General anaesthesia

General anaesthesia can encompass several different end points, the most critical of which can be defined as unconsciousness. A loss of sensation including that of any painful stimulus, and muscle relaxation are also desirable endpoints of general anaesthesia. Modern-day anaesthesia is accomplished using a balanced combination of different drugs to confer these different endpoints. 

General anaesthetics have been in use for the last 100 years, yet their mechanism of action are still not yet clearly defined. For many years it was thought that general anaesthetics exerted their effects by dissolving in cell membranes and perturbing the lipid environment in a non-specific manner. This theory derived from the observation that for a number of drugs which induced anaesthesia, their potency correlated with their oil-water partition coefficients. This Meyer-Oveiton correlation was accepted for a number of years, however in the last 15-20 years evidence has shown that a more likely theory is that of specific interactions of anaesthetics with proteins, particularly those within the CNS that mediate neurotransmission [1]. 

Franks NP, Lieb WR (1994) Molecular and cellular mechanisms of general anaesthesia. Nature 367 607-614... 

Fornazzari L, Wilkinson DA, Kapur BM, et al Cerebellar, cortical and functional impairment in toluene abusers. Acta Neurol Scand 67 319—329, 1983 Franks NP, Lieb WR Molecular and cellular mechanisms of general anaesthesia. Nature 367 607-614, 1994... 

Dybendal X Guttormsen AB, Elsayed S, Askeland 48 B, Harboe T, Florvaag E Screening for mast cell tryptase and serum IgE antibodies in 18 patients with anaphylactic shock during general anaesthesia. 

Direct eye contact with liquid produces injury, generally transient, to the corneal epithelium. The liquid is mildly irritating to the skin due to the degreasing effect repeated contact may cause dermatitis. Ingestion of substantial quantities of liquid can damage the mucous membranes, and produce acute effects ranging from mild discomfort to profound anaesthesia. 

Humphrey JH, McClelland M. 1944. Cranial-nerve palsies with herpes following general anaesthesia. Br MedJ 1 315-318. 

Casele-Rondi, G. (1996). Perceptual processing during general anaesthesia... 

Manipulations of any kind in untrained carnivores require general anaesthesia. Sample collection was therefore performed on anaesthetised individuals. Depending on the preferences of the veterinarian, the fossas were either caught with nets or anaesthetised by a combination of injectable narcotics (Xylazine 2.5-5.0 mg/kg, Ketam-inhydrochloride 10.5-20 mg/kg and Diazepam 0.5-1.0 mg/kg) prior to anaesthesia by inhalation gases (Isoflurane 1.5-3 Vol. % and Oxygen l-2L/min) applied by facial mask. 

Although the topic of anaesthesia is hugely complicated, it is clear that the physiological effect of the compounds depends on their entrapment in the blood. Once dissolved, the compounds pass to the brain where they promote their narcotic effects. It is now clear that the best anaesthetics dissolve in the lipids from which cell membranes are generally made. The anaesthetic probably alters the properties of the cell membranes, altering the rates at which neurotransmitters enter and leave the cell. 

Metformin is a biguanide used to treat diabetes mellitus. It is contraindicated in patients undergoing general anaesthesia since anaesthesia can interfere with renal function. The risk of lactic acidosis associated with metformin increases in patients with renal impairment. Metformin should be stopped before and during surgery where anaesthesia is indicated. Metformin should only be restarted after the renal function has returned to normal. 

Most studies were performed with hyperosmolar solutions. Hypertonic disruption is under clinical evaluation for enhanced delivery of small molecular weight cytostatic agents to brain tumours. Technically, the procedure is performed as a high-flow short-term infusion of 25% mannitol or arabinose under general anaesthesia. The underlying mechanism is a sequelae of endothehal cell shrinkage, disruption of tight junctions and vasodilatation by osmotic shift [72]. 

The muscarinic cholinergic receptor is competitively blocked by hyoscine and hyoscyamine. Both of these alkaloids are more active than atropine. The applications of these alkaloids for clinical purposes are connected with the induction of general anaesthesia. Clinical consideration should be paid to the fact that these alkaloids also affect the brain and thereby the central nervous system. Atropine crosses the blood-brain barrier. Hyoscine and hyoscyamine depress the motor areas of the cerebral cortex. 

Fig. 15. Relationship between the alfentanil plasma concentrations and the probability of needing naloxone to restore adequate spontaneous ventilation. The diagram at the upper part shows the alfentanil plasma concentrations of the patients who required naloxone (upward deflection) or did not require naloxone (downward deflection). The plasma concentration-effect curve for this clinical endpoint (lower part) was defined from the quantal data shown in the upper diagram using logistic regression. Bars indicate SE of C5o%. (From Ausems ME, Hug CC, Stanski DR, Burm AGE. Plasma concentrations of alfentanil required to supplement nitrous oxide anaesthesia for general surgery. Anaesthesiology 1986 65 362-73, reproduced by permission.)...

Nerve block anaesthesia in general practice mostly concerns finger or toe blocks. Lidocaine or prilocaine, 10 mg/ml without adrenaline (norepinephrine), is used and is injected on each side of the finger or toe, in two portions by the four nerve branches. Injection of larger volumes than 1-2 ml/side carries a risk of ischaemia because of the firm tissue. The transport through the nerve sheath takes a few minutes, and for a satisfactory result one should wait 5-10 minutes before the planned intervention starts. 

Nitrous oxide is used for induction and maintenance of anaesthesia. It is widely used as carrier gas for other volatile agents in general anaesthesia. The usual concentra-... 

Barbiturates are the derivatives of barbituric acid. They are general CNS depressants. They can cause sedation, hypnosis and general anaesthesia depending upon the particular barbiturates used and its dose. 

Anaesthesia The ultra short acting barbiturates produce general anaesthesia (details are given in chapter General anaesthetics ). 

For general anaesthesia Ultra short acting barbiturates are used. 

It is indicated for preoperative sedation, conscious sedation prior to short diagnostic or endoscopic procedures, induction of general anaesthesia prior to administration of other anaesthetic agents. 

It is indicated as a narcotic analgesic supplement in general or regional anaesthesia, as an anaesthetic agent with oxygen and skeletal relaxant in selected high risk patients (e.g. open heart surgery). 

As a adjuvant to general anaesthesia (specially in major surgical procedures e.g. abdominal and thoracic surgery, orthopaedic procedures, intubation etc). 

Pre-anaesthetic medication These agents reduce the salivary and respiratory secretion and are administered half an hour before general anaesthesia. They also prevent laryngospasm. Atropine is given in combination with morphine as a preanaesthetic medication to antagonize the central depressant action of morphine on respiration. 

It is indicated in prophylaxis or treatment of ventricular arrhythmias associated with Ml, digitalis intoxication, ventricular tachyarrhythmia, in patients predisposed to ventricular arrhythmias during general anaesthesia. 

---