Negative pressure ventilation (NPV) is a method of mechanical breathing assistance that operates externally, without the need for tubes inserted directly into the patient’s airway. This non-invasive approach works by applying a controlled, cyclical vacuum to the outside of the chest and torso. This external subatmospheric pressure causes the chest wall to expand, which facilitates lung inflation. This action helps patients who are unable to move air effectively on their own, essentially mimicking the body’s natural breathing mechanism.
The Physics of Creating Respiration
The fundamental principle governing negative pressure ventilation is Boyle’s Law, which states that pressure and volume are inversely related for a fixed amount of gas. In natural human breathing, the contraction of the diaphragm and external intercostal muscles increases the volume of the thoracic cavity. This increase in volume lowers the pressure inside the lungs, creating a pressure gradient relative to the atmosphere.
The pressure inside the lungs becomes lower than the air pressure outside the body, causing air to rush in through the nose and mouth to equalize the pressure. Negative pressure ventilators artificially recreate this physiological gradient by decreasing the pressure surrounding the patient’s chest and abdomen. When the machine creates a vacuum, it pulls the chest wall outward, increasing the volume of the lungs.
The resulting drop in pressure within the lungs draws in atmospheric air, forcing an inhalation. When the external negative pressure is released, the natural elastic recoil of the lungs and chest wall passively pushes the air out, causing exhalation. This external mechanism simulates how the body’s own respiratory muscles normally generate breathing.
Historical Role of the Iron Lung
The concept of external negative pressure ventilation was initially developed in the late 1920s, leading to the invention of the tank respirator, commonly known as the Iron Lung. The first widely used version was developed by Philip Drinker and Louis Shaw in 1928, followed by a more accessible model from John Haven Emerson in 1931. This device was a large, horizontal, airtight cylinder that enclosed the patient’s entire body, with only the head exposed through a sealed collar.
The Iron Lung became synonymous with the polio epidemics of the mid-20th century. Poliovirus often caused paralysis of the respiratory muscles, leaving patients unable to breathe on their own. The tank respirator provided continuous, life-saving support for thousands of people during this period.
The machine worked by mechanically varying the air pressure inside the sealed cylinder. By periodically lowering the pressure inside the chamber, it forced the patient’s chest to expand, drawing air into the lungs. Though cumbersome and eventually replaced by other technologies, the Iron Lung played an important role in keeping patients with respiratory muscle paralysis alive before the widespread availability of the polio vaccine.
Differences from Positive Pressure Ventilation
Negative pressure ventilation (NPV) differs substantially from the method used by most modern ventilators, known as positive pressure ventilation (PPV). PPV devices force air directly into the patient’s lungs through an artificial airway, such as an endotracheal tube or a tight-fitting mask. This inward push of air inflates the lungs, which is the opposite of the chest-pulling mechanism of NPV.
A key physiological difference is the effect on the cardiovascular system. PPV increases the pressure within the chest cavity, which can impede the return of blood to the heart, potentially affecting circulation. Because NPV mimics natural breathing, it tends to have a more physiological effect on chest pressure, which is more favorable for blood flow.
PPV offers greater control over the volume and flow of air delivered, which is necessary for managing critically ill patients. NPV devices, particularly the full-body Iron Lung, made patient access and nursing care difficult and carried a risk of causing upper airway obstruction. Furthermore, the high internal pressures of PPV can lead to barotrauma, a physical injury to the lungs, a complication less common with the external mechanism of NPV.
Current Specialized Clinical Uses
While positive pressure ventilation largely supplanted the Iron Lung, modern negative pressure devices still have specialized applications. Today’s NPV technology is much smaller, utilizing devices like the chest cuirass or jacket ventilators, which only enclose the torso. These smaller, portable devices are often used for chronic respiratory failure, where non-invasive, intermittent support is preferred.
Patients with neuromuscular diseases, such as muscular dystrophy or spinal cord injuries, often benefit from NPV at home, particularly for night-time breathing assistance. The external, non-invasive nature allows the patient to speak, swallow, and avoid the need for a permanent artificial airway. NPV has also been explored in cases of acute-on-chronic respiratory failure, where it can provide rest for fatigued respiratory muscles.
The primary advantage is that NPV supports breathing without requiring intubation, which reduces the need for heavy sedation and lowers the risks associated with an invasive airway. This approach allows for a more comfortable, physiological ventilation pattern, making it a viable option for long-term support outside of an intensive care setting.