Monday, June 19, 2017

1937-80: The evolution of mechanical ventilation

Inventor Holger Hess
(from Ambu.com)ega
Note: Here is my attempt to piece the history of mechanical ventilation together. If you have access to information or pictures to help tell this history please contact me. This is all done pro bono. As you can see there are no adds on this blog.

Non-invasive negative pressure ventilators (iron lungs) were a godsend in intensive care units during the first half of the 20th century. Yet their day in the sun was about to end, mainly due to two medical developments.
  1. The realization that iron lungs made it very challenging to clear secretions from airways. The patient's bed had to be pulled from the tank by one person, another person had to provide manual artificial respiration to the patient, a third had to rotate the patient, while another wiped away secretions. Considering there were sometimes copious secretions, caregivers sometimes had trouble keeping up. Since excessive secretions obstructed airways, caregivers yearned for a better system. 
  2. Improved anesthetics made it possible to perform complicated operating procedures, particularly upper abdominal surgeries. This made it necessary to develop an apparatus that would provide artificial respiration without covering the patient.  
These two developments lead to a paradigm shift away from noninvasive ventilation and toward invasive ventilation. They also lead to a shift away from negative pressure ventilation and towards positive pressure ventilation. These transitions were made possible due to the following innovations.

1923: Waters to and fro bag and canister:  This is not a history of anesthesia, although at times our history intertwines with theirs. Roger Waters devised a system that includes a canister filled with sodalime. The canister is placed between a rubber mask and a rebreather bag.  A gas flow (possibly oxygen with anesthesia) entered the system between the canister and the mask. Inspiration and expiration then went through the canister (to and fro). The soda lime would collect CO2 and create moist heat. The bag would collect exhaled gas flow. During inhalation the patient would get humidified gas. (28, page 433) (29, page 49)

This created a closed circuit system whereby the anesthesia gas used would not leak into the atmosphere. This was important because surgeons, anesthesiologists, nurses and other operating room attendants did not want to inhale the gas, but also because some anesthetics were flammable. So it made sense to make sure the gas was delivered to the patient in a closed loop system. This system was also nice because it was cheap, sterile, and easy to operate. (28, page 433) (29, page 49)

The anesthetic Waters created this system to deliver was Cyclopropane, a gas that is explosive when it comes into contact with oxygen. Other anesthetics are also flammable, so the system allowed them to be contained. (30, page 147)

I just mention this device here because it is used in 1952 as part of a study that is pertinent to our own history.

1930s: First use of Positive End Expiratory Pressure (PEEP). Applying a constant pressure during expiration was first used during open chest surgeries to prevent lung collapse. A circle system anesthesia apparatus was used, where the exhalation outlet was connected with rubber tubing to a glass tube. The tube was inserted into a glass of water. The deeper the tube was inserted the higher the PEEP. This could usually generate 3-5 cwp of PEEP.  (24, page 91)

1937:  Suction pump invented:  Iron lungs were nice in that they gave caregivers the ability to keep children infected with paralytic poliomyelitis alive. Yet caregivers could not prevent some of these kids from drowning in the copious secretions they produced, secretions that obstructed their airways. This challenge was met in 1935 by Dr.  Hess, who said he had "a dream of developing products to save lives." (17) (18)

In 1937 he formed Testa Laboratories.  He invented a suction pump so secretions could easily be cleared (or sucked) from the patient's airway.  The pump was connected to rubber tubing. The distal end of the tubing was connected to a catheter that was inserted into the patient's airway. The pump acted like a vacuum and sucked secretions from the airway.  (17) (18)

One end of a rubber tube was attached to the pump, and the opposite end was inserted into the patient's airway.  When the pump was turned on a negative pressure was created, and any loose secretions in the patient's oral cavity would be sucked up.  This was a significant improvement over manually clearing the airway or feeling helpless as a patient suffocated on his own phlegm. (17) (18)

1945?: Bennett Clinical Research Model X2 Respirator (Bennett Valve):  During WWII, Dr. V. Ray Bennett worked with Dr. Cournand and Dr. Motley to invent the BRx2 Resuscitator for the United States Aeromedical Laboratory. (25, page 1)

According to The Journal of the American Association of Nurse Anesthetists, "This device featured the Bennett clinical research model (Ben X-2, or the Bennet Valve) valve and was used for administration of intermittent pressurized oxygen inhalation during high-altitude flights in unpressurized aircraft."  (24, page 90)

The system was connected to rubber corrugated tubing and a comfortable mask created by Bennett for other breathing units he invented. It was meant to control breathing, or assist breathing, for pilots at high altitudes. Yet as the war ended soon after it was invented, it was never used by the military.  (25, page 1) (26, AARC Virtual Museum).

A picture and write up on the device used here can be viewed at the AARC Virtual Museum. (http://museum.aarc.org/gallery/ippb/)

The device was then used medically to supply positive pressure ventilation, with or without PEEP, to patients in respiratory failure. (25, page 1) (26, AARC Virtual Museum).

John Dillon, of Los Angeles County Hospital, writing for the Anesthesia History Association Newsletter in July of 1990, explained how Bennett came to his office in the autumn of 1976 with his "gadget." Dr. Dillon said:
I took it into an operating room, along with Ray (Bennett), where a patient was under general anesthesia, and we slipped it into the circuit as an assistor; it worked very nicely. The depth of ventilation was readily controllable. Our outstanding characteristic, which was important at that time, was that it was not controlled by electricity and hence not a danger with ethyl ether. (26, page 1)
They then decided to try out the device on patients in the "infectious disease" section of the hospital where there was several poliomyelitis patients in Drinker Collin's Respirators, or iron lungs. The received the permission from a patient who was paralyzed due to the disease to try out the device. Dillon said:
We told him that we were going to put a mask on his face, and that it was connected to a device that would breathe for him while he was in the lung, and for him when the lung was opened. I assured him that we would not let him become distressed and that, if he were uncomfortable, he should blink his eyes rapidly, which was about all he could do, and we would put him back in the lung immediately."  (25, page 1,9)
We put a mask on and connected the "Valve" to a cylinder of compressed air. The unit synchronized with the lung. We then had the lung shut off and cracked open. The unit now provided "Controlled Respiration" to the patient at the rate and depth which had been set by the lung. The patient showed no sign of distress. His chest moved freely. As soon as it was evident that respiration was adequate, a couple of nurses, or attendants, gave the patient a sponge bath and some passive exercise on his limbs." (25, page 9)
We were all impressed by the action of the "Bennett Valve" as a ventilator. The patient was kept out of the lung for almost a half hour, and when he was returned to the lung and could talk, told us that he had been perfectly comfortable." (25, page 9)
The Bennett Valve, later known as the Puritan Bennett Valve, was therefore the first such device to supply controlled respiration with positive pressure on both a patient under anesthesia and paralyzed due to poliomyelitis.

1946: First Positive Pressure Inhalation Device:  Dr. Forrest Bird was an experienced pilot by the time WWII started. A week after the bombing of Pearl Harbor he signed up for the Army Air Corp. He was responsible for transporting many of the planes used in the war from factories to aircraft carriers, and was in an ideal position to study aerodynamics. (21)

He studied high altitude breathing problems that prevented pilots at this time from flying higher than 28,000 feet. He obtained a German regulator and made adjustments to it that allowed pilots to receive intermittent positive pressure breaths, allowing them to travel as high as 37,000 feet by the end of the war. This was important because it allowed American pilots to fly higher than enemy planes, giving them an advantage over enemy pilots.  (21)

The device he invented was effectively the first prototype of an intermittent positive pressure breathing machine, and would prove "invaluable" when he later worked on devising respirators for medical purposes. (21) (23)

1948:  Continuous Positive Airway Pressure (CPAP):  The first studies using crude CPAP devices were done during the 1930s. Alvin Barach performed the first modern studies on CPAP. This is a continuous flow of pressure applied to the airway by a mask. During WWII Alvin Barach supervised experiments whereby CPAP was used on a variety of pilots who traveled to high altitudes.

After the war he studied the use of CPAP on a variety of patients, although his work was relatively ignored until the 1980s when studies would confirm CPAP was was useful in treating patients with sleep apnea or lung diseases. CPAP was studied during the 1980s as a means of preventing patients from needing intubation.  (v2)

1948: Bennett Flow-Sensing Pressure Breathing Unit TV-2P: V. Ray Bennett's valve (The Bennett Valve) was featured in the Bennett TV-2P Respirator.   It was flow sensing, and time cycled, meaning a breath ended after a set time was met. It could provide controlled mechanical breaths to treat patients in respiratory failure. (24, page 90) (7, page 1318)

Patient's could also control the rate, making it also an ideal device for delivering intermittent positive pressure breathing. A nebulizer could be added to the circuit for the delivery of bronchodilators during IPPB therapy. It was run by electricity, batter, hand pump, or air. (22, page 94)

1948:  First Study Using IPPB:  Dr. Albert Bower noted a near epidemic number of cases of respiratory failure among bulbar poliomyelitis patients using Drinker Collin's Respirators at Los Angeles County Hospital. Several TV-2P's were used to provide assisted ventilation to several of these patients with noted respiratory failure. However, they proved less than ideal for long term IPPB.

Dr. Bower worked with Biomedical engineer V. Ray Bennett, who used his attachments (BR X2 valves) to make the Drinker-Collin's Respirators...
...capable of supplying "intratracheal" intermittent positive pressure ventilation (IPPV), supplementary to its NPV (negative pressure ventilation). Together with their teams, Bower and Bennett used this attachment for 73 of 1949's 130 "respirator cases", to establish the first-ever large-scale long-term success of IPPV for respiratory failure in acute polio. (22, page 91)
The device was powered by the motor of the negative pressure ventilator. Positive pressure could be supplied to these patients by a mask that covered both the mouth and nose, or a tracheotomy adapter created by Bower. "The special exhalation was installed adjacent to the mask or tracheotomy connector." A bellows adapter was also added to the system. A humidifier could also be added. Likewise, supplemental oxygen or helium could also be added to the system.  (22, page 94-95)

Now, these negative pressure ventilators were still negative pressure ventilators, although positive pressure breaths could be applied to patients when needed. While the study did not report how often ventilation was augmented by IPPB, "The authors do describe IPPV as providing more effective AV, so presumably, they would favour it."

A year later, Bower and Bennett "demonstrated the superiority of (supplemental) IPPV over (negative pressure ventilation) alone, achieving a survival rate of 83.7 (108/129)-- compared with the 21.1% survival rate in 1946 among the 38 patients ventilated that year. (22, page 91)

This study was the first that used positive pressure ventilation, and the first to study the long term effects of intermittent positive pressure breathing. The study showed that use of IPPV lowered the incidence of mortality among those inflicted with paralytic bulbar poliomyelitis. It was essential to the transition from negative pressure ventilators to positive pressure ventilators. According to Critical Care Resuscitation, "Bower and Bennett deserve greater recognition of their pioneering merit than they currently receive in the written history of intensive care medicine."   (22, page 91, 99)

The lack of success of the TV-2P for long term ventilation and the success of this study highlighted the need for a more effective IPPB machine for general patient care. (22, page 94)

The Bennett Valve was also featured in the Bennett BA-2 Anesthesia Ventilator. It too was used by Los Angeles County Hospital during the 1948 polio epidemic.  (7, page 1318)

1950s-1970s:  Rubber masks:  Positive pressure breaths were often provided by using a rubber mask over the patient's mouth and nose. One of the major complications of the rubber masks used at this time was that they were opaque and concealed aspiration or foaming pulmonary edema, and this was noted as a major disadvantage of such masks.

Another disadvantage was prolonged use caused facial skin breakdown. And yet another disadvantage was that air would often leak around the masks. Masks also required persons to hold them onto the face, and this could get very tiresome for both the patient and the caregiver.

 When used on polio patients, nursing assistants or respiratory therapists would often work in two hour shifts. The disadvantages of these masks could be compensated for by tracheotomies and inserting a catheter, and later by intubation (see below).  (v3)

Mark 4 Resuscitator
Introduced by Dr. Forrest Bird
in 1958? as an anesthesia ventilator.

1952: The Bird Mark I:  Pressure ventilation was found useful during operations to prevent respiratory failure, although they posed two pretty significant problems.
  • There was no way to monitor accurate pressures and tidal volumes delivered to patients, and so uncuffed endotracheal tubes were needed to prevent barotrauma from excessive pressures and tidal volumes. This left airways unprotected and subject to aspiration of stomach contents.
  • Some early anesthesia ventilators were powered by electricity. This posed a problem because sparks from them could trigger an explosion due to flammable gases present in operating rooms at the time.   (24, page 91)
This created a need for a pneumatic respirator, or one that was powered by air instead of electricity. 

Roger Manley was an anesthesiologist from Westminster, London. He worked with Dr. Forrest Bird, who incorporated the technology he
invented in 1946 into an anesthesia ventilator. The first Bird Medical Universal Respirator prototype was introduced in 1950, and a second was introduced in 1951.

All that was needed to operate it was a 50 PSI source. By this time most operating rooms had piped in oxygen, so this should not have been a problem. It was pressure triggered and cycled, meaning breaths began when the patient generated a negative pressure, and ended when a preset pressure was met.

It was marketed in 1952 as the Manley Respiratory, although it was later refined and re-branded as the Mark II. It was used by anesthesiologists for the next 40 years.

The machine provided some clear advantages to older ventilators. Patients were given anesthesia, and once the paralyzing effect occurred, patients were intubated. Tubing from the machine was connected to an endotracheal tube. The tube made it easy to keep airways patent and free from secretions. The ventilator provided a hands free method of providing artificial respiration until the anesthetic was withdrawn and patients were spontaneously breathing.

Previous anesthesia ventilators only provided the preset rate and inspiratory time, what was referred to as "controlled ventilation." This meant that the patient could not trigger a breath. This worked fine in operating rooms where patients were paralyzed and sedated due to anesthesia. But it was not acceptable for the general population, particularly patients who were awake and alert who could fight the machine, thereby creating what is referred to as patient-ventilator asynchrony.

So, while initially made as an anesthesia ventilator, the Mark 1 was also found to be a useful ventilator by physicians in emergency rooms and critical care units. This was because it sensed a patient's inspiratory effort, and then provided a positive pressure breath to "assist" ventilation. This became known as "assisted ventilation."

And, of course, when used outside operating rooms, they were operated by inhalation therapists. This created a new market for pneumatic positive pressure ventilators, a market that Bird would tap into with a later model.

Bennett PR II
(AARC.org gallary of ICU Ventilators.)
Check out this link for pictures
of most ventilators on this page.
One of the pictures shows
external PEEP with the MA1. 
1952:  Bennett Pressure Breathing Unit:  After the BR X2 Respirator attachment was successfully studied in 1948, Dr. V. Ray Bennett incorporated into the PR I and later the PR II.  Like the Mark I and Mark II, it was a pressure cycled ventilator. It had a nebulizer cup for the nebulizatiion of Isuprel and Alevaire. It could be used as a ventilator or for IPPB treatments.

Pneumatic positive pressure machines were increasingly used as ventilators instead of iron lungs when suctioning of the airway was required. These units were also increasingly used for IPPB therapy, and often referred to as IPPB machines.

Iron Lungs and IPPB machines were used as ventilators until the 1960s when volume ventilators (such as the Emerson and MAI ventilators below) were proven to be safer and more effective as ventilators. However, IPPB units remained a viable ventilator option when called upon.

Bennett later refined this machine and re-branded it as the Bennett Bennett PR 2 in 1963. Both of these machines (the BR1 and BR2) ) were still mentioned in respiratory therapy texts through the 1990s.

1952-1953: Second Study of Long Term IPPB:  Dr. Bjorn Aage, an anesthesiologist, Ibsen performed a large scale study using intermittent positive pressure ventilation in Copenhagen, Denmark. During the polio epidemic of 1952-1953, Ibsen was called to an emergency meeting, where he recommended providing ventilation manually using a bag when polio patients. Despite criticism, Ibsen's idea went on to become the largest study up to that time using Intermittent positive pressure breaths. (24, page 97) (27, page 398)

Isben was met with criticism because the hospital had access to an Emerson Tank (iron lung) and six cuirass respirators. And here came along an anesthesiologist who proposed "ventilating patients with breathing failure, somehow without respirators." (27, page 399)

Waters to and fro system (anesthesia.med)
His first patient was a "moribund 12 year old, Vivi Ebert (her name has been in the public domain). Per Ibsen's request, she had a tracheotomy, and then she was ventilated with manual positive pressure ventilation. The device used was a "Waters to and fro bag and (soda lime packed) canister." This was an early anesthesia device for providing positive pressure ventilation to patients while anesthesia was administered. (27, page 399)

It should be noted that "The treatment succeeded, and she lived until 1971." This success garnished the excitement of all involved, and this lead to a two year study of other similar patients with poliomyelitis, and the hospital had plenty of trainees, students, nurses, and anesthesiologists to perform the duties. About 277 patients were provided IPPV with the device, and this was about four times as many patients that were trialed with IPPV in Los Angeles a in 1948. (24, page 97)(37, page 399)

This study showed that IPPV could prove useful when used on a large scale for poliomyelitis patients in respiratory failure.

For the record here, Ibsen is often referred to as the father of intensive care for his work in this study, and for his work in establishing the first intensive care unit in the world in 1953 at Copenhagen. (27, page 398)

1953:  Bag-valve mask (self inflating resuscitator) invented. Surgeons are unable to perform surgeries in the upper abdominal region mainly due to the inability to provide artificial resuscitation. They were also faced with the daunting challenge of performing artificial resuscitation for kids infected with paralytic poliomyelitis when they were outside their iron lungs. Surely there were a few mechanical or pneumatic devices, although these were all new and relatively primitive at this time. Plus they wouldn't have been available at many hospitals. To tackle these challenges, Holger Hess joined forces with anesthesiologist Henning Ruben.

AMBU resuscitator
Also shows opaque rubber mask.
 (from AMBU.com)
This Hess invented the bag valve mask (BVM), or what is also known as the first self inflating resuscitator. Now it is most commonly referred to as an AMBU-bag. There has often been speculation as to what the  acronym A-M-B-U stands for, although no one knows for sure. There are a couple accepted ones, such as ambulatory manual breathing unit, or air mask breathing unit. Either way, the device gave anesthesiologists the ability to give paralytics during complicated surgeries, allowing surgeons to save lives.

The device was a lightweight black rubber bag. On one end was rubber tubing that connected to an air or oxygen tank (preferably oxygen). The other end of the bag was connected to a rubber mask that was placed over the patient's face. Once the flow was turned on, the caregiver would squeeze the bag to provide a breath. Once the caregiver released the bag, it self inflated. This was an evolutionary breakthrough that quickly gained in popularity throughout the hospital.

Initially mailed in wooden boxes out of a small factory in Copenhagen, the company soon had to move to larger facilities. It even changed its name to AMBU.  While initially used only by anesthesiologists, when other physicians learned of the product they wanted one. Soon one was available in nearly every corner of every hospital, particularly in operating rooms, emergency rooms, and critical care units.

Anesthesiologists now had the means of providing  providing artificial resuscitation during operations. Physician, anesthesiologists, nurses aides, orderlies and inhalation therapists would take turns providing artificial respiration to kids infected with polio until an iron lung was available.
A picture of the Bird Mark 7,
sometimes referred to
simply as "The Bird."
(From independenceplus.com)

1958:  Bird Mark 7 Universal Respirator:
Dr. Bird continued working on positive pressure technology, and in 1955 came up with a prototype of the Bird Mark 7.

Dr. Bird showcased it at various teaching hospitals, allowing it to be used in the most critically ill patients who were suffering from respiratory failure where other accepted therapies had failed to work. The Bird Mark 7 Respirator was quickly accepted by the medical community, and in 1958 it was introduced to the market.

This respirator was ideal for inhalation therapists, as it was portable, durable, and easy to operate. It was connected to a 50 PSI source and was operated by a flow of air. It could be used to provide controlled positive pressure breaths long term by connecting it to an endotracheal or tracheotomy tube. It was one of the first respirators to provide continuous controlled mechanical breaths for patients who were unable to spontaneously breathe. (20, pages 66-67).

Dr. Forrest Bird demonstrating
the Bird prototype introduced in 1950,
and the Bird Mark 7 introduced in 1958.
Like the Mark I and Bennett respirators, it was pressure cycled. Like the other pressure breathing machines. Likewise, tidal volumes were determined by resistance, meaning they were variable from breath to breath, patient to patient. When set to "air-mix" the fraction of inspired oxygen was 40%, although could also get as high as 80%. (20, page 67)

It was also easy to repair and maintain, and for this reason, Dr. Bird once quipped that it was the Model T of respirators.

It was initially intended to be used as a ventilator. Yet two problems were observed:
  1. No manometers or devices to indicate how much of a tidal volume was being delivered. 
  2. No alarms to indicate when too much pressure was being given, or if the patient became disconnected (20, page 67)
These disadvantages required patients ventilated with these machines to be located close to nurses stations, where they could be frequently monitored by nurses, doctors, and inhalation therapists. The machines made a distinct sound during inhalations, so it would be easy to hear if they were working.

The conveniences offered by these machines made them ideal for IPPB therapy. So, while IPPB therapy was first described in 1947, and while there were other pressure machines that did the same thing, the Bird Mark 7 made it famous. It would become the most commonly used IPPB machine, and was generally responsible for the IPPB Revolution that followed.

They were very commonly found in hospitals during the 1960s and 1970s, although some hung around until the 2000s.  Some of these machines can still be found in respiratory therapy equipment storage rooms collecting dust, although most have been discarded or sent to Africa.

A Bird Mark 7 service manual can be viewed here.

John "Jack" Haven Emerson
(polioplace.org)
1964:  Emerson Volume Ventilator The Bennett and Bird Respirators were the two main ventilators used in the first decade after the decline of negative pressure ventilators. Yet their weaknesses soon became apparent. This particularly became evident during the 1960s as critical care units were on the rise. So this made apparent the need for volume ventilators, the first of which to enter the scene was created by the already famous Dr. John Emerson.

According to a 1998 article by Richard Branson in Respiratory Care, "Jack Emerson: Notes on his life and contributions to respiratory care," John Emerson was the second person to come up with a volume ventilator. Branson described this ventilator.
"This simple devise resembled a green washing machine and used a piston to deliver precise volumes. Oxygen was added into a ‘trombone-shaped’ accumulator connected to the intake of the piston for delivery of elevated FIO2. The tidal volume was changed by a crank on the front of the machine, which controlled the stroke of the piston. Respiratory rate and inspiratory-to-expiratory-time ratio (I:E) were adjustable. The humidifier was a modified pressure cooker and was known as the Emerson Hot Pot. A belt, connected to a DC motor and pulley wheel, served to move the piston. In case of failure of the existing belt, a spare was hung inside each cabinet. The belts were similar to those used to circulate air in forced air gas furnaces in homes. On numerous occasions I have heard the story of the belt becoming loose or breaking and the spare found to be missing. Under these circumstances, the resourceful respiratory therapist would run to the parking lot and obtain the belt from a Volkswagen Beetle (the old one) and place it in the Emerson to restore it to working order. I’ve never looked to see whether the two belt sizes are compatible because it’s such a good story. In any event, the Emerson Post op Volume Ventilator was reliable and would allow ventilation of patients when other devices failed. Emerson’s device was not the first of the piston ventilators (M¨orch and Engstrom preceded him), but it was the first device to allow independent control of I:E." (9, page 568)
This was the first volume ventilator.  Unlike the IPPB units that could be used with either an endotracheal tube, trach tube, mask or mouthpiece, volume ventilators required that the patient either be intubated or have a tracheotomy tube.

1967:  The Puritan Bennett MA1 Ventilator:  In 1940 Ray Bennet produced a gas delivery system that involved a "jewled pneumatic valve - the 'valve that breathes with the patient." It was this concept that allowed the Puritan Bennett company to create the the MA1. (a10) (b11)

It was a compact unit that could easily be carted to the patient's bedside. Knobs on the machine allowed the therapist to set in a desired rate, tidal volume, sigh depth, sigh rate, and maximum pressure.

The machine also added a feature from the IPPB machines called sensitivity. This allowed the therapist to control how hard or easy it was for patients to trigger breaths. When set high, such as 10cwp, patients would have to work very hard to start a breath. This was ideal for patients who were not breathing on their own. It would be a fright show as the patient started to wake up and breathe spontaneously. This resulted in patient anxiety and stress and patient-ventilator assynchrony.

When the sensitivity was set to 2-3 cwp the patient could easily trigger a breath. This was an ideal feature, making mechanical ventilation much more comfortable for patients. This feature combined "controlled ventilation" with "assist ventilation, and it became known as "assist-control mode." This made it so the patient would receive the dialed in rate, although could trigger spontaneous breaths in between machine breaths.

Once the patient was intubated, tubing from the machine could easily be hooked up to the ETT (it could also be hooked up to a trach tube). It gave caregivers much better control compared to negative pressure and positive pressure breathing units.

Most therapists who were familiar with these machines say they were very durable. Even after other ventilators were purchased and broke down, the MA1s continued to function. Even after the MA2s were introduced, most facilities preferred to stick with their reliable MA1. This tended to be the case until more reliable volume ventilators hit the market.

One problem with the MA1 is that it didn't come with much needed alarms. This was my problem in my brief experience with the ventilator. Most alarms were external alarms, and were complicated to work, or at least I thought so. External alarms included high rate, low rate, high pressure, and low pressure.

Another knock on the MA1 was that it had no way showing rate and tidal volume. To assure that these were adequate, an external manometer was needed.

Still, even with these annoying alarms, the machine was relatively easy to operate. If you needed to make a change all you did was turn a knob.

When I was an RT student in 1995 there was an old MA1 in the classroom. We were vaguely educated on this machine, mainly because most hospitals that still had one were phasing them out. When I was first hired at Memorial Medical Center we had one that sat in the storage room. I only had to use it one time before it was taken out of circulation sometime around the year 2000.

Another neat thing about these machines is that they were so durable, and lasted so long, that nearly every time a ventilator was featured on TV, or in a movie, it was an MA1. This continued to be the case for years after they weren't even in circulation.

In fact, when I took my registry test back in 1997, nearly all the ventilator questions were regarding the MA1. So, in preparing for the test, we had to become pseudo MA1 experts.  But that was in an era before computers, back when it wasn't easy to update a test with the click of a keystroke.

1967: Positive End Expiratory Pressure (PEEP): This feature was added to ventilators as a feature in order to keep airways open, or to prevent airways from collapsing at end expiration. It was initially used to improve oxygenation in patients suffering from Acute Respiratory Distress Syndrome (ARDS). This was initially added as an external feature, although newer ventilators came with PEEP capabilities. (31, page 9)

1971: Intermittent Mandatory Ventilation:

Servo 900C set up and ready to go.
1971:  Servo 900 Ventilator:  It was small, slightly larger than two shoe boxes, and all the knobs were on the front. It became known as a minute ventilation ventilator because the respiratory therapist would set the minute ventilation and the tidal volume and respiratory rate would be secondary. In this way, in volume control mode, the tidal volume could be set.

This machine had a sensor to make sure the tidal volume was adjusted with changes in patient compliance so the patient was guaranteed to get the set tidal volume. This was the first ventilator to have this function.

In order to set tidal volume, though, you had to do a little math. I know because when I did my clinical rotation in 1996 at Blodgett Hospital, in Grand Rapids Michigan, they still used this ventilator on a regular basis. It was also commonly used on cardiac patients at Mott's Children's Hospital at the University of Michigan when I did a rotation.

Many students were afraid to use it, and so was I at first. Yet once you were used to it, it was a very nice ventilator. However, students were thankful when it was ultimately replaced by the Servo 300 Ventilator.

It was the first ventilator to have all the alarms you needed right on the machine, and it was also the first ventilator to allow for the addition of a device so you could see pressure and flow curves. This was nice because you could see what you were doing and make changes based on the needs of the patient. (d12)

It was also the first ventilator to provide both volume control and intermittent positive ventilation (IMV), which was later improved to synchronized intermittent positive ventilation. These new modes improved the physician's ability to wean patients from the ventilator. (e13)

There were some features added to the servo 900 series that most respiratory therapists would understand, but probably not the lay person.  The first addition came in 1978, and this was the ability to measure exhaled carbon dioxide (CO2).  This was nice because, prior to this, the only way to determine CO2 was by drawing an arterial blood gas (ABG), which is an invasive blood draw.

This was nice because it allowed clinicians the ability to change settings based on a noninvasive number, as opposed to having to do an invasive blood draw. It meant that a change could be made right now, as opposed to waiging up to 30 minutes for the results of the ABG. (e14)

A couple other features may be a bit more complicated to explain. Positive End Expiratory Pressure (PEEP) is essentially the same thing as CPAP, and is a continuous flow during expiration. It works to keep airways patent to ensure adequate oxygenation.

Prior to 1981 various devices were used to provide PEEP. One was by placing the expiratory circuit in a bottle of water. Today this system is still used in neonatal units and is referred to as bubble CPAP. Non-electronic PEEP was fine, although there was not way to know how much you were giving, and it sometimes applied external resistance to breathing. These problems were solved in 1981 as PEEP was introduced to the Servo 900 series.  Now all you had to do was dial it in.

Most initial modes of ventilating patients gave mandatory breaths to patients. This essentially means that the caregiver dialed in a rate and tidal volume, and this is what was given to the patient. It is called control ventilation, meaning that the ventilator controls everything. The patient can try to take in a breath, but nothing will come.

This was fine when patients were sedated and paralyzed, but once they started to wake up, you can imagine this would be quite uncomfortable. This often resulted in patient-ventilator asynchrony, where the machine would try to force a breath into the patient when the patient wasn't ready for it, and the patient would want to take a breath and no breath would come.

A new mode called Pressure Support solved this dilemma. PS means that the patient's spontaneous inspirations were supported with a small amount of pressure. This would assure that the breath was adequate, and was important for those patients who were too week, or too sick, to take in an adequate tidal volume. It was also a useful tool as far as weaning patients off the ventilator.

Pressure Control mode was also later added. This essentially means that the patient is guaranteed a certain pressure, while the tidal volume may vary with each breath. There are certain disease conditions that benefit from this mode. Neonates who use uncuffed ETTs also benefit from pressure control mode.  (e15)

1975: Synchronized Intermittent Mandatory Ventilation:

1977: Mandatory Minute Volume Ventilation:



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