U
U wave. Low-amplitude positive deflection following the T wave of the ECG, possibly representing slow repolarisation of papillary muscle. Seen best in the right chest leads and at slow heart rates but not always present. Made more prominent by hypokalaemia. Reversed polarity may indicate myocardial ischaemia.
U–D interval. During caesarean section, the time between incision of the uterus and delivery of the baby. As the interval increases, so fetal wellbeing is compromised, probably due to disruption of placental blood flow. Fetal acidosis is thought to be unlikely at U–D intervals of 1.5–3 min.
UEMS, European Union of Medical Specialties (Union Européenne des Médecins Spécialistes; UEMS), see European Board of Anaesthesiology
Ulcerative colitis, see Inflammatory bowel disease
Ulnar artery. A terminal branch of the brachial artery, arising at the apex of the antecubital fossa. Lies superficial to flexor digitorum profundus and deep to the superficial flexors in the forearm. Then passes deep to flexor carpi ulnaris, lateral to the ulnar nerve. At the wrist, it lies between flexor carpi ulnaris and flexor digitorum profundus tendons. Passes anterior to the flexor retinaculum to end lateral to the pisiform bone. Branches supply the deep extensor and ulnar muscles of the forearm, the wrist and elbow joints and the deep and superficial palmar arches. May be cannulated for arterial BP measurement but the radial artery is preferred because the latter is thought to be associated with a lower risk of digital ischaemia.
Ulnar nerve (C7–T1). A terminal branch of the medial cord of the brachial plexus. Descends on the medial side of the upper arm, first in the anterior, and later in the posterior compartment. Passes behind the medial epicondyle to enter the forearm; descends on the medial side deep to flexor carpi ulnaris, medial to the ulnar artery. Divides into cutaneous branches 5 cm above the wrist. Dorsal and palmar cutaneous sensory branches supply the skin of the ulnar half of the hand and palm, and the medial 
fingers. Also supplies the elbow joint, flexor carpi ulnaris and the ulnar side of flexor digitorum profundus. In the hand, it supplies the hypothenar muscles, interossei, medial two lumbricals and adductor pollicis.
fingers and the ulnar side of the hand. Paralysis of the small muscles of the hand results in clawing.May be blocked at various sites.
See Brachial plexus block; Elbow, nerve blocks; Wrist, nerve blocks
Ultrafiltration. Process by which water is removed from the blood during various forms of dialysis. Water passes across the semipermeable membrane as a result of positive pressure on the blood side of the membrane (e.g. the patient’s BP or use of a pump), negative pressure on the other side, or an osmotic gradient from use of dialysate fluid. Rates of up to 1.5 l/h may be removed by intermittent isolated ultrafiltration (IIUF) in which a haemodialysis circuit is used but without dialysate, but more controlled removal of fluid (100–150 ml/h) may be achieved in slow continuous ultrafiltration (SCUF), with greater haemodynamic stability and without the need for fluid replacement. Effective for treatment of decompensated heart failure. Combination with dialysis may also be employed (SCUF-D) but removal of larger molecules is less efficient than with continuous haemofiltration.
Ultra-rapid opioid detoxification, see Rapid opioid detoxification
a transducer uses a piezoelectric material to convert electrical energy into intermittent pulses of high frequency (3–15 MHz) sound waves.
transducer probes may have linear, curvilinear or phased arrays. Linear arrays tend to emit at a higher frequency than curvilinear arrays (e.g. 10 vs. 3 MHz), delivering greater resolution, but poorer tissue penetration; they are therefore suitable for imaging superficial structures, e.g. for internal jugular venous cannulation. Curvilinear arrays produce a signal that spreads out within the body, allowing imaging of deeper and larger structures (e.g. fetus). Phased arrays deliver electronically angulated beams that can be ‘swept’ through the body, providing relatively large sector images from a small probe ‘footprint’ (e.g. allowing passage of the beam between ribs for echocardiography).
assessment of blood flow (using the Doppler effect): e.g. in cardiac output measurement; transcranial Doppler ultrasound; assessing patency of peripheral vessels.
– real-time guided needle placement for blockade of peripheral nerves and nerve plexuses.
– identification of vertebral anatomy and estimation of the depth of the epidural space during central neuraxial blockade.
confirmation of normal anatomy and/or real-time guided needle placement during central venous cannulation (recommended by NICE).
Marhofer P, Harrop-Griffiths W, Willschke H, Kirchmair L (2010). Br J Anaesth; 104: 673–83
See also, Doppler effect; Imaging in intensive care; Transoesophageal echocardiography
Units, SI. System of units (Système Internationale d’Unités) introduced in 1960 by the General Conference of Weights and Measures (Conférence Générale des Poids et Mesures) and based on the metric system. There are seven base (or fundamental) units: metre, second, kilogram, ampere, kelvin, candela and mole. Derived units include the newton, pascal, joule, watt and hertz. Standard terms denote multiples and divisions of units, e.g. kilo- (× 103), mega- (× 106), giga- (× 109), tera- (× 1012) and peta- (× 1015); and milli- (× 10−3), micro- (× 10−6), nano- (× 10−9), pico- (× 10−12) and femto- (× 10−15) respectively.
Universal gas constant. Constant (symbol R) in the ideal gas law equation PV = nRT, where P = pressure, V = volume, n = number of moles of gas and T = temperature of a perfect gas. Equals 8.3144 J/K/mol (1.987 cal/K/mol).
Uraemia. Strictly, a plasma urea exceeding 7.0 mmol/l; the term was formerly used to describe the clinical picture in renal failure.
Urapidil. α1-Adrenergic receptor antagonist with central 5-HT1A receptor agonist activity. Causes reduction in preload and afterload by causing arterial and vasodilatation with little reflex tachycardia. Has little effect on the coronary vessels. Although studied experimentally with promising results, it is not available commercially.
Urea (NH2CONH2). A product of hepatic amino acid breakdown to ammonia. Produced in the urea cycle from hydrolysis of arginine; ornithine is also produced and reacts with carbamoyl phosphate and then aspartate to reform arginine. Ammonia and CO2 are introduced into the cycle by ‘carrier’ molecules. Freely filtered at the glomerulus of the nephron; about 50% is reabsorbed in the proximal tubule. Excretion in the urine accounts for 85% of daily nitrogen excretion. Normal plasma levels: 2.5–7.0 mmol/l. Increased production (e.g. from increased protein intake or catabolism) or dehydration may increase plasma urea slightly, but levels above 13 mmol/l usually represent impaired renal function. Creatinine measurement or clearance studies may aid diagnosis.
Urinary retention. Inability to pass urine. May be either acute or chronic (the latter often leading to retention with overflow). May occur with prostatic enlargement, urethral stricture, spinal or epidural anaesthesia (including use of spinal opioids), after abdominal or pelvic surgery and following administration of drugs with anticholinergic effects. Neurological causes are rarer but include spinal cord injury, cauda equina syndrome, Guillain–Barré syndrome and autonomic neuropathies, e.g. diabetic. Signs include a full bladder, tender to palpation and dull to percussion. Can be confirmed with bladder ultrasound. May cause agitation and confusion postoperatively; can cause hypertension, tachycardia and raised ICP in unconscious patients. May cause acute pyelonephritis.
Urinary tract infection (UTI). Most common nosocomial infection seen in critically ill patients and a common cause of generalised sepsis. In hospitals, almost always associated with urinary catheterisation, with the risk increasing the longer the catheter is in place. Gram-negative organisms (e.g. Escherichia coli, pseudomonas) are commonly involved, gaining entry to the bladder either through the catheter’s lumen or along its surface. Diagnosis is confirmed by the presence of white blood cells and > 100 000 organisms/mm3 on urine microscopy. Urinary catheterisation should only be performed when necessary, aseptic technique used (often accompanied by a single prophylactic dose of gentamicin) and the catheter removed as soon as possible. Treatment of established UTI is with antibacterial drugs according to the results of urine culture.
Hooton TM, Bradley SF, Cardenas DD, et al (2010). Clin Infect Dis; 50: 625–63
Urine. Liquid containing urea and other waste products, excreted by the kidneys. Normal output in temperate climates is 800–2500 ml/day. Coloured yellowish by the pigments urochrome and uroerythrin, it darkens on standing by oxidation of urobilinogen to urobilin (colour does not necessarily reflect urine’s concentration). Coloured red by haemoglobin or myoglobin and purple in porphyria. Specific gravity is normally 1.002–1.035. Osmolality may range between 30 and 1400 mosmol/kg, depending on fluid and hormonal status. pH is usually below 5.3. Normally contains under 150 mg protein/24 h. Abnormal constituents include glucose, ketones, bilirubin, erythrocytes, large numbers of leucocytes and casts.
Despite the kidneys’ ability to concentrate the urine, a minimum of 500 ml/day is required to eliminate urea and other electrolytes. Oliguria is usually defined as less than 0.5 ml/kg/h and may indicate hypovolaemia or renal failure; anuria is complete cessation of urine flow and may indicate obstruction or urinary retention in addition. Polyuria occurs in diabetes insipidus, renal failure, diuretic therapy, diabetes mellitus (because of the osmotically active glucose load) and excessive water intake (water diuresis).
Urine output is routinely measured during critical illness and major surgery, since it reflects tissue perfusion and volume status of the circulation (assuming normal renal and cardiac function). Although renal blood flow is often reduced and circulating levels of vasopressin are high during surgery, urine output is usually maintained. An hourly urine output of > 0.5 ml/kg/h is regarded as the minimum acceptable during critical illness or surgery by most anaesthetists.
Urokinase. Enzyme extracted from male human urine, used as a fibrinolytic drug mainly for thrombolysis in the eye and arteriovenous shunts, although also indicated in PE or DVT. Acts via activation of plasminogen.
sympathetic motor preganglionic fibres from T1–L2 and parasympathetic motor preganglionic fibres from S2–4 via the paracervical plexus. Actions are variable, depending on the stage of the menstrual cycle and pregnancy.
• Actions of drugs on the pregnant uterus:
α-adrenergic receptor agonists, e.g. noradrenaline: increase uterine tone and strength of contraction.
β-adrenergic receptor agonists, e.g. adrenaline, salbutamol: decrease uterine tone and strength of contraction. Agonists specific for β2-receptors are used as tocolytic drugs to delay premature labour.
oxytocin and ergometrine: produce powerful contraction. Atosiban (oxytocin antagonist) causes uterine relaxation.
prostaglandins PGE2 and PGF2α: stimulate uterine contraction.
volatile inhalational anaesthetic agents: cause dose-related reduction of uterine tone.
others: acetylcholine, bradykinin, histamine and 5-HT increase contraction. Smooth muscle relaxants (e.g. amyl nitrite, GTN and papaverine) cause relaxation. Alcohol has a direct relaxant action and suppresses oxytocin secretion from the pituitary gland.
Utstein style. Uniform system of reporting data for out-of-hospital cardiac arrests, arising from a meeting of representatives of international Resuscitation Councils in Utstein Abbey on the Island of Mosteroy off Norway in June 1990. A second meeting (the Utstein II conference) in London the same year resulted in the publication of recommended guidelines for uniform reporting of such data, the ‘Utstein style’. This ‘style’ has been recommended for in-hospital CPR attempts, paediatric CPR, laboratory CPR research and trauma.
