Hello there! In this video lesson from the “Anaesthesia Unravelled” web series we will be talking about “Intravenous General Anaesthetics”. General anaesthesia promotes a generalized depression of the brain, either by intravenous or inhalation anaesthesia, due to the action on GABAergic receptors, prolonging their action. An ideal anaesthetic agent should be stable in solution, promote a swift induction, rapidly achieve the equilibrium between blood and brain drug concentrations, preserve cardiopulmonary stability, be easily metabolised and enable a rapid and smooth recovery.
Intravenous anaesthesia has been tested as an anaesthetic method since the 17th century, but it was only in the beginning of the 20th century that the first intravenous anaesthetics could be successfully synthesized. In this video lesson we’ll be covering barbiturates, etomidate, propofol and alfaxalone. We will also talk about the guaiacol glyceryl ether (guaifenesin), which is not a general anaesthetic but frequently used as a co-inducer in large animals.
Barbiturates were the first intravenous general anaesthetics used on a large scale, mainly during the first half of the 20th century. Anaesthesia by thiopental lasts up to 15 minutes and by pentobarbital up to 90 minutes. One of the main problems of barbiturates is cardiovascular depression, with a decrease in stroke volume, hypotension and a reduction in the cardiac output. Compensatory tachycardia may occur, but it is not enough to balance the depressant effects. Another problem is their cumulative effect, which occurs due to the deposition of the agents in adipose tissue and slow biotransformation. So, barbiturates should be avoided in patients with cardiac, pulmonary, liver and weight disorders.
Etomidate is an excellent anaesthetic for cardiac patients. Its cardiovascular depression is practically inexistent in clinical doses. Another interesting feature is that it is biotransformed by liver hydrolysis and plasma esterases, being recommended for patients with liver disease. However, an undesired effect of etomidate is the inhibition of cortisol synthesis, even after a single dose. Therefore, it is used only as an induction agent. It also promotes insufficient muscle relaxation after induction.
Propofol was introduced in the 1970s, quickly gaining prominence due to its rapid biotransformation. In addition to being an ideal anaesthetic for patients with liver disease, it became the basis for general intravenous anaesthesia (TIVA). However, it should be noted that propofol promotes more severe cardiovascular depression than that of thiopental, detail that must be considered in cardiac patients.
Alphaxalone came back to prominence, as it was recently reintroduced to the market with a diluent that does not promote an adverse effect. The pharmacokinetics are similar to propofol, but this drug is much safer for the cardiovascular system, even at high doses. Another advantage is the possibility of intramuscular administration.
Guaifenesin is a central muscle relaxant, widely used as a co-inducer in the anaesthesia of horses. It is a physiologically safe medication, but its preparation can be quite laborious. Currently, it has been used more frequently as a basis for continuous infusions in horses than as an induction agent.
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– Cattai et al. The incidence of spontaneous movements in dogs undergoing total intravenous anaesthesia with propofol. Vet Anaesth Analg, 42:93-98, 2015.
– Goodchild CS; Serrao JM. Propofol-induced cardiovascular depression: science and art. Br J Anaesth, 10.1093/bja/aev320.
– Gozalo-Marcilla M, Aguiar AJA. Anestesia Total Intravenosa. In: Luna SPL, Carregaro, AB. Anestesia e Analgesia de Equideos, Ruminantes e Suínos. 437-460, 2019.
– Kashtan et al. Comparative evaluation of propofol and thiopentone for total intravenous anaesthesia. Can J Anaesth, 37:2, 170-6, 1990.
– Maney et al. A comparison of cardiopulmonary and anesthetic effects of an induction dose of alfaxalone or propofol in dogs. Vet Anaesth Analg, 40:237-244, 2013.
– Taboada FM, Murison PJ. Induction of anaesthesia with alfaxalone or propofol before isoflurane maintenance in cats. Vet Rec., 167:85-89, 2010.