Anaesthetic Techniques and Stages

Anaesthetic Techniques

• There are four main types: General Anaesthetic, Regional Anaesthetic, Peripheral Nerve Block, Monitored Anaesthesia Care.

Selecting an Anaesthetic Technique

• Patient considerations
• Surgeon considerations
• Patient safety
• Intraoperative/postoperative monitoring

Optimal Anaesthetic Technique

• Patient safety
• Patient satisfaction
• Excellent operating conditions for the surgeon
• Rapid recovery
• Avoidance of postoperative side effects
• Postoperative pain control

The Stages of Anaesthesia

• **

Stage 1: Induction/Stage of Analgesia

• Starts from the induction period until the patient loses consciousness.
• The patient may appear drowsy or dizzy during this stage.
• It is important to maintain a quiet environment.
• **

Stage 2: Excitement/Delirium

• This stage lasts from the time the patient loses consciousness until certain reflexes such as swallowing, gag, and eyelid reflexes are lost.
• The patient may appear excited and breathe irregularly during this stage.
• The patient is sensitive to external stimuli at this time.
• Silence should be maintained.
• **

Stage 3: Surgical Anaesthesia

• This stage occurs from the period the patient loses certain reflexes until respiratory paralysis occurs.
• The patient will have regular respiration, constricted pupils, relaxed jaws, and loss of auditory sensation.
• **

Stage 4: Stage of Danger

• This stage is reached when too much anaesthesia has been given and the patient has not been observed carefully.
• Death may result from respiratory or cardiac arrest if not resuscitated properly.
• The patient will not be breathing, with little to no heartbeat.

General Anaesthesia

• Defined as a drug's capacity to induce and sustain, as needed, a state of unconsciousness, amnesia, analgesia, and immobility.
• Association pathways are broken in the cerebral cortex, producing a lack of sensory and motor perception.
• Pain is controlled by general insensibility so the patient is unconscious and cannot hear, feel, or move their body.

Goals of General Anaesthesia

• Rapid induction
• Use of the minimal level of anesthetic that will give analgesia, unconsciousness, and amnesia
• Minimal depression of the cardiovascular system
• Rapid recovery

Common Practice Today

• Anesthesiologists use a combination of drugs, including premedication (e.g., sedatives, opioid analgesics), IV anesthetics for induction, muscle relaxants, a mixture of volatile anesthetic gases for maintenance.
• Initiate inhaled anesthetic at a concentration greater than MAC (Minimum Alveolar Concentration), then reduce for maintenance.

Four Aspects of General Anaesthesia

• Inhalation Induction
• Intravenous Induction
• Rapid Sequence
• Maintenance of Anaesthesia

Pharmacodynamics - Inhaled Anaesthetics

• Mechanism of Action, which involves:
  • Lipid solubility being directly proportional to potency.
  • Reversal of anaesthetic effect being achieved with the application of pressure.
  • No common chemical structure for the variety of compounds is capable of producing anaesthesia.
  • Molecular structure changes responsible for producing anaesthesia must occur within seconds and be reversible.
  • A reduction in body temperature lowers anaesthetic requirements.

Anaesthetic Mechanism of Action

• Anaesthetic sites of action
  • Supraspinal and spinal anatomic structures
  • Spinal and cerebral GABAa receptors
    • GABA receptors are the primary receptor within the CNS to modulate anaesthetic effects
    • Agonism of the GABA receptor by volatile agents results in enhanced CI¯ conductance leading to inhibitory actions on local neurons
  • Reticular formation within the brain stem, cerebral cortex, and hippocampus

Inhalational Anesthetics

• Blood solubility can be low, intermediate, or high.
• Muscles and lipids have high affinity for anesthetic agents.

MAC-Minimum Alveolar Concentration

• The amount of anesthetic gas (end-tidal concentration) will provide surgical anesthesia so 50% of subjects will not respond to the surgical incision.
• Alveolar concentration represents brain concentration after a short period of equilibration.
• MAC-awake is the minimal alveolar concentration at which 50% of subjects will respond to the command "open your eyes" and is generally between 0.4-0.5 MAC.
• MAC-BAR is the minimal alveolar concentration necessary to block the adrenergic response to skin incision, generally 1.5 MAC.

Factors That Affect MAC: Increase MAC

• Hyperthermia
• Hypernatremia
• Sympathoadrenal stimulation
• Chronic alcohol abuse
• Chronic opioid abuse
• Increases in ambient pressure
• Hypercapnia
• Decreasing age
• Hyperthyroid
• Acute alcohol abuse

Factors That Affect MAC: No Change in MAC

• Sex
• Weight
• Duration of anesthesia
• Hypo/hyperkalaemia
• PO2 > 40 mmHg
• MAP > 40 mmHg

Factors That Affect MAC: Decrease MAC

• Nitrous oxide
• Hypothyroid/myxoedema
• Hypocapnia
• Hypothermia - decrease is roughly linear
• Hyponatremia
• Increasing age
• Hypoxemia
• Hypotension
• Anaemia
• Pregnancy
• CNS depressant drugs - opioids, benzodiazepines, major tranquilizers
• Other drugs - lithium, lidocaine, magnesium

Pharmacokinetics - Second Gas Effect

• Occurs when two anaesthetics of varying onset speeds are administered together.
• A high concentration of a fast anaesthetic (such as nitrous oxide) is administered with a slower second anaesthetic gas (such as desflurane or sevoflurane).
• The slower gas achieves anaesthetic levels more quickly than if it had been given alone.
• The slower agent has a better uptake in the alveoli that is even greater in the arterial blood gas
• Second gas effect accelerates the rate of increase of alveolar pressure.
• Consequence of the concentration effect (increasing the inspired anaesthetic partial pressure accelerate the rise of the alveolar partial pressure)

Pharmacokinetics - Diffusion Anoxia

• This is otherwise known as the Fink effect or "third gas effect".
• Occurs during emergence when a high concentration of N2O is given.
• Completed anaesthesia, N20 exits the body quickly through the lungs and is replaced by less soluble nitrogen or air.
• Results transient dilution of normal respiratory gases (O2,CO2).

Pharmacokinetics - Components of Anaesthesia

• Compartment of anaesthesia:
  • Blood
  • Lung
  • CNS
• The speed at which an anaesthetic concentration can be obtained in the CNS determines how rapidly a patient will be anaesthetized.
• It depends not only on the anaesthetic concentration delivered to the alveoli, but also on the relatively low capacity for the anaesthetic in the blood and other tissues, which must be saturated prior to significant blood-to-brain transfer.

Cardiovascular - Systemic Hemodynamics

• Volatile Agents: ↓ MAP (via a reduction in SVR), ↓CO, ↓CI (all dose-dependent)
• N20: 个SVR (activates the sympathetic nervous system) – lead to 个CVP and arterial pressure BUT....in combination with volatile agents 个SVR and support arterial pressure
• N2O with opioids: augments cardiac suppression (N20 produces a direct negative inotropic effect)........What can this unmask?
• Cellular level: reduces intracellular free calcium in cardiac and vascular smooth muscle – What is the end result?????

Cardiovascular - Heart Rate (HR)

• Changes in HR are relative to the concentration of anaesthetic agents.
• Result of several variables: antagonism of SA node automaticity, modulation of baroreceptor reflex activity, and sympathetic nervous system activation.
• Sevoflurane causes minor alteration in HR.
• Isoflurane & Desflurance increases HR.

Cardiovascular – Coronary Blood Flow

• Can cause vasodilation – where Sevo causes the least
• In presence of hypotension – a steal phenomenon can occur
• Sevo, Des, Iso can be used in patients with a history of ischemic heart disease BUT.....
• Anaesthetic Preconditioning (APC)

Cardiovascular – Arrhythmias

• Generally not seen with some agents
• Desflurane and Sevoflurane affects action potential duration (APD)

Properties of Specific Anaesthetic Agents: Nitrous Oxide (Non-Volatile)

• Not used alone, except when full anaesthesia is not necessary (e.g. dental procedures) because it causes more nausea/vomiting.
• Direct negative inotropic effect.
• Uptake and elimination are rapid due to the low blood/gas partition coefficient (0.47).
• Contraindicated in patients with air-filled cavities (e.g. air embolus, pneumothorax, etc), pulmonary hypertension, and vitamin B12 deficiency.

Properties of Specific Anaesthetic Agents: Sevoflurane (Volatile)

• Expensive
• Used for outpatient surgery and induction in children.
• Bronchodilator

Properties of Specific Anaesthetic Agents: Isoflurane (Volatile)

• Has a pungent odour
• Not used for induction
• Bronchodilator

Properties of Specific Anaesthetic Agents: Desflurane (Volatile)

• Commonly used today for inpatient and outpatient surgery.
• Very rapid induction
• Bronchial irritant

Maintenance

• Isoflurane (ISO): MAC = 1.1
• Desflurane (DES): MAC = 6
  • Most insoluble, so the fastest to equilibrate, but a respiratory irritant, making it unsuitable for gaseous induction.
• Sevoflurane (SEVO): MAC = 2.2
  • Used for gaseous induction
• Nitrous Oxide: MAC = 105 (a gas)

Concentration Factors

• Inspired concentration (FI)
• Alveolar concentration (FA)
  • Blood solubility
  • Cardiac output
  • Tissue solubility (Fat, muscle, other)

Strategies to Increase FA (Alveolar Concentration)

• Increase minute ventilation.
• Increase delivered concentration (concentration effect).
• Increase fresh gas flow.
• Second gas effect.

Gases Ideal for Induction: Sevoflurane

• Non-pungent
• Used with/without Nitrous Oxide
• Used in paediatrics, especially if IV is not in place.
• Used when difficult intubation is suspected.
  • Absence of salivation
  • Preservation of spontaneous breathing

Characteristics of Sevoflurane

• Loss of consciousness occurs approximately 1 minute after breathing Sevoflurane at 8%.
• Compliments with benzodiazepines.
• Opioids may complicate.

Technique for Inhalation Induction: Gradual

• 7LPM N20(66%)/3LPM 02(33%)
• Sevo started at 1%, then every 2 breaths increased by 1% until the patient is asleep

Technique for Inhalation Induction: Vital Capacity Single-Breath Technique

• Prime circuit with Sevoflurane and a fresh gas flow of 8LPM (approx 60 sec)
• Sevoflurane set at 8%.
• 7LPM N20(66%)/3LPM 02(33%).
• Apply face mask to patient with deep breathing, loss of consciousness in 1 min.
• If LMA or face mask, continue with inhaled gases.
• If intubation is required, a neuromuscular-blocking drug is given to relax skeletal muscles.

Intravenous Induction

• Used for induction and maintenance of general anaesthesia.
• Allows the patient to experience a pleasant loss of consciousness while rapidly achieving surgical levels of anaesthesia.
• Adjunct to inhalation type.
• Valuable in allaying anxiety.
• Decrease the amount of inhalation anaesthesia required.

Intravenous Induction Agents

• IV anaesthetic agents that produce a rapid onset of unconsciousness, such as:
  • Propofol
  • Thiopental (not used anymore)
  • Etomidate
• Ketamine (dissociative anaesthesia)
• Neuromuscular Blocking agent - Muscle Relaxants:
  • Succinylcholine
  • Rocuronium
  • Vecuronium
  • Pancoronium Bromide
• Benzodiazapam
  • Midazolam
  • Diazepam (Valium)
• Opiotes
  • Fentanyl

Propofol - Diprivan: Primary Uses

• Induction
• Anaesthesia for short procedures (boluses every ~5 min)
• Sedation

Propofol - Diprivan: Advantages

• Rapid onset/recovery, even after repeated injections
• Anti-emetic properties

Propofol - Diprivan: Disadvantages

• Pain on injection
• Involuntary muscular movement
• Respiratory depression
• Decreases blood pressure
• Allergy (sulfite sensitivity, lecithin)
• Propofol infusion syndrome

Etomidate: Primary Use

• Induction in patients with cardiovascular problems

Etomidate: Advantages

• Rapid induction
• Ultra-short acting (5-15 min)
• No cardiovascular depression
• Minimal respiratory depression

Etomidate: Disadvantages

• Pain on injection
• Involuntary muscular movement
• Nausea and vomiting
• Hiccups
• Not analgesic
• Adrenocortical effect

Ketamine: Primary Uses

• Induction or anaesthesia in at-risk patients with cardiovascular problems
• Sedation or general anesthesia in children
• Asthma

Ketamine: Advantages

• Cardiovascular stimulant
• Bronchodilator
• Profound analgesia and amnesia

Ketamine: Disadvantages

• Emergence reactions (not in children 15; adults 65)
• Increases intracranial pressure
• Suppresses respiration (less severe than other anaesthetics)

Opioids: Primary Uses

• Analgesia

Opioids: Advantages

• Profound analgesia
• Relative cardiovascular stability
• High potency, short duration (15-30 min.; Remifentanil; 5 min) except morphine
• Reduces emergence phenomena
• Reversible by opioid receptor antagonists

Opioids: Disadvantages

• Nausea
• Slow gastric emptying
• Respiratory depression at high doses (assisted ventilation required).

Benzodiazepines: Primary Uses

• Anti-anxiety agent pre-op.
• Sedation

Benzodiazepines: Advantages

• Relatively rapid onset
• Cause amnesia
• Relatively little cardiovascular effect
• Anti-convulsant

Benzodiazepines: Disadvantages

• Not analgesic
• Cause respiratory depression
• Long-acting (diazepam or repeated inj. of midazolam)

Sequence for IV Induction

• Preoxygenation:
  • Breathing O2 via 3-5 LPM through face mask
  • Intention: replace nitrogen in the pts FRC with O2
• To increase safety during induction
• IV insertion – may happen before preO2
• Administer anaesthetic agents to induce a rapid onset of unconsciousness.
• Potentially muscle relaxant prior to airway insertion
• Airway insertion or face mask ventilation
• Maintenance of anaesthesia

Procedure Checklist

• Check Equipment
  • Ensure all equipment is working and within reach
  • Complete a check of the anaesthesia equipment at the start of each day and before each new case
  • Ensure suction is working
• Prior to Patient Entering OR Suite
  • Patient is identified
  • Consent is reviewed
  • Test results & Diagnostic studies are checked
  • I.v infusion started preop. Or on arrival
  • All drugs are given i.v.
• Once in OR Suite
  • Intraoperative monitoring
  • IV

Procedure: Airway

• Decide the induction type – IV, Inhalation, or combo
• When the patient becomes apneic and the eyelid reflex is gone, the airway is checked for patency by ventilating the patient with a mask
• Depending on several factors such as an adequate airway and the type and duration of surgery, oxygen and anesthetic gases may be delivered to a spontaneously breathing patient via a mask that is held in place with a head strap
• Positioning of the head and oral or nasal airway is used to maintain a patent airway
• If mask anesthesia is not suitable, then the endotracheal tube may be used to facilitate ventilation or to protect the airway from aspiration
• Verify correct location of the endotracheal tube by listening for equal bilateral breath sounds with a stethoscope.
• Use absence of sounds over stomach, appropriate level and waveform of ETCO2
• Observe expansion with each breath look at the chest
• Check condensation of moisture from expired air in the endotracheal tube and breathing circuit
• Auscultate chest for breath sounds and/or a capnoraphic tracing on the monitor to ensure end tidal CO2
• Allow patient to breathe spontaneously with intermittent assistance after paralysis from (succinylchiline) has worn off, or additional muscle relaxant may be given and ventilation controlled mechanically
• If the procedure is an emergency or the patient is at risk for aspiration (for example, in cases of intestinal obstruction, full stomach, hiatal hernia)
• Anesthesia personnel may undertake a rapid sequence induction with assistance for cricoid pressure
• If performing rapid intubation, access difficulty, choose awake intubation, using a fiber optic bronchoscope, or other techniques
• Maintain anesthesia with either intravenous or inhalational routes plus/minus additional muscle relaxant

Maintenance of Anaesthesia: Objectives

• Amnesia
• Analgesia
• Skeletal muscle relaxation
• Control of sympathetic nervous system responses evoked by noxious stimulations

How to Meet Objectives in Maintenance of Anaesthesia

• Combine inhaled or intravenous administered drugs, with or without neuromuscular blocking drugs.
• Each drug must have a specific goal.
• Neuromuscular Blocking Drugs - ensure no movement and need to be in combination with anaesthetic gases or other agents ("Light Anaesthesia").

Types of Regional Anaesthesia

• Loss of sensation in limited areas of body without loss of consciousness
• Stabilizes or elevates the threshold of the excitation of the nerve cell membrane
• Decrease cell membrane permeability to all ions thus depolarization and transmission of nerve impulses are prevented
• Spinal – Epidural - Local – Infiltration

Types of Anaesthesia

• Peripheral Nerve Block
• Monitored Anaesthesia Care
  • Anaesthesia assistant will assist the Anaesthetist to monitor the patient during the procedure including:
    • Treatment of clinical problems
    • Support vital functions
    • Administration of sedatives, analgesics, hypnotics, anaesthetic drugs etc
    • Psychological support and physical comfort