W
Wake-up test. Intraoperative awakening to allow assessment of spinal cord function during spinal surgery. Has also been used to assess cerebral function during basilar artery clipping.
Warfarin sodium. Oral anticoagulant drug, first synthesised in 1944. Rapidly absorbed by mouth and almost totally protein-bound. Competes with vitamin K in the synthesis of coagulation factors II, VII, IX and X in the liver; therefore requires 1–2 days for its effect to develop. Also inhibits protein C and S. Metabolised in the liver and excreted in urine and faeces. Half-life is about 30 h. Dosage is adjusted according to results of coagulation studies: the International Normalised Ratio (INR) is maintained at about 2–3 for prophylaxis and treatment of DVT, PE, transient ischaemic attacks and in patients with atrial fibrillation at high risk of embolisation; 3–4.5 for recurrent DVT/PE, cardiac and arterial prostheses. The usual maintenance dose is 3–9 mg/day. The INR is usually checked daily or on alternate days initially, but thereafter up to every 2 months, depending on the response.
Drugs causing hepatic enzyme induction (e.g. rifampicin, phenytoin) reduce its effect. If the second drug is withdrawn without reducing the dose of warfarin, haemorrhage may occur. Effects may be enhanced by drugs that displace it from protein-binding sites, e.g. sulphonamides, NSAIDs. Emergency treatment of haemorrhage due to excessive warfarin effect involves use of vitamin K injection (up to 5 mg iv) and the administration of factors II, VII, IX and X (prothrombin complex concentrate, or fresh frozen plasma).
heart valves: maintain warfarin therapy for short (under 30 min) surgery, with fresh frozen plasma available. Otherwise, stop warfarin 3 days preoperatively, and start heparin infusion 24 h later (about 15 000 units/12 h), maintaining activated partial thromboplastin time (APTT) at 2–3 times normal. Stop heparin 6 h preoperatively, and check INR and APTT 1 h preoperatively. Surgery may be delayed, or plasma administered, if INR exceeds 1.5. Restart warfarin as soon as possible postoperatively, or heparin if nil by mouth for over 48 h. Extra precautions have been suggested for prosthetic mitral valves, since the risk of emboli is greater than for other valves: aspirin 75 mg or dipyridamole 300 mg/day is started when warfarin is stopped. Heparin is restarted 6–12 h postoperatively until able to take warfarin. Increasingly, low-mw heparin is being used to ‘bridge’ coagulation perioperatively instead of unfractionated heparin.
[Wisconsin Alumni Research Foundation, where warfarin was developed]
Warren, John C (1778–1856). Professor of Surgery and Anatomy at Harvard Medical School. It was at Warren’s invitation that Wells gave his demonstration of N2O anaesthesia, which ended in failure. Later, at Morton’s first public demonstration of diethyl ether, Warren performed the surgery.
Washout curves. Graphs displaying the exponential decline in concentration of a substance that is continuously being removed from a system. The substance may be ‘washed out’ by blood flow, in the case of dye dilution cardiac output measurement, or by ventilation of the lungs, in the case of nitrogen washout. The term is sometimes used to describe any negative exponential process.
Water, see Fluid balance; Fluids, body
Water balance, see Fluid balance
Water diuresis. Diuresis occurring about 15 min after the intake of a large volume of hypotonic fluid. Absorption of the fluid is followed by inhibition of vasopressin secretion and by increased urinary water loss.
Water intoxication, see Hyponatraemia
Waterhouse–Friderichsen syndrome, see Adrenocortical insufficiency
Waters bag, see Anaesthetic breathing systems
Waters, Ralph Milton (1883–1979). US anaesthetist; became Assistant Professor of Surgery in charge of anaesthetics at University of Wisconsin, leading to his appointment as the first university Professor of Anaesthesia in the USA (1933). Was the first to establish a resident training programme in anaesthesia and the first to use cyclopropane clinically (1930). Re-examined chloroform toxicity, advocated the use of inflatable cuffs on tracheal tubes, and was involved in many aspects of anaesthesia, including the use of thiopental and endobronchial intubation. Designed his ‘to-and-fro’ cannister for CO2 absorption in anaesthetic breathing systems, and the Waters airway, a metal oropharyngeal airway with a side-arm for attachment to a gas supply.
Waterton, Charles (1783–1865). Squire of Walton Hall, Yorkshire; made his first voyage to South America in 1812. Described the preparation of curare and the blowpipes, darts, bows and arrows used by the Indians of the Amazon and Orinoco basins. Experimented with the drug on his return to England, and maintained life in a paralysed donkey by employing IPPV. Published details of his work and travels in Wanderings in South America (1825).
Watt. Unit of power. One watt (W) = 1 joule per second (J/s).
Waveforms. Repetitive patterns plotted against time produce waveforms that may be complex (e.g. ECG) or simple, as in the sine wave. All complex waveforms may be mathematically deconstructed into component sine waves (Fourier analysis). For any sine wave, there is oscillation about a mean value, the maximal displacement from which is the amplitude. The number of complete oscillations per second is the frequency, and the distance between successive points at the same stage of the cycle is the wavelength. Waveform monitoring is very common in anaesthesia and intensive care, e.g. cardiovascular (ECG, intravascular pressures, plethysmography), respiratory (rate, depth, pattern), ventilatory (gas flow, pressure), neurological (intracranial pressure, EEG, nerve conduction studies).
Weaning from ventilators. Process of gradual withdrawal of ventilatory support. Usually presents no problems after less than a few days’ ventilation; following longer periods or poor baseline respiratory function, rapid weaning is less likely.
• Criteria for beginning weaning vary considerably; the following have been suggested:
absence of major organ or system failure, particularly CVS.
precipitating illness is successfully treated.
absence of severe infection or fever.
minimal sedation with absence of severe pain.
– arterial blood gases are near premorbid values.
– respiratory rate < 35 breaths/min.
– maximal negative inspiratory airway pressure attainable exceeds –25 cmH2O.
– airway occlusion pressure greater than 6 cmH2O below atmospheric.
– tidal volume > 5 ml/kg.
– minute ventilation < 10 litres.
– vital capacity > 10–15 ml/kg.
– FRC > 50% of predicted value.
