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
