Download MBBS (Bachelor of Medicine, Bachelor of Surgery) Neuroanaesthesia PPT 4 Inhalational Anaesthetics Lecture Notes
INHALED
ANAESTHETICS
Introduction
? These are the most common drug used for G/A
? Popularity is based on their
? Ease of administration
? Ability to monitor their effects
? Relatively inexpensive
? Prevents recalls and provides MR also
History
? The discovery of anaesthetic properties of N2O, diethyl
ether and chloroform in 1840s
? Long duration of 80 years before other inhaled
anaesthetic were introduced. In 1950, all were flammable
toxic exception of N2O
? Halothane was synthesized in 1951
? Introduced for clinical use in 1956
? Due to enhance dysarrhythmogenic effect of epenephrine
led to search for new derivative
History (contd...)
? Enflurane
? Introduced in clinical use in 1973
? Nephrotoxicity seems less likely
? Does not enhance dysarrhythmogenic effect of epinephrine
? It has epileptogenic potential
? Isoflurane, isomer of enflurane, introduced in 1981. Resistant
to metabolism making organ toxicity unlikely
History (contd...)
? Desflurane was introduced in 1993
? Sevoflurane was introduced in 1995
? Low blood gas solubility of these agents
? Rapid induction and rapid recovery
? Precise control of anaesthetic concentration
Inhalational agents
Classification
A. Volatile anaesthetics
B. Anaesthetic gases
1. Diethyl Ether (CH
1.
3CH2-OCH2CH3)
Nitroux oxide
2. Divinyl Ether [(C
2.
2H3)2 O]
Cyclopropane
3. Ethyl chloride (C
3.
2H5Cl)
Ethylene
4. Chloroform (CHCl
4.
3)
Xenon, Argon
5. Trichloroethylene (CCl
5.
2CHCl)
Sulphur hexafluoride
6. Halothane (CF3CHClBr)
7. Methoxyflurane
8. Enflurane
9. Isoflurane
10. Desflurane
11. Sevoflurane
Uptake and Distribution
? Liquid anesthetic is vaporized and mixed with oxygen
? Mixture is delivered to the patient via a mask or
endotracheal tube (ET tube)
? Mixture travels to lungs (alveoli) and diffuses into the
bloodstream
? Diffusion rate is dependent on concentration gradient
(alveoli/capillary) and lipid solubility of the
anesthetic gas
? Concentration gradient is greatest during initial induction
ANESTHETIC TRANSFER
Physical and Chemical Properties
of Inhalant Anesthetics
? Important properties to consider
? Vapor pressure
? Partition coefficient
? Minimum alveolar concentration (MAC)
? Rubber solubility
Vapor Pressure
? Is the amount of pressure exerted by the gaseous
form of a substance when in equilibrium
? i.e. ? it's ability to evaporate
? Determines how readily an inhalation anesthetic
will evaporate in the anesthetic machine vaporizer
? Dependent upon temperature and anesthetic
agent
Blood:Gas Partition Coefficient
? The measure of the solubility of an inhalation
anesthetic in blood as compared to alveolar gas (air)
? Indication of the speed of induction and recovery for
an inhalation anesthetic agent
? Low blood:gas partition coefficient
? Agent is more soluble in alveolar gas than in blood at
equilibrium
? Agent is less soluble in blood
? Faster expected induction and recovery
MINIMUM ALVEOLAR CONCENTRATION
(MAC)
? It is the steady state expired gas concentration of an
anesthetic
? At 1 atm pressure
? That prevents movement
? In response to surgical stimulus
? In 50% patients
Analogous to ED 50
? Best measure of anesthetic potency as it mirrors the brain
partial pressure.
? MAC values of different anesthetic are roughly additive.
MAC
MAC BAR- MAC that blunts adrenergic response to
noxious stimulus (1.5MAC)
MAC UNCONSCIOUS- MAC at which pt loses
consciousness (0.4-0.5MAC)
MAC AWAKE- MAC at which patient opens his or her
eyes to command (0.15-0.5MAC)
Increasing
Anesthetic Depth
MAC Fraction
MAC Fraction
MAC
?MAC of inhalational agents
N2O 104
Halothane 0.75
Isoflurane 1.17
Desflurane 6.6
Sevoflurane 1.8
? Roughly 1.3 MAC of any of the volatile anesthetic can
prevent movement in 95% pts during surgical stimuli.
FACTORS AFFECTING MAC
INCREASING MAC
? CNS metabolism
? CNS neurotransmission
? Hyperthermia
? Chronic alcohol abuse
? Hyponatremia
? Drugs - MAO I
- Amphetamine
- Cocaine
- Ephedrine
- L -DOPA
Decreasing MAC
? CNS metabolism
? CNS neurotransmission
? age
? Hypothermia
? Acute alcohol
? Hypotension(50mmhg MAP)
? Hypoxemia(38mmhg)
? Pregnancy
? Narcotics
? Ketamine
? Benzodiazepines
? Lithium
? Local anesthetics
NO EFFECTS ON MAC
?
Gender
?
Duration of anesthesia
?
Hypertension
?
Anemia
?
Thyroid status
?
Hypo or hypercarbia
?
Metabolic alkalosis
?
Hyperkalemia
?
Magnesium levels
Diethyl Ether (CH3CH2-O-CH2CH3)
History
? Prepared originally by Valerius Cordus- Sweet oil of vitriol
? Introduced in profession by W.T.G. Morton of Boston on Oct
16, 1846
? Classic stages and planes of anesthesia described using
ether
Diethyl Ether (CH3CH2-O-CH2CH3) (contd..)
Manufacture
? By heating together conc H2SO4 and 95% ethyl alcohol at
130?C
Physical properties
? Colorless, pungent volatile liquid
? Blood / gas solubility 12, MAC 3.04
? Relatively inert
? Acetaldehyde and ether peroxide as impurities, greater the
EP Lesser potency
? Stored in dark cool place
? Unaltered in the body 85-90% - Lungs, 15% metabolized in
liver
? inflammable in air and explosive in O2
EFFECTS ON ORGAN SYSTEM
A. Circulatory system
? Heart rate First increased Unaltered
Blood pressure
? Decreased BP after 1st hour ? below phase II
? Vaso Motor Centre paralysis in deep plane
? Functioning Sympathetic Nervous System BP
? Ether increase in sympathetic adrenal activity
? Cardiac output
? Lighter Plane of Anaesthesia CO increases
? Deep Plane of Anaesthesia CO decreases
? Arrhythmia ? rare, adrenaline safer with ether
B. Respiratory system
? RR increase Ist then decrease in deeper plane
? Ether vapour ? Irritant Laryngospasm
? Ether dilates bronchial musculature
? Hence induction ? Gradual
C.Nervous system
? Central nervous system
? Induce analgesia Excitement Anaesthesia
? Medullary depression Late, precedes the serious
cardiac depression
? CBF increases increases CSF pressure
? Sympathetic nervous system
? Ether
? Central stimulation increase blood catecholamine level
? Increase in HR
? Increased production of glycogen increased BS level
? Centration of spleen
? Dilatation ? Gut and inhibition of movements
? Dilatation of coronary arteries
? Dilation of pupils
? Parasympathetic ? NS central depression
D. Alimentary system
? PONV (>50% patients)
? Salivary gland stimulation ? Induction and depressed later on
? Gastrointestinal atony
? Liver function decreased, decreased sec of bile and bile salts
E. Urinary system
? Urine flow ? diminished
? Dec in plasma volume and renal Vaso-Constriction
Advantages of Ether
? Relatively non-toxic, safe and potent
? Relatively cheap and can be used without sophisticated
apparatus
? Excellent relaxation
? Respiratory depression not accompanied by serious cardiac
damage in A/o hypoxia
? Maintained BP, no tendency to arrhythmias
? Thus ether ? very safe, less experienced anaesthetist. Having
wide safety margin
Disadvantages of Ether
? Induction and recovery slow
? Mucous secretion from upper airway
? Causes albumin urea
? Inflammable: Explodes, sparks flames
? Ether convulsion : Triad
? Deeper ether anaesthesia
? Hyperthermia
? Hypocapnea
HALOTHANE
? It is halogenated alkene.
? Least expensive
? 2 bromo-2-chloro 1,1,1-
F
Br
trifluroethane
? Non-flammable and non explosive
? Non irritant vapors
F
C
C
Cl
? Decomposed by light
(0.01%thymol,amber bottles)
? Absorbed by rubber
F
H
? Corrodes metals
? B:G -2.54
? 20-46% metabolized in the liver
? MAC- 0.87-1.19
EFFECTS ON ORGAN SYSTEM
1. CARDIOVASCULAR:
Dose dependent reduction of arterial blood pressure
by direct myocardial depression.
It is a coronary artery vasodilator.
It causes slowing of SA node conduction resulting in
bradycardia.
Sensitizes heart to catecholamine and induces
arrhythmias
2. RESPIRATORY SYSTEM:
Causes rapid ,shallow breathing.
Decrease in alveolar ventilation and Paco2 elevated.
Potent bronchodilator.
3. CEREBRAL:
Increased cerebral blood flow
Increased temperature- malignant hyperthermia-
Dantrolene is used for treatment
4. NEUROMUSCULAR:
? Relaxes skelatal muscle and potentiates non
depolarizing neuro-muscular blocking agents.
5.RENAL:
? Reduces renal blood flow, glomerular filtration
rate and urinary output.
6. HEPATIC:
? Decreases hepatic blood flow.
CONTRAINDICATION
? Unexplained liver dysfunction.
? Intra-cranial mass lesions.
? Hypo-volemic patient with severe cardiac
diseases.
ISOFLURANE
F
Cl
F
F
C
C
O
C
F
F
H
H
? 1-chloro-2,2,2-trifluoroethyl difluoromethyl
ether
? Colorless volatile liquid
? Pungent
? No preservative
? Does not react with metals
Isoflurane
? It is non flammable volatile with a pungent
smell.
? Physical Properties
? High vapor pressure: need a precision vaporizer
? Low blood:gas partition coefficient (1.4): rapid induction and
recovery
? Good for induction with mask or chamber ???
? MAC = 1.3% to 1.63%: helps determine initial vaporizer setting
? Low rubber solubility
? Stable at room temperature; no preservatives needed = no build
up in the machine
? Almost completely eliminated through the lungs- 0.2% metabolized
by the liver
EFFECTS ON ORGAN SYSTEM
CARDIOVASCULAR:
Causes minimal cardiac depression.
Maintains cardiac output, heart rate, and rhythm
Fewest adverse cardiovascular effects
Rapid increase in MAC lead to increase in HR and BP.
Dilates coronary arteries. (Coronary Steal)
2. RESPIRATORY SYSTEM:
Respiratory depression .
Irritant to upper airway
3. CEREBRAL:
Maintains cerebral blood flow
If conc > 1 MAC causes increase in CBF and Intracranial
pressure.
4. NEUROMUSCULAR:
Induces adequate to good muscle relaxation
5. RENAL:
Decreases renal blood flow , glomerular filtration rate and
urinary output.
6. HEAPTIC:
Reduces hepatic blood flow.
INDICATIONS
? For Cardiac and Neuro- Surgery
? In patients with hepatic or renal compromise
CONTRAINDICATION
? No such contraindication.
? Caution in asthmatics
SEVOFLURANE
? Methylpropylether
F
? Nonflammable
F C
pleasant smell
F
H
? MAC is higher in
children (2.6%in O2
H
C
O
C
F
and 2.0%in
N2O)and neonates
F
H
(3.3%)
F C
? Stable
F
Sevoflurane
High vapor pressure: need a precision vaporizer
Low Blood:gas partition coefficient (0.65)
= rapid induction and recovery
Good for induction with a mask or chamber. Easier to mask a
patient, more pleasant smelling
High controllability of depth of anesthesia
MAC = 2.34% to 2.58%
Cost about 10x more than Isoflurane
Eliminated by the lungs, minimal hepatic metabolism- 2-5%
Can react with potassium hydroxide (KOH) or sodium hydroxide
(NaOH) in desiccated CO2 absorbent to produce a chemical
(Compound A) that causes renal damage
EFFECTS ON ORGANS
1. CARDIOVASCULAR SYSTEM:
? Mildly depresses myocardial contractility.
? May prolong QT interval, but no significance.
2. RESPIRATORY SYSTEM:
? Depresses respiratory rate.
? It reverses broncho-spasm
3. CEREBRAL:
? Maintains cerebral blood flow
? Increases CBF and intra-cranial pressure.
? Some paddling and excitement during recovery
? No post-op analgesia
4. RENAL SYSTEM:
? Slightly decreases renal blood flow. Higher Conc
Causes Nephro-toxicity
5. HEPATIC:
? Decreases portal vein blood flow but increases
hepatic artery blood flow thus maintaining total
hepatic blood flow.
6.NEUROMUSCULAR:
? Adequate muscle relaxation.
INDICATION
? For induction
? Especially useful in children
? In patients with reactive upper airway
CONTRAINDICATION
? No such contraindication
? Caution in severe hypo-volemia.
DESFLURANE
? Fluorinated methyl
ethyl ether
F
F
F
? Colorless, without
preservative
F
C
C
O
C
F
? Non flammable
? Special heated
F
H
H
vaporizer
Desflurane
Structure much similar to that of isoflurane.
Recovery time are approximately 50 % less than those of
Isoflurane.
Pungent Smell
? Expensive
? Lowest blood:gas partition coefficient: very rapid induction and
recovery
? Used with a special heated electronic precision
vaporizer (TEC 6)
? MAC = 7.2% and 9.8%
? Least potent inhalant agent
? Eliminated by the lungs- 0.02% metabolized in liver
EFFECTS ON ORGAN SYSTEM
1. CARDIOVASCULAR SYSTEM:
? Similar to Isoflurane ( Increases HR and BP when increased
MAC rapidly)
? Dilates coronary arteries.
2. RESPIRATORY SYSTEM:
? Causes decrease in tidal volume and increase in resp rate.
? Pungency and airway irritation so causes coughing and
sometime bronchospasm.
? Strong vapors cause coughing and holding the breath=
difficult to mask
2. 3. CEREBRAL:
? Increases CBF and Intracranial pressure.
4. NEUROMUSCULAR:
? Relaxes skeletal muscle.
5. RENAL AND HEPATIC SYSTEM:
? No any evidence has been documented.
INDICATION- For Hepatic and Renal Surgery
CONRAINDICATION ? Same as isoflurane
NITROUS OXIDE
Physical properties:
?It is a laughing gas.
?It is only inorganic anesthetic gas in clinical use.
?Colorless and odorless
?Non Explosive and Non Infammable
?Gas at room temperature and can be kept as a
liquid under pressure.
?It is relatively inexpensive.
Effects of Nitrous Oxide on Organ System
1. CARDIOVASCULAR SYSTEM
? Stimulate sympathetic nervous system.
? Directly depresses myocardial contractility.
? Arterial blood pressure ,heart rate and cardiac
output are slightly increased.
2. RESPIRATORY SYSTEM:
? Increases respiratory rate with decreases tidal volume.
? Minimal change in minute ventilation.
3. CEREBRAL:
? Increases CBF thus increasing intracranial pressure.
4. RENAL SYSTEM:
? It decreases renal blood flow thus leads to drop in
glomerular filtration rate and urinary output.
5. HEPATIC SYSTEM:
? Decreases the Hepatic blood flow but to a lesser
extent than other inhalation agents.
6. GASTROINTESTINAL:
? It causes post operative Nausea and Vomiting.
CONTRAINDICATION OF N2O
? Air embolism
? Pneumothorax
? Acute Intestinal Obstruction
? Tension Pneumocephalus
? Tympanic membrane grafting
Uses of N2O
Mixed with oxygen at 40-67%, then delivered
to patient
Reduces MAC 20-30%
Used with Halothane and Methoxyflurane
to reduce the adverse effects of these gases
This post was last modified on 07 April 2022