 900-002-043) which is designed to provide a gas pressure reduction from 3000 psi at the tank to 60-70 psi at its outlet, and weighs 17 oz. The tank connector for the reducer consists of a brass fitting to which two brass bars are welded so as to provide handles tor manual attachment. A 0-3000 psi pressure gauge is mounted on the reducer to indicate the quantity of oxygen in the cylinder. An auxiliary port is added to the reducer to permit the insertion of a slip-ring hose connector which, when in place, provides a constant flow of 1 liter/nun of oxygen to supply a sleeping mask.
A piece of polyurethane tubing 3 feet long, 5/16 inches O.D.,? inch I D., and weighing 2.5 oz (Molded Products Company, Easthampton, Massachusetts), is used between the reducer and the diluter-demand regulator to supply the latter with oxygen at 60-70 psi. The tubing was selected for its ability to withstand pressure, for its ability to retain flexibility in the cold, and for its resistance to abrasion.
The diluter-demand regulator (Robertshaw Controls Company, Anaheim, California, P/N 900-700-036-24), already described, weighs 6.5 oz.
A standard military A-14 oxygen mask assembly weighing 14 oz (Sierra Engineering Company, Sierra Madre, California, P/N 500-254) was selected because of its long history of reliable performance and excellent properties in the cold. The assembly is made of silicone rubber and does not harden in sub-freezing environments. In addition, the small mask dead-space and the placement of the expired-air valve minimize problems of moisture accumulation and icing within the mask. The mask is readily crushable in place by hand to free ice from the exhaust valve and ports.
A commercially available oxygen breathing mask set (Ohio Medical Company, Berkeley, California, P/N 304-5075-600), consisting of a light, molded plastic mask, a rebreather bag and a 5-foot length of polyethylene tubing, was procured for use as a sleeping mask. The end of the tubing was fitted with a slip-ring insert connector for attachment of the mask to the pressure reducer when desired.
Oxygen System Performance during the Expedition. On the whole, the oxygen system performed exceptionally well, and met all of the objectives in terms of maintaining the climbers in a good physiological state while at the same time realizing oxygen economy. Some minor and readily correctible faults emerged and may be enumerated.
All climbers recommended lighter and less bulky oxygen cylinders, even if shorter lifetimes resulted, because of some interference with balance. However, it must be noted that any tank will cause difficulty in balance when the degree of technical climbing characteristic of this expedition is encountered.
The high-pressure polyurethane tubing connecting the pressure reducer and the diluter-demand regulator did stiffen somewhat with the cold temperatures, although it functioned well otherwise. Consideration might be given to the use of protected silicone rubber tubing in its place.
One climber who was experienced with other oxygen systems felt that there was some resistance to breathing. This is probably attributable to the one-inch diameter corrugated rubber tubing connecting the mask and the diluter-demand regulator. It is well known that one-inch tubing is restrictive at high ventilatory rates; thus, substitution of 1¼Vi-inch diameter tubing would be desirable.
It was necessary to eliminate the rebreather bag from the sleeping mask because of low-temperature cracking. Also, the polyethylene tubing stiffened noticeably in the cold, but was not a serious problem because it was used only during sleep. Some climbers simply put the hose directly into the sleeping bag with satisfactory results.
The positive results, on the other hand, were impressive. Only one oxygen cylinder out of 300 taken on the expedition was found to have leaked. It may be added that each cylinder was encased in “Dorvon” polystyrene foam material (Wilshire Foam Products, Inc., Torrance, California) for transport from the United States to the expedition base camp, which undoubtedly accounted for the exceptionally low oxygen loss rate in transit.
The pressure reducers and the diluter-demand regulators received heavy use and performed without difficulties. A single instance of apparent icing of the diluter-demand regulator occurred, which was readily cleared by brief hand warming of the regulator.
The A-14 mask proved to be entirely satisfactory. Moisture accumulation and icing were no problem, and the mask was found to fit well on all individuals who wore it. The broad face flaps gave protection from wind and cold; in addition, the flaps helped to provide an adequate gas seal even on a bearded face.
Most important of all, however, was the finding that a single 1200-liter oxygen cylinder consistently provided 6 to 8 hours of supply for heavy climbing above 23,000 feet. In all, 7 climbers utilized the diluter-demand system for a total of 75 man-days under these conditions.
The effectiveness of the system is best demonstrated by the fact that two of the expedition members, Dougal Haston and Don Whillans, spent 23 consecutive days above 25,000 feet, and attained an altitude of 27,300 feet at the highest point of their arduous climb. This feat is the longest recorded continuous sojourn by man at these extreme elevations and was made possible only through the nearly continual use of oxygen to prevent the physiological deterioration which otherwise would have set in. There is little doubt, therefore, that the diluter-demand oxygen system used during the 1971 International Himalayan Expedition represents a significant improvement over other systems used previously.
Acknowledgement. The development of the oxygen system was supported by NASA Grant NGL 05-003-024.
1Bert, P. La Pression Barométrique. Paris: G. Masson, 1878.
2Bruce, C.G. The Assault on Mount Everest, 1922. London: Edward Arnold & Co., 1923.
3Ullman, J.R. Americans on Everest. Philadelphia: J. B. Lippincott Co.. 1964.
4Hunt, J. The Conquest of Everest. New York: E. P. Dutton & Co., 1954.
5Pugh, L.G.C.E. “Physiological and Medical Aspects of the Himalayan Scientific and Mountaineering Expedition. 1960-61,” British Medical Journal, 1962, Vol. 2 pp 621-627.