Conventional mechanical ventilation via endotracheal intubation or tracheostomy in the treatment of acute respiratory failure (ARF) is a lifesaving procedure. Yet it exposes patients to severe complications, including upper airway trauma and nosocomial pneumonia, and may prolong the length of stay in the intensive care unit (ICU) and hospital because additional time may be necessary for weaning.1–4
Negative-pressure ventilation (NPV) works by exposing the surface of the thorax to subatmospheric pressure during inspiration. This pressure causes thoracic expansion and a decrease in pleural and alveolar pressures, creating a pressure gradient for air to move from the airway opening into the alveoli. When the pressure surrounding the thorax increases and becomes atmospheric or greater, expiration occurs passively owing to the elastic recoil of the respiratory system. The inspiratory changes with NPV, in pleural and alveolar pressures, replicate those during spontaneous breathing. On the contrary, positive-pressure ventilation (PPV) causes an increase in intrathoracic pressures during inspiration (Fig. 16-1).
Airway and intrathoracic pressures during positive-pressure ventilation (upper panel) and during negative-pressure ventilation (bottom panel). Palv, Alveolar pressure; PCW, elastic recoil pressure of the chest wall; PL, elastic recoil pressure of the lung; Paw, airway pressure; Pmus, muscle pressure; Ppl, pleural pressure; Ptank, tank ventilator pressure.
All NPVs have two major components: an airtight, rigid chamber that encloses the rib cage and abdomen and a pump that generates pressure changes in the chamber.5,6
Tank ventilators enclose the body up to the neck. The advantage is that chest wall expansion is not limited by contact with the sides of the chamber, and only one airtight seal is required around the neck. Most modern tank ventilators are constructed of aluminium and plastic and are lighter than previously. The patient’s body rests on a thin mattress, and the head protrudes through a porthole at one end of the ventilator. A head and neck rest is provided in most designs to ensure comfort and to prevent upper airway collapse. Most tank ventilators have windows allowing patient observation and portholes for catheters and monitor leads and where procedures can be performed (Fig. 16-2).7
Microprocessor-based iron lung (Coppa CA 1001, Coppa, Biella, Italy).
Jacket Ventilator (Pulmo-Wrap, Poncho-Wrap)
This ventilator is a windproof, water-permeable nylon parka suspended over a rigid grid that includes the rib cage and abdomen. It allows the application of negative pressure over the anterior portion of the chest wall.8 Airtight seals around the neck, arms, and hips are required to prevent air leakage. The jackets do not restrain expansion of the rib cage and ...