Mechanical ventilation historical perspective
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Vesalius was the first person to describe mechanical ventilation by inserting a reed or cane into the trachea of animals and then blowing into this tube. The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation.
Negative Pressure Machines
- The iron lung, also known as the Drinker and Shaw tank, was developed in 1929 and was one of the first negative-pressure machines used for long-term ventilation.
- It was refined and used in the 20th century largely as a result of the polio epidemic that struck the world in the 1950s.
- The machine is effectively a large elongated tank, which encases the patient up to the neck.
- The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air.
- Gaseous exchange in the lungs takes place by process of simple diffusion, however, air must be drawn into the lungs to make it available for exchange.
- In spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates air flow.
- In the iron lung, vacuum is created inside a tank by pumping out the air, thus generating a pressure gradient to allow flow of air.
- After the air is drawn into the lungs via negative tank pressure, the vacuum is released leading to equalization of pressure inside the tank and the ambient air pressure. This leads the air to exit the lungs due to natural elastic recoil of the lung and thoracic cage.
- Due to the vacuum created in the tank, the abdomen also expands along with the chest cavity leading to decreased venous return and pooling of blood in the extremities.
- The patients can talk and eat normally and can see the world through a well-placed series of mirrors.
- Some could remain in these iron lungs for years at a time quite successfully.
- Today, negative pressure mechanical ventilators are still in use, notably with the Polio Wing Hospitals in England such as St Thomas' (by Westminster in London) and the John Radcliffe in Oxford.
- The prominent device used is a smaller device known as the cuirass.
- A cuirass is a shell-like unit, creating negative pressure only to the chest using a combination of a fitting shell and a soft bladder. Its main use is in patients with neuromuscular disorders who have some residual muscular function.
- However, it was prone to falling off and caused severe chafing and skin damage and was not used as a long-term device.
- In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high-pressure oscillation pump in order to carry out biphasic cuirass ventilation.
Positive Pressure Machines
- The design of the modern positive-pressure ventilators was mainly based on technical developments by the military during World War II to supply oxygen to fighter pilots at high altitude.
- Such ventilators replaced the iron lungs as safe endotracheal tubes with high volume/low pressure cuffs were developed.
- The popularity of positive-pressure ventilators rose during the polio epidemic in the 1950s in Scandinavia and the United States.
- Positive pressure through the manual supply of 50% oxygen through a tracheostomy tube led to a reduced mortality rate among patients with polio and respiratory paralysis.
- However, because of the sheer amount of manpower required for such intervention, positive-pressure ventilators became increasingly popular.
- Positive-pressure ventilators work by increasing the patient's airway pressure through an endotracheal or tracheostomy tube. The positive pressure allows air to flow into the airway until the ventilator breath is terminated.
- Subsequently, the airway pressure drops to zero, and the elastic recoil of the chest wall and lungs push the tidal volume out through passive exhalation.
- Chamberlain D (2003) "Never quite there: A tale of resuscitation medicine" Clinical Medicine, Journal of the Royal College of Physicians' 3 6:573-577