When we breathe in, the outward movement of the chest increases the volume of the lungs and the pressure in them falls below that of the outside world. Therefore, air is drawn in automatically. When we breathe out, some air exits because of the normal elastic recoil of the lungs, but we also force air out by using the muscles of the chest and the DIAPHRAGM. Replicating this artificially involves using a device to produce intermittent positive or negative pressure ventilation as described below.
The simplest form of IPPV is mouth-to-mouth resuscitation (see APPENDIX 1: BASIC FIRST AID), where an individual blows his or her own expired gases into the lungs of a non-breathing person via the mouth or nose. Similarly gas may be blown into the lungs via a face mask or down an endotracheal tube (see ENDOTRACHEAL INTUBATION) and a self-inflating bag or an anaesthetic circuit containing a bag which is inflated by the flow of fresh gas from an anaesthetic machine, gas cylinder, or piped supply. In all these examples expiration is passive.
For more prolonged artificial ventilation it is usual to use a specially designed machine or VENTILATOR to perform the task. The ventilators used in operating theatres when patients are anaesthetised and paralysed are relatively simple devices, usually consisting of bellows which fill with fresh gas and are then mechanically emptied (by means of a weight, piston, or compressed gas) via a circuit or tubes attached to an endotracheal tube into the patient's lungs. Adjustments can be made to the volume of fresh gas given with each breath and to the length of inspiration and expiration. Expiration is usually passive, back to the atmosphere of the room via a scavenging system to avoid pollution.
In intensive-care units, where patients are not usually paralysed, the ventilators are more complex. They have electronic controls which allow the user to programme a variety of pressure waveforms for inspiration and expiration. There are also programmes that allow the patient to breathe between ventilated breaths or to trigger ventilated breaths, or inhibit ventilation when the patient is breathing.
Indications for artificial ventilation are when patients are unable to achieve adequate respiratory function even if they can still breathe on their own. This may be due to injury or disease of the central nervous, cardiovascular, or respiratory systems, or to drug overdose. Artificial ventilation is performed to allow time for healing and recovery. Sometimes the patient is able to breathe but it is considered advisable to control ventilation – for example, in severe head injury. Some operations require the patient to be paralysed for better or safer surgical access and this may require ventilation. With lung operations or very unwell patients, ventilation is also indicated.
Artificial ventilation usually bypasses the physiological mechanisms for humidification of inspired air, so care must be taken to humidify inspired gases. It is important to monitor the efficacy of ventilation – for example, by measuring BLOOD GASES, PULSE OXIMETRY, end tidal carbon dioxide (the amount identified at the end of an expiration of air), and airways pressures.
Artificial ventilation is not without its hazards. The use of positive pressure raises the mean intrathoracic pressure. This can decrease venous return to the heart and cause a fall in CARDIAC OUTPUT and blood pressure. Positive-pressure ventilation may also cause PNEUMOTHORAX, but this is rare. While patients are ventilated, they are unable to breathe and so accidental disconnection from the ventilator may cause HYPOXIA and death.
is a relatively modern form of ventilation which utilises very small tidal volumes (see LUNGS) from a high-pressure source at high frequencies (20–200/min). First developed by physiologists to produce low, stable intrathoracic pressures whilst studying CAROTID BODY reflexes, it is sometimes now used in intensive-therapy units for patients who do not achieve adequate gas exchange with conventional ventilation. Its advantages are lower intrathoracic pressures (and therefore less risk of pneumothorax and impaired venous return) and better gas mixing within the lungs.