Term Paper on "Anesthesia Inhalation Agents Effects on Human Physiology"

Term Paper 6 pages (1587 words) Sources: 1+

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Anesthesia Inhalation Agents Effects on Human Physiology variety of anesthetic techniques are available in clinical practice today for different surgical situations. A trained anesthetist decides the most appropriate method for each case and has the necessary skills to use the technique of choice. Volatile anesthetic agents are commonly used and have an important safety feature in that agents that enter the circulation via the lungs may leave by the same route. Thus, the concentration of anesthetic at the brain can be rapidly reduced as long as the patient is breathing adequately. General anesthetics are used to produce unconsciousness during surgery. Unlike local anesthetics that are used in dentistry and minor surgery, general anesthetics circulate throughout the body, which results in a stronger action on the nervous system and a greater potential for side effects.

Basic Pharmacology of Anesthetic Agents

An agent inhaled into the lungs will enter the circulation and be carried to all tissues of the body. The concentration that reaches the brain produces the state of anesthesia. The exact mechanism of anesthesia is poorly understood, but it seems that the nerve cells absorb the agent thereby reducing the ability to conduct impulses to each other. The more soluble the agent is in blood, the longer it takes to build up an effective concentration in the brain and the slower the onset of unconsciousness. Thus, with a very soluble agent, the induction of anesthesia is prolonged. On the other hand, with an agent that is relatively insoluble in blood, the blood becomes saturated quickly, the brain concentration rises quickly and the effect is seen rapidly. The degree of solubility of an agent in blood is indicated by its blood gas solubility coefficient.

Commonly-used Anesthetic Agents

Halothane

Halothane is a well-tolerated, non-irritant potent agent giving rapid induction, low dose maintenance and rapid recovery. There is a predictable, dose-related depression of respiration and cardiac function. As halothane is a very potent agent, it is not suitable for use by untrained anesthetic staff. Its poor analgesic properties necessitate deep planes of anesthesia before surgery can be tolerated. For this reason, it is generally not suitable as a sole agent without an analgesic supplement, e.g. nitrous oxide, trichloroethylene, local anesthetic block or other analgesic, especially during spontaneous respiration. The depression of the cardiovascular system may cause bradycardia, hypotension and a reduction in cardiac output (Desalu, Kushimo, & Odelola, 2004). The combined depressant effects on the circulation and respiration mean that during anesthesia and the early recovery phase supplemental oxygen should always be given.

Indications for halothane include almost all general anesthesia. Contraindications for use include simultaneous administration with adrenaline, especially during spontaneous breathing, or a history of hepatitis following a previous anesthetic. Common dosages of halothane include inspired concentrations of up to 3%. A maintenance dose is 1-2% for spontaneously breathing patients and 0.5-1% during controlled ventilation (Preckel et al., 2004).

Overall, halothane use alone in general anesthesia is not ideal: because it has no analgesic properties and high concentrations are needed to abolish reflex activity. This becomes expensive and may also be unsafe. The combination of high concentrations of halothane, oxygen and air, high levels of carbon dioxide (from respiratory depression) and heart disease is potentially very hazardous for the patient, especially if the pulse is not adequately monitored for arrhythmias (Gurkan, Canatay, Agacdiken, Ural, & Toker, 2003).

Enflurane

Induction of and recovery from anesthesia with enflurane is rapid. Enflurane provides a mild stimulus to salivation or tracheobronchial secretions. Pharyngeal and laryngeal reflexes are readily obtunded. The level of anesthesia can be changed rapidly by changing the inspired enflurane concentration. Enflurane reduces ventilation as depth of anesthesia increases. High PaCO2 levels can be obtained at deeper levels of anesthesia if ventilation is not supported. There is a decrease in blood pressure with induction of anesthesia, followed by a return to near normal with surgical stimulation (Zhou, Zhao, & Chen, 2003). Elevation of the carbon dioxide level in arterial blood does not alter cardiac rhythm.

Desflurane

Desflurane is one of a family of halogenated methylethylethers, which is administered by inhalation producing a dose-related, reversible loss of consciousness and pain sensations, suppression of voluntary motor activity, modification of autonomic reflexes and sedation of respiration and the cardiovascular system (Ting et al., 2004).

Isoflurane

Isoflurane is a halogenated volatile anesthetic that induces and maintains general anesthesia by depression of the central nervous system and resultant loss of consciousness (Vivien, Lecarpentier, Riou, & Coirault, 2005). Age significantly affects blood-gas partition coefficients for all anesthetics; the lower blood-gas partition coefficients in children explain in part the more rapid increase in alveolar anesthetic partial pressures in this group. Throughout maintenance of anesthesia, a high proportion of the inspired isoflurane is eliminated by the lungs. When administration is stopped and inspired concentration becomes zero, the bulk of the remaining isoflurane is eliminated unchanged from the lungs. In keeping with its low solubility, recovery from isoflurane anesthesia in man is rapid.

Nitrous Oxide

Nitrous oxide is a colorless, odorless, tasteless, nonflammable, and non-irritating gas used for general anesthesia during surgery and provides pain relief during vaginal child deliveries (Smith & Gardner, 2004). Nitrous oxide is a powerful pain reliever and a weak anesthetic (producing loss of consciousness and feeling). During anesthesia nitrous oxide diffuses into any body cavity, which contains gas. This includes air spaces in the gut, middle ear, endotracheal tube cuff and pneumothorax. Diffusion hypoxia may occur at the end of anesthesia when nitrous oxide rapidly leaves the blood and tissues and passes out through the lungs. This may result in a dilution of the oxygen in the lungs for a few minutes and is prevented by administering extra oxygen at the end of anesthesia.

Advantages of nitrous oxide use are a rapid onset and recovery. It is a good analgesic supplement for halothane and reduces the incidence of awareness. Nitrous oxide produces minimal cardiovascular and respiratory effects. Contraindications for nitrous oxide use include a patient with an untreated pneumothorax or a patient who has been scuba diving within the previous 24 hours due to the potential for decompression sickness (Marczin, 2004).

Sevoflurane

Sevoflurane is a nonflammable and non-explosive liquid administered by vaporization. Sevoflurane has been demonstrated to be a fast-acting, non-irritating anesthetic agent in a variety of animal species and in humans. Administration has been associated with a smooth, rapid loss of consciousness during inhalation induction and a rapid recovery following discontinuation of anesthesia.

Induction is accomplished, with a minimum of excitement or of signs of upper respiratory irritation, no evidence of excessive secretions within the tracheobronchial tree and no central nervous system stimulation (Macario et al., 2005; Ng, 2005; Turner, McCulloch, Kennedy, & Baker, 2004). Like other potent inhalational anesthetics, sevoflurane depresses respiratory function and blood pressure in a dose-related manner. Sevoflurane is an appropriate agent for use in neurosurgery, caesarean section, coronary artery bypass surgery and in non-cardiac patients at risk for myocardial ischemia. Sevoflurane has minimal effect on intra-cranial pressure and preserves CO2 responsiveness. Sevoflurane does not affect renal concentrating ability, even after prolonged anesthetic exposure of up to approximately 9 hours. As with other halogenated agents, minimum alveolar concentration (MAC) decreases with age and with the addition of nitrous oxide.

Conclusion

In summary, many different types of anesthetics are currently used in practice. Although the general mechanism of action is similar among all anesthetics, one may be indicated for a particular case, but not another. Considerations for use should include subject medical history and presentation, length and depth of desired sedation, and potential side effects.

References

Desalu, I., Kushimo, O.T., & Odelola, M.A. (2004). Cardiovascular changes during halothane induction in children. Niger Postgrad Med J, 11(3), 173-178.

Gungor, I., Bozkirli, F., Celebi, H., & Gunaydin, B. (2003). Comparison of the effects of neuroleptanesthesia and enflurane or sevoflurane anesthesia on neuromuscular blockade by rocuronium.… READ MORE

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