In most NPVs, the negative pressure is applied to the patient's torso, or entire body below the neck, to cause their chest to expand, expanding their lungs, drawing air into the patient's lungs through their airway, assisting inhalation. When negative pressure is released, the chest naturally contracts, compressing the lungs, causing exhalation. In some cases, positive external pressure may be applied to the torso to further stimulate exhalation. Another form of NPV device is placed at the patient's airway, and alternates negative pressure with positive pressure to pump air into their lungs, then suck it back out.
Usage
Negative pressure ventilators, while widely used in the early-to-mid 20th Century, are now largely replaced by Positive-pressure airway ventilators, which force air directly into the patient's airway. However, researchers and clinicians still find some uses for NPVs, owing to their specific advantages.
Advantages and disadvantages
Research and developments in artificial ventilation, both negative-pressure and positive-pressure, result in evolving assessments of the benefits and hazards of negative-pressure ventilators. Different researchers and clinicians have made varying assessments, over time, about the primary positive and negative aspects of NPVs. A sampling includes:
Advantages
Generally, NPVs are best with patients who have neuromuscular diseases, but normal lung compliance. They are effective for various conditions, especially neuromuscular and skeletal disorders, particularly for long-term night-time ventilation. They are effective in patients who have severe respiratory acidosis, impaired consciousness, are unable to tolerate a facial mask, and in children. Continuous external negative pressure ventilation was found in a 2015 study to " oxygenation under physiological conditions", concurrent with lower "airway," "transpulmonary," and "intra-abdominal" pressures, than experienced with continuous positive pressure ventilation, in study of Adult respiratory distress syndrome patients, possibly reducing high ARDS mortality.
Disadvantages
NPVs do not work well if patient's lung compliance is decreased, or their lung resistance is increased. They result in a greater vulnerability of the airway to aspiration such as inhalation of vomit or swallowed liquids, than with intermittent positive pressure ventilation. They exacerbate obstructive sleep apnea. The device is not portable and its installation may be difficult. Patients must sleep in a supine position.
Types of NPVs
There are several types of NPVs, including:
Iron lung, also known as atank ventilator, Drinker tank or Emerson tank;
Cuirass ventilator, also known as a chest shell, turtle shell or tortoise shell;
exovent;
jacket ventilator, also known as a poncho or raincoat ventilator;
Pulmotor.
Iron lung
The iron lung, also known as the tank ventilator, Drinker tank or Emerson tank, was the first common pure-NPV device when it was developed in the 1920s by Drinker, Shaw and Mason. It is a large, sealed horizontal cylinder in which the patient lays, with their head protruding from a sealed opening at one end of the tank. An air pump or flexible diaphragm varies the air pressure inside the tank, in continuous alternations, lowering and raising the air pressure in the cylinder. This causes the patient's chest to rise and fall, stimulating inhalation and exhalation through the patient's nose and mouth.
Cuirass ventilator
The cuirass ventilator, also known as the chest shell, turtle shell or tortoise shell, is a more compact variation of the iron lung, which only encloses the patient's torso, and is sealed around their neck and waist, and depressurized and repressurized by an external pump or portable ventilator.
Exovent
The exovent is a modern device similar to the cuirass ventilator, but developed in 2020, in response to the COVID-19 pandemic.
Jacket ventilator
The jacket ventilator, also known as a poncho or raincoat ventilator, is a lighter version of the iron lung or the cuirass ventilator, constructed of an airtight material arranged over a light metal or plastic frame, or screen, and depressurized and repressurized by a portable ventilator.
Positive-and-negative pressure ventilator
Pulmotor
The Pulmotor is a device developed in the early 1900s which was the forerunner of modern mechanical ventilators. It used pressure from a tank of compressed oxygen to operate a valve system that alternately forced air into and out of a person's airway, using alternating positive and negative air pressure. Although portable, and able to be used by lay persons and non-medical emergency responders, some medical personnel criticized it as dangerous and inefficient.
