|
In medicine, mechanical ventilation is a method to assist or replace spontaneous breathing. Mechanical ventilation can be life-saving and is a mainstay of CPR, intensive care medicine, and anesthesia. History Vesalius was the first person to describe mechanical ventilation by inserting a reed or cane into the trachea of animals and then blowing into this tube. The iron lung was used through much of the middle 20th century, mostly for long-term ventilation. It was refined and used largely as a result of the polio epidemic that struck the world in the 1950s. The machine is effectively a large elongated tank, which encases the patient up to the neck. The neck is sealed with a rubber gasket so that the patient's face (and airway) are exposed to the room air. By means of a pump, the air is withdrawn mechanically to provide inspiration and released to room pressure to allow expiration. Thus the patient inhales room air by a means of negative pressure applied to the patient's thoracic area. There are large portholes for nurse or home assistant access. Patients could remain in these iron lungs for years at a time quite successfully. Some are still in use, notably with the Polio Wing Hospitals in England such as St Thomas' (by Westminster in London) and the John Radcliffe in Oxford. The patients can talk and eat normally, and can see the world through a well-placed series of mirrors. A smaller device known as the cuirass was invented to place onto the chest wall like a giant plumber's suction plunger. It was prone to falling off and caused severe chafing and skin damage and was not used as a long term device. In recent years this device has re-surfaced as a modern polycarbonate shell with multiple seals and a high pressure oscillation pump. It has mostly been effective with children and is still in use in domiciliary ventilation in West England and Wales. Clinical use Mechanical ventilation is used when natural (spontaneous) breathing is absent (apnea) or insufficient. This may be the case in cases of intoxication, cardiac arrest, neurological disease or head trauma, paralysis of the breathing muscles due to Guillain-Barré syndrome, Myasthenia Gravis, spinal cord injury, or the effect of anaesthetic and muscle relaxant drugs. Various pulmonary diseases (i.e. pulmonary edema, COPD) or chest trauma (i.e. ARDS, broken ribs), cardiac disease such as congestive heart failure, sepsis and shock may also necessitate ventilation. Prolonged ventilation can lead to ventilator associated pneumonia. Techniques Invasive ventilation Invasive ventilation refers to ventilation via an artificial airway. This airway may be an endotracheal tube ( a tube passed through the mouth or nose into the trachea) or a tracheostomy tube ( a tube inserted through the neck into the trachea). Invasive ventilation is required when a patient is unable to breathe for themselves or they are unable to protect their own airway. This may be due to a disease process such as pneumonia, trauma to the chest wall, during an operation due to anaesthetic drugs or a neuromuscular problem such as a high spinal injury (see Christopher Reeve). Positive and negative pressure ventilation While the exchange of oxygen and carbon dioxide between the bloodstream and the pulmonary airspace works by diffusion and requires no external work, air must be moved into and out of the lungs to make it available to the gas exchange process. In spontaneous breathing, a negative pressure is created in the pleural cavity by the muscles of respiration, and the resulting gradient between the atmospheric pressure and the pressure inside the thorax generates a flow of air. This is imitated by the negative-pressure ventilation that is employed in iron lungs. An iron lung works by creating an underpressure in a chamber which encloses the body and is sealed at the neck. With the patient's airways open, the resulting gradient to the atmospheric pressure serves to inflate the lungs. All other techniques of ventilation are positive pressure ventilation techniques, meaning that air is forced into the lungs by an external overpressure. Modes of ventilation The ventilation mode describes the characteristics of the inspiratory pressure or flow program and determines if a patient may augment the volume or breathing rate through his or her own efforts. There are several ventilation modes, the most important are: Non-invasive ventilation This refers to all modalities that assist ventilation without the use of an endotracheal tube. Indications Initially this technique was used for patients with COPD to avoid intubation, but recent studies have suggested its merit in facilitating weaning, early extubating and cardiogenic pulmonary edema. Generally speaking patients elligable for this treatment should be: Modalities Several types of interfaces are possible. A nasal, oronasal (i.e. venturi mask), or full-face mask. The oronasal masks frequently cause nasal bridge ulcers which is a clear disadvantage. Connection to ventilators There are various procedures and mechanical devices that provide protection against airway collapse, air leakage, and aspiration: Sources | |||||||
|
| ||||||||
 |
|
| |