artificial ventilation The body requires a certain volume of air to be inhaled and exhaled to maintain the correct levels of oxygen and carbon dioxide within the tissues. Tissue damage, which leads eventually to death, occurs if the level of oxygen becomes too low or the amount of carbon dioxide becomes too high. The body is therefore critically dependent on breathing to maintain life. When this ‘bellows’ function of the lungs, moving air in and out, becomes inadequate or stops altogether, the patient must receive artificial ventilation to survive. The ‘kiss of life’ — mouth-to-mouth breathing — is a well-known ‘first-aid’ form of artificial ventilation, and is described under resuscitation. Mechanical systems, for more prolonged use, are considered here.

Negative pressure ventilation

The original devices for artificial ventilation developed negative pressure around the patient's chest. One example of these breathing machines was the ‘iron lung’ or tank ventilator which was used widely during the poliomyelitis outbreak in the early 1950s, although an early version had been invented long before. The patient was placed inside the tank and a seal completed an airtight fit around the neck. Air was rhythmically sucked out from the tank and blown back in. Thus alternating negative and positive pressure in the tank caused air to flow in and out of the patient's lungs.

Positive pressure ventilation

Since the mid twentieth century the application of drugs such as curare (muscle relaxants) to cause complete paralysis of the voluntary muscles, including those of breathing, and the development of special rubber tubes which could be placed in the patient's windpipe, has led to the use of a different type of ventilation called intermittent positive pressure ventilation (IPPV). This is much more invasive than the negative pressure form of ventilator just described. The patient is first anaesthetized, and given a muscle relaxant which stops the breathing. The endotracheal tube is then passed down between the vocal cords and into the patient's windpipe (trachea). A machine then delivers gas in rhythmic bursts at positive pressure which forces gas into the lungs. Each time the positive pressure is withdrawn the patient breathes out because of the passive recoil of the lung tissue. This type of artificial ventilation is now the most common type in medical practice. The gas used can be air, or it can be enriched with a higher proportion of oxygen.

Variations on this simple form of artificial ventilation have been introduced which enable the blood to receive higher levels of oxygen, such as applying pressure for a greater part of the breathing cycle (Positive End Expiratory Pressure (PEEP)) or which are designed to make it more comfortable for the patient to be gradually taken off the artificial ventilation and to breathe for themselves (types of supported ventilation).

Use of artificial ventilation

The most frequent use of IPPV occurs during many types of surgical operations where the anaesthetist administers a drug to paralyze the patient's muscles after inducing anaesthesia. An endotracheal tube is put in place and this is connected to a positive pressure ventilator. The use of the paralyzing drug may be necessary to permit the surgeon to undertake the surgery. An example of this is where the surgeon requires to work inside the abdomen: if paralyzing drugs were not used to relax the abdominal muscles, access to the internal organs would be difficult or impossible. All of the drugs used to produce anaesthesia cause some depression of respiration. Where the surgery is expected to be prolonged, patients may receive artificial ventilation to ensure that they receive adequate delivery of oxygen and removal of carbon dioxide. During the operation, the anaesthetist will monitor the condition of the patient to ensure that the correct volume of air is administered. This is done by analyzing the level of carbon dioxide in the patient's expired air and by measuring the level of oxygen in the blood using a sensor clipped to the finger. At the end of the operation, different drugs are injected by the anaesthetist to reverse the paralysis and the patient then starts to breathe spontaneously. Once normal breathing has returned, the endotracheal tube is withdrawn, but oxygen monitoring continues until the patient has recovered fully.

IPPV may also be used when patients cannot breathe sufficiently themselves to maintain delivery of oxygen and removal of carbon dioxide from the body. The use of artificial ventilation of this type may be undertaken as a planned procedure — after major heart or brain surgery, for example, when the patient is maintained in a deeply sedated state to permit special treatment to be provided in the period immediately after the surgery. Other examples are in cases of severe brain damage due to head injury or stroke, which interfere with the normal regulation of breathing by the brain stem, and damage by injury or disease to the parts of the spinal cord involved in breathing movements. When IPPV is required for longer than a week or two, it is usual to establish a tracheostomy: a hole made surgically in the front of the trachea, so that a tube for connection with the ventilator can be inserted below the larynx.

Some patients may become unable to breathe sufficiently after severe infection or other disturbance of lung function. In these situations, IPPV must be provided as an emergency. Similar drugs to those used to induce anaesthesia are given and an endotracheal tube is put in place. Patients receiving artificial ventilation are usually looked after in an intensive care unit where specially trained medical and nursing staff monitor the patient continually to ensure that the ventilator functions correctly and that it is set to deliver the correct amount of air with the necessary proportion of oxygen.

While the majority of patients requiring such assistance receive some form of positive pressure ventilation, there is renewed interest in the use of negative pressure ventilation for some who suffer from chronic breathing difficulties, since this form of ventilation does not require paralysis and an endotracheal tube. This form of respiratory support may only be required overnight and systems have been developed for use in the home.

Gavin Kenny

See also brain stem; breathing; coma; curare; respiration; resuscitation.