Anesthesia

Inhalation Anesthetic- Nitrous Oxide, Halothane, Isoflurane, Sevoflurane, Desflurane

There are four types of anesthesia.  They are general, regional, sedation, and local anesthesia. Among that the general anesthesia is a type of sleep-like state of a being, for carrying out any surgeries or medicinal procedures. Unlike other kinds of anesthesia, the patient is completely put in a state of unconsciousness. During this process, inhalation anesthetics (nitrous oxide, halothane, isoflurane, sevoflurane, desflurane, etc.), are used to ease the pain of the patient. Here, we will discuss the properties, uses, advantages, disadvantages, induction time, and handling, of these inhalation anesthetics. Besides that, we will also discuss and compare the MAC value of the given inhalation anesthetics. 

 

1. Nitrous Oxide as Inhalation Anesthetic

Nitrous oxide (laughing gas) in the anesthesia machine, is used for two main reasons.

  • As a carrier gas (fresh gas flow) that carries other potent anesthetic drugs
  • As a type of inhalation anesthetic

Properties of Nitrous Oxide

  • Colorless
  • Sweet taste
  • Non-flammable

Advantages of Nitrous Oxide

  • Dissociation:- loss of memory, feelings, one’s identity
  • Sedation
  • Analgesia:- loss of pain
  • Laughter
  • Anxiolytic:- loss of anxiety

Disadvantages of Nitrous Oxide

(Side effects of Nitrous Oxide)

  • Amnesia:-loss of memory
  • Nausea
  • Headache
  • Vomiting

Time for Induction

  • The blood gas partition coefficient is low i.e. 0.47.
  • This gas works within a matter of seconds of administration and ends shortly (2-5 minutes) after the administration stops.

Sources of Nitrous Oxide

There are two main sources in the hospital.

  • Pipeline system
  • N2O Cylinder:-N2O cylinder is internationally color-coded with blue color. The ‘E’ and ‘G’ type cylinders have a vapor pressure of about 750 psi at 250C.

 

2. Halothane as Inhalation Anesthetic

Properties of Halothane

  • Simple Name:- C2HBrClF3
  • Colorless
  • Saturated Vapor Pressure:- 243 mmHg at 200C
  • Boiling Point:- 50.20C
  • In the presence of moisture, halothane is highly corrosive to aluminum, brass, and lead. However copper is an exception. It is also corrosive to some types of plastics and rubbers. So, due to this corrosive nature, there is a chance of degradation of the breathing circuit of the anesthesia machine.
  • It has got a sweet smell. So, there is a low chance of airway irritation.
  • Non-flammable
  • Chemically unstable in the presence of light.
  • Do not form carbon monoxide with the soda lime.

Advantages of Halothane

  • Low airway irritation in comparison to isoflurane.
  • Economical in comparison to other inhalation anesthetics.
  • High potency (low MAC value)
  • Bronchodilation
  • Reduction in salivary and bronchial secretion.

Disadvantages of Halothane

(Side effects of Halothane)

  • There is a high rate of metabolism of halothane (20%). Due to this high metabolism, there is a high chance of renal and hepatic toxicities. Due to this drawback, it has been least preferred among all the other inhalation anesthetics. However, due to the low price, it is still being used in third-world countries like Nepal.
  • Can reduce the tidal volume. A decrease in the tidal volume may increase the respiratory rate and the concentration of CO2 in the blood.
  • There is a chance of triggering malignant hyperthermia in some patients.
  • Nausea and vomiting after the end of the operation can occur.
  • Cardiac arrhythmia is a common problem for halothane.
  • Hypotension is possible at the time of induction of anesthesia.

Time for Induction

  • The blood gas partition coefficient is higher (2.5) than isoflurane, sevoflurane, and desflurane. Thus the induction and clearance time for this drug is slower.
  • For induction of surgical anesthesia, 1% to 3% halothane in air or oxygen, or 0.8% in 65% nitrous oxide can be used.

Storage and Handling of Halothane

  • You can find halothane in an amber color bottle (250 ml and 100ml). You can store it at a normal room temperature (250C).
  • This agent is not stable in the presence of light. So, to stabilize this agent, thymol (0.01%) is added as a preservative.
  • Thymol is not volatile in nature. So, halothane should not be stored in a vaporizer or bottle for a longer period of time. In a long run, the anesthetic agent will evaporate but thymol will accumulate at the bottom. It will impose yellow color on the vaporizer or the anesthetic liquid.
  • You need to clean the vaporizer from time to time by using diethyl ether. Make sure to clean the diethyl ether at the end.
  • You need to protect it from light and heat.

 

3. Isoflurane as Inhalation Anesthetic

Properties of Isoflurane

  • Simple Name:- C3H2F5O
  • Saturated Vapor Pressure:- 238 mmHg at 200C
  • Boiling Point:- 48.50C
  • Do not react to metals and the breathing circuit of the anesthesia machine.
  • Colorless
  • Mild pungent smell
  • Non-flammable
  • Chemically stable at room temperature

Advantages of Isoflurane

  • Low toxicity
  • High potency (low MAC value)
  • Low airway irritants in comparison to desflurane.
  • Economical in comparison to desflurane and sevoflurane.
  • Low metabolism (0.2%):- This reduces the chances of renal and hepatic injuries.
  • Bronchodilation
  • Useful for coronary perfusion studies because of its better potency for coronary vasodilation.

Disadvantages of Isoflurane

(Side effects of Isoflurane)

  • Depresses cortical activities of EEG.
  • Can reduce the tidal volume. A decrease in the tidal volume may increase the respiratory rate and the concentration of CO2 in the blood.
  • May lower the blood pressure during the time of induction.
  • Respiratory depression due to airway irritation.
  • There is a chance of triggering malignant hyperthermia in some patients
  • Different animal models have shown the possible chances of neurotoxicity, fetal growth retardation, cleft palate, etc.
  • There is a slight chance of the formation of carbon monoxide with dry isoflurane.

Time for Induction

  • The blood gas partition coefficient is higher (1.4) than sevoflurane and desflurane but lower than halothane.
  • For induction of surgical anesthesia, 1.5% to 3% of isoflurane with 65% nitrous oxide or 3% to 4% in air or oxygen can be used. The induction triggers after 7 – 10 mins of the administration of the drug.
  • The pungency also delays the time for induction.
  • For maintenance, the dose lies between 1% and 2.8% with nitrous oxide or 1.5% and 3.8% with oxygen alone.

Storage of Isoflurane

You can find isoflurane in an amber color bottle (250 ml and 100ml) or an aluminum bottle (250 ml). You can store it at a normal room temperature.

Agent specific vaporizer for isoflurane

  • Tec-5 vaporizer is one of them

 

4. Sevoflurane as Inhalation Anesthetic

Properties of Sevoflurane

  • Simple Name:- C4H3F7O
  • Saturated Vapor Pressure:- 159 mmHg at 200C
  • Boiling Point:- 58.60C
  • Do not react to metals and the breathing circuit of the anesthesia machine.
  • Colorless
  • Non-pungent smell
  • Non-flammable
  • Chemically stable at room temperature

Advantages of Sevoflurane

  • Low toxicity
  • Low airway irritants in comparison to desflurane and isoflurane. So masks can be used by patients of all age groups.
  • Economical in comparison to desflurane
  • Low blood solubility
  • High potency (low MAC value)
  • Low metabolism (3 to 5%)

Disadvantages of Sevoflurane

(Side effects of Sevoflurane)

The probability of the following side effects using sevoflurane is as follows.

  • Nausea:- 25%
  • Vomiting:- 18%
  • Hypotension:- 4% to 11%
  • Agitation:- 7% to 15%

An increase in the concentration of this anesthetic agent can also cause

  • A decrease in blood pressure and cardiac output
  • Apnea
  • Laryngospasm

Sevoflurane reaction with the CO2 absorber

  • Sevoflurane is quite unstable in soda-lime producing Compound A.  Experiments have shown that Compound A may be responsible for organ toxicity such as hepatic and renal. Thus avoid fresh gas flow (FGF) at 1 L/min for no more than 2 MAC-Hours (2 L/min FGF can be used indefinitely).
  • There is a chance of the formation of carbon monoxide with dry sevoflurane.

Time for Induction

  • The blood gas partition coefficient is low (0.69).
  • A low partition ratio indicates that the agent is not much soluble in blood. Thus, the undissolved gas goes to the brain and diffuses quickly. Also, the recovery rate will be high.
  • 5% dose in adults and 7% dose in children will cause induction for surgical anesthesia within 2 minutes.

Storage of Sevoflurane

You can find sevoflurane in an amber color bottle (250 ml). It has to be stored at a low temperature (150C to 250C) due to its high volatility.

Agent specific vaporizer for Sevoflurane

  • Drager Vapor 2000 vaporizer is one of them

 

5. Desflurane as Inhalation Anesthetic

Properties of Desflurane

  • Simple Name:- C3H2F6O
  • Saturated Vapor Pressure:- 669 mmHg at 200C
  • Boiling Point:- 22.80C
  • Does not react to metals
  • Colorless
  • Pungent smell

Advantages of Desflurane

  • Non-toxic
  • Economical during long procedures
  • Does not show convulsive activity on EEG
  • Low blood solubility
  • High potency (low MAC value)
  • Low metabolism (0.02%)

Disadvantages of Desflurane

(Side effects of Desflurane)

  • Irritation to the patient
  • Poor analgesic property
  • Nausea
  • Vomiting
  • Cough
  • Breath-holding
  • Apnea
  • Costly than other inhalation anesthetics.

Time for Induction

  • The blood gas partition coefficient is 0.42.
  • The gas works within 1 to 2 mins of administration.

Storage of Desflurane

You can find (250 ml) of desflurane in an amber color bottle. Since the gas is highly volatile you need to store it at a low temperature.

Desflurane is an airway irritant

Desflurane, when used at a higher concentration, causes irritation to the respiratory tract due to its pungent smell. So, nowadays, it is hardly used for patients showing spontaneous breathing. In other words, it is mostly in use for patients who need mandatory breathing.

Why is desflurane heated?

 

Minimum Alveolar Concentration (MAC)

Minimum Alveolar Concentration (MAC) is the concentration of inhaled anesthetics (in %), that can cause immobility among 50% of patients under noxious stimulus (e.g. skin incision) at 1 atmospheric pressure. It shows the potency of the inhalation anesthetic which is inversely proportional to the potency. In other words, higher MAC (or lower potency) means that more inhalation anesthetic needs to be administered to the patient for induction or maintenance of anesthesia.

To induce and maintain the anesthesia in the patient, there should be a certain concentration of inhaled anesthetic in the central nervous system (CNS). For this to happen there should be equilibrium in the partial pressure of gases in the alveoli, blood, and CNS.

Factors that affect the value of Minimum Alveolar Concentration

  • low capacity of tissues to absorb the anesthetics
  • low metabolism, and excretion
  • rapid bidirectional flow of gases between alveoli, blood, and CNS.

Calculation of Minimum Alveolar Concentration

Based on Mapleson’s equation, we have

MACage = MAC40 x 10-0.00269 (age-40)

Where MAC40 = MAC value for the patient of age 40

             MACage = MAC value to be calculated for a patient of a certain age

The age of 40 is taken as a standard age for the calculation of MAC value.

Minimum Alveolar Concentration for Nitrous Oxide, Halothane, Isoflurane, Sevoflurane, Desflurane
Minimum Alveolar Concentration

Here, k = MACage, anesthetic / MACage, N2O. This ratio is almost constant for any age of the patient.

From the above table, we can conclude that

  • MAC value for nitrous oxide is 104% which is really high. Thus, it is not alone useful for general anesthesia.
  • With an increase in the age of the patient, there is a decrease in the MAC value. This is because the rate of metabolism decreases along with the age.
  • MAC value for patients below 1 year is not consistent. Thus, you cannot calculate the value.
  • The administration of nitrous oxide in the form of carrier gas will decrease the MAC value.
  • More is the dose of nitrous oxide; less will be the MAC value of the inhalation anesthetics.

 

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button