Ventilator Support

1. Negative pressure ventilators, such as the iron lung and Porta-lung, enclose the body below the neck in a tank within which negative pressure is generated. Passive inspiration is induced by the pull on the chest wall. Expiration, commonly, occurs passively as well, although it can be augmented by external positive pressure. There are also more portable versions of these ventilators, such as the pneumowrap (pulmowrap), tortoise shell-shaped cuirass, and the plastic pneumowrap, also known as the raincoat or poncho ventilator.
   • Most effective in patients with normal lungs and normal chest compliance.
   • Not used for patients in a wheelchair or with upper airway obstruction or significant bulbar weakness.

2. Positive pressure ventilators consist of two basic types: pressure regulated and volume regulated. In pressure regulated ventilators, air volume changes if resistance to air flow changes, such as in leaks in the ventilator or leaks around the tracheostomy tube. In volume regulated ventilators, a specific volume of air is delivered with each cycle, independent of leaks or any other cause of change in resistance to the air flow. Advantages of positive pressure ventilators over negative pressure ventilators include:
   • Air volume, humidity, flow rates, and fraction of inspired oxygen can be more accurately controlled
   • Less bulky, more portable, and battery operated - can be used for patients in a wheelchair
   • Airway protection is provided when used in combination with invasive means, such as endotracheal or tracheostomy tubes - can be used in patients with bulbar involvement
   • More effective in patients with lung pathology (where ventilatory impairment is due to intrinsic pulmonary disease), with swallowing dysfunction that predisposes to aspiration, and patients in chronic care facilities because of lack of family support or care providers
   • Greater access to the patient for medical and nursing care
   • Support noninvasive ventilatory assistance, such as mouth intermittent positive pressure ventilation (IPPV)

3. Noninvasive IPPV (Intermittent Positive Pressure Ventilation) can achieve satisfactory alveolar ventilation for cooperative patients with compliant respiratory systems. In patients with chronic ventilatory insufficiency secondary to neuromuscular conditions, noninvasive IVVP can improve verbal communication and appearance and lower the incidence of tracheal problems, such as tracheomalacia and stenosis, hemoptysis, and recurrent infections. This therapeutic technique can prevent or treat atelectasis when continued until the patient's predicted inspiratory capacity is achieved. Noninvasive IPPV devices, such as nasal masks, nasal pillows, and oronasal interfaces, are most commonly used for nocturnal ventilatory assistance. Daytime ventilatory assistance is commonly provided by mouthpiece IPPV or IAPVs (Intermittent Abdominal Presssure Ventilators) (Viroslav, et al 1996).

4. Diaphragmatic assist devices, which assist ventilation by increasing movement of the diaphragm, include:
   • Pneumobelt, also known as the intermittent abdominal pressure ventilator (IAPV) - a motorized, inflatable bladder that is secured over the abdomen. The bladder alternately expands and contracts (as air is forced into and released from it), which intermittently compresses the abdomen, causing the diaphragm to move upwards and augment expiration. Since inhalation with the pneumobelt is largely passive and dependent on gravity, the device is only useful in the sitting or standing position as an alternative to tracheostomy-dependent means of ventilatory support
   • Rocking bed, a motorized bed that moves continuously in the longitudinal plane. When the head is higher than the rest of the body, the diaphragm is pulled down and inhalation is assisted. When the head is lower, the abdominal contents are pulled down and expiration is assisted.