Introduction
Mechanical ventilators are life-saving devices in patients with respiratory failure. A negative pressure ventilator is a non-invasive mechanical ventilator. It provides ventilatory support without the use of endotracheal intubation. Negative pressure ventilators were used in the 20th century. They are now widely replaced with positive pressure ventilators. But, negative pressure ventilators are also used in certain conditions because of their benefits.
What Is Negative Pressure Ventilation?
A negative pressure ventilator applies negative air pressure to the entire body below the neck. This initiates the inhalation of air due to the expansion of the lungs. The chest contracts and compresses the lung for exhalation when the negative pressure is released. The negative pressure ventilation causes positive recoil of the thorax. Positive external pressure may be applied to the entire body to stimulate exhalation. One form of negative pressure ventilator placed in the airway uses both positive pressures for inhalation of air and negative pressure for exhalation.
What Is the Difference Between Positive and Negative Pressure Ventilation?
Positive pressure ventilators are invasive or non-invasive devices that deliver air to the lungs. It delivers air through a face mask with a tight seal (non-invasive) or through endotracheal tubes (invasive). It forces air into the lungs, expands, and delivers oxygen to the air sacs in the lungs. Negative pressure ventilators apply negative pressure around the body, cause expansion of the lungs by the contraction of the diaphragm and intercostal muscles, and stimulate the inspiration of air. Negative pressure ventilators are non-invasive devices.
What Are the Types of Negative Pressure Ventilators?
There are various types of negative pressure ventilators which include:
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Iron Lung - It was the first developed negative pressure ventilator. It is also known as a tank ventilator. It is a large, sealed horizontal cylinder. The patient lies down with the head protruding from the opening at one end of the cylinder. An air pump varies the air pressure inside the cylinder. It lowers and raises the air pressure in continuous alterations and causes the patient’s chest to rise and fall. This stimulates inhalation and exhalation through the patient’s nose and mouth, which are outside the cylinder.
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Cuirass Ventilator - It is also known as chest shell, turtle shell, or tortoiseshell ventilator. It consists of a plastic model of the trunk, the edges are padded airtight, and the cuirass is attached to the patient with a back strap. They are easy to wear and suitable for home use. It has a variety of negative pressure pumps that provide preset pressure within the cuirass.
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Exovent - It is a newly developed ventilator similar to cuirass in response to COVID (coronavirus disease). Exovent is larger than cuirass but smaller than an iron lung ventilator. It has its own internal mattress. The patient lies on it, and the cover is placed over the entire body and arms and fitted with neck and hip seals.
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Jacket Ventilator - It is also known as a poncho or raincoat ventilator. It has an inner metal or plastic framework covered with an airtight anorak-like jacket that seals around the neck, arms, and thighs. The air within the jacket is intermittently evacuated to provide ventilation action.
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Pulmotor - It is a positive and negative pressure ventilator. It uses pressure from a tank of compressed oxygen and operates a valve system to alternatively force air into the patient’s airway using alternative positive and negative pressure. But it is considered dangerous by some medical professionals due to the risk of barotrauma (damage to the body tissues due to the continuous changes in the air pressure) and vomiting.
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Hayek Oscillator - It is a versatile ventilator with a lightweight, flexible, airtight chest enclosure with a soft seal around the chest and abdomen. It rests on a backplate or a flexible cushion. This ventilator is attached to a piston pump providing a baseline and high-frequency negative pressure. It behaves as a conventional ventilator at 2 hertz frequency. Above 2 hertz, it behaves as a high-frequency ventilator and delivers oxygen by diffusion.
What Are the Indications for Negative Pressure Ventilators?
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They are used in patients with neuromuscular disorders with normal lung compliance (ability to expand and contract).
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They are used for long-term nighttime ventilation in neuromuscular and skeletal disorders.
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They are used in patients with respiratory acidosis, impaired consciousness, and if patients are unable to wear a face mask due to facial deformity, claustrophobia, or airway secretions.
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It is used in pediatric intensive therapy units or neonatal units for continuous negative pressure ventilation or intermittent ventilation to facilitate weaning from positive pressure ventilation.
What Are the Disadvantages?
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Negative pressure ventilators do not work if lung compliance is decreased.
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They increase the risk of aspiration, such as vomit or swallowing liquids.
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They worsen obstructive sleep apnea.
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The patient should always lie in the supine position.
What Are the Effects of Negative Pressure Ventilation?
Cuirass ventilator improves cardiac output by lowering mean intrathoracic pressure and maximizing the venous return to the heart. There is a decrease in cerebral blood flow and a decrease in oxygenated and deoxygenated hemoglobin compared with positive pressure ventilation. This suggests increased venous drainage from the brain during negative pressure ventilation. There is a decrease in the mechanical and diaphragmatic activity of the diaphragm (a respiratory muscle).
What Are the Complications of Negative Pressure Ventilation?
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Extra thoracic upper airway obstruction is due to the suppression of inspiratory action of respiratory muscles that makes it floppier and causes passive closure.
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Venous obstruction at the contact points of the skin.
Conclusion
Positive pressure ventilation has the advantage of convenience, comfort, and portability compared with a negative pressure ventilator. But negative pressure ventilators have produced an alternative strategy for intensive care problems. The negative pressure ventilation at a high frequency that aids gas exchange by diffusion is beneficial. Long-term ventilation should depend on the patient’s needs. Improvements in the techniques of negative pressure ventilation simultaneously with other non-invasive ventilatory methods provide choices in ventilation. The development of new forms of negative pressure ventilators overcame the drawbacks of their predecessors and had a revival in ventilation for children.
