Dr rowan molnar anaesthetics study guide part iv Identifies (hopefully confirms!) anaesthetic agent being used Measures inspiratory & expiratory concentrations Expiratory (alveolar) concentration enables calculation of MAC fraction or multiple – i.e. estimation of anaesthetic depth. Now mandatory when inhalational anaesthetic agents are used. Dr Rowan Molnar, Dr Rowan Molnar Anaesthetics, Dr Rowan

1. DR ROWAN MOLNAR ANAESTHETICS
STUDY GUIDE PART IV
Anaesthetic Drugs: Pharmacology, Use
&
Related Issues

2. CLASSIFICATION OF DRUGS USED FOR
ANAESTHESIA
“The Big Five “
 Inhalation anaesthetic agents – gasses/vapours
 IV anaesthetic agents alias “Hypnotics” or “induction
agents”
 Narcotic (& other) analgesics
 Muscle relaxants – neuromuscular blocking agents
 Local anaesthetic agents
Other agents are often given as part of anaesthesia – e.g.
antiemetics & autonomic agents, but are not
conventionally regarded as anaesthetic agents per se.

3. PHARMACOLOGY 1:
INHALATIONAL ANAESTHETIC AGENTS
 Inhaled – therefore delivered via apparatus
 Gasses or volatile liquids
 Moderate to high lipid solubility – “solvents”
 Effects related to physical properties
(rather than to a generic chemical structure)
 Effects on multiple organ systems
 Actual mode of action not yet fully elucidated, but
thought to be by dissolving into cell membranes &
causing secondary changes in configuration of ion
channels.

4. CURRENTLY USED INHALATIONAL ANAESTHETICS
(SHOWN IN THEIR INTERNATIONAL COLOUR CODES):
 Nitrous oxide (N20)– a gas. Insufficiently potent
to produce full anaesthesia on its own, but is rapid
acting, pleasant to inhale & is the only currently
used agent that is also analgesic.
 Sevoflurane
 Desflurane all liquids that are flourinated
ethers
 Isoflurane
Earlier volatile agents such as ether, chloroform &
halothane have been superceded due to issues
such as flammability, slow recovery, & toxicity.

5. PRACTICAL PHARMACOLOGY OF
INHALATIONAL AGENTS
 Used for induction sometimes (predominantly in children) &
maintenance of anaesthesia in the majority of cases - either
alone, or in combination with narcotics & muscle relaxants.
 Modern flourinated agents are good hypnotics, & provide a
degree of muscle relaxation at high doses, but not analgesia.
 In contrast, nitrous oxide is analgesic, but doesn’t decrease
muscle tone, and is a poor hypnotic except at very high (i.e.
hypoxic) concentrations.
 The combination of a volatile agent, e.g. sevoflurane, with a
50:50 nitrous oxide/oxygen mix is a useful combination that
combines the attributes of both agents.

6. PRACTICAL PHARMACOLOGY OF
INHALATIONAL AGENTS (2)
 Sevoflurane has superceded isoflurane as probably the
most widely used agent, & has also superceded halothane
as the agent of choice for inhalational induction in children.
 All currently used agents have relatively low solubility in
blood & tissue – meaning that their partial pressures rise &
fall quickly, producing more rapid induction & emergence.
 The classical stages of anaesthesia are still seen with
modern agents – including the delerium phase –
characterised by restlessness & risk of laryngospasm. This
stage is usually seen on emergence, or with inhalational
inductions in children.

7. PRACTICAL PHARMACOLOGY OF
INHALATIONAL AGENTS (3)
 Nitrous oxide, as a gas is delivered by a flowmeter (as are O2
& air – the 3 flowmeters on a typical modern anaesthetic
machine). A linkage between the N20 & oxygen flowmeters
stops the delivery of any mixture <25%O2. Most anaesthetic
machines also only allow delivery of either N20/O2 or air/O2,
not all 3 & none allow air/N2O (a hypoxic mixture).
 Volatile agents are delivered by vapourisers – devices which
add a precise percentage of vapour to the gas mixture.
Modern vapourisers are agent specific and colour
coded/labelled accordingly. They have numerous
mechanisms to ensure accurate delivery, plus safety
measures such as “keyed” filling systems that match only the
correct bottle; and machines that can have more than one
vapouriser fitted must have interlocks that prevent more than
one being turned on.

8. PHARMACOLOGY 2:
IV ANAESTHETIC “INDUCTION”
AGENTS
Used for:
 Induction of anaesthesia
 Sole agent for brief procedures
 By infusion for longer procedures - in place of
inhaled agents – i.e. total intravenous
anaaesthesia “TIVA”

9. CLASSIFICATION OF INTRAVENOUS
AGENTS
Barbiturates – Thiopentone
Benzodiazepines – Midazolam
Dissociative agents – Ketamine
Others- Propofol
+ Alpha-2 agonists – Dexmetomidine . . . maybe “the
next big thing”

10. GENERAL FEATURES OF IV AGENTS
 Lipid soluble
 High volume of distribution (Vd)
 Initial distribution to VRG
 Offset of (initial) effect predominantly by
redistribution
 More complex when used as infusions
(Computerised multicompartment
pharmacokinetic modelling required)

11. PROPOFOL
 Most widely used agent now
 Rapid(ish) onset & offset
 Shorter elimination halftime
 Less CVS & respiratory depression
 Doesn’t predispose to laryngospasm
 ED50 for induction: ~ 2 mg/kg
 Suitable kinetics for infusion

12. OTHER IV AGENTS
THIOPENTONE
 First widely used agent
 Rapid onset & initial offset
by redistribution
 Long elimination halftime
 CVS & resp depressant
 Laryngospasmogenic
 ED50: ~ 5mg/kg
 Still used for RSI
“The correct dose of
thiopentone is enough” (and
no more!!)
MIDAZOLAM
 Low CVS & resp depressant
 Anxiolytic, good initial adjuvant
agent, not often used as sole
agent
KETAMINE
 “Dissociative” agent
 Phencyclidine derivative
 Cardiorespiratory stimulant (in
vivo)
 Maintains airway reflexes
 Analgesic in subanaesthetic
doses
“The disaster anaesthetic”

13. TOTAL INTRAVENOUS ANAESTHESIA
“TIVA”
 Not practical until introduction of propofol ,
with its short elimination half life, meaning
minimal accumulation with infusion.
 Usually target controlled infusion using
computerised algorithm in syringe pump
software. Operator enters patient weight, age,
& desired blood level.
 Often used in combo with remifentanil &
cisatracurium infusions for long cases (these
also have good kinetics for use by infusion).

14. TIVA –GOOD & BAD
Advantages
 Good for cases of long
or uncertain duration
 Less effects on CBF &
ICP than volatile
agents
 Less likely to cause
PONV then either
volatiles or N2O.
Disadvantages
 Long setup time
 More expensive
 Multiple syringe pumps
required
 No direct measure of
blood or effect site
concentration

15. PHARMACOLOGY 3: NARCOTIC
ANALGESICS & ACUTE PAIN MANAGEMENT
A Definition of Pain:
“An unpleasant localised sensory experience
perceived as actual or potential tissue
damage.”
May be acute or chronic

16. CLASSIFICATION OF ANALGESICS
Conduction blockade
Opiods
Paracetamol
NSAIDs & COX2s
Miscellaneous agents
 Complementary/Non pharmacological

17. AN OPIOD IS A DRUG THAT EXHIBITS AGONIST ACTIVITY
AT OPIATE (ENDORPHIN/ENKEPHALIN) RECEPTORS. A
CLASSIFICATION OF OPIODS INCLUDES:
 Opiates (naturally occuring constituents of opium)
& their derivatives:
e.g. morphine, codeine, diamorphine
(heroin)
 Synthetic opiods
e.g. pethidine, fentanyl cogeners,
oxycodone
 Partial agonists
e.g. pentazocine “Fortral”, buprenorphine
N.B. This classification does not include the narcotic
antagonists e.g. naloxone “Narcan” & naltrexone;
however these are closely related, being n-allyl
substituted derivatives (hence their names)of opiods

18. PROPERTIES OF OPIODS
 Analgesia
Spinal ( μ/κ) & supraspinal (μ)
 Respiratory depression
 Sedation/euphoria (addiction potential)
 Emesis
 Depression of GI motility
 Pruritis
 Urinary retention
No difference in respiratory depression
between equi-analgesic doses of any
narcotic agonists
} Neuraxial route
predominantly

19. SO THE DIFFERENCES BETWEEN OPIODS
ARE LESS IN THEIR ANALGESIC EFFICACY
THAN IN:
 Onset
 Duration
 Potency/dose
 Histamine release
 Autonomic effects
 Chest wall rigidity
 Effective routes of administration

20. ROUTES OF ADMINISTRATION OF OPIODS:
 Intravenous: (a) Boluses – titrated to effect – e.g recovery pain
protocol (b) Infusions – require close monitoring due to potential
for overdose as narcotic requirements fall away.
(c) PCA – now widely used. Intrinsically safer than infusions, plus
positive psychological effect of patient knowing they are in control.
 Neuraxial - Epidural or intrathecal (spinal) – usually in combination
with regional anaesthesia, but may also be stand alone technique
for postoperative analgesia. Risk of late onset respiratory
depression if agent migrates into intracranial CSF in significant
amount (highest with morphine, but this is also the longest acting)
 IM/SC – decreasing importance with availability of PCA & better
oral agents, & multimodal therapy.
 Oral – variable bioavailability: e.g. oxycodone high, morphine ~
15% due to first pass metabolism.
 Sublingual(buprenorphine) /Intranasal(fentanyl) – lipid soluble
agents fairly rapidly absorbed & this route avoids first pass effect
(& injection)
 Transcutaneous – e.g. fentanyl patches for chronic pain

21. PROBLEMS WITH OPIODS
 Respiratory depression/cough suppression
 Tolerance
 Abuse/addiction potential
 Accountability/access/supply difficulties
- consequent to abuse potential.
 Nausea & vomiting
 Constipation

22. MULTIMODAL ANALGESIA OPTIONS
 Regional/local blockade (if possible)
 Paracetamol
 NSAID or COX2
 Basal opiod (e.g. oxycontin); or tramadol
(or both)
 prn or PCA opiod
 Other
Clonidine or ketamine

23. PHARMACOLOGY 4:
NEUROMUSCULAR BLOCKERS
 Purely paralysing agents – no analgesic or
hypnotic activity.
 Two types based on modes of action:
Depolarising (Suxamethonium)
Versus
Nondepolarising
(NDNMBs, several available)

24. WHY USE PARALYSING DRUGS AT ALL?
 Permit procedures at a lighter plane of anaesthesia –
hence less CVS depression
 Intubation & ventilation
 Surgery
 Permit IPPV without interference
 Lower airway pressures by increasing chest wall
compliance.
 Lower O2 consumption in critical periods