Guyton and Hall Physiology Chapter 45 - Physiology of Deep-Sea Diving and Other Hyperbaric Conditions PDF

CHAPTER 45 UNIT VIII Physiology of Deep-Sea Diving and Other Hyperbaric Conditions When people descend benea...

CHAPTER 45 UNIT VIII Physiology of Deep-Sea Diving and Other Hyperbaric Conditions When people descend beneath the sea, the pressure NITROGEN NARCOSIS AT HIGH NITROGEN around them increases progressively as they go to greater PRESSURES depths. To keep the lungs from collapsing, air must be About four-fifths of the air is nitrogen. At sea level pressure, supplied at very high pressure to keep them inflated. This the nitrogen has no significant effect on bodily function, but maneuver exposes the blood in the lungs to extremely at high pressures, it can cause varying degrees of narcosis. high alveolar gas pressures, a condition called hyperba- When the diver remains beneath the sea for 1 hour or more rism. Beyond certain limits, these high pressures cause and is breathing compressed air, the depth at which the first major alterations in body physiology and can be lethal. symptoms of mild narcosis appear is about 120 feet. At this level, the diver begins to exhibit joviality and loss of many Relationship of Pressure to Sea Depth. A column of of his or her cares. At 150 to 200 feet, the diver becomes seawater 33 feet (10.1 meters) deep exerts the same pres- drowsy. At 200 to 250 feet, the person’s strength wanes con- sure at its bottom as the pressure of the atmosphere above siderably, and the diver often becomes too clumsy to per- the sea. Therefore, a person 33 feet beneath the ocean sur- form the work required. Beyond 250 feet (8.5 atm pressure), face is exposed to 2 atmospheres (2 atm) pressure, with 1 the diver usually becomes almost useless as a result of nitro- atm of pressure caused by the weight of the air above the gen narcosis if he or she remains at these depths too long. water and the second atmosphere caused by the weight of Nitrogen narcosis has characteristics similar to those of the water. At 66 feet, the pressure is 3 atm, and so forth, in alcohol intoxication, and for this reason it has frequently accord with the table in Figure 45-1. been called “raptures of the depths.” The mechanism of this narcotic effect is believed to be the same as that of most other Effect of Sea Depth on the Volume of Gases—Boyle’s gas anesthetics. That is, it dissolves in the fatty substances Law. Another important effect of depth is the compres- in neuronal membranes and, because of its physical effect sion of gases to smaller and smaller volumes. The illustra- on altering ionic conductance through the membranes, it tion in Figure 45-1 shows a bell jar at sea level containing reduces neuronal excitability. Ascent to a shallower depth 1 liter of air. At 33 feet beneath the sea, where the pressure reverses the narcosis within a few minutes, with no known is 2 atm, the volume has been compressed to only a half- long-term effects if the ascent is not too rapid.  liter, and at 8 atm (233 feet) it has been compressed to one- eighth liter. Thus, the volume to which a given quantity of gas is compressed is inversely proportional to the pressure. OXYGEN TOXICITY AT HIGH PRESSURES This principle of physics is called Boyle’s law, and it is ex- Effect of Very High Po2 on Blood Oxygen Transport. tremely important in diving physiology because increased When the Po2 in the blood rises above 100 mm Hg, the pressure can collapse the air chambers of the diver’s body, amount of O2 dissolved in the water of the blood increases especially the lungs, and may cause serious damage. markedly. This effect is shown in Figure 45-2, which de- Often in this chapter it is necessary to refer to actual vol- picts the same O2-hemoglobin dissociation curve as that ume versus sea level volume. For example, we might speak shown in Chapter 41 but with the alveolar Po2 extended of an actual volume of 1 liter at a depth of 300 feet; this is to more than 3000 mm Hg. Also depicted by the lowest the same quantity of air as a sea level volume of 10 liters. curve in the figure is the volume of O2 dissolved in the fluid of the blood at each Po2 level. Note that in the normal EFFECT OF HIGH PARTIAL PRESSURES range of alveolar Po2 ( ≈2 atm Po2), the 0 hemoglobin-O2 buffering mechanism fails, and the tissue 0 760 1560 2280 3040 Po2 can then rise to hundreds or thousands of mm Hg. At Oxygen partial pressure in lungs (mm Hg) these high levels, the amounts of oxidizing free radicals Figure 45-2. Quantity of O2 dissolved in the fluid of the blood and in literally swamp the enzyme systems designed to remove combination with hemoglobin at very high PO2 values. them, and now they can have serious destructive and even 562  Chapter 45 Physiology of Deep-Sea Diving and Other Hyperbaric Conditions lethal effects on the cells. One of the principal effects is same high pressure as that in the alveolar breathing mixture, to oxidize the polyunsaturated fatty acids that are essen- and over several more hours, enough nitrogen is carried to tial components of many of the cell membranes. Another all the tissues of the body to raise their tissue nitrogen partial effect is to oxidize some of the cellular enzymes, thus dam- pressure to equal the nitrogen pressure in the breathing air. UNIT VIII aging the cellular metabolic systems severely. The nervous Because nitrogen is not metabolized by the body, it tissues are especially susceptible because of their high remains dissolved in all the body tissues until the nitro- lipid content. Therefore, most of the acute lethal effects of gen pressure in the lungs is decreased back to some lower acute O2 toxicity are caused by brain dysfunction. level, at which time the nitrogen can be removed by the reverse respiratory process. However, this removal often Chronic Oxygen Poisoning Causes Pulmonary Disa- takes hours to occur and is the source of multiple prob- bility. A person can be exposed to only 1 atm pressure of lems, collectively called decompression sickness. O2 almost indefinitely without developing the acute oxy- gen toxicity of the nervous system just described. How- Volume of Nitrogen Dissolved in the Body Fluids at ever, after only about 12 hours of 1 atm O2 exposure, lung Different Depths. At sea level, almost exactly 1 liter of passageway congestion, pulmonary edema, and atelectasis nitrogen is dissolved in the entire body. Slightly less than caused by damage to the linings of the bronchi and al- one-half of this nitrogen is dissolved in the water of the veoli begin to develop. The reason for this effect in the body, and a little more than one-half is dissolved in the fat lungs but not in other tissues is that the air spaces of the of the body, because nitrogen is five times as soluble in fat lungs are directly exposed to the high O2 pressure, but O2 as in water. is delivered to the other body tissues at almost normal Po2 After the diver has become saturated with nitrogen, because of the hemoglobin-O2 buffer system. the sea level volume of nitrogen dissolved in the body at different depths is as follows: CARBON DIOXIDE TOXICITY AT GREAT DEPTHS IN THE SEA Feet Liters 0 1 If the diving gear is properly designed and functions 33 2 properly, the diver has no problem due to toxicity because depth alone does not increase the CO2 partial 100 4 pressure in the alveoli. This is true because depth does 200 7 not increase the rate of CO2 production in the body, and 300 10 as long as the diver continues to breathe a normal tidal volume and expires the CO2 as it is formed, alveolar CO2 Several hours are required for the gas pressures of pressure will be maintained at a normal value. nitrogen in all the body tissues to come nearly to equilib- In certain types of diving gear, however, such as the rium with the gas pressure of nitrogen in the alveoli. The diving helmet and some types of rebreathing appara- reason for this requirement is that the blood does not flow tuses, CO2 can build up in the dead space air of the appa- rapidly enough, and the nitrogen does not diffuse rapidly ratus and be rebreathed by the diver. Up to an alveolar enough, to cause instantaneous equilibrium. The nitrogen CO2 pressure (Pco2) of about 80 mm Hg, twice that in dissolved in the water of the body comes to almost com- normal alveoli, the diver usually tolerates this buildup by plete equilibrium in less than 1 hour, but the fat tissue, increasing the minute respiratory volume a maximum which requires five times as much transport of nitrogen of 8- to 11-fold to compensate for the increased CO2. and has a relatively poor blood supply, reaches equilib- Beyond 80 mm Hg alveolar Pco2, however, the situation rium only after several hours. Thus, if a person remains becomes intolerable, and eventually the respiratory cen- under water at a deep level for only a few minutes, not ter begins to be depressed, rather than excited, because much nitrogen dissolves in the body fluids and tissues, of the negative tissue metabolic effects of high Pco2. The whereas if the person remains at a deep level for several diver’s respiration then begins to fail rather than com- hours, both the body water and body fat become satu- pensate. In addition, the diver experiences severe respi- rated with nitrogen. ratory acidosis and varying degrees of lethargy, narcosis, and finally even anesthesia, as discussed in Chapter 43. Decompression Sickness (Also Known as Bends, Com- pressed Air Sickness, Caisson Disease, Diver’s Paraly- sis, Dysbarism). If a diver has been beneath the sea long DECOMPRESSION OF THE DIVER AFTER enough that large amounts of nitrogen have dissolved in EXCESS EXPOSURE TO HIGH PRESSURE the body, and the diver then suddenly comes back to the When a person breathes air under high pressure for a long surface of the sea, significant quantities of nitrogen bub- time, the amount of nitrogen dissolved in the body fluids bles can develop in the body fluids, either intracellularly increases. This is because blood flowing through the pul- or extracellularly, and can cause minor or serious damage monary capillaries becomes saturated with nitrogen to the in almost any area of the body, depending on the number 563 UNIT VIII Aviation, Space, and Deep-Sea Diving Physiology Pressure Outside Body vessels are affected. Tissue ischemia and sometimes tissue Before After sudden death result. decompression decompression In most people with decompression sickness, the O2 = 1044 mm Hg O2 = 159 mm Hg symptoms are pain in the joints and muscles of the legs N2 = 3956 N2 = 601 and arms, affecting 85% to 90% of persons who experience Total = 5000 mm Hg Total = 760 mm Hg decompression sickness. The joint pain accounts for the term “bends” that is often applied to this condition. In 5% to 10% of people with decompression sickness, nervous system symptoms occur, ranging from dizziness in about 5% to paralysis or collapse and unconsciousness in as many as 3%. The paralysis may be temporary, but in some cases, damage is permanent. Finally, about 2% of people with decompression sick- Body Body ness experience “the chokes,” caused by massive numbers of Gaseous pressure Gaseous pressure microbubbles plugging the capillaries of the lungs. This con- in the body fluids in the body fluids dition is characterized by serious shortness of breath, often H2O = 47 mm Hg H2O = 47 mm Hg followed by severe pulmonary edema and, occasionally, death. CO2 = 40 CO2 = 40 O2 = 60 O2 = 60 N2 = 3918 N2 = 3918 Nitrogen Elimination From the Body; Decompression Total = 4065 Total = 4065 Tables. If a diver is brought to the surface slowly, enough A B of the dissolved nitrogen can usually be eliminated by Figure 45-3. Gaseous pressures inside and outside the body show- expiration through the lungs to prevent decompression ing (A) saturation of the body to high gas pressures when breathing air at a total pressure of 5000 mm Hg and (B) the great excesses of sickness. About two-thirds of the total nitrogen is liber- intrabody pressures responsible for bubble formation in the tissues ated in 1 hour, and about 90% is liberated in 6 hours. when the lung intra-alveolar pressure body is suddenly returned from Tables that detail procedures for safe decompression 5000 mm Hg to the normal pressure of 760 mm Hg. have been prepared by the US Navy. To give the reader an idea of the decompression process, a diver who has and sizes of bubbles formed. This phenomenon is called been breathing air and has been on the sea bottom for 60 decompression sickness. minutes at a depth of 190 feet undergoes decompression The principles underlying bubble formation are shown according to the following schedule: in Figure 45-3. In Figure 45-3A, the diver’s tissues have
t
10 minutes at 50-feet depth become equilibrated to a high dissolved nitrogen pressure
t
17 minutes at 40-feet depth (PN2 = 3918 mm Hg), about 6.5 times the normal amount
t
19 minutes at 30-feet depth of nitrogen in the tissues. As long as the diver remains deep
t
50 minutes at 20-feet depth beneath the sea, the pressure against the outside of the
t
84 minutes at 10-feet depth body (5000 mm Hg) compresses all the body tissues suffi- Thus, for a work period on the sea bottom of only 1 ciently to keep the excess nitrogen gas dissolved. However, hour, the total time for decompression is about 3 hours. when the diver suddenly rises to sea level (Figure 45-3B), the pressure on the outside of the body becomes only 1 Tank Decompression and Treatment of Decompres- atm (760 mm Hg), while the gas pressure inside the body sion Sickness. Another procedure widely used for de- fluids is the sum of the pressures of water vapor, CO2, O2, compression of professional divers is to put the diver into and nitrogen, or a total of 4065 mm Hg, 97% of which is a pressurized tank and then gradually lower the pressure caused by the nitrogen. Obviously, this total value of 4065 back to normal atmospheric pressure, using essentially mm Hg is far greater than the 760-mm Hg pressure on the the same time schedule as noted earlier. outside of the body. Therefore, the gases can escape from Tank decompression is even more important for treating the dissolved state and form bubbles, composed almost people in whom symptoms of decompression sickness develop entirely of nitrogen, both in the tissues and in the blood, minutes or even hours after they have returned to the surface. where they plug many small blood vessels. The bubbles In this case, the diver undergoes recompression immediately may not appear for many minutes to hours because some- to a deep level, and then decompression is carried out over a times the gases can remain dissolved in the “supersatu- period several times as long as the usual decompression period. rated” state for hours before bubbling. “Saturation Diving” and Use of Helium-Oxygen Mix- Symptoms of Decompression Sickness (“Bends”). The tures in Deep Dives. When divers must work at very deep symptoms of decompression sickness are caused by gas levels—between 250 feet and nearly 1000 feet—they fre- bubbles blocking many blood vessels in different tissues. quently live in a large compression tank for days or weeks at a time, remaining compressed at a pressure level near At first, only the smallest vessels are blocked by minute that at which they will be working. This procedure keeps bubbles, but as the bubbles coalesce, progressively larger 564  Chapter 45 Physiology of Deep-Sea Diving and Other Hyperbaric Conditions the tissues and fluids of the body saturated with the gases to which they will be exposed while diving. Then, when they return to the same tank after working, there are no significant changes in pressure, so decompression bubbles Mask do not occur. UNIT VIII In very deep dives, especially during saturation diving, Hose helium is usually used in the gas mixture instead of nitro- gen for three reasons: (1) it has only about one-fifth the narcotic effect of nitrogen; (2) only about one-half as much Demand valve First-stage volume of helium dissolves in the body tissues as nitrogen, valve and the volume that does dissolve diffuses out of the tissues during decompression several times as rapidly as does ni- trogen, thus reducing the problem of decompression sick- ness; and (3) the low density of helium (one-seventh the density of nitrogen) keeps the airway resistance for breath- ing at a minimum, which is very important because highly compressed nitrogen is so dense that airway resistance can become extreme, sometimes making the work of breathing beyond endurance. Finally, in very deep dives, it is important to reduce the O2 concentration in the gaseous mixture because other- wise O2 toxicity would result. For example, at a depth of 700 feet (22 atm pressure), a 1% O2 mixture will provide all Air cylinders the O2 required by the diver, whereas a 21% mixture of O2 (the percentage in air) delivers a Po2 to the lungs of more than 4 atm, a level very likely to cause seizures in as little as 30 minutes. Figure 45-4. Open-circuit demand type of SCUBA (self-contained underwater breathing apparatus). SELF-CONTAINED UNDERWATER BREATHING APPARATUS (SCUBA) the mask. Then, on expiration, the air cannot go back into DIVING the tank but, instead, is expired into the water. The most important problem with SCUBA is the lim- Before the 1940s, almost all diving was done using a diving ited amount of time a diver can remain beneath the water helmet connected to a hose through which air was pumped surface; For example, only a few minutes are possible at to the diver from the surface. Then, in 1943, the French a 200-foot depth. The reason for this limitation is that explorer Jacques Cousteau popularized a self-contained tremendous airflow from the tanks is required to wash underwater breathing apparatus, known as SCUBA. The CO2 out of the lungs—the greater the depth, the greater type of SCUBA used in more than 99% of all sports and the airflow in terms of quantity of air per minute required commercial diving is the open-circuit demand system because the volumes have been compressed to small sizes. shown in Figure 45-4. This system consists of the follow- ing components: (1) one or more tanks of compressed air Special Physiological Problems in Submarines or some other breathing mixture; (2) a first-stage “reduc- ing” valve for reducing the very high pressure from the Escape From Submarines. Essentially the same prob- lems encountered in deep sea diving are often found in re- tanks to a low pressure level; (3) a combination inhala- lation to submarines, especially when it is necessary to es- tion “demand” valve and exhalation valve that allows air cape from a submerged submarine. Escape is possible from to be pulled into the lungs with slight negative pressure of as deep as 300 feet without use of any apparatus. However, breathing and then to be exhaled into the sea at a pressure proper use of rebreathing devices, especially when using level slightly positive to the surrounding water pressure; helium, can theoretically allow escape from as deep as 600 and (4) a mask and tube system with small “dead space.” feet or perhaps more. The demand system operates as follows. The first-stage One of the major problems of escape is prevention of reducing valve reduces the pressure from the tanks so air embolism. As the person ascends, the gases in the lungs that the air delivered to the mask has a pressure only a expand and sometimes rupture a pulmonary blood vessel, few mm Hg greater than the surrounding water pressure. forcing the gases to enter the vessel and cause air embolism The breathing mixture does not flow continually into the of the circulation. Therefore, as the person ascends, he or she must make a special effort to exhale continually. mask. Instead, with each inspiration, slight extra negative Health Problems in the Submarine Internal Environ- pressure in the demand valve of the mask pulls the dia- ment. Except for escape, submarine medicine generally phragm of the valve open, and this action automatically centers on several engineering problems to keep hazards releases air from the tank into the mask and lungs. In this out of the internal environment. First, in atomic subma- way, only the amount of air needed for inhalation enters 565 UNIT VIII Aviation, Space, and Deep-Sea Diving Physiology rines, there is the problem of radiation hazards, but with 70 mm Hg. Therefore, hyperbaric oxygenation of the tis- appropriate shielding, the amount of radiation received by sues can frequently stop the infectious process entirely and the crew submerged beneath the sea has been less than the thus convert a condition that formerly was almost 100% normal amount of radiation received above the surface of fatal into one that is cured in most cases by early treatment the sea from cosmic rays. with hyperbaric therapy. Second, poisonous gases, on occasion, escape into Other conditions in which hyperbaric O2 therapy has the atmosphere of the submarine and must be controlled been either valuable or possibly valuable include decom- rapidly. For example, during several weeks’ submergence, pression sickness, arterial gas embolism, carbon monoxide cigarette smoking by the crew can liberate enough carbon poisoning, osteomyelitis, and myocardial infarction. monoxide, if not removed rapidly, to cause carbon monox- ide poisoning. On occasion, even Freon gas has been found to diffuse out of refrigeration systems in sufficient quantity Bibliography to cause toxicity. Brubakk AO, Ross JA, Thom SR: Saturation diving; physiology and pathophysiology. Compr Physiol 4:1229, 2014. Hyperbaric Oxygen Therapy Castellini M: Life under water: physiological adaptations to diving and living at sea. Compr Physiol 2:1889, 2012. The intense oxidizing properties of high-pressure O2 (hy- Doolette DJ, Mitchell SJ: Hyperbaric conditions. Compr Physiol 1:163, perbaric oxygen) can have valuable therapeutic effects in 2011. several important clinical conditions. Therefore, large pres- Fitz-Clarke JR: Breath-hold diving. Compr Physiol 8:585, 2018. sure tanks are now available in many medical centers into Leach RM, Rees PJ, Wilmshurst P: Hyperbaric oxygen therapy. BMJ which patients can be placed and treated with hyperbaric 317:1140, 1998. O2. The O2 is usually administered at Po2 values of 2 to 3 Pendergast DR, Lundgren CE: The underwater environment: car- atm pressure through a mask or intratracheal tube, whereas diopulmonary, thermal, and energetic demands. J Appl Physiol the gas around the body is normal air compressed to the 106:276, 2009. same high-pressure level. Pendergast DR, Moon RE, Krasney JJ, et al: Human physiology in an The same oxidizing free radicals responsible for O2 toxicity aquatic environment. Compr Physiol 5:1705, 2015. Poff AM, Kernagis D, D’Agostino DP: Hyperbaric environment: oxy- are also believed to be responsible for at least some of the ther- gen and cellular damage versus protection. Compr Physiol 7:213, apeutic benefits. Some of the conditions in which hyperbaric 2016. O2 therapy has been especially beneficial are described next. Rostain JC, Lavoute C: Neurochemistry of pressure-induced nitrogen One successful use of hyperbaric O2 has been for treat- and metabolically inert gas narcosis in the central nervous system. ment of gas gangrene. The bacteria that cause this condi- Compr Physiol 6:1579, 2016. tion, clostridial organisms, grow best under anaerobic con- Vann RD, Butler FkK, Mitchell SJ, Moon RE: Decompression illness. ditions and stop growing at O2 pressures greater than about Lancet 377:153, 2011. 566