Summary

Inhalational anaesthetics exhibit favourable pharmacokinetic properties but their therapeutic indices make it necessary to know,

• Clinical considerations for each drug
• Properties and clinical efficacy of an ideal halogenated/gaseous agent

Inhalational anaesthetics agents signify a basic drug used in modern balanced anaesthesia as they possess excellent hypnotics and provide varying degrees of analgesia and skeletal muscle relaxation at higher concentrations. Although, multiple agents can be utilised, be it inhaled or administered intravenously (IV), inhalational anaesthesia is still the most common modality utilized today in hospital settings.

In present surgical practices and in many clinical situations, anaesthetic agents such as sevoflurane and desflurane have proved their increased efficacy. Inhaled anaesthetic agents possess excellent characteristics, but their use may be limited in some patients and certain clinical applications.

Inhalational anaesthetic agents & clinical considerations

Nitrous oxide

It is one of the common inhalational agents, used for surgeries. Due to its extremely high MAC value (104%), it is often co-administered with oxygen and the other volatile anaesthetics. Once released, the liquid changes phases into a gas and is then delivered in amalgamation with oxygen for general anaesthesia.

Sevoflurane
Sevoflurane is the most employed non-irritating inhalational anaesthetic with a MAC of 2.4%. It is used in mask induction techniques with or without nitrous oxide, as it bears minimal effects on cardiovascular output and minimal irritation to the lungs and respiratory tract.

Desflurane

This potent inhalation agent has extremely low blood:gas coefficient and enables extremely rapid emergence with a MAC of 6%. However, this agent is not suitable for use as it bears a pungent aroma and is extremely irritating to pulmonary tissues and the respiratory tract.

Isoflurane

Exhibiting the lowest MAC of 1.15%, the agent possesses potent peripheral vasodilating properties with a resultant reduction in systemic vascular resistance.

Characteristic features of an ideal inhaled anaesthetic agent

An ideal inhalational anaesthetic agent must possess the following properties:

Ample potency: Minimum alveolar concentration (MAC) provides a correlation between anaesthetic dose and immobility. One MAC is the concentration of the anaesthetic at 101.3 kPa that produces immobility in 50% of patients exposed to a noxious stimulus. While administering a combination of inhaled anaesthetics, it is important to note a roughly additive effect of MAC.

Low solubility in blood and tissues: It is the ratio between the concentration of an anaesthetic in the blood phase to the concentration of the anaesthetic in the gas phase when the anaesthetic is in equilibrium between the two phases. The solubility of an anaesthetic agent in blood is quantified as the blood:gas partition coefficient. A low ratio indicates a low affinity of blood for the anaesthetic as it envisages a more specific control over the anaesthetic state and a more rapid recovery from anaesthesia

Resistance to degradation: In order to reduce wastage, inhaled anaesthetic agents are administered in a circular absorption system containing divalent and monovalent absorbents (sodium hydroxide, potassium hydroxide) bases in combination with water. This removes carbon dioxide and allows rebreathing of the inhaled anaesthetic. These bases can degrade potent inhaled anaesthetics. The degradation depends on the nature of the absorbent whether it is moist or desiccated and the type of inhaled anaesthetic. For instance, desiccated absorbents can degrade all inhaled anaesthetics to carbon monoxide (mostly with desflurane).

Clinical efficacy of inhalational anesthesia

1. Absorption and distribution: Anaesthetic agents are absorbed and distributed because of the pressure gradients and are equilibrated when tensions of inspired gas equal in alveoli, blood, and tissues. Inhalational agents with low solubility in blood and adipose tissue equilibrate more rapidly and offer a driving force for these agents to enter the brain, where their anaesthetic action occurs.
2. Systemic influences: Anaesthetic agents and their mixtures exhibit similar influence on the vital organs like lungs, heart, and circulation and less apparent action on other organs. Inhalational agents reduce mean arterial pressure and myocardial contractility. This decrease in mean arterial pressure reduces renal and hepatic blood flow.
3. Respiratory effects: Being respiratory depressants, inhalational anaesthetic agents have a inclination to increase respiratory rate but decrease tidal volume. Their net influence is depicted as the degree of hypercapnia that occurs during administration.
4. Cardiovascular effects: A dose-dependent reduction in mean arterial pressure (MAP) is exhibited by inhalation anaesthetics. These agents vary with respect to the precise physiologic parameters they target like myocardial contractility, and systemic vascular resistance.
5. Relaxation of muscles: Influence of inhalation anaesthetics can be detrimental to the skeletal muscles. The agents may have different influences on the relaxation of skeletal muscles. For example isoflurane, desflurane, and sevoflurane have substantial skeletal muscle relaxation, but nitrous oxide does not affect the muscle tone.

In conclusion, inhaled anaesthetics are both safe and effective in inducing and maintaining anaesthesia as they differ in potency and adverse-effect profile. Since inhalational anaesthetics possess a relatively low margin of safety with therapeutic indices of 2 to 4, it is crucial to know the kinetics of these drugs. Therefore, during the selection of inhalation anaesthetics, patient characteristics, duration and type of procedure or surgery must be taken into consideration.