Thiopental anesthesia induction

Various barbiturates such as the short acting agent pentobarbital and the ultra-short acting agents thiopental and methohexital are used for anesthesia induction. They produce loss of consciousness without analgesia and with little effects on the cardiovascular system. Unconsciousness is combined with respiratory depression as the barbiturates produce non-selective CNS depression. 

Anesthesia induction with propofol causes a significant reduction in blood pressure that is proportional to the severity of cardiovascular disease or the volume status of the patient, or both. However, even in healthy patients a significant reduction in systolic and mean arterial blood pressure occurs. The reduction in pressure appears to be associated with vasodilation and myocardial depression. Although propofol decreases systemic vascular resistance, reflex tachycardia is not observed. This is in contrast to the actions of thiopental. The heart rate stabilization produced by propofol relative to other agents is likely the result of either resetting or inhibiting the baroreflex, thus reducing the tachy-cardic response to hypotension. 

The answer is c. (Hardman, p 323.) Induction of anesthesia by parenteral administration of thiopental sodium and other barbiturates is... 

Thiopental and methohexital belong to the barbiturates which, depending on dose, produce sedation, sleepiness, or anesthesia. Barbiturates lower the pain threshold and thereby facilitate defensive reflex movements they also depress the respiratory center. Barbiturates are frequently used for induction of anesthesia. 

Thiopental remains the most popular IV induction agent. Its rapid and pleasant induction of anesthesia and its relatively low cost are among the reasons for its high acceptance rate by both the patient and the practitioner. Also, it does not induce obstructive secretions in the airway, produces little or no emesis, and does not sensitize the myocardium to endogenous catecholamines that may be released in response to the stress of surgery. It can, however, cause cardiovascular depression. 

The general pharmacology of the barbiturates is discussed in Chapter 22. Thiopental is a barbiturate commonly used for induction of anesthesia. Thiamylal is structurally almost identical to thiopental and has the same pharmacokinetic and pharmacodynamic profile. 

The induction of unconsciousness may be the result of exposure to excessive concentrations of toxic solvents such as carbon tetrachloride or vinyl chloride, as occasionally occurs in industrial situations (solvent narcosis). Also, volatile and nonvolatile anesthetic drugs such as halothane and thiopental, respectively, cause the same physiological effect. The mechanism(s) underlying anesthesia is not fully understood, although various theories have been proposed. Many of these have centered on the correlation between certain physicochemical properties and anesthetic potency. Thus, the oil/water partition coefficient, the ability to reduce surface tension, and the ability to induce the formation of clathrate compounds with water are all correlated with anesthetic potency. It seems that each of these characteristics are all connected to hydrophobicity, and so the site of action may be a hydrophobic region in a membrane or protein. Thus, again, physicochemical properties determine biological activity. 

When utilized as sedative hypnotics, barbiturates are administered orally. They are rapidly and completely absorbed by this route with nearly 100% bioavailability and an onset of action ranging from 10 to 60 min.3 Sodium salts are more rapidly absorbed than free acids. Intramuscular injections of sodium salts should be made deep into the muscle to prevent pain and tissue damage. Some barbiturates are also administered rectally barbiturates utilized for the induction and maintenance of anesthesia (thiopental) or for treating status epilepticus (phenobarbital) are administered intravenously. 

Effects on respiration are similar to those of thiopental at usual anesthetic doses. However, propofol causes a marked decrease in systemic blood pressure during induction of anesthesia, primarily through decreased peripheral resistance. In addition, propofol has greater negative inotropic effects on the heart than etomidate and thiopental. Apnea and pain at the site of injection are common adverse effects of bolus administration. Muscle movements, hypotonus, and (rarely) tremors have also been reported following its use. Clinical infections due to bacterial contamination of the propofol emulsion have led to the addition of antimicrobial adjuvants (eg, ethylenediaminetetraacetic acid and metabisulfite). 

Barbiturates are classified according to their duration of action (Figure 9.7). For example, thiopental [thye oh PEN tal], which acts within seconds and has a duration of action of about 30 minutes, is used in the intravenous induction of anesthesia. By contrast, phenobarbital [fee noe BAR bi tal], which has a duration of action greater than a day, is useful in the treatment of seizures (see p. 148). Pentobarbital [pen toe BAR bi tal], secobarbital [see koe BAR bi tal] and amobarbital [am oh BAR bi tal] are short-acting barbiturates, which are effective as sedative and hypnotic (but not antianxiety) agents. 

Malignant hyperthermia has been described in a 10-year-old boy who received thiopental and suxamethonium for induction of anesthesia, followed by desflurane for maintenance of anesthesia (15). 

The effects of desflurane and sevoflurane on bronchial smooth muscle reactivity have been compared in a randomized study of 40 patients (36). Anesthesia was induced with thiopental, followed by muscle relaxation and ventilation. Airway pressures were recorded during administration of desflurane or sevoflurane at one minimal alveolar concentration (MAC). Airway resistance increased by 5% in the desflurane group and fell by 15% in the sevoflurane group. The increase in airways resistance was greater in smokers and with desflurane, but did not differ with sevoflurane. The result was a surprise, given that desflurane stimulates the sympathetic nervous system. Thiopental also increased airways resistance by 10%. The result is important, because induction of anesthesia can cause bronchospasm and desflurane can exacerbate this. 

A 2-year-old girl with a past history of asthma, developmental delay, short neck, and lumbar lordosis, but no known genetic defect or syndrome underwent anesthesia with midazolam and paracetamol premedication, halothane and nitrous oxide induction, and isoflurane plus nitrous oxide for maintenance of anesthesia. Difficulty with mouth opening was noted and endotracheal intubation was difficult. Limb rigidity developed rapidly. Thiopental and cisatracurium were given and the muscle rigidity abated over the next 10 minutes. 

Convulsions have been reported in two patients with no history of epilepsy after induction of anesthesia with propofol (44). However, in a crossover comparison in 20 epileptic patients undergoing cortical resection, in which the effects on the electrocorticogram of either propofol or thiopental during isoflurane + nitrous oxide anesthesia were studied, propofol caused no greater proconvulsive effect than thiopental, which is used to treat status epilepticus (45). In spite of occasional reports, a true epileptogenic effect of propofol remains to be proven. 

A 65-year-old woman, who had had normal preoperative serum electrolytes and a normal QT interval with sinus rhythm, received hydroxyzine and atropine premedication followed by thiopental and vecuronium for anesthetic induction. Endotracheal intubation was difficult and precipitated atrial fibrillation, which was refractory to disopyramide 100 mg. Anesthesia was then maintained with sevoflurane 2% and nitrous oxide 50%. Ten minutes later ventricular tachycardia ensued, refractory to intravenous lidocaine, disopyramide, and magnesium. DC cardioversion resulted in a change to a supraventricular tachycardia, which then deteriorated to torsade de pointes. External cardiac massage and further DC cardioversion were initially unsuccessful, but the cardiac rhythm reverted to atrial fibrillation 10 minutes after the sevoflurane was switched off. Two weeks later she had her operation under combined epidural and general anesthesia, with no changes in cardiac rhythm. 

Immune hemolytic anemia with acute renal insufficiency has been reported in a 55-year-old patient after induction of anesthesia with thiopental 450 mg a specific thiopental antibody was detected the patient recovered fuUy (7). 

Vomiting is common during many types of anesthesia (8). By reducing upper esophageal sphincter pressure during induction, thiopental can contribute to this complication (9). 

In the horse, induction with thiopental usually results in a brief increase in heart rate and a decrease in cardiac output but no significant change in arterial blood pressure. Arterial blood pressure is typically lower after thiopental induction than in ketamine-based anesthetic techniques (Bennett et al 1998, Muir et al 2000). However, when horses are placed on inhalation anesthetics for maintenance of anesthesia, the hemodynamic effects of the induction agents is short lived and the hypotension and reduced cardiac output typical of inhalation anesthesia predominates (Bennett et al 1998, Wagner et al 1996). Respiratory depression is significant with an accompanying increase in Paco2 and decrease in pH. 

Propofol is a nonbarbiturate hypnotic agent and the most recently developed intravenous anesthetic. Its rapid induction and short duration of action are identical to thiopental, but recovery occurs more quickly and with much less nausea and vomiting. Also, propofol is rapidly metabolized in the hver and excreted in the urine, so it can be used for long durations of anesthesia, unlike thiopental. Hence, propofol is rapidly replacing thiopental as an anesthetic agent. 

Trade names Anesthal Hypnostan Intraval Nesdonal Sodipental Thiopental (Baxter) Trapanal Indications Induction of anesthesia Category Barbiturate Half-life 3-12 hours... 

Etomidate is a nonbarbiturate hypnotic without analgesic activity and is used for induction of general anesthesia, for supplementing subpotent anesthetics such as nitrous oxide in oxygen, and for maintenance of anesthesia for short operative procedures. Etomidate lowers cerebral blood flow, and similar to ketamine, methohexital, or thiopental, also reduces cerebral oxygen consumption (see also Table 16). 

Halothane is a potent anesthetic agent with properties that allow a smooth and rather rapid loss of consciousness that progresses to anesthesia. Thiopental is used for induction of anesthesia, and halothane is used for its maintenance (see Table 16). 

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