The iron lung, formally known as the tank respirator or tank ventilator, is a historical life-support device developed in the late 1920s. Created by Philip Drinker and Louis Agassiz Shaw at Harvard University in 1928, this large, horizontal metal cylinder was designed to manage respiratory failure during major public health crises. It provided a way to mechanically sustain breathing for patients whose own muscles had failed. The device’s function relies on a physical principle to force the mechanics of breathing when the body cannot perform the action naturally.
How Negative Pressure Ventilation Works
The iron lung operates by utilizing external negative pressure ventilation to move air in and out of the lungs. To achieve this, the patient’s body is sealed inside the cylindrical chamber, with a tight-fitting rubber gasket around the neck, leaving only the head exposed to room air. The machine contains an electric-powered pump and bellows system that cyclically alters the air pressure within the sealed tank.
This mechanism is a direct application of Boyle’s Law, which dictates that the pressure and volume of a gas are inversely related. When the machine’s pump draws air out of the chamber, it creates a partial vacuum, or negative pressure, around the patient’s chest and abdomen. This drop in external pressure forces the flexible chest wall and diaphragm to expand outward, which in turn increases the volume of the lungs.
As the lung volume increases, the pressure inside the lungs momentarily drops below the atmospheric pressure outside the patient’s mouth, causing ambient air to rush in and fill the lungs. This constitutes the act of inhalation, mimicking the body’s natural breathing process. The pump then releases the vacuum, allowing the pressure inside the tank to return to normal atmospheric levels. The natural elastic recoil of the lungs and chest wall passively pushes the air out, completing the exhalation phase.
The Medical Need It Addressed
The iron lung was developed to address conditions that caused paralysis of the respiratory muscles, with its most widespread use coming during the Polio epidemic. Poliomyelitis, an infectious disease caused by the poliovirus, attacks the central nervous system, specifically targeting motor neurons in the spinal cord and brainstem. While most infections were mild, a small percentage progressed to a paralytic form of the disease.
The most severe form was bulbar polio, which involved the nerve cells controlling involuntary actions, including those responsible for breathing. The virus could destroy the motor neurons of the phrenic nerve, which originates in the cervical spinal cord (levels C3 through C5) and controls the diaphragm, the primary muscle of respiration. When the diaphragm and intercostal muscles were paralyzed, the patient could no longer expand their chest cavity to draw a breath.
Without the ability to move the chest wall, these patients would suffocate, requiring immediate mechanical assistance. The iron lung provided a non-invasive method to sustain life for weeks or months, giving the patient’s body a chance to recover from the acute viral infection. The device served as a temporary muscle replacement, mechanically performing the function of breathing until the patient could potentially regain some independent muscle function.
Why the Iron Lung Became Obsolete
The decline of the iron lung resulted from two major simultaneous developments in public health and medical technology. The most significant factor was the development of effective preventative measures against the disease it primarily treated. The introduction of the Salk inactivated polio vaccine in 1955 and the later Sabin oral polio vaccine drastically reduced the incidence of poliomyelitis worldwide. As the number of new polio cases plummeted, the primary need for the tank respirator largely disappeared.
Technological advancements in mechanical ventilation also provided a superior alternative for treating general respiratory failure. The positive pressure ventilator was invented and rapidly adopted in the 1950s, representing a fundamental shift in respiratory support. Unlike the iron lung, which pulls air into the lungs, positive pressure devices push air directly into the airway, often through a tube inserted into the trachea.
Positive pressure ventilation systems were smaller, more portable, and less expensive to manufacture than the tank respirator. The iron lung was also limiting for patient care, as medical staff had difficulty accessing the patient’s body for nursing and medical procedures without temporarily interrupting the ventilation cycle. The modern alternatives offered increased mobility and allowed for continuous patient access, transforming the management of respiratory failure.