V
Vacuum insulated evaporator (VIE). Container for storage of liquid O2 and maintenance of piped gas supply. An outer carbon steel shell is separated by a vacuum from an inner stainless steel shell, which contains O2. The inner temperature varies between −160 and −180°C, at a pressure of 7–10 bar. Gaseous O2 is withdrawn and heated to ambient temperature (and thus expanded) as required (Fig. 161); a pressure regulator distal to the superheater prevents pipeline pressure from exceeding 4.1 bar. If pressure within the container falls due to high demand, liquid O2 may be withdrawn, vaporised in an evaporator and returned to the system, restoring working pressure. If passage of heat across the insulation causes vaporisation of liquid O2 and a rise in pressure, gas is allowed to escape through a safety valve. The contents are indicated by a weighing device incorporated into the chamber’s supports.
Howells RS (1980). Anaesthesia; 35: 676–98
Fig. 161 Vacuum insulated evaporator
Vagus nerve. Tenth cranial nerve. Arises in the medulla from the:
dorsal nucleus of the vagus (parasympathetic).
nucleus ambiguus (motor fibres to laryngeal, pharyngeal and palatal muscles).
nucleus of the tractus solitarius (sensory fibres from the larynx, pharynx, GIT, heart and lungs, including taste fibres from the valleculae).
Leaves the medulla between the olive and inferior cerebellar peduncle, and passes through the jugular foramen of the skull. Descends in the neck within the carotid sheath between the internal jugular vein and internal/common carotid arteries (see Fig. 113; Neck, cross-sectional anatomy, and Fig. 104a; Mediastinum). Passes behind the root of the lung to form the pulmonary plexus, then on to the oesophagus to form the oesophageal plexus with the vagus from the other side. Both pass through the oesophageal opening of the diaphragm to supply the abdominal contents and GIT as far as the splenic flexure (see Fig. 21; Autonomic nervous system).
to the external auditory meatus and tympanic membrane.
to muscles of the pharynx and soft palate.
The vagi form a major part of the parasympathetic nervous system. Vagal reflexes causing bradycardia, laryngospasm and bronchospasm may occur during anaesthesia. Intense stimulation may result in partial or complete heart block or even asystole. Anal and cervical stretching (e.g. Brewer–Luckhardt reflex) and traction on the extraocular muscles (oculocardiac reflex) are particularly intense stimuli, but it may also follow skin incision and stimulation (e.g. surgical) of the mesentery, biliary tract, uterus, bladder, urethra, testes, larynx, glottis, bronchial tree and carotid sinus. Also involved in the diving reflex.
Anticholinergic drugs antagonise vagal reflexes during surgery. Should they occur, surgical activity should cease, and atropine or glycopyrronium be administered if necessary.
Valproate/valproic acid, see Sodium valproate
• Direct arterial BP tracings in normal subjects show four phases (Fig. 162):
phase I: increase in intrathoracic pressure expels blood from thoracic vessels.
phase II: decrease in BP due to reduction of venous return; activation of the baroreceptor reflex causes tachycardia and vasoconstriction, raising BP towards normal.
Fig. 162 Normal Valsalva response (see text)
‘square wave’ response, seen in cardiac failure, constrictive pericarditis, cardiac tamponade and valvular heart disease, when CVP is markedly raised. BP rises, remains high throughout the manoeuvre and returns to its previous level at the end.
autonomic dysfunction, e.g. autonomic neuropathy, drugs. BP falls and stays low until intrathoracic pressure is released. Pulse rate changes and overshoot are absent.
an exaggerated reduction in BP may be seen in hypovolaemia, e.g. during IPPV.
Useful as a bedside test of autonomic function. Concurrent ECG tracing allows accurate measurement of changes in heart rate. The manoeuvre may be useful in evaluating heart murmurs, and may be successful in terminating SVT (because of increased vagal tone in phase IV).
Valtis–Kennedy effect. Shift to the left of the oxyhaemoglobin dissociation curve during blood storage, originally described in 1954 for acid–citrate–dextrose storage. The shift reflects progressive depletion of 2,3-DPG.
[DJ Valtis, Greek physician; Arthur C Kennedy (1922–2009), Glasgow physician]
Valveless anaesthetic breathing systems. Anaesthetic breathing systems designed to eliminate resistance due to adjustable pressure-limiting valves. In the Samson system, the valve is replaced by an adjustable orifice; in the Hafnia systems, expired gases pass through a port, assisted by an ejector flowmeter.
[Heyman H Samson, South African anaesthetist; Hafnia: Latin name for Copenhagen]
Valvular heart disease. Causes, features and anaesthetic management: as for congenital heart disease and individual lesions. Valve replacement: as for cardiac surgery. Many prosthetic valves are available in different sizes, e.g. Silastic ball-and-cage, metal flaps and porcine valves. Thrombosis may form on prostheses, hence the requirement for long-term anticoagulation. Patients with prosthetic valves may also require prophylactic antibiotics as for congenital heart disease.
Van der Waals equation of state. Modification of the ideal gas law, accounting for the forces of attraction between gas molecules, and also the volume of the molecules:
Van der Waals forces. Weak attractive forces between neutral molecules and atoms, caused by electric polarisation of the particles induced by the presence of other particles.
Van Slyke apparatus. Device used to measure blood gas partial pressures. O2 and CO2 are released into a burette from the blood by addition of a liberating solution. Each gas in turn is converted to a non-gaseous substance by chemical reaction, and the pressure drop in the burette measured for each. The same reagents may be used as in the Haldane apparatus.
[Donald D van Slyke (1883–1971), US chemist]
See also, Carbon dioxide measurement; Gas analysis; Oxygen measurement
Vancomycin. Glycopeptide and antibacterial drug with bactericidal activity against aerobic and anaerobic Gram-positive bacteria (including multi-resistant staphylococci). Usually reserved for severe infections, resistant organisms or penicillin allergy. Despite this restriction in use, resistant species of vancomycin-resistant Staphylococcus aureus and entercocci are increasingly seen. Not absorbed orally.
• Dosage:
125–250 mg orally qds, in pseudomembranous colitis (for 7–10 days).
Vancomycin-resistant enterococci, see Infection control; Vancomycin
Vaporisers. Devices for delivering accurate and safe concentrations of volatile inhalational anaesthetic agents to the patient.
– gas passes through the vaporiser under pressure at the back bar of the anaesthetic machine.
– in most modern types (e.g. ‘Tec’ [temperature-compensated] vaporisers) fresh gas is divided by the control dial into two streams, one of which enters the vaporisation chamber, becomes fully saturated with agent and rejoins the other stream at the outlet (Fig. 163a). The ratio of the two streams (splitting ratio) determines the final delivered concentration (N.B. a different mechanism exists for the Tec 6 desflurane vaporiser: see below). In older vaporisers the delivered concentration was also affected by other factors (see below). In the obsolete ‘copper kettle’ type, a separate supply of O2 was passed through the vaporiser, becoming fully saturated. It was then added to the main fresh gas flow, at a rate calculated according to desired final concentration and vaporiser temperature. The original design included a large mass of copper as a heat sink, hence its name.
– have high resistance, so unsuitable for positioning in a circle system.
– performance is not affected by whether ventilation is spontaneous or controlled.
– include the Tec series of vaporisers. Features of the Mark 4 over the Mark 3 (Fig. 163b):
– flow of liquid agent into the delivery line is prevented if the vaporiser is inverted.
– interlock system prevents use of more than one vaporiser at a time, if mounted side by side.
– fitted with the key filling system (not fitted to all Mark 3 models).
Features of the Mark 5 over the Mark 4:
Features of the Mark 6 (desflurane vaporiser):
– cannot use the above mechanism because desflurane’s BP is very close to room temperature and therefore small variations in the latter result in large changes in SVP.
– requires an electrical power supply for the heating elements and control mechanisms.
Features of the Mark 7 over the Mark 5 (all modern agents but desflurane):
– more accurate and easily controllable output throughout different flow ranges.
– improved filling options and protection against spillage and contamination.
draw-over vaporisers: despite their variable output, often preferred in the battlefield (e.g. triservice apparatus) and developing countries because they are cheap, simple and portable:
– gas is drawn into the vaporising chamber by the patient’s inspiratory effort.
– performance is affected by minute ventilation, the output falling as ventilation increases.
– may be suitable for use within circle systems, e.g. Goldman vaporiser (Fig. 163b), a small, light, uncompensated device with a glass container (originally adapted from a motor vehicle fuel pump). Similar vaporisers were designed by McKesson and Rowbotham, the latter’s containing a wire gauze wick.
– also used for draw-over techniques, e.g. (Fig. 163b):
– EMO (Epstein and Macintosh of Oxford) diethyl ether inhaler: large vaporiser, incorporating a large vaporisation chamber, a water jacket for a heat sink and a temperature-compensating fluid-filled bellows at the outlet.
– obsolete types, used formerly for obstetric analgesia:
– Emotril (Epstein and Macintosh of Oxford/Trilene) trichloroethylene apparatus: incorporated within a metal box.
– Cardiff methoxyflurane inhaler: free-standing on a base.
systems involving addition of liquid volatile agent directly to the fresh gas stream:
– incorporated into computerised anaesthetic machines.
– combined with a carbon filter/evaporator system that fits into the patient’s breathing system, conserving and recycling ~90% of the administered agent. Have been used for sedation on ICU without the need for anaesthetic machines.
• Factors affecting the delivered concentration:
splitting ratio (plenum vaporisers).
temperature of the liquid: affects the SVP. As liquid vaporises, latent heat of vaporisation is lost, and temperature and thus SVP fall. Delivered concentration of agent would therefore fall if not for temperature compensation devices, e.g.:
– fluid-filled bellows at the gas outlet, e.g. EMO inhaler (see below); expands as temperature rises.
– longitudinally expanding metal rod at the gas outlet, e.g. Dräger models.
surface area of the gas/liquid interface: increased with:
– a cowl to direct gas flow on to or into the liquid (e.g. the original Boyle’s bottle).
Vaporisers have been associated with many hazards, and require regular servicing.
Vapour. Matter in the gaseous state below its critical temperature; i.e. its constituent particles may enter the liquid state. As liquid vaporises, heat is required (latent heat of vaporisation); as vapour condenses, an equal amount of heat is produced. These processes occur continuously above the surface of a liquid at equilibrium.
Vapour pressure. Pressure exerted by molecules escaping from the surface of a liquid to enter the gaseous phase. When equilibrium is reached at any temperature, the number of molecules leaving the liquid phase equals the number entering it; the vapour pressure now equals SVP. Raising the temperature of the liquid increases the molecules’ kinetic energy, allowing more of them to escape and raising the vapour pressure. When SVP equals atmospheric pressure, the liquid boils.
Vaptans, see Vasopressin receptor antagonists
Variance. Standard deviation squared. Thus an indicator of spread of values within a sample. Although standard deviation is commonly used when describing data, many statistical calculations employ its square, hence the use of variance as a meaningful term (e.g. analysis of variance, ANOVA).
Vascular access, see Central venous cannulation; Intravenous fluid administration
Vascular resistance, see Pulmonary vascular resistance; Systemic vascular resistance
Vasculitides. Group of conditions characterised by inflammation of blood vessels. All except giant cell arteritis are uncommon and associated with connective tissue diseases or drug hypersensitivity. Diagnosis requires biopsy, which often shows granuloma formation associated with vessel inflammation.
group 1: systemic necrotising arteritis of small/medium arteries:
– connective tissue disease-associated arteritis.
group 2: small vessel vasculitis:
– Henoch–Schönlein purpura: usually occurs in childhood following an upper respiratory tract infection. Features include fever, headache, macular/urticarial rash becoming purpuric, over the buttocks and limbs. Inflammatory synovitis is common. Focal glomerulonephritis may lead to nephrotic syndrome and, rarely, renal failure.
– mixed cryoglobulinaemic vasculitis.
– connective tissue disease-associated vasculitis.
group 3: giant cell arteritis/large artery vasculitis:
– temporal arteritis: usually affects the elderly and often associated with polymyalgia rheumatica. Headache, often localised, is the predominant symptom. Blindness may result if corticosteroids are not given promptly.
– Takayasu’s arteritis: rare large vessel arteritis affecting young women. Affects the aorta and its branches, causing inflammation and then stenosis of affected vessels. Features include cerebrovascular insufficiency (fainting, dizziness) and reduced peripheral pulses. Treatment depends on the underlying condition but usually includes immunosuppressive drugs.
Vasoconstrictor drugs, see Vasopressor drugs
Vasodilator drugs. Drugs causing vasodilatation as their primary effect (cf. isoflurane, morphine). The term is sometimes reserved for drugs acting directly at vascular smooth muscle. Nitric oxide is thought to be a common end pathway for most drugs.
• May be divided according to their main site of action, although considerable overlap occurs:
venous system: GTN, isosorbide.
arterial system: hydralazine, calcium channel blocking drugs, salbutamol, diazoxide, minoxidil, adenosine.
venous and arterial systems: sodium nitroprusside, α-adrenergic receptor antagonists, angiotensin converting enzyme inhibitors, ganglion blocking drugs, potassium channel activators.
• Used to reduce SVR and thus:
systemic BP, e.g. in hypotensive anaesthesia, hypertensive crisis, pre-eclampsia. Their effect is sometimes offset by reflex tachycardia.
afterload and ventricular work, e.g. in cardiac failure, shock. Increase stroke volume and reduce myocardial O2 demand.
Also reduce preload via venous dilatation. Also used to reduce pulmonary vascular resistance in pulmonary hypertension, although the systemic circulation is usually affected too.
Vasomotor centre. Group of neurones in the ventrolateral medulla, involved in the control of arterial BP. Projects to sympathetic preganglionic neurones in the spinal cord. Normal continuous discharge causes partial contraction of vascular smooth muscle (vasomotor tone) and resting sympathetic stimulation of the heart.
chemoreceptor discharge.
hypoxia (causes direct stimulation initially, but depression follows).
baroreceptor discharge.
Thus responds to hypotension (reduced baroreceptor discharge) by increasing sympathetic activity.
Dorsal and medial neurones functionally constitute the cardioinhibitory centre, stimulation of which inhibits the vasomotor centre and increases vagal activity.
Vasopressin (Arginine vasopressin, AVP; Antidiuretic hormone, ADH). Neuropeptide synthesised in the cell bodies of the supraoptic and paraventricular nuclei of the hypothalamus. Transported down their axons to the posterior lobe of the pituitary gland, from where it is secreted. Metabolised in the kidney and liver, it has a circulatory half-life of 10–30 min. There are at least three types of vasopressin receptor, all of which are G protein-coupled receptors: V1 receptors are Gq-coupled and located primarily on vascular smooth muscle and platelets; V2 receptors (Gs-coupled) mediate vasopressin’s antidiuretic actions on the kidney; and V3 receptors (coupled to multiple G proteins) are located centrally and involved in vasopressin’s neurotransmitter actions.
water retention by the kidney, acting via V2 vasopressin receptors. These increase adenylate cyclase activity and cAMP levels, triggering insertion of water channels (aquaporins) into the luminal membranes of cells in the renal collecting ducts. This results in water reabsorption from the renal tubules. Urine volume decreases; its concentration increases. Conversely, plasma volume increases; its concentration decreases.
has a role in temperature regulation, control of circadian rhythm and memory function.
increased plasma levels of coagulation factor VIII.
increased plasma osmolality; detected by osmoreceptors in the anterior hypothalamus.
decreased ECF volume and hypotension (e.g. in haemorrhage); detected by baroreceptors.
pain, nausea, hypoxaemia, emotional and physical stress.
drugs, e.g. morphine, barbiturates.
drugs, e.g. alcohol, butorphanol.
• Used therapeutically in various forms:
