Inhalation Agents

INHALATION AGENT

INTRODUCTION Inhalation agents are substances that are

brought into the body via the lungs and are distributed with the blood into the different tissues Requires conversion from liquid phase to

vapor phase by vaporizer The main target of inhalation : the brain A combination on inhalation anesthetics +

intravenous drug is called = balanced

HISTORY Last century : Chloroform, ether,

Nitrous Oxide , Ethyl Chloride 1930 – 1940’s : Ethylene, Cyclopropane, Trichloroethylene, Isopropenyl Vinyl ether 1956 : Halothane 1960’s : Methoxyflurane (pull

from the market = nephrotoxic potential)

1970’s : Enflurane, Isoflurane 1990 : Sevoflurane 1992 : Desflurane

HOW IT IS DELIVERED TO PATIENT Pipeline/ tank 02, o2/N2o, or O2/air -

Flowmeter (o2, N2o, air : L/mmin) Vaporizer (dialled %) Breathing circuit Airway (ETT / LMA) Lungs (trachea > Bronchioles > alveoli) Pulmonary veins L Atrium, L Ventricle, Aorta Brain, Other Organs

HOW DOES IT ACT ON THE BRAIN TO CAUSE SLEEP MULTIPLE THEORIES - Action on cell membrane : - inhibition of ligand gated channels - NMDA receptor blockade - GABA receptor potentiated - Voltage gated Na, Ca, P channels effect - Axonal transmission depressed - Synaptic transmission altered = Final effect suppression of the brain activity

MECHANISM OF ACTION Theories suggest that they work by altering the lipids in cell membranes since the potency of anesthetics correlates extremely closely with the solubility of the drug in oil.

CURRENTLY USED INHALED ANESTHETICS Halothane Isoflurane Enflurane Desflurane Sevoflurane Nitrous Oxide

HALOTHANE

HALOTHANE Colorless, pleasant smell liquid Non flammable halogenated alkene Vapor pressure of 244mmHg at 20 degree

Celsius and boils at 50.2 degree Celsius Is susceptible to decomposition = stored in amber-colored bottles + thymol is added as preservative Not recommended for obstetric anesthesia except uterine relaxation is required. It crosses placenta barrier = fetal depression =resulting hypotension, hypoxemia and acidosis Excellent hypnotic but no analgesic properties

Advantage :

- Rapid, smooth induction - Minimal stimulation of salivary and bronchial secretions - Bronchodilatation - Muscle relaxation - Relatively rapid recovery

Disadvantage :

- Relatively slower induction compared to isoflurane and sevoflurane - Myocardial depression - Bradycardia - Arrhythmias - Uterine relaxation - Post- operative shivering - Liver toxicity

ISOFLURANE

ISOFLURANE Nonflammable halogenated Vapor pressure of 239mm Mercury at 20 degree

Celsius and boils at 48.5 degree Celsius Colorless liquid, pungent odor, stable in liquid Does not react with metal or other substances Advantage :

-

Rapid induction and recovery Little risk of liver and renal toxicity Cardiovascular stability Muscle relaxation

Disadvantage :

- Expensive - Pungent odor, makes inhalation induction unpleasant - Causes coronary vasodilatation and may aggravate coronary ischemia

ENFLURANE

ENFLURANE Nonflammable, clear and colorless liquid Pleasant ethereal smell Stable in soda lime and metal Vapor pressure of 172 mm Mercury at 20

degree Celsius and boils at 56.5 degree Celsius

Advantage :

-

Rapid induction and recovery Little risk of liver toxicity Muscle relaxation Low incidence of arrhythmias

Disadvantage :

- Expensive - Seizure activity on EEG

DESFLURANE

DESFLURANE Nonflammable Vapor pressure of 673mm Mercury at 20

degree Celsius and boils at 23.5 degree Celsius Requires the use of electrically heated vaporizers Little effect on cardio-respiratory system Pungent and may cause airway irritation manifested by salivation, coughing and laryngospasm Might as well increase heart rate - not to be used in patients with aortic valve stenosis

SEVOFLURANE

SEVOFLURANE Nonflammable, clear, colorless liquid Pleasant ethereal smell Stable in soda lime and metal Vapor pressure of 39000mm Mercury at 20

degree Celsius and boils at minus 88 degree Celsius

Advantage :

-

Rapid induction and recovery Little risk of liver and renal toxicity Muscle relaxation Low incidence of arrhythmias

Disadvantage :

- Expensive - Seizure activity on EEG

Nitrous Oxide

Nitrous Oxide

INTRODUCTION Nitrous oxide, is a chemical compound with

chemical formula N20

Nitrous Oxide Is an organic nonflammable gas that support

combustion Also known as laughing gas Vapor pressure 0f 39 000 mm Mercury at 20 Degree Celsius and boils at minus 88 degree Celsius Colourless, sweet smelling, non-irritating A gas at room temperature and ambient pressure Inexpensive A week analgesic but good analgesic agent

Entonox is preparation of 50% nitrous in

oxygen = produce good analgesic effects

Side effect :

- Myocardial depression - Effect on closed gas spaces; causes increase in pressure in middle ear, bowel lumen, pleural, pericardial cavities - Prolonged exposure of more than 8 hours affect vitamin B12 synthesis and may result in megaloblastic anemia - Teratogenic changes, should be avoided in early pregnancy

MINIMUM ALVEOLAR CONCENTARTION (MAC) The measure of potency is the minimum alveolar concentration (MAC). DEFINITION ?

MAC is the concentration of anesthetic that causes immobility in 50% of subjects exposed to the agent at one atmosphere.

The lower the MAC the more potent the general anesthetic agent

10 0

Alveolar concentration (percent of inspired concentration)

N2 Isoflur ane

5 0

0

Percent of initial Alveolar concentration

10 0

5 0

0

Inducti

Enflur ane Haloth ane 2 Minut 0 es

4 0

Haloth ane Enflur ane Isoflur ane N

Recov

2

Rate of induction and rate of recovery from anesthesia

Rate Of Induction & Rate Of Recovery 1. The more soluble the agent is in blood, the more drug it takes to saturate the blood and the more time it takes to raise the partial pressure and the depth of anesthesia. 2. The less soluble the agent is in blood, the less drug it takes to saturate the blood and the less time it takes to raise the partial pressure and depth of anesthesia.

OXYGEN

INTRODUCTION Oxygen must be present in every breathing

gas. This is because it is essential to the human body’s metabolic process, which sustains life.

If the body is deprived of oxygen for more

than a few minutes, unconsciousness and death result.

OXYGEN At standard temperature and pressure, oxygen

is a colorless, odorless and tasteless gas With the molecular formula O2, n which the two oxygen atoms are chemically bonded to each other with spin triplet electron configuration Has poor solubility in water. Specific gravity of 1.105 makes it slightly heavier than air. When cooled to its boiling point of -297o F (-183o C), oxygen becomes a transparent, pale blue liquid that is slightly heavier than water.

OXYGEN Piped O2 is supplied from a liquid O2 reserve,

where it’s stored under pressure (10-12 bar, 1200 Kpa) at approximately -180ºC in a vacuum-insulated evaporator (VIE), effectively a thermos flusk

O2, are delivered to the anaesthetic room

at a pressure of 400 kilopascal (Kpa) (4 bar, 60 pounds per square inch (psiJ) )

OXYGEN The gas reach the anesthetic

machine via flexible reinforced hoses, color coded throughout the length These attached to the wall

outlet via a specific probe to the anesthetic machine via a specific nut and union.

OXYGEN A smaller cylinder is

attached directly to the anesthetic machine as an emergency The pressure in a full

cylinder is 12000 Kpaand this falls in direct proportion to the cylinder content.

o 2

TUBING COLOR CODES BRITISH AMERICAN STANDARD STANDARD OXYGEN NITROU S OXIDE