Breath Taking. Michael J. Stephen

Чтение книги онлайн.

СКАЧАТЬ resident came up with a plan to lower the driving pressure of air from the ventilator while giving the air a longer time to get in, easing the flow into Mr. Joseph’s rigid lungs. We also had the nurse give him a paralyzing level of sedation, calming any involuntary fighting he was doing with his own respiratory muscles.

      The modifications seemed to work, and the ventilator alarms went quiet. But this only told us that the machine was happy. The resident instructed me to check the level of gases (oxygen and carbon dioxide) in the patient’s blood to see if his body was happy. A few minutes later the results were relayed from the lab. Our patient’s oxygen level was just above 60 mmHg, and his carbon dioxide was around 48, corresponding to a pH of 7.30. These numbers were not great, but good enough to get him through what remained of the night.

      For the rest of that month, I saw Mr. Joseph every morning at six o’clock, my first patient of the day. I would analyze how he was doing with oxygenation and ventilation, looking for any improvements. In the evening, at home, I would thumb through John B. West’s book. By the end of the month, I began to understand some of the nuances of how gas exchange works, how different parts of the lung receive very different amounts of oxygen and blood flow. Specifically, the lower lobes generally get both a lot more inhaled air and more blood flow. Some of this is likely due to the effect of gravity.

With this knowledge of variable blood flow and air flow within the lungs, researchers eventually came up with the idea to minimize airflow in stiff lungs like Mr. Joseph’s. The reasoning behind this idea is that, since blood flow and circulation are almost certainly compromised given the level of inflammation in the lungs, there is no need for a normal amount of air in each breath. Before this protocol was established, physicians had been blowing too much air into diseased lungs, creating too much stretch, and that extra stress was creating more inflammation. John B. West helped us appreciate that air flow and blood flow can be variable, and attempts should be made to match them. The breakthrough 2000 New England Journal of Medicine study showed that deaths were significantly fewer in ARDS patients who received less air when on the ventilator.35 Practice in medical intensive care units all over the world changed overnight. No study before this, and no study since, has had such a dramatic impact on what we do in the medical intensive care unit.

      Mr. Joseph was admitted to the hospital in January 2002, so this article was fresh in everybody’s mind, and throughout the month we kept the air flow in Mr. Joseph’s lungs to the absolute minimum possible while still maintaining ventilation. To compensate for a very low amount of air with each intake, we turned up his respiratory rate from the normal twelve breaths per minute to thirty, even thirty-four at times. Normally, a respiratory rate of thirty-four breaths per minute is not sustainable, but with a machine it is. We considered proning, or flipping Mr. Joseph onto his stomach, while he was on the ventilator, to further decrease stress on his lungs by reducing the effect of gravity, thus giving them a rest and a chance to heal. The front of the lungs is where alveoli are often less affected in diseases like ARDS. (Most recently, proning is commonly being used for patients with COVID-19 as the inflammation from pneumonia almost always begins in the lower part of the lungs.)

      Throughout the month, Dr. West taught me about theory, while Mr. Joseph taught me about the real world. Slowly but surely he made progress, his stiff ARDS lungs loosening up enough that he could breathe on his own during the day, while using the ventilator at night. He eventually went to a rehabilitation facility, and from there, presumably, home to the wilds of Maine. As happens so often in the practice of medicine, all we did was keep him alive until he was able to heal himself.

      Today, even though specific drug treatments for ARDS are nonexistent, other therapies have shown progress. Foremost among these is extracorporeal membrane oxygenation (ECMO), where blood is taken out of the body, run through a machine that removes carbon dioxide and adds oxygen, and then returned. It functions, in essence, like an artificial lung. It is not a long-term solution and only serves to buy time for the lungs to heal themselves. Studies of this treatment in adults with ARDS have had conflicting results, but it is an option for those who fail on the traditional ventilator.

Looking further into the future, the promise of stem cells is not just a dot on the horizon, but a viable therapy that is now making its way through clinical trials. Stem cells have the ability not only to transform into different cell types but also to mitigate inflammation. A phase 2 study, which analyzes mostly safety, was recently completed in patients with ARDS and showed positive findings.36 Further studies are underway, and the entire pulmonary community is holding its breath to see whether the first treatment to improve outcomes for ARDS patients is on its way.

      John B. West thought a lot about the issues of oxygenation and ventilation throughout his long career, from the quiet of his laboratory to the windy heights of cold Mount Everest. For the first five decades of his career, Dr. West studied the same class of animal—mammals. But then he turned his attention to a completely different species—birds—and brought awareness to important aspects of breathing.

Today, some ten thousand species of birds exist on Earth, about twice the number of mammal species. They colonize many different habitats and are able to maintain incredibly high workloads, or metabolic rates. One species that stands out is the hummingbird, which, with a wing beat frequency of up to 70 beats per second and a heart that can go to over 1,200 beats per minute, has a metabolic rate thirty times higher than that of humans. Another remarkable bird is the bar-headed goose, which is able to fly up to thirty thousand feet. These are feats of physiology that humans could not think of matching, and Dr. West believes it is their bird lungs, radically different from human lungs, that allow them to sustain these very high workloads.37

      Dr. West’s interest in birds was piqued in 1960, when he spent six months with a team of researchers on Mount Everest. The project was dubbed the Silver Hut Expedition for the tin house in which they lived, on the Mingbo Glacier, at nineteen thousand feet (the nearby peak of Mount Everest is at twenty-nine thousand feet). From this perch, West and the other scientists investigated the effects of high altitude on the human body. After some time in this environment, West had become frightfully tired and thin from the stress of altitude. One morning, as he struggled to get going, he looked out of the window of the Silver Hut, drawn by a quacking noise. Way above his head, at about twenty-one thousand feet, was a gaggle of twelve rather ordinary-looking tan geese flying effortlessly in skies normally reserved for jet airplanes. How could West explain the difference between his own extreme fatigue and the bird’s easy flight?

      The answer to his question lay in the design of their lungs. Despite all the obvious differences between birds and humans, one of the most important distinctions is not immediately apparent, though it is the likely key to birds’ success colonizing so many habitats: their lungs have separated out the jobs of oxygenation and ventilation. Our lungs are simple in that they have combined these jobs into a single unit. They expand and contract to provide movement of air, or ventilation, much like a fireplace bellows. The same areas that expand and contract to provide ventilation also house the gas exchange areas that allow oxygen to move into the blood and carbon dioxide to be released.

      Figure 5: Anatomy of a bird’s lung with air sacs for ventilation and air capillaries for gas exchange.

      But as West’s observations led him to point out, if an engineer was designing a breathing machine, the functions of gas exchange and air movement would be separated. Birds have such a system. With each breath they take, air moves into air sacs, which are large, easily distensible organs in which no gas exchange takes place. This air then gets shunted to a separate area for gas exchange, termed air capillaries. There, because there is no need for the gas exchange units to bend when a breath occurs, the distance between the air and the blood vessels is incredibly thin, much less than the one third of a micron in mammals, making the exchange even easier. A final difference is that air movement in the bird lung goes around in a circle, much as blood does, so birds get fresh air with both СКАЧАТЬ