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.
