How anaesthesia works

1. General and Local anaesthetic
agents
ALICE

2. Introduction
• Anesthetics are drugs used to cause complete or partial loss of
sensation. These drugs are subdivided based on site of action and
can either be general or local.
• General anesthetics can cause central nervous system (CNS)
depression to produce loss of pain sensation and consciousness
• local anesthetics cause the same sensation and feeling in a
certain area of the body without producing the systemic effects
related to severe CNS depression.

3. Introduction
• General anesthesia involves the administration of combined
different general anesthetic agents with the fewest adverse effects
to achieve analgesia (loss of pain perception),
unconsciousness (loss of awareness of one’s own
surroundings), and amnesia (inability to recall what took place).

4. Introduction
• It also blocks the autonomic reflexes governing involuntary reflex
response of the body to injury which can compromise cardiac,
respiratory, gastrointestinal, and immune status.
• Muscle reflexes are also blocked to prevent jerking movements
that might interfere with surgical procedures.

5. Risk Factors Associated with General Anesthesia
• Widespread CNS depression can occur in individuals with the
following risk factors:
• CNS Factors: neurological diseases that may produce an
abnormal reaction to the CNS-depressing and muscle-relaxing
effects of general anesthetic agents like epilepsy, stroke and
myasthenia gravis.
• Cardiovascular (CV) Factors: underlying CV diseases can be
worsened by severe reactions to anesthesia (shock, hypotension,
dysrhythmia, and ischemia) like coronary artery disease (CAD).

6. Risk Factors Associated with General Anesthesia
• Respiratory Factors: obstructive pulmonary diseases that can
complicate delivery of gas anesthetics, intubation, and mechanical
ventilation like asthma, chronic obstructive pulmonary disease
(COPD), and bronchitis.
• Renal and hepatic function: conditions that interfere with
metabolism and excretion of anesthetics that could lead to
prolonged anesthesia like acute renal failure and hepatitis.

7. Administration of General Anesthesia
• Patients undergo predictable stages during administration of
anesthesia.
• These steps are referred to as the depth of anesthesia

8. Administration of General Anesthesia
STAGES OF ANESTHESIA ( Also referred to as the Guedel's signs)
Stage I (analgesia): The patient initially experiences analgesia without amnesia.
Later in Stage I, both analgesia and amnesia are produced.
Stage II (excitement); During this stage, the patient often appears to be
delirious and may vocalize but is definitely amnesic. Respiration is irregular
both in volume and rate, and retching and vomiting may occur if the patient is
stimulated. For these reasons, efforts are made to limit the duration and severity
of this stage, which ends with the re-establishment of regular breathing.

9. Administration of General Anesthesia
STAGES OFANESTHESIA ( Also referred to as the Guedel's signs)
Stage III (surgical anesthesia): This stage begins with the recurrence of regular respiration and extends
to complete cessation of spontaneous respiration (apnea). Four planes of stage III have been described in
terms of changes in ocular movements, eye reflexes, and pupil size, which under specified conditions
may represent signs of increasing depth of anesthesia.
Stage IV (medullary depression): This deep stage of anesthesia includes severe depression of the
vasomotor center in the medulla, as well as the respiratory center. Without circulatory and respiratory
support, death rapidly occurs.

10. Administration of General Anesthesia
• Administration of general anesthesia is divided into three phases:
• Induction: from beginning of anesthesia to stage 3. The most
dangerous period of induction is stage 2 because of the systemic
stimulations that occur.
• Maintenance: from stage 3 to completion of surgical procedure.
• Recovery: from discontinuation of anesthetics to regained
consciousness, movement, and ability to communicate by the
patient.

11. Overview of General Anaethetics
 Intravenous Anesthetics
 Several drugs are administered intravenously, alone or in combination with other anesthetic drugs, to
achieve an anesthetic state or to sedate patients in intensive care units (ICUs) who must be
mechanically ventilated.
 These drugs include:
• Barbiturates (eg, thiopental, methohexital)
• Benzodiazepines (eg, midazolam, diazepam)
• Propofol
• Ketamine
• Opioid analgesics (morphine, fentanyl, sufentanil, alfentanil, remifentanil)
• Miscellaneous sedative-hypnotics (eg, etomidate, dexmedetomidine).

12. Overview of General Anaethetics
Inhaled Anesthetics
 The most commonly used inhaled anesthetics are;Nitrous Oxide, Halothane,
Isoflurane, Desflurane, Sevoflurane, Enflurane, Methoxyflurane
 These compounds are volatile liquids.
 Nitrous oxide, a gas at ambient temperature and pressure, continues to be an
important adjuvant to the volatile agents.

13. Local Anesthesia
• Refers to a loss of sensation in limited areas of the body.
• Can be achieved by different methods:
• Topical administration: involves the application of cream,
lotion, ointment, or drop of local anesthetic to traumatized skin to
relieve pain.
• It can also involve application of these forms to the mucous
membranes in the eyes, nose, throat, mouth, urethra, anus, or
rectum.

14. Local Anesthesia
• Infiltration: involves injecting the anesthetic directly into the
tissues to be treated.
• It brings the anesthetic into contact with the nerve endings in the
area and prevents them from transmitting nerve impulses to the
brain.
• Field Block: involves injecting the anesthetic all around the area
that will be affected by the surgery.
• Anesthesia comes in contact with all of the nerve endings
surrounding the area. It is often used for tooth extraction.

15. Local Anesthesia
• Nerve Block: involves injecting the anesthetic at some point
along the nerve(s) that run to and from the region in which the
loss of pain sensation or muscle paralysis is desired.
• Several types of nerve blocks include: peripheral nerve block,
central nerve block, epidural anesthesia, caudal block, and spinal
anesthesia.

16. General Anesthetic Agents

17. General Anesthetic Agents
• General anesthetic agents can be classified into: barbiturate
anesthetics, nonbarbiturate general anesthetics, anesthetic gases,
and volatile liquids.

18. General Anesthetic Agents
• Classification
• Barbiturate Anesthetics
• Non-Barbiturate Anesthetics
• Anesthetic Gases
• Volatile requids

19. Barbiturate anesthetics
Barbiturate anesthetics are intravenous drugs used to induce
rapid anesthesia, which is then maintained with an inhaled drug.
Drugs under this class include;-methohexital , thiopental

20. Barbiturate anesthetics
• Therapeutic Action
• The desired and beneficial actions of barbiturate anesthetics are
as follows:
• Barbiturate anesthetics depress the CNS to produce hypnosis and
anesthesia without analgesia.

21. Barbiturate anesthetics
• Indications
• Thiopental is the most widely used intravenous anesthetic.
• Both thiopental and methohexital do not have analgesic
properties so patients would require analgesics post-operation.
• These drugs are indicated for induction and maintenance of
anesthesia as well as for induction of hypnosis.

22. Barbiturate anesthetics
• Use in children
• At greater risk for complications after anesthesia (e.g.
laryngospasm, bronchospasm, aspiration, etc.)
• Care should include support and reassurance; assessment of child
for any skin breakdown related to immobility, and safety
precautions.

23. Barbiturate anesthetics
• Use in dults
• They should receive education about what will happen during
administration of anesthesia. Expected body reactions should also
be explained.
• Continuously reassure adult patients during the time that they are
aware of their surroundings but they are unable to speak.
• Most general anesthetics are not recommended during pregnancy
because of potential adverse effects to the fetus.
• A period of 4-6 hours after receiving anesthetics is required for
lactating women before they can start nursing.

24. Barbiturate anesthetics
• Older adults
• They are more susceptible to adverse effects (e.g. CNS, CV, and
dermatological effects).
• At risk for developing toxicity because of possible hepatic and renal
impairment.
• Safety measures should be instituted (e.g. side rails, call light,
ambulation assistance, and skin care).
• Longer monitoring and regular orienting and reassuring is essential.
• After general anesthesia, it is important for nurses to promote vigorous
pulmonary toilet to decrease the risk of pneumonia.

25. Barbiturate anesthetics
• Pharmacokinetics-Characteristics
Thiopental
• Rapid onset and rapid recovery (bolus dose)—slow recovery following
infusion
• Standard induction agent; cardiovascular depression; avoid in
porphyrias

26. Barbiturate anesthetics
• Pharmacokinetics
Route Onset Peak Duration
IV 1 min N/A 20-30 min
T1/2: 3-8 h
Metabolism: liver
Excretion: kidney (urine)

27. Barbiturate anesthetics
• Contraindications and Cautions
• Silicon (e.g. Rubber stoppers, disposable syringes).
Methohexital will cause an immediate breakdown of the silicone.
• Pregnancy, lactation. CNS depressive effects to baby and fetus.
• Neither drug should be used until the anesthesiologist or staff are
ready and equipped for intubation and respiratory support.
• The rapid onset of action of these drugs can cause respiratory
depression and apnea.

28. Barbiturate anesthetics
• Adverse Effects
• CNS: CNS suppression
• CV: bradycardia, hypotension
• Respiratory: respiratory depression
• GI: decreased GI activity
• Nausea and vomiting are common after recovery.

29. Barbiturate anesthetics
• Interactions
• Theophylline, oral anticoagulants, beta-blockers, corticosteroids,
hormonal contraceptives, phenylbutazones, metronidazole,
quinidine, carbamazepine: decreased effectiveness of these drugs
with barbiturates
• Narcotics: increased risk for apnea

30. Nonbarbiturate Anesthetics

31. Nonbarbiturate General Anesthetics
• Droperidol, etomidate, fospropofol, ketamine,
midazolam(available only in generic form) propofol

32. Nonbarbiturate anesthetics
• Indications
• Midazolam, the prototype nonbarbiturate anesthetic, is widely
used to produce amnesia or sedation for many diagnostic,
therapeutic, and endoscopic procedures.
• It can also be used to induce anesthesia and to provide
continuous sedation for intubated and mechanically ventilated
patients.

33. Nonbarbiturate anesthetics
• Indications
• Droperidol produces marked sedation and produces a state of
mental detachment. It also has antiemetic effect which reduces
the incidence of nausea and vomiting in surgical and diagnostic
procedures.

34. Nonbarbiturate anesthetics
• Indications
• Ketamine is useful in situations when cardiac depression is
dangerous because it causes sympathetic stimulation with
increase in blood pressure and heart rate.

35. Nonbarbiturate anesthetics
• Indications
• Propofol is used for short procedures because it has a very rapid
clearance and produces much less of a hangover effect and allows
for quick recovery.

36. Nonbarbiturate anesthetics
• Use in children
• At greater risk for complications after anesthesia (e.g.
laryngospasm, bronchospasm, aspiration, etc.)
• Propofol is widely used for diagnostic tests and short procedures
in children older than 3 years of age because of its rapid onset and
metabolism.
• Etomidate is not recommended for use in children younger than
10 years of age.

37. Nonbarbiturate anesthetics
• Use in adults and older adults
• Same as in barbiturate analgesics

38. Nonbarbiturate anesthetics
• Pharmacokinetics-characteristics
Etomidate,
• Rapid onset and moderately fast recovery
• Cardiovascular stability; decreased steroidogenesis; involuntary muscle
movements
Ketamine
• Moderately rapid onset and recovery
• Cardiovascular stimulation; increased cerebral blood flow; emergence
reactions impair recovery

39. Nonbarbiturate anesthetics
• Pharmacokinetics-characteristics
Midazolam
• Slow onset and recovery;
• flumazenil reversal available, Used in balanced anesthesia and conscious sedation;
cardiovascular stability; marked amnesia
Propofol
• Rapid onset and rapid recovery
Used in induction and for maintenance; hypotension; useful antiemetic action
Fentanyl
• Slow onset and recovery; naloxone reversal available
• Used in balanced anesthesia and conscious sedation; marked analgesia

40. Nonbarbiturate anesthetics
• Pharmacokinetics
Route Onset Peak Duration
IM 15 min 30 min 2-6 h
IV 3-5mins <30 min 2-6 h
T1/2: 1.8-6.8 h
Metabolism: liver
Excretion: kidney
(urine)

41. Nonbarbiturate anesthetics
• Contraindications and Cautions
• Conditions that can be compromised by vomiting.
Midazolam is more likely to cause nausea and vomiting than other
anesthetics.
• Renal or hepatic failure, prolonged QT intervals.
Contraindicated with use of droperidol.
• Respiratory depression and arrest is associated with use of
nonbarbiturate anesthetics so life support equipment should be
readily available always.

42. Nonbarbiturate anesthetics
• Adverse Effects
• Midazolam: CNS suppression, respiratory depression
• Droperidol: chills, hypotension, hallucinations, drowsiness, QT
prolongation
• Etomidate: myoclonic and tonic movements

43. Nonbarbiturate anesthetics
• Adverse Effects
• Ketamine: hallucinations, dreams, psychotic episodes (can cross
blood-brain barrier)
• Propofol: local burning on injection sites, bradycardia,
hypotension, pulmonary edema
• Fospropofol: sensation of perianal burning, stinging, tingling, rash
(do not usually require intervention and will usually pass)

44. Nonbarbiturate anesthetics
• Interactions
• Ketamine and halothane can cause severe cardiac depression
with hypotension and bradycardia.
• NMJ Blockers. Potentiated muscular blocking of NMJ if paired
with ketamine
• Inhaled anesthetics, other CNS depressants, narcotics,
propofol, thiopental. Increased toxicity and length of recovery
with use of midazolam.

45. Anesthetic Gases

46. Anesthetic Gases
• Anesthetic gases enter the bronchi and alveoli, pass through the
capillary system and are transported to the heart to be pumped
throughout the body.
• Have high affinity for fatty tissue (including the lipid membrane
of the nerves in the CNS), and are lipophilic.
• Passes quickly to the brain and causes CNS depression.
• Very flammable and associated with toxic adverse effects.

47. Anesthetic Gases
• Examples :Anesthetic Gases nitrous oxide (blue)

48. Anesthetic Gases
• Therapeutic Action
• Moves quickly in and out of the body, can accumulate in closed
body compartments (e.g. sinuses) and cause pressure.
• Very potent analgesic.
• Do not cause muscle relaxation.

49. Anesthetic Gases
• Indications
• Nitrous oxide is usually used for dental surgery. It is also
combined with other agents for anesthetic use.
• Only one anesthetic gas, nitrous oxide (blue cylinder), is still
used.

50. Anesthetic Gases
• Use in children
• At greater risk for complications after anesthesia (e.g.
Laryngospasm, bronchospasm, aspiration, etc.)
• Children need to be cautioned not to bite themselves when
receiving dental anesthesia.

51. Anesthetic Gases
• Use in adults and older adults
• Similar to non-barbiturate anaesthetics

52. Anesthetic Gases
• Pharmacokinetics
• Onset 1-2mins Peak N/A Duration
• T1/2: minutes
Metabolism: not metabolized
Excretion: lungs

53. Anesthetic Gases
• Contraindications and Cautions
• Conditions which are at risk for hypoxia. Oxygen is always
given with nitrous oxide because the drug can block the reuptake
of oxygen after surgery.
• Monitor for signs of hypoxia, chest pain, and stroke.
• Pregnancy. Potential adverse effects to the fetus. Lactation.
Should wait 4 hours after administration of nitrous oxide before
nursing a baby.

54. Anesthetic Gases
• Adverse Effects
• Respiratory: pneumothorax
• GI: bowel obstruction
• EENT: acute sinus pain, middle ear pain
• Inactivates Vitamin B12.

55. Anesthetic Gases
• Interactions
• Ketamine and halothane can cause severe cardiac depression
with hypotension and bradycardia.

56. Volatile Liquids

57. Volatile liquids
• Volatile liquids are also inhaled anesthetics because they are
unstable at room temperature and release gases.
• Most volatile liquids are halogenated hydrocarbons.

58. Volatile liquids
• Examples: desflurane , enflurane, halothane , isoflurane

59. Volatile liquids
• Therapeutic Action
• Depresses the CNS, causing anesthesia.
• Relaxes muscles.
• Sensitizes the myocardium to the effects of norepinephrine and
epinephrine

60. Volatile liquids
• Indications
• Halothane, the prototype drug, is used for maintenance of
anesthesia and can be effective as an induction agent.
• Desflurane is widely used to in/outpatient surgery because of its
rapid onset and quick recovery time.
• Isoflurane is widely used to maintain anesthesia after induction.
Can cause muscle relaxation.

61. Volatile liquids
• Use in children
• At greater risk for complications after anesthesia (e.g.
Laryngospasm, bronchospasm, aspiration, etc.)
• Halothane is widely used for children, especially those with
respiratory dysfunction because it tends to produce bronchial
dilation. However, it is contraindicated for those with increased
intracranial pressure (ICP).

62. Volatile liquids
• Use in adults
• Expected body reactions should also be explained.
• Continuously reassure adult patients during the time that they are
aware of their surroundings but they are unable to speak.
• A period of 4-6 hours after receiving anesthetics is required for
lactating women before they can start breastfeeding.

63. Volatile liquids
• Use in older adults
• They are more susceptible to adverse effects (e.g. CNS, CV, and
dermatological effects).
• Institute safety measures
• Longer monitoring and regular orienting and reassuring is
essential.
• Promote vigorous pulmonary toilet after anaesthesia to decrease
the risk of pneumonia.

64. Volatile liquids
• Route Inhaled Onset Rapid Peak Rapid Duration End of
inhalation
• T1/2: Unknown
Metabolism: liver
Excretion: kidney (urine)

65. Volatile liquids
• Contraindications and Cautions
• Hepatic impairment. Can contribute to hepatic toxicity.
• Cardiovascular disease. Associated with bradycardia and
hypotension
• Respiratory depression and increased sensitivity. Has
tendency to cause respiratory depression and it is an irritant to the
airways.

66. Volatile liquids
• Contraindications and Cautions
• Pregnancy, lactation. Potential adverse effects to the fetus and
the baby.
• All of these drugs have the potential to trigger malignant
hyperthermia. In such cases, dantrolene is the preferred
treatment and should be readiily available.

67. Volatile liquids
• Adverse Effects
• Halothane’s recovery syndrome: fever, anorexia, nausea,
vomiting, hepatitis (can progress to fatal hepatic necrosis). It is
not used for more than 3 weeks to reduce the patient”srisk.
• Desflurane is associated with a collection of respiratory
reactions: cough, increased secretions, laryngospasms.

68. Volatile liquids
• Interactions
• Caution should be used when any of these drugs is combined with
other CNS suppressants.

69. Stages/phases of general anaesthesia
• Induction: from beginning of anesthesia to stage 3. The most
dangerous period of induction is stage 2 because of the systemic
stimulations that occur.The drugs used during this phase are;
HALOTHANE- Can cause halothane hepatitis and ISOFLULANE-
Currently used more than halothane due to its reduced side
effects.
• Barbiturates e.g. Sodium thiopentone (Thiopental)- induces
sleep very quickly i.e. in 10 seconds

70. Stages/phases of general anaesthesia
• Maintenance: Phase in which the patient is in surgical
anaesthesia;To maintain anesthesia oxygen+Nitrous oxide+one
volatile agent are given.
• Muscle.Short acting (Depolarizing) relaxants e.g.
suxamethonium (scoline) is used in intubation.Long acting
(Non- Depolarizing) relaxants e.g. Pancuronium, Tracurium and
Nimbex.- used to maintain anesthesia.

71. Stages/phases of general anaesthesia
• Recovery/reversal: from discontinuation of anesthesia to
regained consciousness, movement, and ability to communicate.
• Anesthesia is reversed by administering the following drugs:-
Neostigmine 2.5 mgs IV and Atropine 1.2 mgs IV.

72. IN SUMMARY
 The physiologic state induced by general anesthetics typically includes analgesia,
amnesia, loss of consciousness, inhibition of sensory and autonomic reflexes, and
skeletal muscle relaxation.
 The extent to which any individual anesthetic agent can exert these effects varies
depending on the drug, the dosage, and the clinical situation.
 An ideal anesthetic drug would induce loss of consciousness smoothly and rapidly,
while allowing for prompt recovery of cognitive function after its administration is
discontinued.
 The drug would also possess a wide margin of safety and be devoid of adverse
effects.
 No single anesthetic agent is capable of achieving all of these desirable effects
without some disadvantages when used alone.

73. IN SUMMARY
 The modern practice of anesthesiology most commonly involves the use of combinations of intravenous and
inhaled drugs, taking advantage of their individual favorable properties while minimizing their adverse reactions.
 The anesthetic technique will vary according to the proposed type of diagnostic, therapeutic, or surgical
intervention.
 For minor procedures, oral or parenteral sedatives may be used in conjunction with local anesthetics .These
techniques provide profound analgesia, but with retention of the patient's ability to maintain a patent airway and
to respond to verbal commands.
 For more extensive surgical procedures, anesthesia frequently includes preoperative benzodiazepines, induction
of anesthesia with an intravenous anesthetic (eg, thiopental or propofol), and maintenance of anesthesia with a
combination of inhaled (eg, volatile agents, nitrous oxide) and intravenous (eg, propofol, opioid analgesics)
drugs.

74. ACTIVITY
• Watch the video on anaesthesia https://youtu.be/k84SpHki7EI
• Optional: https://youtu.be/AkKXrD_-S2o
• Review the dosage table.