L
Labetalol hydrochloride. Combined β- and α-adrenergic receptor antagonist, with ratio of activities usually quoted between 2 : 1 and 5 : 1 respectively. Selective for α1-receptors, but non-selective at β-receptors, with some intrinsic sympathomimetic activity. Used to treat severe hypertension and pre-eclampsia, and in hypotensive anaesthesia. 90% protein-bound. Half-life is 4 h. Metabolised in the liver and excreted in urine and faeces. Undergoes extensive first-pass metabolism when given orally.
Labour, active management of. Term referring to a collection of medical interventions and management, including strict diagnostic criteria for the onset and course of labour, artificial rupture of membranes, early use of oxytocic drugs and continuous obstetric input so that the duration of labour is limited. Despite claims of improved outcome, evidence is at best conflicting.
Cephalopelvic disproportion may cause delay at any stage, but especially secondary arrest. Most other causes are treated successfully with oxytocic drugs (with continuous fetal monitoring).
Lack breathing system, see Coaxial anaesthetic breathing systems
β-Lactams. Group of substances containing the 4-atom β-lactam ring (Fig. 96). Include the penicillins, cephalosporins, the monobactam aztreonam and the carbapenems. The β-lactam ring is a site of breakdown by bacterial β-lactamase, leading to bacterial resistance.
Fig. 96 Structure of β-lactam ring
Lactate. Byproduct of anaerobic metabolism of the products of glycolysis. Hypoxia prevents aerobic metabolism of pyruvate to CO2 and water (via the tricarboxylic acid cycle); instead lactate is formed, with consequent increases in plasma lactate/pyruvate ratio (normally 10) and plasma lactate (normally 0.3–1.3 mmol/l). The liver removes 70% of lactate and the rest is converted to pyruvate by mitochondria-rich skeletal and cardiac muscle.
Lactic acidosis. Metabolic acidosis accompanied by raised plasma lactate levels.
those with overt tissue hypoxia and anaerobic respiration (type A):
– severe hypoxaemia.
– shock/haemorrhage/hypotension.
those without apparent initial tissue hypoxia (type B), further subdivided into:
– hepatic failure/renal failure (delayed clearance of lactate).
– severe infection, thiamine deficiency, alcoholic and diabetic ketoacidosis (pyruvate dehydrogenase dysfunction).
– cyanide poisoning, biguanides, salicylates, sodium valproate (uncoupling of oxidative phosphorylation).
– exercise, severe infection, seizures, β-adrenergic receptor agonists, fructose and sorbitol in TPN, malignancies (increased glycolysis such that the aerobic pathway is overwhelmed).
directed at the underlying cause (with supportive therapy).
minimisation of drugs that may exacerbate the problem (e.g. adrenaline).
cautious use of bicarbonate (increased lactate levels have followed its use).
Laevobupivacaine, see Bupivacaine
Lambert–Beer law, see Beer–Lambert law
Lamotrigine. Anticonvulsant drug, blocks voltage-gated sodium channels and thus inhibits the presynaptic release of glutamate and other excitatory neurotransmitters. Used for partial or secondary generalised seizures, either alone or in combination with other anticonvulsants. Has also been used for the treatment of bipolar disorders and in chronic pain management. Rapidly absorbed after oral administration with peak plasma concentrations at 2.5 h; half-life is 24–36 h. Plasma concentration is increased by sodium valproate but decreased by drugs causing enzyme induction (e.g. other anticonvulsants). It also induces its own metabolism.
Lanreotide. Long-acting somatostatin analogue; actions and effects are similar to those of octreotide. Used mainly in long-term management of acromegaly, by deep sc or im injection.
Lansoprazole. Proton pump inhibitor; actions and effects are similar to those of omeprazole.
LAP, see Left atrial pressure
– subcutaneous/mediastinal emphysema and pneumothorax.
– bradycardia or asystole due to vagal stimulation.
– gas may splint the diaphragm and reduce lung expansion.
– raised intra-abdominal pressure may increase risk of regurgitation and aspiration of gastric contents. High incidence of PONV.
– ICP is increased.
– explosion through ignition of intestinal methane by diathermy has been reported.
– reduced FRC and diaphragmatic splinting, with risk of atelectasis, hypoxaemia and hypoventilation.
– increased risk of regurgitation.
– bleeding may go unnoticed if not in the immediate area being worked on.
– large amounts of irrigating fluid may be used, with the risk of hypothermia if not warmed.
Other considerations include patient co-morbidity; e.g. obesity in patients for gastric banding. The above complications lead many anaesthetists to choose tracheal intubation and IPPV as the technique of choice, although the LMA is also commonly used for short gynaecological procedures in suitable patients.
Gerges FJ, Kanazi GE, Jabbour-Khoury SI (2006). J Clin Anesth; 18: 67–78
Laplace’s law. For a hollow distensible structure:
where P = transmural pressure
R1 = radius of curvature in one direction
R2 = radius of curvature in the other direction
For a cylinder, one radius = infinity, therefore 
For a sphere, both radii are equal, therefore 
• Physiological/clinical importance:
– arteriolar smooth muscle response to fluctuating intraluminal pressure: wall tension varies in order to maintain constant radius and blood flow (one theory of the mechanism of autoregulation).
– as intraluminal pressure in arterioles or airways falls, or external pressure rises, there is a critical closing pressure across the wall at which collapse may occur. In the lungs, this may occur during forced expiration, limiting expiratory air flow.
– ventricular cardiac muscle must generate greater tension when the heart is dilated than when of normal size, in order to produce the same intraventricular pressure. Thus an enlarged failing heart must contract more forcibly to sustain BP, hence the benefit of reducing preload.
– in the lungs, alveoli would tend to collapse as they became smaller, were it not for surfactant, which reduces surface tension.
– if the outlet of an anaesthetic breathing system is obstructed, the reservoir bag distends, thus limiting the dangerous build-up of pressure that would occur within a non-distensible bag.
Larrey, Baron Dominique Jean (1766–1842). French surgeon-in-chief to Napoleon. Employed refrigeration anaesthesia in 1807, and again in 1812 during the Russian campaign to allow painless amputations in half-frozen soldiers. Also employed triage. Supported Hickman when the latter presented his experiments on ‘suspended animation’ to the French Academy in 1828.
[Napoleon Bonaparte (1769–1821), French Emperor]
Baker D, Cazalaà J-B, Carli P (2005). Resuscitation; 66: 259–62
Laryngeal mask airway (LMA). Device invented by Brain and introduced into practice in 1988 for supporting and maintaining the airway without tracheal intubation. Consists of an oval head attached to a connecting tube (Fig. 97a). The head is inserted blindly into the pharynx to lie against the back of the larynx, and the circumferential cuff inflated to form a seal. Pressing the junction of the head and tube backward and upward against the palate during insertion has been recommended by the inventor; alternative methods include insertion with the bowl facing upwards and rotating it 180° once inserted. Tolerated at lighter levels of anaesthesia than a tracheal tube. Insertion is easier following propofol induction of anaesthesia than thiopental induction, because of the former’s greater suppression of laryngeal reflexes.
Has been used for spontaneous or controlled ventilation, the latter using inflation pressures of up to 10–25 cmH2O. Does not protect against aspiration of gastric contents. May be removed before the patient wakes, or left in position. A bite block is required to prevent obstruction or damage by the teeth.
routine inhalational anaesthesia.
airway maintenance in difficult intubation, in both previously unsuspected and known cases.
emergency management of failed intubation.
CPR.
Also available with reinforced tubes to prevent kinking. More recent developments include: the ProSeal LMA, which features a larger cuff (providing a better seal against the glottis), and a gastric drainage port that opens at the tip of the cuff (Fig. 97b); and the intubating LMA, in which the tube is shorter and rigid with a sharp angle. In the latter, the bars covering the laryngeal aperture are replaced by a single flap that lifts the epiglottis when a tracheal tube (a soft silicone one specifically provided for the purpose) is passed blindly through it (Fig. 97c). The intubating and standard LMAs have been used (with or without a fibreoptic ’scope) in known and unsuspected cases of difficult intubation. A version of the intubating LMA has also been introduced that incorporates a small display screen mounted at the proximal end of the airway so that intubation can be observed in real time.
Laryngeal nerve blocks, see Intubation, awake
Laryngeal nerves. Derived from the vagus nerves:
– arises at the base of the skull and passes deep to the carotid arteries.
– the internal laryngeal nerve pierces the thyrohyoid membrane with the superior laryngeal vessels, supplying the mucous membrane of the larynx down to the vocal cords.
– on the left, arises anterior to the ligamentum arteriosus, passes below and behind it and the aorta and ascends in the neck (see Fig. 113; Neck, cross-sectional anatomy).
– in the neck, ascends in the groove between the oesophagus posteriorly and trachea anteriorly.
superior laryngeal: slack cord and weak voice.
if one side only is affected, the contralateral cord may move across and restore the voice.
Branches may be damaged during surgery (e.g. thyroidectomy) and tracheal intubation, especially if undue force is used or the cuff is inflated within the larynx. The recurrent laryngeal nerve may be involved by lesions in the neck, thorax or mediastinum (on the left).
The superior laryngeal nerve may be blocked to allow awake tracheal intubation.
[Sir Felix Semon (1849–1921), German-born English laryngologist]
Laryngeal reflex. Laryngospasm in response to stimulation of the laryngeal/hypopharyngeal mucosa. Afferent pathway is via the laryngeal nerves, vagus and brainstem.
Laryngoscope. Instrument used to perform laryngoscopy. The first direct-vision laryngoscope was invented by Kirstein and later developed by Jackson; the principle was later modified by Magill, Macintosh and others.
– contains a battery power source (originally connected to mains electricity).
– short or adjustable handles are available; smaller lighter handles are usually used for paediatric anaesthesia. The Anderson laryngoscope handle bears a hook for the left index finger, allowing laryngoscopy using only the thumb and index finger whilst the other fingers of the left hand are free to apply pressure over the front of the infant’s larynx.
– usually set at right angles to the handle.
– many different laryngoscope blades have been described, most of them interchangeable when standard attachments are used.
Devices incorporating viewing channels or with video chips at the distal end allow either placement of the device in the trachea under visual control, with advancement of the tracheal tube over it, or identification of the glottis and observation of the tube’s passage through the vocal cords. The image may be viewed by looking through an eyepiece, attachment to a camera/video system or via a screen incorporated into the device itself (Fig. 98). Flexible fibreoptic instruments are also available.
Concerns over cross-infection, especially transmission of variant Creutzfeldt–Jakob disease, have led to widespread use of disposable laryngoscope blades, laryngoscopes or blade covers/sheaths.
Laryngoscope blades. Parts of laryngoscopes inserted into the mouth.
base for attachment to handle.
flange: parallel to the tongue; usually only present for the proximal one- to two-thirds of the blade.
Most are designed for use with the laryngoscope handle held in the left hand; i.e. the tongue is pushed to the left side of the patient’s mouth by the flange and web.
• Common varieties (Fig. 99a):
Macintosh (1943): tongue, web and flange form a reverse Z shape in cross-section. The most commonly used blade in the UK; also popular in the USA. Available in large adult, adult, child and baby sizes; the latter size was not designed by Macintosh and was criticised by him as being anatomically incorrect. A ‘left-handed’ version is available, for use when anatomical features of the airway require insertion of the tracheal tube from the left side of the mouth instead of the right. The McCoy blade (1993) is hinged at the tip, and is controlled by a lever on the laryngoscope handle. It allows elevation of the epiglottis whilst reducing the amount of force required during laryngoscopy. Although it may make a difficult laryngoscopy easier, it may also make an easy one more difficult. Another blade with a similar function to the McCoy is actually flexible throughout its length; its curvature is increased by a lever on the handle.
Magill (1926): U-shaped in cross-section.
• Specific paediatric blades (Fig. 99b):
• Others:
Guedel and Flagg (1928): similar to Magill’s, but with the bulb at the tip. Guedel’s is set at an acute angle to the handle.
Siker (1956): angled blade incorporating a mirror at the angle.
Laryngoscopy. Act of viewing the larynx. Indirect laryngoscopy was first described in 1855 in London by Garcia using a mirror. Direct laryngoscopy was pioneered by Kirstein, Killian and Jackson in the late 1800s/early 1900s, and is now the technique most commonly used for tracheal intubation. The view of the larynx during direct laryngoscopy is shown in Fig. 100.
Anaesthesia for diagnostic or therapeutic laryngoscopy must provide relaxation of the jaw and vocal cords, with rapid recovery of laryngeal reflexes without laryngospasm. Problems include sharing of the airway, the hypertensive response to laryngoscopy and contamination of the airway with blood and debris. Usually performed under general anaesthesia, with IPPV through a special 5–6 mm ‘microlaryngoscopy’ cuffed tracheal tube (resistance is too high for spontaneous ventilation). Other methods include injector and insufflation techniques as for bronchoscopy. Spraying the cords with lidocaine reduces postoperative laryngospasm but at the expense of diminished laryngeal reflexes.
[Manuel Garcia (1805–1906), Spanish singing teacher]
See also, Intubation, complications of; Intubation, difficult; Intubation, tracheal
Laryngospasm. Reflex closure of the glottis by adduction of the true and/or false cords. May persist after cessation of its stimulus. The precise mechanism is controversial; the lateral cricoarytenoid muscles are thought to be most important in adducting the cords whilst cricothyroid tenses them. The extrinsic muscles of the larynx may also have a role.
response to other stimulation, e.g. surgery, movement, stimulation of anus, cervix (Brewer–Luckhardt reflex).
May cause complete or partial airway obstruction, the latter often presenting as inspiratory stridor. Causes hypoxaemia and hypoventilation; pulmonary oedema has been reported.
when laryngospasm has subsided, depth of anaesthesia may be deepened.
laryngeal muscle relaxation may be achieved with suxamethonium (as little as 8–10 mg may suffice) or propofol, followed by ventilation with O2 and tracheal intubation if necessary.
local anaesthetic spray to the larynx and laryngeal nerve blocks.
use of neuromuscular blocking drugs and tracheal intubation.
protects the tracheobronchial tree and lungs, e.g. during swallowing.
Extends from the root of the tongue to the cricoid cartilage, i.e. level with C3–6 (at higher level in children).
length: 45 mm (men); 35 mm (women).
anteroposterior: 35 mm (men); 25 mm (women).
transverse: 45 mm (men); 40 mm (women).
• Composed of hyoid bone, and epiglottic, thyroid, cricoid, arytenoid, corniculate and cuneiform cartilages, joined by several muscles and ligaments (Fig. 101):
– attached posteriorly to the pharynx by the middle constrictor muscle.
– attached to the arytenoid laterally by the aryepiglottic membrane.
– attached superiorly to the hyoid bone by the thyrohyoid membrane and muscle.
– attached via its superior surface to the thyroid cartilage by the cricothyroid membrane.
– attached via its posterior surface to the arytenoids by the posterior cricoarytenoid muscles.
– attached inferiorly to the trachea by the cricotracheal membrane.
– pyramid-shaped, the bases articulating with the back of the cricoid.
– also attached to the cricoid by posterior and lateral cricoarytenoid muscles.
– the vocal cords pass from the vocal processes anteriorly to the back of the thyroid cartilage.
– attached to the epiglottis superomedially via the aryepiglottic folds and muscles.
– attached anterolaterally to the back of the thyroid cartilages by the thyroarytenoid muscles.
– attached to each other by the transverse arytenoid muscle.
cuneiform cartilages: lie anterior to the corniculate cartilages, in the aryepiglottic folds.
• Membranes and areas of the larynx:
– passes from the anterior arytenoid to lateral epiglottis.
thyrohyoid membrane: lateral borders are thickened to form the lateral thyrohyoid ligaments.
The entrance to the larynx slopes downwards and backwards, bounded anteriorly by the epiglottis, laterally by the aryepiglottic folds and posteriorly by the arytenoid cartilages. The piriform fossa is the recess on each side, between the aryepiglottic folds medially and thyroid cartilage and thyrohyoid membrane laterally. The rima glottidis is the narrowest part of the airway in adults; the cricoid is the narrowest in children.
• Epithelium: squamous above the cords, columnar below. Mucosa of the cords is closely adherent.
the cords are tensed by cricothyroid and relaxed by thyroarytenoid and vocalis muscles.
