1. Inhalation Anesthetics
Done by: Abdallah Amjad Riyalat

2. • Anesthesia :defined as the abolition of sensation
characterized by:
1 reversible loss of consciousness
2 analgesia of the entire body
3 amnesia
4 some degree of muscle relaxation .

3. Stages of Anesthesia
Stage I (analgesia stage) – Conscious, Perception of pain is diminished ( Mild CNS depression)
Stage II (delirium stage) – Unconscious – Body responds reflexively– Breath holding, pupils dilated – Muscle tone intact (An excited state resulting from cortical motor depression), can be avoided with rapidly acting, potent anesthetics
Stage III (surgical anesthesia) – Increasing degrees of muscular relaxation – (disappearance of muscle tone)
IV (medullary depression) – Depression of cardiovascular and respiratory centers

4. The. course. of. a. general anesthetic. can. be. divided. into three
The course of a general anesthetic can be divided into three phases: (1) induction (2) maintenance (3) emergence
Inhalation anesthetics , are useful in the induction in patients whom it may be difficult to start an intravenous line
Regardless of the patient’s age, anesthesia is often maintained with inhalation agents

5. Inhalational Anesthetic Agents
Inhalational anesthesia, refers to the delivery of gases or vapors from the respiratory system to induct or maintain anesthesia
Exposure to the pulmonary circulation allows a more rapid appearance in arterial blood than IV administration
Advantages:
1. Completely painless induction
2.No IV (intravenous) access needed
3. Rapid appearance of drug in arterial blood
4.Safe.

6. Dose
While intravenous agents are given in mg (or mcg)/kg doses , the inhaled agents are given in volume percent concentrations
This brings us to the concept of Minimum Alveolar Concentration (MAC).
The concentration of a gas in the alveoli creates an alveolar partial pressure of gas which in turn reflects its partial pressure in the active site (brain).
MAC refers to the concentration of the inhaled agent in alveolar gas necessary to prevent movement of 50% of patients when a standard incision is made.
The rationale for this measure of anaesthetic potency is: a. alveolar concentration can be easily measured b. near equilibrium, alveolar and brain tensions are virtually equal c. the high cerebral blood flow produces rapid equilibration b. individual variability is small c. sex, height, weight & anaesthetic duration do not alter MAC

7. MAC is a useful measure. because: 1. it. mirrors
MAC is a useful measure because: 1. it mirrors brain partial pressure 2. allows comparisons of potency between agents 3. provides a standard for experimental evaluations
Many factors influence MAC, and therefore influence the concentrations required to maintain anesthesia.

10. FACTORS AFFECTING INSPIRATORY CONCENTRATION (FI)
1.the fresh gas flow rate
2 the volume of the breathing system,
3 any absorption by the machine or breathing circuit
The fresh gas leaving the anesthesia machine mixe with gases in the breathing circuit before being inspired by the patient. Therefore, the patient is not necessarily receiving the concentration set on the vaporizer
The greater the fresh gas flow rate , the smaller the breathing system volume, and the lower the circuit absorption, the closer the inspired gas concentration will be to the fresh gas concentration

11. FACTORS AFFECTING ALVEOLAR CONCENTRATION (FA)
1.The uptake
If there were no uptake of anesthetic agent by the body, the alveolar gas concentration (FA) would rapidly approach the inspired gas concentration (FI)
The greater the uptake, the slower the rate of rise of the alveolar concentration and the lower the FA:FI ratio.
So initially, FA:FI=O CUZ there is no agent in the lung yet
EQ when , FA:FI=1
Fast induction is defined as FA:FI=1 quickly
FA is proportional to PA=Pblood=Pcns
Therefore, the greater the uptake of anesthetic agent, the greater the difference between inspired and alveolar concentrations, and the slower the rate of induction

12. factors. affect anesthetic
factors affect anesthetic uptake( Factors determining how quickly the inhalational agent reaches the brain from the alveoli )
solubility in the blood
alveolar blood flow
the difference in partial pressure between alveolar gas and venous blood

13. solubility in the blood
the blood/gas partition coefficient (λb/g)
A higher solubility (λb/g)>1… more agent in blood than in gas so need to dissolve more gas to get certain partial pressure
A lower solubility (λb/g)<1… more agent in gas than in blood so need to dissolve less gas to get certain partial pressure
The higher the blood/gas coefficient, the greater the anesthetic’s solubility and the greater its uptake by the pulmonary circulation
As consequence of this increased solubility, alveolar partial pressure rises to a steady state more slowly
A higher solubility =more uptake =more time for induction(slow induction)

14. alveolar blood flow essentially equal to cardiac output
As cardiac output increases, anesthetic uptake increases, the rise in alveolar partial pressure slows, and induction is delayed

15. 2.ventilaion
The lowering of alveolar partial pressure by uptake can be countered by increasing alveolar ventilation
Constantly replacing anesthetic taken up by the pulmonary bloodstream results in better maintenance of alveolar concentration
So increasing the ventilation will increase the absorption of agent into the lung so accelerating the EQ and faster induction

16. 3.Concentration two phenomena:
1.Concentrating effect augmented gas inflow

17. FACTORS AFFECTING ARTERIAL CONCENTRATION (Fa)
Ventilation/Perfusion Mismatch
Normally, alveolar and arterial anesthetic partial pressures are assumed to be equal, but in fact, the arterial partial pressure is consistently less than end expiratory gas would predict
Reasons for this may include
1 venous admixture
2 alveolar dead space
3. nonuniform alveolar gas distribution

18. the existence. of. ventilation/perfusion. mismatching. will. increase
the existence of ventilation/perfusion mismatching will increase the alveolar–arterial difference
Mismatch acts as a restriction to flow: It raises the pressure in front of the restriction, lowers the pressure beyond the restriction, and reduces the flow through the restriction
The overall effect is an increase in the alveolar partial pressure (particularly for highly soluble agents) and adecrease in the arterial partial pressure (particularly for poorly soluble agents).

19. Thank you