Extreme Medicine (17 page)

We also know that there are receptors that sense mechanical stretch in the lungs and the muscles that do the work of breathing. But what weight each of these indicators carries, how they interplay, and to what extent the mechanisms vary from one individual to another remain a matter of educated guesswork.

It is interesting that in this age we talk about the search for a grand unified theory of physics but have still to achieve a well-unified theory of that most fundamental of physiological functions: what it is that makes us breathe. In this field ordinary men and women have been conducting their own experiments and confounding the prediction of scientists for decades.

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G
IORGIOS
H
AGGI
S
TATTI WAS,
at first sight, an unremarkable Italian fisherman. He was of diminutive height with skin darkened by days spent laboring under Mediterranean skies. His build was slight; his pulse and rate of respiration were regular. His heart sounds, too, were normal. An Italian physician, searching for something out of the ordinary that could explain Statti's extraordinary feat, recorded these details. In 1913 Statti became something of a local celebrity, after diving to recover the anchor of a warship lost in the harbor depths. Holding a single breath of air, they say he reached a depth in excess of seventy meters.

Skin diving itself wasn't anything new. The exploits of the Japanese amas had been known for centuries: These remarkable female divers remained submerged in coastal waters for minutes at a time collecting shellfish, sea cucumbers, and pearls. However, nothing in the history of the amas suggests that they dived much beyond a depth of twenty meters—certainly nothing comparable to the depth reached by Statti.

Intriguingly, Statti claimed that he was capable of even deeper dives. But nothing about the Italian gave a clue as to how he was able to achieve such feats. When tested on land, he could hold his breath for only forty seconds. Only one feature of his physique stood out: a seemingly overinflated, barrel-shaped chest. The only other evidence of his exposure to the great pressure of depth was his impaired hearing. One eardrum was entirely absent and the other damaged and perforated. Statti was bemused by the interest in his salvage dive. To him it was nothing out of the ordinary: He was a fisherman, and this was just something he knew that he was capable of. He was baffled by the doctor's questions and annoyed at being asked to hold his breath on dry land. To Statti it was a meaningless test; for him everything was different once he was in the water.

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I
N 1968 A PHYSIOLOGIST NAMED
Albert B. Craig, working at Rochester Hospital, wrote a paper reviewing the predicted physiological limits for breath-hold diving. With the body of knowledge in hand, it appeared that human beings shouldn't be able to free-dive beyond around thirty-four meters from the surface. This, scientists had estimated, was the depth below which the increased pressure of the surrounding water would crush the lungs and reduce their volume to the point at which blood would pour into their airspaces.

A theoretical limit on the length of time for which a person could hold the breath had also been set at around three minutes. This wasn't just an arbitrary number. Physiologists had calculated it based upon the amount of oxygen left in the lungs of a person of average build after maximal inspiration and balanced that against the resting rate of oxygen consumption by the body. When the graphs were drawn and extrapolated, the scientists saw that the levels of oxygen in the bloodstream would fall to a point at which unconsciousness was inevitable in less than the time it would take to boil an egg.

Craig was aware of many cases of divers who had dived well beyond those depths and times. While science had drawn neat lines in laboratories, delineating the theoretical limits of survival, breath-hold divers had busied themselves swimming right past them. It's not clear whom this delighted more, the scientists or the divers.

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B
Y THE MID-1980S, THE RECORD
breath-hold dive stood at one hundred meters. Neoprene, rubber fins, and eye goggles had lent this modern sport of free diving a new dimension, but among physiologists, it remained poorly understood. In a series of international symposia, the dangers of free diving were hotly debated: the plummeting oxygen reserves and climbing carbon dioxide levels, the theorized volume changes, and mounting pressures. But of one thing scientists were sure. There was some depth beyond which breath-hold divers would not survive. As Craig himself said, when queried on the subject, “I think the limit will be reached when one of these breath-hold divers comes up coughing blood.”

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T
HE KEY TO THESE SEEMINGLY
superhuman performances lies in the mammalian diving reflex. When under the water, nerve endings in the skin of your face sense that you are immersed and kick off a series of adaptive responses, which include slowing of the heart and constriction of the blood vessels in your peripheral circulation.

This restricts blood supply to nonessential organs and tissue beds, reducing their oxygen demand and conserving the supply for the heart and brain. This constriction of the blood vessels is in a different league from the gentle blanching of the skin that might, for example, happen when you arrive at the airport check-in desk only to realize that your passport is still at home. In the diving reflex, it appears to be a hard clamping down of much larger vessels, among them important arterioles and veins, and this leads to a massive increase in blood pressure.

We measure blood pressure in millimeters of mercury (mm Hg). Assuming that you're fit and well, your peak blood pressure is equivalent to a column of mercury probably no more than 120 mm in height. A general practitioner might think about putting you on medication to control your blood pressure if it exceeded 140 mm Hg. In an accident and emergency department, a reading above 180 mm Hg might warrant immediate intervention. But for otherwise healthy breath-hold divers, pressures above 230 mm Hg have regularly been recorded.

These huge pressures reflect the centralization of blood that happens as the peripheral circulation shuts down. The rush of blood to the body's central compartment has further benefits. It is thought to engorge the tissues of the chest with blood, allowing airspaces to be compressed beyond the physiologists' theoretically defined limits while at the same time protecting them from damage.

This selective diversion of blood is just one measure that reduces global oxygen demand while increasing oxygen supply to the heart and brain. Another mechanism relies upon the spleen, a fist-size organ that sits just below the ninth rib on the lower left side of the chest. The spleen mainly acts to filter old and damaged red blood cells from the circulation—like a quality control officer monitoring a production line. It also garrisons a population of white blood cells that help the immune system in the fight against disease by identifying and destroying bacteria and compromised cells. But in extremis, when oxygen levels are perilously low, the spleen can also function as a kind of oxygen reservoir. Within it a pool of red blood cells is held—like the tin of money you might tuck away on a kitchen shelf for a rainy day. The drop in oxygen levels that accompanies breath-hold diving forces the spleen to contract, spilling its supply of rainy-day blood cells back into the circulation and, in theory, further increasing the supply of available oxygen.

Having made more oxygen available for vital organs through these mechanisms, the brain sends signals to the heart instructing it to reduce its rate, thereby further reducing demand. The heart is among the most metabolically active tissues in the body. Reducing its rate of work reduces its own oxygen consumption. In free divers, heart rates as low as twenty beats per minute have been recorded during deep dives.

All of this is an elaborate scheme for managing supply and demand under extraordinarily challenging conditions. These in part explain why the current free-diving depth record stands at 214 meters (held by an Austrian, Herbert Nitsch), with the longest breath hold a staggering eleven minutes and thirty-five seconds (held by a Frenchman, Stéphane Mifsud).

But these extreme feats do not come without risk or penalty. As the circulation shuts down at the edges and becomes more sluggish, muscles and other less vital organ systems are forced to work largely without oxygen. This anaerobic respiration causes waste products to build up in the bloodstream, steadily acidifying it. This is like the body taking a high-interest, short-term loan from its metabolic bank. In fact it's more like borrowing from the worst kind of loan shark. Except in this instance, failure to promptly repay the debt
always
leads to death.

The oxygen debt is called in at the end of the dive. After surfacing, divers take long, gasping breaths that serve to replenish their plundered reserves and rebalance their physiology. For those who misjudge the dive, however, oxygen levels will fall to the point at which consciousness can no longer be maintained. If this occurs with no one present to immediately assist and return the diver to the surface, death by drowning is inevitable.

But those who free-dive regularly appear to undergo a series of adaptive changes that seem to reduce the risk of fatal misadventure. The dive reflex becomes more pronounced with training: Experienced breath-hold divers exhibit lower heart rates and higher blood pressures on immersion. The tolerance for accumulating carbon dioxide also improves as the bodies of these divers reset their expectations. The cells that detect changes in carbon dioxide levels in the bloodstream respond more laconically, sending signals to the brain less urgently when breathing stops. Lungs become more compliant, and their volumes also increase. This, as the doctor examining Giorgios Statti in 1913 noted, leads to a more expanded chest. Most impressive of all, experienced free-divers train themselves to resist the reflexive and primal urge to breathe—a remarkable feat of mind over matter allowing them to push human physiology well beyond its default limits.

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U
NTIL THE MIDDLE
of the twentieth century, diving was a pursuit that demanded umbilical connection to the surface: a tube through which the atmosphere above could be pumped down to a diver. The Siebe Gorman standard diving dress—a helmet of brass bolted to a waterproof canvas suit, given ballast by shoes and weights made of lead—became the iconic image of the diving industry. The helmet was supplied with air via a hose from the surface, driven first by hand pumps and later by diesel motors. The link with the surface could not be broken.

Later, self-contained underwater breathing systems were invented, which allowed the diver to swim without being encumbered by an umbilical hose. Thousands of liters of air could be compressed under massive pressure and held in a metal cylinder small enough to be carried on a diver's back. A valve mounted on these tanks reduced the pressure of the cylinder supply to something that could be breathed safely. Without this, breathing from them would be like inflating party balloons from the air hose at a gas station—the uncontrolled rush of high-pressure gas would soon injure the lungs.

As well as reducing the pressure, valves were also used to allow air to be supplied on demand—sipped from the tank like a drink from a bottle. This self-contained underwater breathing apparatus became known as SCUBA and allowed underwater explorers and workers to greatly extend the time for which they could remain immersed. But having solved the problem of how to maintain oxygen supply, divers encountered a new problem. Staying safe at depth is about more than the availability of fresh air. Maintaining your supply of oxygen is only the first challenge. With depth comes pressure and with pressure physiological alteration.

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A
BOVE WATER, YOU ARE SWIMMING
at the bottom of a different ocean: an ocean of air. You're not aware of its weight above you, but it's there nevertheless, pushing on your body: 14.7 pounds for every square inch. We call that one atmosphere—appropriately so, because it is the pressure exerted by the weight of the single atmosphere that stands above us.

Water is far denser than air. The kilometers of gas above exert a pressure equivalent to just ten meters of sea water. Every ten meters that you sink into the water adds another whole atmosphere of pressure. So at ten meters the pressure is double that at sea level. At twenty meters, it's three times as great; at thirty, it's four times, and so on.

Now water, of which the many trillions of cells of our body are largely composed, cannot be compressed, and so for the most part, our bodies do not significantly deform as we descend into the deep. But the same is not true of the pockets of air held in soft body cavities like the gut and the lungs.

Volumes of gas trapped in the body are reduced as pressure rises. If you blew up a balloon at the surface and managed to pull it down to ten meters, it would halve in volume. At twenty meters below the surface, where the pressure would have tripled, the balloon would have shrunk to a third of its original size.

The reverse is also true. A balloon inflated under pressure at twenty meters will grow to three times that size by the time it has floated up to the surface. If the skin of the balloon can't accommodate that increase in size, then the balloon will burst.

This is among the most immediate threats that divers face. The lungs are effectively clusters of millions of airspaces—alveoli—that behave like tiny balloons. To inflate them at depth, you need to take a breath of pressurized air sufficient to overcome the increased pressure of the surrounding water. Once inflated, they'll behave exactly like balloons, expanding as you rise toward the surface with walls that are in parts a single cell thick, so they'll burst just as easily too.

A rupture of the lung's alveolar air sacs will push air into places that it doesn't belong. The real risk is of air expanding out of these burst balloons and entering the blood vessels that run over their surfaces. If this happens, the bubbles can travel through the veins of the lung back to the heart. From there they will be fired off around the body, blocking essential routes of blood supply. If the circulation of the brain or the heart is involved, the effect can be instantly fatal.