D
δ wave, see Wolff–Parkinson–White syndrome
DA examination (Diploma in Anaesthetics). First specialist examination in anaesthetics; first held in 1935 in London. Originally intended for anaesthetists with at least 2 years’ experience and 2000 anaesthetics, later reduced to 1 year’s residence in an approved hospital. A two-part examination was introduced in 1947, becoming the FFARCS examination in 1953; the single-part DA remained separate until 1984, when the DA (UK) became the first part of the new three-part FFARCS. Replaced by the new FRCA examination in 1996.
Dabigatran etexilate. Direct thrombin inhibitor, preventing the conversion of fibrinogen to fibrin; thus impairs coagulation and platelet activation. Prolongs the activated partial thromboplastin time (APTT). A prodrug, rapidly metabolised to active metabolite (dabigatran) by non-specific esterases; oral bioavailability is 6.5%. Onset of action is 0.5–2 h, with steady state achieved within 3 days. 80% of drug is renally excreted unchanged; half-life is 8–14 h, increased to > 24 h in severe renal failure. Also metabolised by the liver and excreted in the bile.
Licensed for DVT prophylaxis in adults undergoing total knee or hip replacement surgery, and for prevention of CVA in patients with AF with certain risk factors. Under investigation for the treatment of DVT and acute coronary syndrome. Developed as an alternative to warfarin, over which it has several advantages:
no therapeutic monitoring required.
fixed dosage (modified according to age, indication, renal function and drug interactions).
faster onset and shorter duration of action.
stable, predictable pharmacokinetics, with fewer drug interactions.
Disadvantages include: higher cost; no specific antidote in case of overdose (although can be removed by dialysis); unsuitable for patients with severe renal impairment (GFR < 30 ml/min).
• Dosage:
CVA prophylaxis in patients with AF: 150 mg orally bd, reduced to 75 mg in renal impairment.
• Drug interactions: potentiated by concomitant verapamil and amiodarone; reduced dosing regimen is used. Inhibited by rifampicin, ketoconazole and quinidine.
Main side effect is bleeding; risk is similar or lower than equivalent treatment with warfarin.
Dalteparin sodium, see Heparin
Damage control resuscitation. Approach to major trauma, developed in modern military settings. Aims to reduce the incidence and severity of the combination of acute coagulopathy, hypothermia and metabolic acidosis that commonly accompany major haemorrhage. Consists of:
‘permissive hypotension’: restricted fluid resuscitation to maintain a radial pulse rather than a normal or near-normal BP, accepting a period of organ hypoperfusion (except in head injury, where maintenance of cerebral perfusion pressure is thought to be vital).
‘haemostatic resuscitation’: early use of blood products to prevent/treat acute coagulopathy, including platelets (in a 1:1 ratio with packed red cells [1 pool of platelets for every 5 units of red cells]), fresh frozen plasma (in a 1:1 ratio) and cryoprecipitate; tranexamic acid, eptacog alfa and calcium have also been advocated. Use of more recently donated packed red cells rather than older units may be beneficial.
Jansen JO, Thomas R, Loudon MA, Brooks A (2009). Br Med J; 338: b1778
Damping. Progressive diminution of amplitude of oscillations in a resonant system, caused by dissipation of stored energy. Important in recording systems such as direct arterial BP measurement. In this context, damping mainly arises from viscous drag of fluid in the cannula and connecting tubing, compression of entrapped air bubbles, blood clots within the system and kinking. Excess damping causes a flattened trace which may be distorted (phase shift). The degree of damping is described by the damping factor (D); if a sudden change is imposed on a system, D = 1 if no overshoot of the trace occurs (critical damping; Fig. 49a). A marked overshoot followed by many oscillations occurs if D 
1 (Fig. 49b), and an excessively delayed response occurs if D 
1 (Fig. 49c). Optimal damping is 0.6–0.7 of critical damping, and produces the fastest response without excessive oscillations. D depends on the properties of the liquid within the system and the dimensions of the cannula and tubing.
Danaparoid sodium. Heparinoid mixture, containing no heparin, used for the prophylaxis of deep vein thrombosis. Useful as a heparin alternative in cases of heparin-induced thrombocytopenia.
Dandy–Walker syndrome, see Hydrocephalus
Dantrolene sodium. Hydantoin skeletal muscle relaxant that acts by binding to the ryanodine receptor and limits entry of calcium into myocytes. Used to treat spasticity, neuroleptic malignant syndrome and MH.
Krause T, Gerbershagen MU, Fiege M, et al (2004). Anaesthesia; 59: 364–73
Darrow’s solution. Hypotonic solution designed for iv fluid replacement in children suffering from gastroenteritis. Composed of sodium 122 mmol/l, chloride 104 mmol/l, lactate 53 mmol/l and potassium 35 mmol/l.
Data. In statistics, a series of observations or measurements.
– interval: does not include zero, e.g. Celsius temperature scale (the ‘zero’ is an arbitrary point in the scale; thus 10°C does not represent half as much heat as 20°C). The Kelvin scale is a ratio scale since 0 K does represent absence of heat and thus 10 K represents half as much heat as 20 K.
If they have a normal distribution, continuous data may be described by the mean and standard deviation as indicators of central tendency and scatter respectively (described as for ordinal data if not normally distributed).
– ordinal, e.g. ASA physical status. The difference between scores of 2 and 3 does not equal that between scores of 4 and 5, and a score of 4 is not ‘twice as unfit’ as a score of 2. Ordinal data are described by the median and percentiles (plus range).
– nominal, e.g. diagnosis, hair colour. May be dichotomous, e.g. male/female; alive/dead. Nominal data are described by the mode and a list of possible categories.
Different kinds of data require different statistical tests for correct analyses and comparisons: parametric tests for normally distributed data (most continuous data) and non-parametric tests for non-normally distributed data (categorical and some continuous data). The latter may often be ‘normalised’ by mathematical transformation, allowing application of the more sensitive parametric tests.
See also, Clinical trials; Statistical frequency distributions
Davenport diagram. Graph of plasma bicarbonate concentration against pH, useful in interpreting and explaining disturbances of acid–base balance. Different lines may be drawn for different arterial PCO2 values, but for each line, bicarbonate falls as pH falls and increases as pH increases (Fig. 50).
• Can be used to demonstrate what happens in various acid–base disorders:
line BAD represents part of the titration curve for blood.
point A represents normal plasma.
point B represents respiratory acidosis; i.e. a rise in arterial PCO2, reducing pH and increasing bicarbonate. In order to return pH towards normal, compensatory mechanisms (e.g. increased renal reabsorption) increase bicarbonate; i.e. move towards point C.
point D represents respiratory alkalosis; compensation (increased renal bicarbonate excretion) results in a move towards point E.
point G represents metabolic alkalosis; compensatory hypoventilation causes a move towards point C.
The same relationship may be displayed in different ways, e.g. the Siggaard-Andersen nomogram, which contains more information and is more useful clinically.
[Horace W Davenport (1912–2005), US physiologist]
Fig. 50 Davenport diagram (see text)
Davy, Sir Humphrey (1778–1829). Cornish-born scientist, inventor of the miner’s safety lamp. Discovered sodium, potassium, calcium and barium. Suggested the use of N2O (which he named ‘laughing gas’) for analgesia in 1799, whilst director of the Medical Pneumatic Institution in Bristol. Later became President of the Royal Society.
Riegels N, Richards MJBM (2011). Anesthesiology; 114: 1282–8
reduced psychological upheaval for patients, especially children.
reduced requirement for nursing care and hospital services, and thus cheaper.
Standards of anaesthetic and surgical care and equipment (including preoperative preparation, monitoring and recovery facilities) are as for inpatient surgery.
• Patients may be managed within:
• Patient selection: traditional rigid criteria (e.g. relating to age, ASA physical status, BMI) are now generally considered unnecessary, three main criteria being:
medical condition: patient should be fit or any chronic disease should be stable/controlled.
premedication is usually omitted, although short-acting drugs (e.g. temazepam) are sometimes used.
general principles are as for any anaesthetic, but rapid recovery is particularly desirable. Thus short-acting drugs are usually used, e.g. propofol, fentanyl, alfentanil. Sevoflurane and desflurane are commonly selected volatile agents because of their low blood gas solubility and rapid recovery. Tracheal intubation is acceptable but is usually avoided if possible. Suxamethonium is often avoided as muscle pains are more common in ambulant patients.
• Postoperative assessment must be performed before discharge; the following are usually required:
full orientation and responsiveness.
ability to walk, dress, drink and pass urine.
D-dimer, see Fibrin degradation products
Dead space. Volume of inspired air that takes no part in gas exchange. Divided into:
anatomical dead space: mouth, nose, pharynx and large airways not lined with respiratory epithelium. Measured by Fowler’s method.
alveolar dead space: ventilated lung normally contributing to gas exchange, but not doing so because of impaired perfusion. Thus represents one extreme of 
mismatch.
Physiological dead space equals anatomical plus alveolar dead space. It is calculated using the Bohr equation. Assessment may be useful in monitoring 
mismatch in patients with extensive respiratory disease, especially when combined with estimation of shunt fraction. Normally equals 2–3 ml/kg; i.e. 30% of normal tidal volume. In rapid shallow breathing, alveolar ventilation is reduced despite a normal minute ventilation, because a greater proportion of tidal volume is dead space.
Apparatus dead space represents ‘wasted’ fresh gas within anaesthetic equipment. Minimal lengths of tubing should lie between the fresh gas inlet of a T-piece and the patient, especially in children, whose tidal volumes are small. Facemasks and their connections may considerably increase dead space.
– mortality/survival prediction and allocation of resources; triage.
– relief of symptoms, e.g. pain management, palliative care.
– CPR.
– diagnosis of brainstem death.
– informed consent before treatments (i.e. the risk of death).
– withdrawing treatment or resisting aggressive interventions when they are likely to be futile.
– do not attempt resuscitation orders.
– reporting of deaths to the coroner.
– claims of negligence or manslaughter.
– adequate preparation and support of patients, relatives and staff.
– counselling of staff after a patient’s death, especially when unexpected.
– organisational/educational: audit; surveys of ICU and anaesthetic morbidity and mortality; risk management.
Debrisoquine sulphate. Antihypertensive drug, acting by preventing noradrenaline release from postganglionic adrenergic neurones. Similar in effects to guanethidine, but does not deplete noradrenaline stores. No longer available in UK.
Decamethonium dibromide/diiodide. Depolarising neuromuscular blocking drug, introduced in 1948. Blockade lasts 15–20 min. No longer in use in the UK.
Decerebrate posture. Abnormal posture resulting from bilateral midbrain or pontine lesions (below the level of the red nucleus). Composed of internal rotation and hyperextension of all limbs, with hyperextension of neck and spine, and absent righting reflexes. Similar posturing may be seen in severe structural brain damage or coning.
Declamping syndrome, see Aortic aneurysm, abdominal
Decompression sickness (Caisson disease). Syndrome following rapid passage from a high atmospheric pressure environment to one of lower atmospheric pressure (e.g. surfacing after underwater diving), especially if followed by air transport at high altitude. Caused by formation of nitrogen bubbles as the gas comes out of solution, which it does readily because of its low solubility. Bubbles may embolise to or form in various tissues, giving rise to widespread symptoms in: the joints (‘the bends’); the CNS (‘the staggers’); the skin (‘the creeps’); the lungs (‘the chokes’). Treated by immediate recompression followed by slow, controlled depressurisation.
[Caisson: pressurised watertight chamber used for construction work in deep water]
Vann RD, Butler FK, Mitchell SJ. Moon RE (2011). Lancet; 377: 153–64
Decontamination of anaesthetic equipment, see Contamination of anaesthetic equipment
Decubitus ulcers (Pressure sores). May result from a number of factors:
malnutrition, including mineral deficiency.
predisposing conditions, e.g. diabetes mellitus, immunodeficiency.
diarrhoea or urinary incontinence.
[Judy Waterlow, British nurse]
Keller BP, Wille J, van Ramshorst B, van der Werken C (2002). Intensive Care Med; 28: 1379–88
Deep vein thrombosis (DVT). Thrombus formation in the deep veins, usually of the leg and pelvis. A common postoperative complication, especially following major joint replacement, colorectal surgery and surgery for cancer. The true incidence is unknown but may exceed 50% in high-risk patients. Carries high risk of PE. May rarely cause systemic embolisation via a patent foramen ovale (present in 30% of the population at autopsy).
• Triad of predisposing factors described by Virchow in 1856:
venous stasis, e.g. low cardiac output (e.g. cardiac failure, MI, dehydration), pelvic venous obstruction, prolonged immobility.
vessel wall damage, e.g. direct trauma, inflammation, varicosities, infiltration.
increased blood coagulability, e.g. trauma, malignancy, pregnancy, oestrogen or antifibrinolytic administration, hereditary hypercoagulability, smoking (see Coagulation disorders).
More common in old age, sepsis and obesity. Increased risk after surgery is thought to be related to increased platelet adhesiveness and activation of the coagulation cascade caused by tissue trauma, exacerbated by possible venous damage and immobility.
Clinical diagnosis is unreliable but suggested by tenderness, swelling and increased temperature of the calf, and pain on passive dorsiflexion of the foot (Homans’ sign). The upper leg may also be involved. Accompanying superficial thrombophlebitis may be absent. Investigations include: Doppler ultrasound; impedance plethysmography; thermography; uptake of radiolabelled fibrinogen or platelets; and venography (the ‘gold standard’ test). Measurement of FDPs (e.g. D-dimer) is also used, a normal level excluding DVT.
Treated by systemic anticoagulation with unfractionated or low-molecular-weight heparin; the latter are more commonly used since they can be given sc once a day in fixed doses based on the patient’s weight, without needing laboratory monitoring. This tends to offset the increased drug cost. In addition, haemorrhagic complications are less frequent than with unfractionated heparin. Low-molecular-weight heparins are thus used to treat DVT without admission to hospital in selected patients. Warfarin is administered orally, and heparin discontinued when a prothrombin time of 2–3 times normal is achieved. Long-term interruption of leg blood flow may not be prevented, and prevention of PE has never been proven. Initial bed rest, leg elevation and analgesia are also prescribed, although there is no evidence that these measures affect outcome. Optimal duration of warfarin therapy is controversial but the usual duration ranges from 6 weeks to 6 months, depending on underlying risk factors. Surgical removal of thrombus has been performed, but reaccumulation usually occurs, possibly due to vessel wall damage. Use of fibrinolytic drugs is controversial.
– intermittent pneumatic compression of the calves peri- and postoperatively.
– graduated compression stockings.
– encouragement of postoperative leg exercises and early mobilisation.
reduction of intravascular coagulation:
– fibrinolytic drugs: not shown to prevent DVT.
– antiplatelet drugs: have been shown to prevent DVT and PE.
– heparin and warfarin anticoagulation: effective but with risk of haemorrhage. Low-dose heparin is widely used (5000 units sc 2 h preoperatively then bd/tds) and has been shown to be effective in reducing DVT and fatal PE. Low-molecular-weight heparin reduces DVT with reduced haemorrhagic side effects (for doses, see Heparin). Dihydroergotamine has been used together with heparin, and is thought to reduce the incidence further, possibly via venous vasoconstriction.
– discontinuation of oral contraceptives preoperatively.
– epidural/spinal anaesthesia is thought to be associated with increased fibrinolysis and reduced risk of DVT.
– statins reduce the incidence of DVT.
Low-molecular-weight heparin and compression stockings are most commonly used.
[Rudolph LW Virchow (1821–1902), German pathologist; John Homans (1877–1954), US surgeon]
Defibrillation. Application of an electric current across the heart, to convert (ventricular) fibrillation to sinus rhythm. Use of electricity was described in the late 1700s/early 1800s (e.g. by Kite), but modern use arose from experiments in the 1930s–1950s, largely by Kouwenhoven. The first successful resuscitation of a patient from VF was by Claude Beck in 1947. Direct current is more effective and less damaging than alternating current.
Modern defibrillators contain a capacitor, with potential difference between its plates of 5000–8000 V (Fig. 51a). Stored charge is released during discharge; the energy released is proportional to the potential difference. An inductor controls the shape and duration of the delivered electrical pulse. With traditional monophasic defibrillation up to 400 J is used for external defibrillation (360 J actually delivered to the patient because of energy losses), and 20–50 J for internal defibrillation. The current pulse (up to 30 A) causes synchronous contraction of the heart muscle followed by a refractory period, thus allowing sinus rhythm to occur. Modern devices deliver lower-energy biphasic waveforms, which are more effective at terminating VF; current is delivered in one direction followed immediately by a second current in the opposite direction (Fig. 51b). More sophisticated developments include the ability to measure and correct for the transthoracic impedance by varying the shock delivered.
Firm application of paddles (one to the right of the sternum, the other over the apex of the heart – taking care to avoid any implanted devices) using conductive jelly increases efficiency, although self-adhesive pads are now preferred. They may also be placed on the front and back of the chest. Thoracic impedance is reduced by the first shock, so a second discharge at the same setting will deliver greater energy to the heart. For children, 4 J/kg is used. Lower energy levels and R-wave synchronisation are used in cardioversion. Repeated shocks may result in myocardial damage.
Hazards include electrocution of members of the resuscitation team and fire. Standing clear of the patient and disconnection of the patient’s oxygen supply (moving it 1 m from his/her chest) during defibrillation is mandatory.
Implantable cardioverter defibrillators (ICDs) are used for recurrent VF/VT or predisposing conditions (see Defibrillators, implantable cardioverter).
[William Kouwenhoven (1886–1975), US engineer; Claude S Beck (1894–1971) US thoracic surgeon]
See also, individual arrhythmias; Cardiac pacing; Cardiopulmonary resuscitation
Defibrillators, implantable cardioverter (ICD). Specialised pacemakers designed to detect and treat potentially life-threatening arrhythmias. Have similar principles to pacemakers but are described by a different coding system (Table 18). Depending on the patient’s arrhythmia, they may respond with anti-tachycardia pacing, anti-bradycardia pacing, a low-energy synchronised shock (< 5 J) or a high-energy unsynchronised shock. A particular concern during surgery is the potential for electromagnetic interference (e.g. from surgical diathermy) to be misinterpreted as VF or VT, causing inappropriate discharge of the defibrillator. Defibrillator function may be disabled either by preoperative reprogramming of the ICD or intraoperative application of a magnet. The latter may have unpredictable effects on the ICD and this should be confirmed with the ICD manufacturer before use.
Anaesthetic management for insertion is similar to that for pacemakers.
Deflation reflex. Stimulation of inspiration by lung deflation, initiated by pulmonary stretch receptors. Of uncertain significance in humans.
Degrees of freedom. In statistics, the number of observations in a sample that can vary independently of other observations. For n observations, each observation may be compared with n – 1 others; i.e. degrees of freedom = n – 1. For chi-squared analysis, it equals the product of (number of rows – 1) and (number of columns – 1).
Dehydration. Reflects loss of water from ECF, alone or with intracellular fluid (ICF) depletion. Sodium is usually lost concurrently, giving rise to hypernatraemia or hyponatraemia, depending on the relative degrees of loss. If ECF osmolality rises, water passes from ICF into ECF by osmosis. A predominant water loss is shared by both ICF and ECF; water and sodium loss is borne mainly by ECF if osmolality is not greatly affected. Thus fever, lack of intake, diabetes insipidus and osmotic diuresis (mainly water loss) may be tolerated for longer periods than severe vomiting, diarrhoea, intestinal obstruction and diuretic therapy (water and sodium loss), although reduced water intake often accompanies the latter conditions.
The physiological response to dehydration includes increased thirst, vasopressin secretion and renin/angiotensin system activation, and CVS compensation for hypovolaemia via osmoreceptor and baroreceptor mechanisms.
• Clinical features are related to the extent of water loss:
5% of body weight: thirst, dry mouth.
5–10% of body weight: decreased intraocular pressure, peripheral perfusion and skin turgor, oliguria, orthostatic hypotension. JVP/CVP are reduced.
Blood urea and haematocrit are increased, and urine has high osmolality (over 300 mosmol/kg) and low sodium content (under 10 mmol/l), assuming normal renal function.
Treatment includes oral and iv fluid administration; in the latter, dextrose solutions are favoured for combinations of ICF and ECF losses and electrolyte solutions for ECF losses alone. Dehydration should be corrected preoperatively.
Delirium tremens (DTs). Alcohol withdrawal syndrome seen in chronic heavy drinkers. Occurs in about 5% of cases. Thought to be caused by reduction in both inhibitory GABAA activity and presynaptic sympathetic inhibition; hypomagnesaemia and hypocalcaemia may contribute to CNS hyperexcitability. Characterised by:
Treatment includes: rehydration; correction of hypoglycaemia and electrolyte imbalance; and sedation. Benzodiazepines are the agents of choice, although carbamazepine and phenobarbital have also been used. Prevention is important and is achieved with the same drugs.
Demand valves. Valves that allow self-administration of inhalational anaesthetic agents; they may form part of intermittent-flow anaesthetic machines, or be used to administer Entonox. The standard Entonox valve was developed from underwater breathing apparatus, and contains a two-stage pressure regulator within one unit that fits directly to the cylinder. The first-stage regulator is similar to those on anaesthetic machines. At the second stage, gas flow is prevented by a rod which seals the valve. The patient’s inspiratory effort moves a sensing diaphragm and tilts the rod, opening the valve. Very small negative pressures are required to produce gas flow of up to 300 l/min. The mouthpiece/mask elbow piece incorporates an expiratory valve and a safety (overpressure) valve. Other demand valves for use with Entonox have the second-stage (demand) regulator attached to the patient’s mask.
Demeclocycline hydrochloride. Tetracycline, used as an antibacterial drug and to treat the syndrome of inappropriate ADH secretion (SIADH), possibly by blocking the renal action of vasopressin.
Demyelinating diseases. Non-specific group of disorders, with abnormality of the axonal myelin sheath as the predominant feature. Defective myelin may be present at birth (dysmyelinating) or may arise in areas of previously normal myelin (demyelinating). Multiple (disseminated) sclerosis (MS) is the commonest of the latter and is considered below; others include post-immunisation or parainfectious encephalomyelitis.
Diagnosed clinically, supported by gadolinium-enhanced MRI findings and the presence of oligoclonal IgG bands in the CSF. Evoked potential testing may support the diagnosis.
Treatment is supportive, but may include corticosteroids. Interferon beta has been shown to decrease the relapse rate in relapsing remitting MS. Hyperbaric O2 therapy has been advocated but results are disappointing.
Anaesthetic implications are unclear, since effects of stress, surgery and anaesthesia cannot be separated from the spontaneity of new lesion formation. Thus case reports are often conflicting in their conclusions. Increases in body temperature should be avoided; avoidance of anticholinergic drugs has been suggested. An abnormal response to neuromuscular blocking drugs, including hyperkalaemia following suxamethonium, has been suggested but without direct evidence. Prolonged muscle spasms may occur, but epilepsy is rare. Increased tendency to DVT has been suggested. Impairment of respiratory and autonomic control has been reported. Although not contraindicated, epidural or spinal anaesthesia has sometimes been avoided for medicolegal reasons, although it is increasingly used as the preferred method of obstetric analgesia and anaesthesia.
Denervation hypersensitivity. Increased sensitivity of denervated skeletal muscle to acetylcholine. Develops approximately 4–5 days after denervation, and is due to proliferation of extrajunctional acetylcholine receptors over the entire muscle membrane, instead of being restricted to the neuromuscular junction. Thought to be the mechanism underlying the exaggerated hyperkalaemic response to suxamethonium seen after peripheral nerve injuries. Its cause is unclear. Also occurs in smooth muscle.
Density. For a substance, defined as its mass per unit volume. Relative density (specific gravity) is the mass of any volume of substance divided by the mass of the same volume of water.
Dental nerve blocks, see Mandibular nerve blocks; Maxillary nerve blocks
Dental surgery. Anaesthesia may be required for tooth extraction, conservative dental surgery or maxillofacial surgery.
• For outpatient ambulatory surgery, the following may be used:
mandibular and maxillary nerve blocks.
iv sedation, e.g. Jorgensen technique and variants.
inhalational anaesthetic agents, including N2O and/or volatile agents. Traditionally administered from intermittent-flow anaesthetic machines, using nasal inhalers, although continuous-flow machines are increasingly used. Quantiflex apparatus is also used. Occupational exposure to N2O is a hazard, especially in small dental surgeries.
• General anaesthetic principles are as for day-case surgery; main problems:
high proportion of children, usually anxious and unpremedicated.
shared airway as for ENT surgery. Airway obstruction, mouth breathing during nasally administered anaesthesia, airway soiling and breath-holding may occur. For these reasons the traditional nasal inhaler (mask-like device held over the nose from behind the patient’s head) is now rarely used by hospital anaesthetists.
arrhythmias may arise from use of adrenaline solutions, stimulation of the trigeminal nerve, anxiety and hypercapnia.
Mouth packs may be employed to prevent airway soiling. They should be placed under the tongue, pushing the tongue back to seal the mouth from the airway. Mouth gags or props are often used to hold the mouth open.
Dental surgery is performed on an inpatient surgery basis if airway obstruction, cardiac or respiratory disease, coagulation disorders or extreme obesity is present. Nasotracheal intubation is usually performed, and a throat pack placed. Use of concentrated adrenaline solutions by dentists is still common, e.g. 1:80 000, despite anaesthetists’ objections.
Depolarising neuromuscular blockade (Phase I block). Follows depolarisation of the postsynaptic membrane of the neuromuscular junction via activation of acetylcholine receptors, but with only slow repolarisation. Effect of depolarisation of presynaptic receptors is unclear, but may be partially responsible for the fasciculation seen. Suxamethonium is the most commonly used and widely available drug; previously used drugs include decamethonium and suxethonium.
• Features of depolarising blockade:
may be preceded by fasciculation.
does not exhibit fade or post-tetanic potentiation.
increased by acetylcholinesterase inhibitors.
potentiated by respiratory alkalosis, hypothermia, hyperkalaemia and hypermagnesaemia.
antagonised by non-depolarising neuromuscular blocking drugs.
development of dual block with excessive dosage of drug.
See also, Neuromuscular blockade monitoring; Non-depolarising neuromuscular blockade
Dermatomes. Lateral walls of somites (segmental units appearing longitudinally early in embryonic development), which form the skin and subcutaneous tissues. Cutaneous sensation retains the somatic distribution, corresponding to segmental spinal levels. Thus areas of skin are supplied by particular spinal nerves; useful in determining the extent of regional anaesthetic blocks and localising neurological lesions (Fig. 52).
Fig. 52 Dermatomal nerve supply
Dermatomyositis, see Polymyositis
Desensitisation block, see Dual block
Desferrioxamine mesylate. Chelating agent used in the treatment of aluminium and iron poisoning.
Desflurane. 1-Fluoro-2,2,2-trifluoroethyl difluoromethyl ether. Inhalational anaesthetic agent, synthesised in the 1960s but only introduced in the UK in 1994. Chemical structure is the same as isoflurane, but with the chlorine atom replaced by fluorine (Fig. 53).
colourless liquid with slightly pungent vapour.
SVP at 20°C 88 kPa (673 mmHg).
MAC 5–7% in adults; 7.2–10.7% in children.
supplied in liquid form with no additive.
may react with dry soda lime to produce carbon monoxide (see Circle systems).
requires the use of an electrically powered vaporiser due to its low boiling point.
• Effects:
– rapid induction (although limited by its irritant properties) and recovery.
– EEG changes as for isoflurane.
– may increase cerebral blood flow, although the response of cerebral vessels to CO2 is preserved.
– ICP may increase due to imbalance between the production and absorption of CSF.
– reduces CMRO2 as for isoflurane.
– has poor analgesic properties.
– respiratory depressant, with increased rate and decreased tidal volume.
– myocardial ischaemia may occur if sympathetic stimulation is excessive.
– arrhythmias uncommon, as for isoflurane. Little myocardial sensitisation to catecholamines.
– renal and hepatic blood flow generally preserved.
– dose-dependent uterine relaxation (although less than isoflurane and sevoflurane).
– skeletal muscle relaxation; non-depolarising neuromuscular blockade may be potentiated.
2–6% is usually adequate for maintenance of anaesthesia, with higher concentrations for induction. Uptake and excretion are rapid because of its low blood gas solubility; thus it has been suggested as the agent of choice in day-case surgery, although this is controversial. Although more expensive than isoflurane, less drug is required to maintain anaesthesia once equilibrium is reached, and equilibrium is reached much more quickly; it may therefore be more economical for use during longer procedures.
Fig. 53 Structure of desflurane
Desmopressin (1-desamino-8-D-argininevasopressin; DDAVP). Analogue of vasopressin, with a longer-lasting antidiuretic effect but minimal vasoconstrictor actions. Used to diagnose and treat non-nephrogenic diabetes insipidus. May also be used to increase factor VIII and von Willebrand factor levels by 2–4 times in mild haemophilia or von Willebrand’s disease. Has also been used to improve platelet function in renal failure
