Breath Taking. Michael J. Stephen
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some advanced maneuvers to help keep oxygen going in and carbon dioxide coming out. The last thing the attending suggested was “proning.” Then he disappeared. The first question I asked was, “What’s proning?” The resident, Kevin, looked at me wearily and said, “Proning is when you flip the patient over and have them lie on their stomach on the bed.”
I still didn’t quite get it. A groan followed. (There are many potential sources of humiliation in the hospital for an intern.) “‘Why?’ To help with oxygenation. And ventilation. You need to go learn about lung physiology. And you need to go read your West.” Kevin shook his head and sat down to go through the mound of material in front of him.
The West my resident was referring to was a book by John B. West. John B. West is not known outside the small circle of lung medicine practitioners, but within it he stands tall, and through his research and textbooks he has cemented a reputation as the leading educator within all pulmonary medicine in the last hundred years.
Dr. West was born in Adelaide, Australia, in 1928 and developed an interest in science at an early age. He moved to England for his PhD, and then to the University of California, San Diego, to continue his research in pulmonary physiology. There he discovered some unique things about how the lungs work, specifically about how different areas of the lung can have very different blood flow and air flow. Later in his career, he wrote a textbook on pulmonary physiology that changed how we educate medical students. And even later, at the age of seventy, he changed his focus to the physiology of the bird lung.
Following Kevin’s advice, I picked up an updated edition of West’s 1974 textbook, Pulmonary Physiology: The Essentials. The volume is slim in the hand, and its two hundred pages of easy type and large diagrams belie the power of its contents—it is still the starting point for learning about modern pulmonary physiology for physicians today.
Dr. West begins his book with a review of the structure of the lung, and right away he makes an extremely important and powerful statement: The architecture of the lung follows its function. Everything about the lungs should be considered within the context of that fact: form follows function.
This statement is one that broadly defines an entire field of biology. The disciplines of comparative and evolutionary biology consider the origins and development of life around us in the context of form following function. The African lion, for example, lives in the grasslands and is almost strictly a meat eater. For that, it needs to be a predator. Its body is powerful and fast, but it can travel only in short bursts. It has big retractable claws and huge powerful teeth to take down its prey. Its niche, diet, body, teeth, and feet with claws all coalesce into one well-defined purpose. The wild dog, by contrast, has a more diverse diet, so its teeth include some canines but also some molars. The dog’s legs and body are built for speed but also for distance, and it has no big claws, since they’re not needed. Looking at the world from the form-follows-function perspective can be a powerful tool for analyzing nature’s systems.
Dr. West states that the main function of the lungs is to facilitate gas exchange. First, oxygen needs to come into the blood and be carried to the cells of the body to keep metabolic processes going. Ventilation—the process of carbon dioxide being released from the bloodstream—needs to follow. Since form follows function, the body must have the ability to augment the work of the lungs to get appropriate amounts of oxygen into the blood and carbon dioxide out even when there are changes in our metabolic demands. Exercise, for example, is a state in which our tissues demand more oxygen and produce more carbon dioxide than usual. Illness from bacteria or viruses is another state that can increase our metabolic demands tremendously as a result of inflammation.
The lungs, for the most part, handle these demands with ease and flexibility. From a starting volume of five liters of air per minute at rest, we can increase our breathing to exchange ten, twenty, and even thirty liters of air per minute, an astonishing amount of gas. Our respiratory rate naturally increases in this situation, but the volume of air with each inhalation also increases, as muscles in our neck and our abdomen not usually used for breathing spring into action, helping to stretch our lungs to accept and release more air. These adjustments are important, since we must exist within a tight physiological space and maintain levels of oxygen and carbon dioxide within a narrow range. Our lungs, with help from the muscles in our chest wall, have the ability to keep us within that range under a wide variety of conditions. The trouble comes when something interferes with this powerful, but at times delicate, system.
Keeping his blood supplied with fresh oxygen, and his ventilation appropriate to expel enough carbon dioxide, was the problem Mr. Joseph was facing that dark January night in Boston. Well past midnight, I could feel the temperature in my body dropping as we prepped our patient for a procedure to put a large-bore intravenous line into his neck. He was on a lot of antibiotics, and unstable on a ventilator, so we deemed that he needed larger intravenous (IV) access to get medicines in more rapidly. For this, we needed to access the internal jugular vein in his neck, and as we carefully scrubbed his neck with cleaning solution, Kevin kept a running commentary.
Kevin spoke about how the patient was in ARDS [acute respiratory distress syndrome], so we would need to keep the pressures in his lung low and try to minimize the work of breathing. He discussed how we would have to keep a close eye on the patient’s oxygenation, and to consider advanced oxygenation and ventilation methods—such as inhaled prostacyclin or maybe even ECMO [extracorporeal membrane oxygenation]—if we couldn’t make some progress in the next few hours. Or perhaps proning.
I did know some of what he was referring to—that oxygen levels needed to be kept at least 60 mmHg in the blood. I also knew that ventilation is measured as the product of the number of breaths in a minute multiplied by how much air is moved with each breath (or tidal volume) and that we care about ventilation because it is the primary determinant of how much carbon dioxide is in the blood. When carbon dioxide builds up in the blood, it dissociates into free hydrogen molecules, which are essentially acid, the amount of which is measured on the pH scale.
The pH stands for potential of Hydrogen, and it is a direct measure of how many hydrogen molecules are present in a solution. The scale typically spans from 0 to 14, with 7 marking the exact middle of the scale. Water at 25 degrees Celsius has a pH of 7 and is considered completely neutral. If there are a lot of hydrogen molecules (H+), we call the solution acidic. On this side of the scale are drinks like black coffee, with a pH of around 5, and tomato juice, with a pH of around 4 (the pH scale is inverse, and a lower pH indicates more acid). On the other side of 7, we call solutions basic, and examples include baking soda–containing liquids, with a pH of 9, or ammonia, with a pH of 11. These solutions have far fewer hydrogen ions than acidic solutions do.
The pH of our blood is 7.40, and it must be kept in the very narrow range between 7.35 and 7.45, with 7.40 being optimal. Living within this pH range is extremely important, because the proteins of our cells—and subsequently metabolism itself—begin to break down when our pH goes too low or too high. The kidneys help to expel or hold onto acid as needed to adjust our pH, but the lungs are a far more powerful system for regulating pH through carbon dioxide, which, as mentioned, breaks down to acid in our blood. We regulate carbon dioxide and acid by simply increasing our breathing and ventilatory rate when too many hydrogen ions are present and the pH is too low, or by slowing it down and letting acid build up when the pH goes too high.
A classic example of when CO2 rises is during exercise, and breathing increases concomitantly to expel the CO2 so our pH doesn’t drop too low. An opposite example is when we hyperventilate at rest without an increase in CO2 production, as can happen during a panic attack. Here one is blowing off too much CO2 and acid, and our pH climbs to dangerous levels, which is why one may be told to breath into a paper bag to inhale the expelled CO2, restoring needed acid to the blood.
As for our patient from Maine, what I didn’t know, but would soon learn, was what we were going to do if ventilation wasn’t adequate and his pH was off, or if we couldn’t maintain the appropriate level of oxygen in his blood. Meanwhile, I stood beside him with a large-bore needle, about to drive it deep into his neck to look for the internal jugular vein, when Kevin somewhat offhandedly mentioned, “Oh, and by the way, if you go in too deep and puncture his lung with that needle, he’s probably going to have a cardiac arrest and die.