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Principles and Practice of Mechanical Ventilation
McGraw-Hill Companies, Inc.
Copyright © 2013 The McGraw-Hill Companies, Inc.
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Chapter One HISTORICAL PERSPECTIVE ON THE DEVELOPMENT OF MECHANICAL VENTILATION
Gene L. Colice
ANATOMISTS OF THE HEART AND LUNGS Early Greeks Renaissance Physicians
CHEMISTS AND PHYSIOLOGISTS OF THE AIR AND BLOOD Understanding Gases Metabolism Blood Gases and Ventilation
EXPLORERS AND WORKING MEN OF SUBMARINES AND BALLOONS Exploration Under Water Exploration in the Air
MECHANICAL VENTILATION OF RESUSCITATION AND ANESTHESIA Vivisection Resuscitating the Apparently Drowned Negative-Pressure Ventilators Positive-Pressure Ventilation Tracheal Intubation Tracheal Anesthesia Differential Pressure Translaryngeal Intubation For the Nonoperative Patient Modern Respirators Intensive Care Adequacy of Ventilation Quality Control of Ventilators Weaning
The history of mechanical ventilation is intimately intertwined with the history of anatomy, chemistry, and physiology; exploration under water and in the air; and of course, modern medicine. Anatomists described the structural connections of the lungs to the heart and vasculature and developed the earliest insights into the functional relationships of these organs. They emphasized the role of the lungs in bringing air into the body and probably expelling waste products, but showed little understanding of how air was used by the body. Chemists defined the constituents of air and explained the metabolic processes by which the cells used oxygen and produced carbon dioxide. Physiologists complemented these studies by exploring the relationships between levels of oxygen and carbon dioxide in the blood and ventilation. Explorers tested the true limits of physiology. Travel in the air and under water exposed humans to extremes in ventilatory demands and prompted the development of mechanical adjuncts to ventilation. Following the various historical threads provided by the anatomists, chemists, physiologists, and explorers provides a useful perspective on the tapestry of a technique modern physicians accept casually: mechanical ventilation.
ANATOMISTS OF THE HEART AND LUNGS
Early Greek physicians endorsed Empedocles' view that all matter was composed of four essential elements: earth, air, fire, and water. Each of these elements had primary qualities of heat, cold, moisture, and dryness. Empedocles applied this global philosophic view to the human body by stating that "innate heat," or the soul, was distributed from the heart via the blood to various parts of the body.
The Hippocratic corpus stated that the purpose of respiration was to cool the heart. Air was thought to be pumped by the atria from the lungs to the right ventricle via the pulmonary artery and to the left ventricle through the pulmonary vein. Aristotle believed that blood was an indispensable part of animals but that blood was found only in veins. Arteries, in contrast, contained only air. This conclusion probably was based on his methods of sacrificing animals. The animals were starved, to better define their vessels, and then strangled. During strangulation, blood pools in the right side of the heart and venous circulation, leaving the left side of the heart and arteries empty. Aristotle described a three-chamber heart connected with passages leading in the direction of the lung, but these connections were minute and indiscernible. Presumably, the lungs cooled the blood and somehow supplied it with air.
Erasistratus (born around 300 BC) believed that air taken in by the lungs was transferred via the pulmonary artery to the left ventricle. Within the left ventricle, air was transformed into pneuma zotikon, or the "vital spirit," and was distributed through air-filled arteries to various parts of the body. The pneuma zotikon carried to the brain was secondarily changed to the pneuma psychikon ("animal spirit"). This animal spirit was transmitted to the muscles by the hollow nerves. Erasistratus understood that the right ventricle facilitated venous return by suction during diastole and that venous valves allowed only one-way flow of blood.
The Greek physician Claudius Galen, practicing in Rome around AD 161, demonstrated that arteries contain blood by inserting a tube into the femoral artery of a dog. Blood flow through the tube could be controlled by adjusting tension on a ligature placed around the proximal portion of the artery. He described a four-chamber heart with auricles distinct from the right and left ventricles. Galen also believed that the "power of pulsation has its origin in the heart itself" and that the "power [to contract and dilate] belongs by nature to the heart and is infused into the arteries from it." He described valves in the heart and, as did Erasistratus, recognized their essential importance in preventing the backward discharge of blood from the heart. He alluded several times to blood flowing, for example, from the body through the vena cava into the right ventricle and even made the remarkable statement that "in the entire body the arteries come together with the veins and exchange air and blood through extremely fine invisible orifices." Furthermore, Galen believed that "fuliginous wastes" were somehow discharged from the blood through the lung. Galen's appreciation that the lungs supplied some property of air to the body and discharged a waste product from the blood was the first true insight into the lung's role in ventilation. However, he failed in two critical ways to appreciate the true interaction of the heart and lungs. First, he believed, as did Aristotle and other earlier Greeks, that the left ventricle is the source of the innate heat that vitalizes the animal. Respiration in animals exists for the sake of the heart, which requires the substance of air to cool it. Expansion of the lung caused the lightest substance, that is, the outside air, to rush in and fill the bronchi. Galen provided no insight, though, into how air, or pneuma, might be drawn out from the bronchi and lungs into the heart. Second, he did not clearly describe the true circular nature of blood flow from the right ventricle through the lungs and into the left ventricle and then back to the right ventricle. His writings left the serious misconception that blood was somehow transported directly from the right to the left ventricle through the interventricular septum.
Byzantine and Arab scholars maintained Galen's legacy during the Dark Ages and provided a foundation for the rebirth of science during the Renaissance. Around 1550, Vesalius corrected many inaccuracies in Galen's work and even questioned Galen's concept of blood flow from the right ventricle to the left ventricle. He was skeptical about the flow of blood through the interventricular pores Galen described. Servetus, a fellow student of Vesalius in Paris, suggested that the vital spirit is elaborated both by the force of heat from the left ventricle and by a change in color of the blood to reddish yellow. This change in color "is generated in the lungs from a mixture of inspired air with elaborated subtle blood which the right ventricle of the heart communicates to the left. This communication, however, is made not through the middle wall of the heart, as is commonly believed, but by a very ingenious arrangement: the subtle blood courses through the lungs from the pulmonary artery to pulmonary vein, where it changes color. During this passage the blood is mixed with inspired air and through expiration it is cleansed of its sooty vapors. This mixture, suitably prepared for the production of the vital spirit, is drawn onward to the left ventricle of the heart by diastole." Although Servetus' views proved ultimately to be correct, they were considered heretical at the time, and he was subsequently burned at the stake, along with most copies of his book, in 1553.
Columbus, a dissectionist to Vesalius at Padua, in 1559 suggested that blood travels to the lungs via the pulmonary artery and then, along with air, is taken to the left ventricle through the pulmonary vein. He further advanced the concept of circulation by noting that the left ventricle distributes blood to the body through the aorta, blood returns to the right ventricle in the vena cava, and venous valves in the heart allow only one-way flow.
These views clearly influenced William Harvey, who studied anatomy with Fabricius in Padua from 1600 to 1602. Harvey set out to investigate the "true movement, pulse, action, use and usefulness of the heart and arteries." He questioned why the left ventricle and right ventricle traditionally were felt to play such fundamentally different roles. If the right ventricle existed simply to nourish the lungs, why was its structure so similar to that of the left ventricle? Furthermore, when one directly observed the beating heart in animals, it was clear that the function of both right and left ventricles also was similar. In both cases, when the ventricle contracted, it expelled blood, and when it relaxed, it received blood. Cardiac systole coincided with arterial pulsations. The motion of the auricles preceded that of the ventricles. Indeed, the motions are consecutive with a rhythm about them, the auricles contracting and forcing blood into the ventricles and the ventricles, in turn, contracting and forcing blood into the arteries. "Since blood is constantly sent from the right ventricle into the lungs through the pulmonary artery and likewise constantly is drawn the left ventricle from the lungs ... it cannot do otherwise than flow through continuously. This flow must occur by way of tiny pores and vascular openings through the lungs. Thus, the right ventricle may be said to be made for the sake of transmitting blood through the lungs, not for nourishing them."
Harvey described blood flow through the body as being circular. This was easily understood if one considered the quantity of blood pumped by the heart. If the heart pumped 1 to 2 drams of blood per beat and beat 1000 times per half-hour, it put out almost 2000 drams in this short time. This was more blood than was contained in the whole body. Clearly, the body could not produce amounts of blood fast enough to supply these needs. Where else could all the blood go but around and around "like a stage army in an opera." If this theory were correct, Harvey went on to say, then blood must be only a carrier of critical nutrients for the body. Presumably, the problem of the elimination of waste vapors from the lungs also was explained by the idea of blood as the carrier.
With Harvey's remarkable insights, the relationship between the lungs and the heart and the role of blood were finally understood. Only two steps remained for the anatomists to resolve. First, the nature of the tiny pores and vascular openings through the lungs had to be explained. About 1650, Malpighi, working with early microscopes, found that air passes via the trachea and bronchi into and out of microscopic saccules with no clear connection to the bloodstream. He further described capillaries: "... and such is the wandering about of these vessels as they proceed on this side from the vein and on the other side from the artery, that the vessels no longer maintain a straight direction, but there appears a network made up of the articulations of the two vessels ... blood flowed away along [these] tortuous vessels ... always contained within tubules." Second, Borelli, a mathematician in Pisa and a friend of Malpighi, suggested the concept of diffusion. Air dissolved in liquids could pass through membranes without pores. Air and blood finally had been linked in a plausible manner.
CHEMISTS AND PHYSIOLOGISTS OF THE AIR AND BLOOD
The anatomists had identified an entirely new set of problems for chemists and physiologists to consider. The right ventricle pumped blood through the pulmonary artery to the lungs. In the lungs the blood took up some substance, evidenced by the change in color observed as blood passes through the pulmonary circulation. Presumably the blood released "fuliginous wastes" into the lung. The site of this exchange was thought to be at the alveolar–capillary interface, and it probably occurred by the process of diffusion. What were the substances exchanged between blood and air in the lung? What changed the color of blood and was essential for the production of the "innate heat"? What was the process by which "innate heat" was produced, and where did this combustion occur, in the left ventricle as supposed from the earliest Greek physician-philosophers or elsewhere? Where were the "fuliginous wastes" produced, and were they in any way related to the production of "innate heat"? If blood were a carrier, pumped by the left ventricle to the body, what was it carrying to the tissues and then again back to the heart?
Von Helmont, about 1620, added acid to limestone and potash and collected the "air" liberated by the chemical reaction. This "air" extinguished a flame and seemed to be similar to the gas produced by fermentation. This "air" also appeared to be the same gas as that found in the Grotto del Cane. This grotto was notorious for containing air that would kill dogs but spare their taller masters. The gas, of course, was carbon dioxide. In the late seventeenth century, Boyle recognized that there is some substance in air that is necessary to keep a flame burning and an animal alive. Place a flame in a bell jar, and the flame eventually will go out. Place an animal in such a chamber, and the animal eventually will die. If another animal is placed in that same chamber soon thereafter, it will die suddenly. Mayow showed, around 1670, that enclosing a mouse in a bell jar resulted eventually in the mouse's death. I the bell jar were covered by a moistened bladder, the bladder bulged inward when the mouse died. Obviously, the animals needed something in air for survival. Mayow called this the "nitro-aereal spirit," and when it was depleted, the animals died. This gas proved to be oxygen. Boyle's suspicions that air had other qualities primarily owing to its ingredients seemed well founded.
In a remarkable and probably entirely intuitive insight, Mayow suggested that the ingredient essential for life, the "nitro-aereal spirit," was taken up by the blood and formed the basis of muscular contraction. Evidence supporting this concept came indirectly. In the early 1600s, the concept of air pressure was first understood. von Guericke invented a pneumatic machine that reduced air pressure. Robert Boyle later devised the pneumatic pump that could extract air from a closed vessel to produce something approaching a vacuum (Fig. 1-1). Boyle and Hooke used this pneumatic engine to study animals under low-pressure conditions. Apparently Hooke favored dramatic experiments, and he often demonstrated in front of crowds that small animals died after air was evacuated from the chamber. Hooke actually built a human-sized chamber in 1671 and volunteered to enter it. Fortunately, the pump effectively removed only about a quarter of the air, and Hooke survived. Boyle believed that the difficulty encountered in breathing under these conditions was caused solely by the loss of elasticity in the air. He went on to observe, however, that animal blood bubbled when placed in a vacuum. This observation clearly showed that blood contained a gas of some type. In 1727, Hales introduced the pneumatic trough (Fig. 1-2). With this device he was able to distinguish between free gas and gas no longer in its elastic state but combined with a liquid. The basis for blood gas machines had been invented.
The first constituent of air to be truly recognized was carbon dioxide. Joseph Black, around 1754, found that limestone was transformed into caustic lime and lost weight on being heated. The weight loss occurred because a gas was liberated during the heating process. The same results occurred when the carbonates of alkali metals were treated with an acid such as hydrochloric acid. He called the liberated gas "fixed air" and found that it would react with lime water to form a white insoluble precipitate of chalk.
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