Anaesthesiology SC004: How Would You Anaesthetize Me? Pharmacology Of Anaesthetic Drugs Flashcards
Types of Anaesthesia
- General anaesthesia (GA)
- Reduced level of consciousness - Regional anaesthesia (RA)
- Loss of sensation to a body part / region
- Patient can be fully awake / sedated / unconscious - Combinations of General + Regional
- Monitored anaesthetic care (MAC) (“Conscious Sedation” / Sedation)
- smaller dose of anaesthetic —> patient maintain some vital functions (e.g airway protection, respiration)
Definition of Anaesthesia
- Anaesthesia = Loss of feeling / sensation
- Reversible
- Providing GA involves more than just dampening sensation (e.g. reflex suppression)
Components of General anaesthesia
- Hypnosis (loss of awareness to surrounding)
- Amnesia (not remembering)
- Analgesia (i.e. Regional anaesthesia)
- Areflexia
- +/- Muscle relaxation (Neuromuscular blockade)
Hypnosis =/= Analgesia: You are not aware of pain —> but body feels the pain —> react by physiological responses (e.g. sympathetic responses)
Sequence of events in anaesthesia
- Identification of patient
- Pre-operative assessment
- Anaesthetic plan
- Type of anaesthetic
- Preparation (patient, staff, equipment, drugs, fluids)
- Intra-operative management
- Post-operative management - Preparation (e.g. equipment, staff, psychological)
- Conduct of anaesthesia
- Post-operative care (e.g. place to transfer)
Pre-operative assessment
Large part can be performed by caring physicians to minimise delays / deferments
- History
- General
- Anaesthetic history
—> Previous anaesthetic experiences
—> Complications:
——> Difficult airway management (e.g. ankylosing spondylitis)
——> Awareness (inadvertent recall of intra-operative events / waking during surgery)
——> Allergies (∵ give a lot of IV drugs in succession systemically —> very important)
——> PONV
——> Delayed emergence from anaesthesia
——> Genetic problems (rare): **Pseudocholinesterase deficiency, **Malignant hyperthermia
——> Nerve damage - P/E
- General (esp. CVS, Resp)
- Airway (mouth, neck etc.)
- Other relevant body parts e.g. L-spine (e.g. in epidural / spinal anaesthetic) - Investigations
- biochemical abnormalities
- conduction abnormalities of heart
Anaesthesiologist then determine:
1. Fitness for surgery
- estimate whether there is sufficient reserve to
—> meet increased metabolic demands from the **stress of surgery + for **healing
—> as there may also be some compromise in cardiorespiratory function from anaesthesia
- Estimate risk of ***exacerbating certain co-morbidities
- IHD
- Respiratory failure - Need for specialist consultations to ***optimise co-existing conditions
- optimise as far as possible for elective patients
- emergency surgery: do best you can with increased monitoring + post-operative care - Anaesthetic care plan
How do co-morbidities in a patient affect the administration of anaesthesia?
- Diseased organs (e.g. Liver failure, Renal failure)
- Adverse effects of anaesthetic drugs (e.g. stroke patient may require less anaesthesia to achieve hypnosis as brain is diseased) - Systemic illness
- Pharmacokinetics
- Pharmacodynamics - Drug interactions
- Affects anaesthetic techniques
***Respiratory system
Anaesthetic agents on Respiratory system:
1. Central control
- ↓ Response to CO2 + Hypoxia
- Airway
- ↓ Protective reflexes
- ↑ Work of breathing (resistance from breathing circuit)
- Retained secretions
- Bronchospasm etc. - Lung mechanics
- ↓ Lung volumes
- ↑ Work of breathing
- Residual paralysis - Gas exchange
- ↑ V/Q mismatch
Pre-existing pulmonary diseases ↑ respiratory complications:
1. Intra-operative
- Coughing on induction —> vomit —> aspiration
- Laryngospasm (body cannot break spasm during anaesthesia unlike awake state)
- Bronchospasm
- Desaturation
- Post-operative
- Mucous retention (∵ ciliary mechanism to clear secretion is impaired during anaesthesia) (in patients with chronic bronchitis —> many mucus)
—> Pneumonia
- Respiratory failure
Optimisation:
1. Lung function optimised before GA for elective surgery
- Stop smoking
- Treat infection
- Bronchodilators, Steroids
- Post-op care to reduce complications
- Supplemental oxygen
- Adequate pain relief
- Post-op physiotherapy, patient education
- Consider post-op ventilatory support
***CVS system
Anaesthetic agents on CVS system:
1. Vasodilation
2. Venodilation
3. ↑/↓ Chronotropy
4. ↓ Ionotropy
—> ↓ CO —> ↓ Tissue perfusion
Surgical stress alters haemodynamics:
- ***↑ Metabolic demands
- Pain of surgery - ***↓ Tissue perfusion
- ↓ CO
- +/-↓ Circulating volume from bleeding
—> May exacerbate myocardial ischaemia
Peri-operative cardiac failure:
Inability of the heart to meet ↑ demand during surgery
- ***↑ Metabolic demand
- May develop arrhythmia
- Fixed CO states (e.g. Aortic stenosis) —> limited compensation
Optimisation:
1. Delineate cardiac risk
2. Judicious investigations
3. Optimise cardiac status
- Lifestyle changes
- Medications
- Valvular surgery
- Revascularisation
4. Appropriate post-op monitoring
5. Effective post-operative pain relief
***Liver
Anaesthetic agents on Liver:
1. ***Reduced synthetic function
- ↓ Protein binding of drugs
- Impaired clotting
- ***Reduced metabolic function
- ↓ Drug metabolism + clearance
- ↓ Clearance of toxins —> Encephalopathy —> response to anaesthesia will be vastly different - ***Circulatory disturbances
- Portal hypertension, Ascites —> High circulating volume —> affect drug distribution
Optimisation:
1. Treat coagulopathy
2. Adjust drug doses
3. Other supportive treatment
***Kidneys
Pre-existing kidney disease on anaesthesia / surgery:
1. Reduced drug excretion
2. Electrolyte abnormalities
3. Hypertension
4. Severe anaemia
5. Platelet dysfunction
Optimisation:
1. Consider prehydration
2. Minimise nephrotoxins
***Diabetes mellitus on Anaesthesia
- Associated CVS + Renal problems
- Fasting + Surgical stress —> upset glucose control
- Diabetogenic hormones ↑ peri-operatively
- Both hyper + hypoglycaemia detrimental to CNS (sugar level + its damage on other body parts)
Optimisation:
1. Clear fasting instructions + treatment plan
2. ***Omit oral hypoglycaemic on morning of surgery
3. May require insulin-dextrose infusion
4. Meticulous peri-operative sugar control
5. Frequent monitoring
Anaesthetic plan
- Type of anaesthetic
- Preparation
- Patient
- Staff
- Equipment
- Drugs
- Fluids - Intra-operative management
- Monitoring
- Positioning
- Fluid management - Post-operative management
- Analgesia
- Disposition
Preparation of patient
- Instructions to patient regarding their co-morbidities in the peri-operative period
- Medications: Avoid / Continue - Fasting instructions
- Other preparations
- e.g. bring their CPAP machine to hospital
Conduct of General anaesthesia
- ***Establish monitoring
- **Induction agent +/- Opioid +/- **Muscle relaxant
- Instrument airway (ET tube, Laryngeal mask)
- ***Maintenance agent (infusion / inhalational)
- Positioning + Protecting patient
- Monitoring, Fluid + drug management
- Emergence +/- ***Reversal agents (only apply to neuromuscular blockade)
- Recovery (Post-anaesthesia care unit / ICU)
***Induction of Anaesthesia
- The act / process of inducing / bringing about anaesthesia —> Putting to sleep
- Use of pharmacological induction agents
—> IV route (fastest, most common)
—> Inhalational route can be used
Common induction agents:
1. IV induction agents
- Propofol
- Thiopentone
- Etomidate
- Ketamine
- “Gas induction” volatile agents (for children / IVDU ∵ hard to get IV access)
- Sevoflurane (yellow)
- Nitrous oxide
Neuromuscular blocking agents (Muscle relaxants)
Non-depolarising
1. Short-acting
- Mivacurium
- Intermediate-acting (most common, onset: 2-3 mins, duration: 20-30 mins, minimal SE)
- **Atracurium, **cis-Atracurium
- ***Rocuronium, Vecuronium - Long-acting
- Pancuronium
Depolarising
4. Suxamethonium (most commonly used in Rapid sequence induction (RIS), onset: 15-30 seconds —> period between hypnosis and intubation is minimised)
Reversal agents:
- Reverse neuromuscular blockade, NOT hypnosis
- Nerve stimulator used to assess degree of neuromuscular blockade
1. Neostigmine (AChE inhibitor) + Anticholinergic (Atropine / Glycopyrrolate: prevent muscarinic effect —> bradycardia, only want ***nicotinic effect)
2. Sugammadex (chelating agent for reversing Rocuronium only)
Rapid sequence induction
Give IV induction agent
—> Quickly give fast onset NMJ blocker (Suxamethonium) before establishing whether patient is unconscious or not
—> Intubate
—> Minimise period between hypnosis and intubation
Indication:
- Full stomach —> prevent regurgitation
Normally in elective surgery:
- IV induction agent is titrated slowly until patient hypnotic enough
—> Intermediate NMJ blocker (onset: 2-3 mins)
Maintenance of anaesthesia
Continuous administration of a maintenance anaesthetic agent
- usually in combination with an analgesic agent
- Inhalational / IV route
Maintenance agent:
1. IV anaesthetic
- Propofol
- Volatile anaesthetic
- Isoflurane (purple)
- Sevoflurane (yellow)
- Desflurane (blue)
- +/- Nitrous oxide (adjuvant to anaesthetic agent ∵ potent analgesic)
Analgesic drugs
Depending scale of surgery + amount of pain
1. Simple
- Paracetamol
- NSAIDs
- IV
- Suppositories - Opioids
- Fentanyl
- Morphine
- Oxycodone
- Pethidine - Others
- Tramadol - Regional technique (help reduce amount of analgesic given during / after surgery)
- Nerve block
- Neuraxial blockade: Spinal anaesthesia, Epidural anaesthesia
Post-operative management
- Analgesia
- Oral
- Parenteral
—> Infusion
—> PCA (Patient controlled analgesia) - Continuation of Regional block
—> Epidural
—> Catheters along major plexus + nerves
- Disposition (where the patient go for right degree of monitoring)
- Home
- Ward
- High dependency unit
- ICU
—> if not sure: ask anaesthetist
(High dependency unit vs ICU: availability of invasive ventilation, nursing staff)
Regional anaesthesia (RA)
Established by deposition of **LA in close proximity to a single nerve / group of nerve fibres / spinal cord
—> LA then block transmission of signals in nerve fibres
—> Reduced sensation from the **anatomical area supplied by blocked nerves
Indication:
- Used alone / in combination with GA
—> Pain relief by nerve blockade
—> Hypnosis by ***anaesthetic agent
Preparation:
- Prepare everything exactly same as GA —> in case of anything going wrong
Contraindication:
1. ***Patient refusal (absolute CI)
2. Bleeding tendency (low platelets, coagulopathy)
3. Infection risk
4. Septic patients (∵ sympathetic block —> further vasodilation —> CVS collapse)
***Types of Regional anaesthesia
- Neuraxial blocks (i.e. block spinal cord below a certain level / segmental part)
- Spinal (aka Subarachnoid block, Intrathecal block) —> CSF fluid
- Epidural —> Epidural fat - Plexus blocks
- e.g. Brachial plexus for arm surgery - Nerve blocks
- e.g. Femoral nerve block - Local infiltration
- e.g. around skin lesion - Topical anaesthesia
- e.g. EMLA cream - IV regional technique
***Neuraxial block
Spinal anaesthesia:
- LA injected into **CSF
- Block sensation supplied by spinal fibres below a certain level depending on type / dose of LA used
—> “Temporary paraplegia”: Dense block
—> Motor + Sensory block
- Last only a few hours —> catheter **not usually used (∵ risk of infection in CSF)
Epidural anaesthesia:
- LA deposited in **epidural space
- Varying degree of motor + sensory block
- Bigger needle —> Bigger hole —> More drug
- Advantage: can leave **catheter in epidural space —> allow ***continuous infusion of LA for days (e.g. after major thoracic surgery —> thoracic epidural —> allow numbness in chest only)
Why need extensive preparation in RA?
May need to convert to GA immediately
1. Total / Partial failure of block
- Operator factors
—> misplaced needle
—> incorrect dosage
- Drug error (including expired drugs)
- Patient factors
—> anatomical variants
- Complications from regional technique
- LA toxicity
- Severe haemodynamic disturbances (from Autonomic block: Vasodilatation)
- Excessive block - Prolonged surgery
- Duration of surgery > Duration of block —> return of feeling during surgery
- Patients restless / becoming uncooperative
LA toxicity
Causes:
1. Inadvertent ***intravascular injection
- Overdose
- Adverse reaction to adjuvant
- Epinephrine
- Preservatives
Spectrum of systemic manifestations:
Neurological S/S:
1. **Lightheadness, Dizziness, Drowsiness
2. **Tingling around lips, fingers / generalised
3. **Metallic taste
4. **Tinnitus
5. BOV
6. Confusion
7. Restlessness
8. Incoherent speech
9. Tremors / Twitching
10. ***Full-blown convulsions with LOC / Coma
CVS S/S:
1. **Bradycardia (can Tachycardia)
2. **Hypotension
3. CVS collapse
4. **Respiratory arrest
5. **QRS + PR prolongation
6. ***AV block
7. Change in T wave amplitude
Severe haemodynamic disturbances
Usually from Neuraxial block
- ***Hypotension
- Sympathectomy (i.e. Sympathetic block)
- Inadequate intravascular volume
- Inappropriate patient choice
—> Cardiac patient
—> Valvular heart disease - ***Bradycardia
- Blockade of cardiac sympathetic fibres
Excessive block
- Excessive ***rostral spread of LA
- Dose: too much
- Position: head down
- Subdural - Respiratory distress
- Motor blockade of intercostal muscles
—> may require ventilatory support - “Total Spinal”
- Supraspinal neurons (Brain, Midbrain, Pons, Medulla) blocked by LA
Summary
- Thorough preparation before surgery requiring GA / RA
- Anticipation + Prevention
- Patients
- Drugs
- Equipment
- Staff (including Surgeons) - Management
As interns:
1. Surgery + GA = Gross disturbances in physiological homeostasis
2. Effects extend well into post-op period
3. Patient assessment + optimisation important before surgery
4. Notify your anaesthetic colleagues early if not sure what to do
