n. (in optometry) see astigmatism.
In its course from the base of the skull to the lumbar region, the cord gives o? 31 nerves on each side, each of which arises by an anterior and a posterior root that join before the nerve emerges from the spinal canal. The openings for the nerves formed by notches on the ring of each vertebra have been mentioned under the entry for spinal column. To reach these openings, the upper nerves pass almost directly outwards, whilst lower down their obliquity increases, until below the point where the cord ends there is a sheaf of nerves, known as the cauda equina, running downwards to leave the spinal canal at their appropriate openings.
The cord is a cylinder, about the thickness of the little ?nger. It has two slightly enlarged portions, one in the lower part of the neck, the other at the last dorsal vertebra; and from these thickenings arise the nerves that pass to the upper and lower limbs. The upper four cervical nerves unite to produce the cervical plexus. From this the muscles and skin of the neck are mainly supplied, and the phrenic nerve, which runs down through the lower part of the neck and the chest to innervate the diaphragm, is given o?. The brachial plexus is formed by the union of the lower four cervical and ?rst dorsal nerves. In addition to nerves to some of the muscles in the shoulder region, and others to the skin about the shoulder and inner side of the arm, the plexus gives o? large nerves that proceed down the arm.
The thoracic or dorsal nerves, with the exception of the ?rst, do not form a plexus, but each runs around the chest along the lower margin of the rib to which it corresponds, whilst the lower six extend on to the abdomen.
The lumbar plexus is formed by the upper four lumbar nerves, and its branches are distributed to the lower part of the abdomen, and front and inner side of the thigh.
The sacral plexus is formed by parts of the fourth and ?fth lumbar nerves, and the upper three and part of the fourth sacral nerves. Much of the plexus is collected into the sciatic nerves, the largest in the body, which go to the legs.
The sympathetic system is joined by a pair of small branches given o? from each spinal nerve, close to the spine. This system consists of two parts, ?rst, a pair of cords running down on the side and front of the spine, and containing on each side three ganglia in the neck, and beneath this a ganglion opposite each vertebra. From these two ganglionated cords numerous branches are given o?, and these unite to form the second part – namely, plexuses connected with various internal organs, and provided with numerous large and irregularly placed ganglia. The chief of these plexuses are the cardiac plexus, the solar or epigastric plexus, the diaphragmatic, suprarenal, renal, spermatic, or ovarian, aortic, hypogastric and pelvic plexuses.
The spinal cord, like the brain, is surrounded by three membranes: the dura mater, arachnoid mater, and pia mater, from without inwards. The arrangement of the dura and arachnoid is much looser in the case of the cord than their application to the brain. The dura especially forms a wide tube which is separated from the cord by ?uid and from the vertebral canal by blood vessels and fat, this arrangement protecting the cord from pressure in any ordinary movements of the spine.
In section the spinal cord consists partly of grey, but mainly of white, matter. It di?ers from the upper parts of the brain in that the white matter (largely) in the cord is arranged on the surface, surrounding a mass of grey matter (largely neurons – see NEURON(E)), while in the brain the grey matter is super?cial. The arrangement of grey matter, as seen in a section across the cord, resembles the letter H. Each half of the cord possesses an anterior and a posterior horn, the masses of the two sides being joined by a wide posterior grey commissure. In the middle of this commissure lies the central canal of the cord, a small tube which is the continuation of the ventricles in the brain. The horns of grey matter reach almost to the surface of the cord, and from their ends arise the roots of the nerves that leave the cord. The white matter is divided almost completely into two halves by a posterior septum and anterior ?ssure and is further split into anterior, lateral and posterior columns.
Functions The cord is, in part, a receiver and originator of nerve impulses, and in part a conductor of such impulses along ?bres which pass through it to and from the brain. The cord contains centres able to receive sensory impressions and initiate motor instructions. These control blood-vessel diameters, eye-pupil size, sweating and breathing. The brain exerts an overall controlling in?uence and, before any incoming sensation can affect consciousness, it is usually ‘?ltered’ through the brain.
Many of these centres act autonomously. Other cells of the cord are capable of originating movements in response to impulses brought direct to them through sensory nerves, such activity being known as REFLEX ACTION. (For a fuller description of the activities of the spinal cord, see NEURON(E) – Re?ex action.)
The posterior column of the cord consists of the fasciculus gracilis and the fasciculus cuneatus, both conveying sensory impressions upwards. The lateral column contains the ventral and the dorsal spino-cerebellar tracts passing to the cerebellum, the crossed pyramidal tract of motor ?bres carrying outgoing impulses downwards together with the rubro-spinal, the spino-thalamic, the spino-tectal, and the postero-lateral tracts. And, ?nally, the anterior column contains the direct pyramidal tract of motor ?bres and an anterior mixed zone. The pyramidal tracts have the best-known course. Starting from cells near the central sulcus on the brain, the motor nerve-?bres run down through the internal capsule, pons, and medulla, in the lower part of which many of those coming from the right side of the brain cross to the left side of the spinal cord, and vice versa. Thence the ?bres run down in the crossed pyramidal tract to end beside nerve-cells in the anterior horn of the cord. From these nerve-cells other ?bres pass outwards to form the nerves that go direct to the muscles. Thus the motor nerve path from brain to muscle is divided into two sections of neurons, of which the upper exerts a controlling in?uence upon the lower, while the lower is concerned in maintaining the muscle in a state of health and good nutrition, and in directly calling it into action. (See also NERVE; NERVOUS SYSTEM.)... spinal cord
In 1819 the French physician, Laennec, introduced the method of auscultation by means of the STETHOSCOPE. Initially a wooden cylinder, the stethoscope has evolved into a binaural instrument consisting of a small expanded chest-piece and two ?exible tubes, the ends of which ?t into the ears of the observer. Various modi?cations of the binaural stethoscope have been introduced.
Conditions affecting the lungs can often be recognised by means of auscultation and the stethoscope. The same is true for the heart, in which disease can, by auscultation, often be identi?ed with striking accuracy. But auscultation is also helpful in the investigation of aneurysms (see ANEURYSM) and certain diseases of the OESOPHAGUS and STOMACH. The stethoscope is also a valuable aid in the detection of some forms of uterine tumours, especially in the diagnosis of pregnancy.... auscultation
Measuring the partial pressure of the gas by taking blood for blood gas estimation provides information on the adequacy of breathing. A high partial pressure may indicate impending or actual respiratory failure.... carbon dioxide (co2)
Astigmatism (See ASTIGMATISM.)
Blepharitis A chronic in?ammation of the lid margins. SEBORRHOEA and staphylococcal infection are likely contributors. The eyes are typically intermittently red, sore and gritty over months or years. Treatment is di?cult and may fail. Measures to reduce debris on the lid margins, intermittent courses of topical antibiotics, steroids or systemic antibiotics may help the sufferer.
Blepharospasm Involuntary closure of the eye. This may accompany irritation but may also occur without an apparent cause. It may be severe enough to interfere with vision. Treatment involves removing the source of irritation, if present. Severe and persistent cases may respond to injection of Botulinum toxin into the orbicularis muscle.
Cataract A term used to describe any opacity in the lens of the eye, from the smallest spot to total opaqueness. The prevalence of cataracts is age-related: 65 per cent of individuals in their sixth decade have some degree of lens opacity, while all those over 80 are affected. Cataracts are the most important cause of blindness worldwide. Symptoms will depend on whether one or both eyes are affected, as well as the position and density of the cataract(s). If only one eye is developing a cataract, it may be some time before the person notices it, though reading may be affected. Some people with cataracts become shortsighted, which in older people may paradoxically ‘improve’ their ability to read. Bright light may worsen vision in those with cataracts.
The extent of visual impairment depends on the nature of the cataracts, and the ?rst symptoms noticed by patients include di?culty in recognising faces and in reading, while problems watching television or driving, especially at night, are pointers to the condition. Cataracts are common but are not the only cause of deteriorating vision. Patients with cataracts should be able to point to the position of a light and their pupillary reactions should be normal. If a bright light is shone on the eye, the lens may appear brown or, in advanced cataracts, white (see diagram).
While increasing age is the commonest cause of cataract in the UK, patients with DIABETES MELLITUS, UVEITIS and a history of injury to the eye can also develop the disorder. Prolonged STEROID treatment can result in cataracts. Children may develop cataracts, and in them the condition is much more serious as vision may be irreversibly impaired because development of the brain’s ability to interpret visual signals is hindered. This may happen even if the cataracts are removed, so early referral for treatment is essential. One of the physical signs which doctors look for when they suspect cataract in adults as well as in children is the ‘red re?ex’. This is observable when an ophthalmoscopic examination of the eye is made (see OPHTHALMOSCOPE). Identi?cation of this red re?ex (a re?ection of light from the red surface of the retina –see EYE) is a key diagnostic sign in children, especially young ones.
There is no e?ective medical treatment for established cataracts. Surgery is necessary and the decision when to operate depends mainly on how the cataract(s) affect(s) the patient’s vision. Nowadays, surgery can be done at any time with limited risk. Most patients with a vision of 6/18 – 6/10 is the minimum standard for driving – or worse in both eyes should
E
bene?t from surgery, though elderly people may tolerate visual acuity of 6/18 or worse, so surgery must be tailored to the individual’s needs. Younger people with a cataract will have more demanding visual requirements and so may opt for an ‘earlier’ operation. Most cataract surgery in Britain is now done under local anaesthetic and uses the ‘phaco-emulsi?cation’ method. A small hole is made in the anterior capsule of the lens after which the hard lens nucleus is liqui?ed ultrasonically. A replacement lens is inserted into the empty lens bag (see diagram). Patients usually return to their normal activities within a few days of the operation. A recent development under test in the USA for children requiring cataract operations is an intra-ocular ?exible implant whose magnifying power can be altered as a child develops, thus precluding the need for a series of corrective operations as happens now.
Chalazion A ?rm lump in the eyelid relating to a blocked meibomian gland, felt deep within the lid. Treatment is not always necessary; a proportion spontaneously resolve. There can be associated infection when the lid becomes red and painful requiring antibiotic treatment. If troublesome, the chalazion can be incised under local anaesthetic.
Conjunctivitis In?ammation of the conjunctiva (see EYE) which may affect one or both eyes. Typically the eye is red, itchy, sticky and gritty but is not usually painful. Redness is not always present. Conjunctivitis can occasionally be painful, particularly if there is an associated keratitis (see below) – for example, adenovirus infection, herpetic infection.
The cause can be infective (bacteria, viruses or CHLAMYDIA), chemical (e.g. acids, alkalis) or allergic (e.g. in hay fever). Conjunctivitis may also be caused by contact lenses, and preservatives or even the drugs in eye drops may cause conjunctival in?ammation. Conjunctivitis may addtionally occur in association with other illnesses – for example, upper-respiratory-tract infection, Stevens-Johnson syndrome (see ERYTHEMA – erythema multiforme) or REITER’S SYNDROME. The treatment depends on the cause. In many patients acute conjunctivitis is self-limiting.
Dacryocystitis In?ammation of the lacrimal sac. This may present acutely as a red, painful swelling between the nose and the lower lid. An abscess may form which points through the skin and which may need to be drained by incision. Systemic antibiotics may be necessary. Chronic dacryocystitis may occur with recurrent discharge from the openings of the tear ducts and recurrent swelling of the lacrimal sac. Obstruction of the tear duct is accompanied by watering of the eye. If the symptoms are troublesome, the patient’s tear passageways need to be surgically reconstructed.
Ectropion The lid margin is everted – usually the lower lid. Ectropion is most commonly associated with ageing, when the tissues of the lid become lax. It can also be caused by shortening of the skin of the lids such as happens with scarring or mechanical factors – for example, a tumour pulling the skin of the lower lid downwards. Ectropion tends to cause watering and an unsightly appearance. The treatment is surgical.
Entropion The lid margin is inverted – usually the lower lid. Entropion is most commonly associated with ageing, when the tissues of the lid become lax. It can also be caused by shortening of the inner surfaces of the lids due to scarring – for example, TRACHOMA or chemical burns. The inwardly directed lashes cause irritation and can abrade the cornea. The treatment is surgical.
Episcleritis In?ammation of the EPISCLERA. There is usually no apparent cause. The in?ammation may be di?use or localised and may affect one or both eyes. It sometimes recurs. The affected area is usually red and moderately painful. Episcleritis is generally not thought to be as painful as scleritis and does not lead to the same complications. Treatment is generally directed at improving the patient’s symptoms. The in?ammation may respond to NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS) or topical CORTICOSTEROIDS.
Errors of refraction (Ametropia.) These will occur when the focusing power of the lens and cornea does not match the length of the eye, so that rays of light parallel to the visual axis are not focused at the fovea centralis (see EYE). There are three types of refractive error: HYPERMETROPIA or long-sightedness. The refractive power of the eye is too weak, or the eye is too short so that rays of light are brought to a focus at a point behind the retina. Longsighted people can see well in the distance but generally require glasses with convex lenses for reading. Uncorrected long sight can lead to headaches and intermittent blurring of vision following prolonged close work (i.e. eye strain). As a result of ageing, the eye becomes gradually long-sighted, resulting in many people needing reading glasses in later life: this normal process is known as presbyopia. A particular form of long-sightedness occurs after cataract extraction (see above). MYOPIA(Short sight or near sight.) Rays of light are brought to a focus in front of the retina because the refractive power of the eye is too great or the eye is too short. Short-sighted people can see close to but need spectacles with concave lenses in order to see in the distance. ASTIGMATISMThe refractive power of the eye is not the same in each meridian. Some rays of light may be focused in front of the retina while others are focused on or behind the retina. Astigmatism can accompany hypermetropia or myopia. It may be corrected by cylindrical lenses: these consist of a slice from the side of a cylinder (i.e. curved in one meridian and ?at in the meridian at right-angles to it).
Keratitis In?ammation of the cornea in response to a variety of insults – viral, bacterial, chemical, radiation, or mechanical trauma. Keratitis may be super?cial or involve the deeper layers, the latter being generally more serious. The eye is usually red, painful and photophobic. Treatment is directed at the cause.
Nystagmus Involuntary rhythmic oscillation of one or both eyes. There are several causes including nervous disorders, vestibular disorders, eye disorders and certain drugs including alcohol.
Ophthalmia In?ammation of the eye, especially the conjunctiva (see conjunctivitis, above). Ophthalmia neonatorum is a type of conjunctivitis that occurs in newborn babies. They catch the disease when passing through an infected birth canal during their mother’s labour (see PREGNANCY AND LABOUR). CHLAMYDIA and GONORRHOEA are the two most common infections. Treatment is e?ective with antibiotics: untreated, the infection may cause permanent eye damage.
Pinguecula A benign degenerative change in the connective tissue at the nasal or temporal limbus (see EYE). This is visible as a small, ?attened, yellow-white lump adjacent to the cornea.
Pterygium Overgrowth of the conjunctival tissues at the limbus on to the cornea (see EYE). This usually occurs on the nasal side and is associated with exposure to sunlight. The pterygium is surgically removed for cosmetic reasons or if it is thought to be advancing towards the visual axis.
Ptosis Drooping of the upper lid. May occur because of a defect in the muscles which raise the lid (levator complex), sometimes the result of ageing or trauma. Other causes include HORNER’S SYNDROME, third cranial nerve PALSY, MYASTHENIA GRAVIS, and DYSTROPHIA MYOTONICA. The cause needs to be determined and treated if possible. The treatment for a severely drooping lid is surgical, but other measures can be used to prop up the lid with varying success.
Retina, disorders of The retina can be damaged by disease that affects the retina alone, or by diseases affecting the whole body.
Retinopathy is a term used to denote an abnormality of the retina without specifying a cause. Some retinal disorders are discussed below. DIABETIC RETINOPATHY Retinal disease occurring in patients with DIABETES MELLITUS. It is the commonest cause of blind registration in Great Britain of people between the ages of 20 and 65. Diabetic retinopathy can be divided into several types. The two main causes of blindness are those that follow: ?rst, development of new blood vessels from the retina, with resultant complications and, second, those following ‘water logging’ (oedema) of the macula. Treatment is by maintaining rigid control of blood-sugar levels combined with laser treatment for certain forms of the disease – in particular to get rid of new blood vessels. HYPERTENSIVE RETINOPATHY Retinal disease secondary to the development of high blood pressure. Treatment involves control of the blood pressure (see HYPERTENSION). SICKLE CELL RETINOPATHY People with sickle cell disease (see under ANAEYIA) can develop a number of retinal problems including new blood vessels from the retina. RETINOPATHY OF PREMATURITY (ROP) Previously called retrolental ?broplasia (RLF), this is a disorder affecting low-birth-weight premature babies exposed to oxygen. Essentially, new blood vessels develop which cause extensive traction on the retina with resultant retinal detachment and poor vision. RETINAL ARTERY OCCLUSION; RETINAL VEIN OCCLUSION These result in damage to those areas of retina supplied by the affected blood vessel: the blood vessels become blocked. If the peripheral retina is damaged the patient may be completely symptom-free, although areas of blindness may be detected on examination of ?eld of vision. If the macula is involved, visual loss may be sudden, profound and permanent. There is no e?ective treatment once visual loss has occurred. SENILE MACULAR DEGENERATION (‘Senile’ indicates age of onset and has no bearing on mental state.) This is the leading cause of blindness in the elderly in the western world. The average age of onset is 65 years. Patients initially notice a disturbance of their vision which gradually progresses over months or years. They lose the ability to recognise ?ne detail; for example, they cannot read ?ne print, sew, or recognise people’s faces. They always retain the ability to recognise large objects such as doors and chairs, and are therefore able to get around and about reasonably well. There is no e?ective treatment in the majority of cases. RETINITIS PIGMENTOSAA group of rare, inherited diseases characterised by the development of night blindness and tunnel vision. Symptoms start in childhood and are progressive. Many patients retain good visual acuity, although their peripheral vision is limited. One of the characteristic ?ndings on examination is collections of pigment in the retina which have a characteristic shape and are therefore known as ‘bone spicules’. There is no e?ective treatment. RETINAL DETACHMENTusually occurs due to the development of a hole in the retina. Holes can occur as a result of degeneration of the retina, traction on the retina by the vitreous, or injury. Fluid from the vitreous passes through the hole causing a split within the retina; the inner part of the retina becomes detached from the outer part, the latter remaining in contact with the choroid. Detached retina loses its ability to detect light, with consequent impairment of vision. Retinal detachments are more common in the short-sighted, in the elderly or following cataract extraction. Symptoms include spots before the eyes (?oaters), ?ashing lights and a shadow over the eye with progressive loss of vision. Treatment by laser is very e?ective if caught early, at the stage when a hole has developed in the retina but before the retina has become detached. The edges of the hole can be ‘spot welded’ to the underlying choroid. Once a detachment has occurred, laser therapy cannot be used; the retina has to be repositioned. This is usually done by indenting the wall of the eye from the outside to meet the retina, then making the retina stick to the wall of the eye by inducing in?ammation in the wall (by freezing it). The outcome of surgery depends largely on the extent of the detachment and its duration. Complicated forms of detachment can occur due to diabetic eye disease, injury or tumour. Each requires a specialised form of treatment.
Scleritis In?ammation of the sclera (see EYE). This can be localised or di?use, can affect the anterior or the posterior sclera, and can affect one or both eyes. The affected eye is usually red and painful. Scleritis can lead to thinning and even perforation of the sclera, sometimes with little sign of in?ammation. Posterior scleritis in particular may cause impaired vision and require emergency treatment. There is often no apparent cause, but there are some associated conditions – for example, RHEUMATOID ARTHRITIS, GOUT, and an autoimmune disease affecting the nasal passages and lungs called Wegener’s granulomatosis. Treatment depends on severity but may involve NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS), topical CORTICOSTEROIDS or systemic immunosuppressive drugs.
Stye Infection of a lash follicle. This presents as a painful small red lump at the lid margin. It often resolves spontaneously but may require antibiotic treatment if it persists or recurs.
Sub-conjunctival haemorrhage Haemorrhage between the conjunctiva and the underlying episclera. It is painless. There is usually no apparent cause and it resolves spontaneously.
Trichiasis Inward misdirection of the lashes. Trichiasis occurs due to in?ammation of or trauma to the lid margin. Treatment involves removal of the patient’s lashes. Regrowth may be prevented by electrolysis, by CRYOTHERAPY to the lid margin, or by surgery.
For the subject of arti?cial eyes, see under PROSTHESIS; also GLAUCOMA, SQUINT and UVEITIS.... eye, disorders of
Aerosols Asthmatic patients (see ASTHMA) ?nd aerosol devices to be of value in controlling their attacks. They provide an e?ective and convenient way of applying drugs directly to the bronchi, thus reducing the risks of unwanted effects accompanying SYSTEMIC therapy. BRONCHODILATOR aerosols contain either a beta-sympathomimetic agent or ipratropium bromide, which is an ANTICHOLINERGIC drug.
ISOPRENALINE was the ?rst compound to be widely used as an aerosol. It did however stimulate beta1 receptors in the heart as well as beta2 receptors in the bronchi, and so produced palpitations and even dangerous cardiac arrhythmias. Newer beta-adrenoceptor agonists are speci?c for the beta2 receptors and thus have a greater safety margin. They include SALBUTAMOL, TERBUTALINE, rimiterol, fenoterol and reproterol. Unwanted effects such as palpitations, tremor and restlessness are uncommon with these, more speci?c preparations. In patients who get insu?cient relief from the beta-adrenoreceptor agonist, the drug ipratropium bromide is worth adding. Salmeterol is a longer-acting choice for twice-daily administration: it is not intended for the relief of acute attacks, for which shorter-acting beta2 stimulants such as salbutamol should be used. Salmeterol should be added to existing corticosteroid therapy (see CORTICOSTEROIDS), rather than replacing it.
Patients must be taught carefully and observed while using their inhalers. It is important for them to realise that if the aerosol no longer gives more than slight transient relief, they should not increase the dose but seek medical help.... inhalants
(See also hyperbaric oxygen treatment.)... oxygen therapy
For more prolonged arti?cial 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.They often consist of bellows which ?ll with fresh gas and which 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 arti?cial 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. Arti?cial 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.
Arti?cial ventilation usually bypasses the physiological mechanisms for humidi?cation of inspired air, so care must be taken to humidify inspired gases. It is important to monitor the e?cacy of ventilation – for example, by using blood gas measurement, pulse oximetry, and tidal carbon dioxide, and airways pressures.
Arti?cial 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.
Negative-pressure ventilation is seldom used nowadays. The chest or whole body, apart from the head, is placed inside an airtight box. A vacuum lowers the pressure within the box, causing the chest to expand. Air is drawn into the lungs through the mouth and nose. At the end of inspiration the vacuum is stopped, the pressure in the box returns to atmospheric, and the patient exhales passively. This is the principle of the ‘iron lung’ which saved many lives during the polio epidemics of the 1950s. These machines are cumbersome and make access to the patient di?cult. In addition, complex manipulation of ventilation is impossible.
Jet ventilation 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 re?exes, 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.... intermittent positive pressure (ipp)
Oxygen is essential for life. It is absorbed via the lungs (see RESPIRATION) and is transported by HAEMOGLOBIN within the ERYTHROCYTES to the tissues. Within the individual cell it is involved in the production of adenosine triphosphate (ATP), a compound that stores chemical energy for muscle cells, by the oxidative metabolism of fats and carbohydrates. HYPOXIA causes anaerobic metabolism with a resulting build-up in LACTIC ACID, the result of muscle cell activity. If severe enough, the lack of ATP causes a breakdown in cellular function and the death of the individual.
When hypoxia occurs, it may be corrected by giving supplemental oxygen. This is usually given via a face mask or nasal prongs or, in severe cases, during ARTIFICIAL VENTILATION OF THE LUNGS. Some indications for oxygen therapy are high altitude, ventilatory failure, heart failure, ANAEMIA, PULMONARY HYPERTENSION, CARBON MONOXIDE (CO) poisoning, anaesthesia and post-operative recovery. In some conditions – e.g. severe infections with anaerobic bacteria and CO poisoning – hyperbaric oxygen therapy has been used.... oxygen
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