By Vicky Pohlen ~
The concept of artificial ventilation dates back to ancient history, although the earliest references to it are vague and conflicting. Stories from Ancient Egypt and the Bible indicate that breathing was understood as essential to life. Two of the clearest early medical anecdotes about our understanding of human ventilation are from Galen and Vesalius.
Galen (130–210 CE), the famous Greco-Roman physician and philosopher, faced the challenge of viewing internal organs in living subjects. In preparing his De usu partium corporis humani, on at least one occasion he blew air into an animal’s lungs using a bellows. A millennium later, Galen’s great critic Andreas Vesalius (1514–1564) addressed this issue by inserting reeds into living animals’ throats through which air could be blown. However, there is no evidence that either Galen or Vesalius used such techniques as therapy for living patients.
Early therapy reminiscent of modern CPR can be found in the written record of the Han Dynasty in China. Zhang Zhongjing (about 208–145 BCE) described a method by which three people could attempt to save a hanged person from death by embracing and methodically moving the victim’s body and limbs to simulate normal breathing. During the Jin Dynasty, Ge Hong (about 284–364 CE) wrote a Handbook of Prescriptions for Emergencies, which details the use of a reed pipe inserted into the throat through the mouth, to be used in conjunction with manipulations of the torso and limbs. H.R. Sylvester in the 1850s and E.A. Schafer in 1903/4 both described CPR-like therapies which were taught widely before eventually being shown to be too inefficient to justify their continued use.
For the treatment of conditions with more complicated causes than the inhalation of water or a minor injury to the throat, we can look at the history of machines that attempted to manipulate a patient’s breathing. Mechanical therapies utilize key principles of biophysics. To better understand the respiratory machines that have been developed since the late 1700s, it is important to recognize the difference between two general classes of respirators: positive pressure machines and negative pressure machines.
Imagine squeezing a sturdy, inflated balloon with a small, stable opening. When the interior volume of the balloon is compressed, it increases the interior pressure. To equalize the pressure inside and outside the balloon, the air inside rushes out rather than exerting pressure on the balloon from within. This relationship can also work in the reverse direction. If the balloon expanded, the external pressure would decrease, and the imbalance between the balloon’s internal and external pressure would create a vacuum, pulling air into the balloon to balance the pressure difference. This is, in simplest terms, how the lungs function. The diaphragm muscles contract and expand the lungs from without; the throat acts as the opening through which air travels to equalize the air pressure differences inside and outside of the body. The difference between positive and negative pressure machines is how each uses this interaction to induce ventilation. Positive pressure ventilation forces air into the lungs. Negative pressure ventilation, however, acts from outside the lungs by decreasing the pressure outside the body. This pressure imbalance between the lungs and the external environment causes the lungs to inflate, resulting in ventilation.
Doctors and scientists in Europe began adding to the canon of biophysical knowledge at an increasing rate in the seventeenth century. In 1640, English scientist John Mayow constructed a system of bellows and a bladder that demonstrated the way air could be moved using pressure. Scottish physician John Dalziel wrote “On Sleep, and an Apparatus for Promoting Artificial Respiration” in 1832 which is often credited with being the first to describe a negative pressure ventilator. In 1864, Alfred Jones developed and patented a body-enclosing box; the patient sat inside, head sticking out of a hole in the top. The operator used an external pump to change the pressure inside the box. Jones advocated for the use of his machine not only for patients with problems breathing, but also for conditions such as deafness or paralysis.
In 1876, a predecessor to the iron lung was developed by Alfred Woillez. Called a spirophore, it supplied negative pressure ventilation and measured the tidal volume of the patient’s lungs. Tidal volume is the change in capacity of a person’s lungs between exhalation and inhalation. By monitoring tidal volume, the needed pressure could be scientifically gauged. Many models were eventually developed, but the iron lung most widely used in the twentieth century was developed in 1928–1929 by Philip Drinker and Louis Agassiz Shaw at the Harvard School of Public Health. It is best known for its use in the treatment of polio. Now rare, but endemic before the Salk and Sabin vaccines, polio leaves many of its victims paralyzed and unable to breath independently. The iron lung could also treat victims of coal gas poisoning, botulism, and paralytic drugs such as barbiturates and tubocurarine.
Until the middle of the twentieth century, there was a strong preference for “negative rather than positive pressure ventilation. All artificial ventilation comes with the risk of permanent injury to the lungs and airways. Positive pressure ventilation as it existed prior to the twentieth century was, if too much pressure was applied, as injurious as no treatment at all. Negative pressure, while perhaps safer, required large, awkward, expensive machines. A polio outbreak in Copenhagen in 1951 became a crisis that negative pressure ventilation could not contain and was a watershed moment in the development of positive pressure therapies. Anesthesiologist Bjorn Ibsen proved that the mortality rate of polio victims under his care plummeted when he used positive pressure therapy as opposed to the standard negative pressure. The hospital grounded its therapeutic strategy on having healthcare workers monitor and provide this treatment by hand. This was possible through the use of tracheostomies. This incident is often regarded as the predecessor to the modern ICU, which today often involves artificial ventilation.
Since the 1950s, mechanical ventilation has mostly transitioned from negative to positive pressure, and the machinery involved has seen several generations of technology. The first machines were simple: they had no alert system and could only be adjusted for tidal volume, which was manually monitored. The next generation of technology arose in the 1970s and involved patient-triggered ventilation, as well as autonomous monitors; however, this technology was still tied to measurements of tidal volume. Since the 1980s the technology has incorporated more flexibilities, safeguards, and alarm systems. Additionally, modern equipment can have two circuits to deliver oxygenated air and other gases, such as those used for anesthetic purposes, to provide ventilation before, during, and after a surgical procedure.
The next generation of hardware will focus on alternatives and supplements to positive-pressure ventilation. Even with advanced monitors and precise control, artificial ventilation can still leave a patient with health complications during and after their time with the machine. Positive-pressure machines can cause long-term damage, despite their flexibility. Research suggests a more diversified approach for such cases as chronic neuromuscular disorders which require long-term therapy and are hazardous to approach with positive-pressure ventilation alone. Newer iterations of negative-pressure ventilation have abandoned large rooms or chambers that encompass the entire body, and are instead being developed as coats or shell-like devices called cuirasses that surround just the torso and are far more portable. As the future of respiratory therapy unfolds, it remains to be seen whether the dichotomy between positive-pressure and negative-pressure ventilation can be bridged, and how this will affect the patients these therapies aim to help.