Anaesthesia - Anaesthetic Drugs, Intubation, Pre-Op Assessment Flashcards
What is the major cause of airway obstruction?
Tongue flopping back
How do you resolve airway obstruction from the tongue?
Head-tilt chin-lift
Then a jaw thrust to move the mandible upwards if necessary
How can you deliver a high percentage of oxygen to an acutely unwell patient?
Non-rebreathe mask with resevoir bag
15L Oxygen
How do you measure an oropharyngeal airway?
From angle of mandible to incisors
OR
From corner of mouth to ear lobe
When do you use an oropharyngeal airway vs nasopharyngeal airway?
Use nasopharyngeal airway:
- If patient is conscious - but would still need to have low GCS
- If oropharyngeal is not tolerated e.g. from gag reflex
- If patient is having seizure - cannot get oropharyngeal airway in
How do you measure a nasopharyngeal airway?
Measure diameter of airway against patient’s little finger
How much oxygen is given through nasal cannulae?
1-6L/min (most commonly 2L/min)
Around 25% oxygen but can be up to 40%
What is the first line airway in cardiac arrest?
Laryngeal mask airway (LMA) - quicker to insert and deliver oxygen
What remains a risk with LMAs?
Risk of aspiration - does not completely block off the oesophagus
What equipment is required to insert an endotracheal tube and how do you insert it?
- Hold laryngoscope in left hand and insert centrally, which sweeps the tongue to the left side and continue inserting until can see epiglottis
- When can see epiglottis, lift it up and outwards and you will see trachea and vocal chords
- Guide endotracheal tube down trachea - the 2 black lines should straddle the vocal chords
- Inflate the cuff to prevent aspiration of gastric contents
- Attach the bag and inflate to check equal chest rising
- Auscultate both apices and both lateral bases and stomach
- Attach capnograph to detect CO2 in expired gas
What sizes of endotracheal tubes are usually used?
Females: size 8; length 21
Males: size 9; length 23
What is used if there is a poor view of the vocal chords?
Bougie - put this into trachea then slide endotracheal tube over it
What is done if you can’t intubate and can’t oxygenate?
Cricothyrotomy - done in the cricothyroid hiatus (between the cricoid and thyroid cartilage)
In which patients is aspiration a greater risk?
Emergency surgery
Pregnant women
Diabetes
Hiatus hernias
What signs of aspiration can be seen in a patient under anaesthetic?
Direct visualisation with laryngoscope Coughing Vomiting Laryngospasm Bronchospasm Decreasing sats Tachypnoea Wheeze and crepitations heard on auscultation
How do you manage aspiration in a patient under anaesthetic?
Apply cricoid pressure
Use suction to clear the mouth of debris
Endotracheal intubation
Refrain from ventilating (if sats remain ok) to prevent dispersion of aspirate
Empty stomach with NG tube
Put patient head down and in left lateral position
Do CXR
What method of intubation is used in the emergency setting?
Rapid Sequence Induction
Give an overview of Rapid Sequence Induction
- Pre-oxygenate with 100% O2 for 3 min or 5 full vital capacity breaths
- Apply cricoid pressure - blcoks off oesophagus to prevent aspiration
2i) Give induction agent - thiopentone or propofol
2ii) Then immediately give muscle relaxant -suxamethonium or rucuronium - Endotracheal intubation - remove cricoid pressure after confirmation of tube position
What is the triad of anaesthesia?
Amnesia - lack of response and recall to noxious stimuli (unconsciousness)
Analgesia - pain relief
Akinesis - paralysis
What are the minimum monitoring standards for anaesthesia?
- ECG
- SpO2
- NIBP (non-invasive BP)
- Expired CO2
- Airway pressure
- A nerve stimulator if a muscle relaxant is used
- Temperature monitoring
What factors make an ideal IV induction agent?
- Act rapidly within one arm-brain circulation (10-20 sec)
- Quick recovery with no hangover effect
- No post-op phenomena
- Painless when given IV
- Non-irritant if injected extravascularly
What is the duration of action of IV induction agents?
4-10 minutes
Give examples of IV induction agents
Propofol - most commonly used
Thiopentone
Ketamine
Etomidate
What are the doses for the four IV induction agents?
Propofol: 1.5-2.5mg/kg
Thiopentone: 4-5mg/kg
Ketamine: 1-1.5mg/kg
Etomidate: 0.3mg/kg
What type of drugs are the four IV induction agents?
Propofol - lipid based (white emulsion)
Thiopentone - barbiturate
Ketamine - phencyclidine derivative
Etomidate - steroid based
What are the pros of propofol?
Excellent suppression of airway reflexes - less likely to have laryngospasm + bronchospasm
Decreases incidence of PONV - anti-emetic properties
Fast acting
What are cons of propofol?
Pain on injection in 40% patients
Involuntary movement
Marked drop of HR and BP
What are the contraindications to propofol?
Extremes of age
<17 years for sedation
Egg or soy allergy
Compromised airway
When is thiopentone predominantly used?
Rapid Sequence Induction
What are the pros of thiopentone?
Faster onset of action than propofol (arm-brain circulation time of 15 sec)
Antiepileptic properties
Protects the brain
What are the cons of thiopentone?
Drop in BP but rise in HR
Bronchoconstriction - does not suppress airway reflexes
Hangover effect - 30% of dose is still present in body after 24h
Intra-arterial injection causes thrombosis and gangrene
Extravascular injection causes severe pain and necrosis
When is thiopentone contraindicated?
Porphyria
Barbiturate allergy
Hypovolaemia
What are the main uses of ketamine in anaesthetics?
Paediatric anaesthesia
In short procedures
Pre-hospital procedural sedation
Why can ketamine be used as the sole anaesthetic for short procedures?
It provides anterograde amnesia and profound analgesia
What are the pros of ketamine?
Causes a rise in BP and HR - good in patients that are shocked, hypovolaemic or anaemic
Bronchodilation - good in asthmatic patients
Can be given IM for profound analgesia
What are the cons of ketamine?
Slower onset - 90 seconds Slower recovery Nausea + vomiting Emergence phenomenon - vivid dreams, hallucinations (does not affect children) Raises ICP and IOP
When is ketamine contraindicated?
- Moderate to severe hypertension, congestive cardiac failure, or a history of cerebrovascular accident
- Acute or chronic alcohol intoxication
- Cerebral trauma, intracerebral mass or haemorrhage or other causes of raised intracranial pressure
- Eye injury and increased intraocular pressure
- Psychiatric disorders such as schizophrenia and acute psychoses
When is etomidate the chosen induction agent?
Acutely unwell patients with trauma/head injuries for whom avoidance of even a brief episode of hypotension is important
What are the pros of etomidate?
Does not affect BP
Cardiovascular stability
Lowest incidence of hypersensitivity reaction
What are the cons of etomidate?
High incidence of PONV
Involuntary muscle movements
Pain on injection - local thrombophlebitis
Adrenal suppression - prolonged use must be avoided
What is used for the maintenance of amnesia?
Propofol infusion using a pump with infusion of fentanyl
Or inhalation agents
- Isoflurane
- Sevoflurane
- Desflurane
- Enflurane
What are the advantages of inhalation agents over IV agents?
No IV access required
More precise control
When is isoflurane used?
Organ retrieval from a donor as it has the least effect on organ blood flow
What are the cons of isoflurane?
It is an irritant - coughing, laryngospasm, breath-holding (use opioids to suppress coughing)
When is sevoflurane used?
Paediatric patient
Cannot gain IV access - it is the agent of choice for inhalational induction of general anaesthesia
When is desflurane used?
Long operations
Obese patients - low lipid solubility so provides quickest recovery post surgery
What is meant by Minimum Alveolar Concentration (MAC)?
Concentration of the vapour that prevents the reaction to a standard surgical stimulus in 50% of subjects
What are the MAC of nitrous oxide, sevoflurane, isoflurane, desflurane and enflurane?
- Nitrous oxide: 104%
- Sevoflurane: 2%
- Isoflurane : 1.15%
- Desflurane : 6%
- Enflurane : 1.6 %
What is the process of muscle contraction at the neuromuscular junction?
- Action potential arrives at neuromuscular junction
- Acetylcholine is released from vesicles at the presynaptic membrane
- Acetylcholine diffuses across the synaptic cleft
- Acetylcholine binds to post-synaptic nicotinic ACh receptors
- Ion channels open allowing influx of Na+ leading to membrane depolarisation
- Muscle contracts
- Acetylcholinesterase hydrolyses ACh at the post-synaptic receptors causing termination of the contraction
What are the two types of neuromuscular blocking agents?
Depolarising
Non-depolarising
How do non-depolarising agents work?
They compete with ACh at the NMJ but without depolarising the membrane
How can the effect of non-depolarising agents be reversed?
Neostigmine - anticholinesterase (inhibit acetylcholinesterase which normally breaks ACh down)
Also give glycopyrolate (antimuscarinic) to prevent side effects of neostigmine (e.g. bradycardia)
What is the use of non-depolarising agents?
Used during balanced anaesthesia to facilitate intermittent positive-pressure ventilation (IPPV) and surgery
Which non-depolarising agents are short acting?
Atracurium
Mivacurium
Which non-depolarising agents are intermediate acting?
Rocuronium
Vecuronium
Which non-depolarising agent is long acting?
Pancuronium
Which non-depolarising agent is used if cardiovascular stability is important?
Vecuronium
Which non-depolarising agent is the drug of choice in renal and liver failure?
Atracurium - is it metbaolised by spontaneous molecular breakdown
What is a reversal agent for rocuronium and vecuronium? What is the benefit of this?
Sugammadex
Rocuronium can now be used in RSI - sugammadex is able to reverse rocuronium faster than the time taken for suxamethonium to wear off
What is the mechanism of action of depolarising agents?
They act similarly to acetylcholine on nicotinic receptors - initially cause fasciculations
They are very slowly hydrolysed by acetylcholinesterase so after muscle contraction the muscle quickly fatigues and relaxes
What is the dose for suxamethonium?
1-1.5mg/kg
What is suxamethonium commonly used for? Why?
Rapid Sequence Induction
Rapid onset - lessens the time between induction and intubation which decreases the risk of aspiration and hypoxia
Rapid offset - if intubation is impossible the patient regains muscle tone and starts protecting their own airway again quickly
Which patients should suxamethonium be avoided in?
Causes raise in plasma K+ so avoid in:
- Renal impairment + dialysis patients
- Burns
What are the adverse effects of suxamethonium?
Fasciculations Hyperkalaemia Muscle pains Rise in ICP and IOP Bradycardia Malignant hyperthermia Increased gastric pressure Prolonged apnoea in pseudocholinesterase deficiency
How can the effects of suxamethonium be reversed?
Dantrolene
Only use this in cases of malignant hyperthermia
Who is resistant to suxamethonium?
Patients with myasthenia
What muscle relaxant should be used in patients with myasthenia?
A small dose of atracurium
Patients with myasthenia are very sensitive to non-depolarising agents
What agents cause lethal paralysis?
Curare - poison used in South America on dart tips for hunting
alpha-Neurotoxoins - snake venom
Organophosphates - e.g. sarin
Botulinum toxin
What class of drugs is used to treat hypotension in anaesthesia? Give examples of the drugs
Vaso-active drugs
- Ephedrine
- Phenylephrine
- Metaraminol
What drugs are used to treat severe hypotension/are used in ICU?
Noradrenaline
Adrenaline
Dobutamine
What is the mechanism of action of ephedrine?
Acts on alpha and beta receptors
Direct and indirect action
Rise in HR and contractility leading to a rise in BP
What is the mechanism of action of phenylephrine?
Direct action on alpha receptors
Rise in BP by vasoconstriction and a drop in HR
What is the mechanism of action of metaraminol?
Predominantly acts on alpha receptors
Direct and indirect action
Rise in BP by vasoconstriction
Which antiemetic agents are used in anaesthesia?
Intraoperative:
1st line - ondansetron 4-8mg (5HT3 blocker)
2nd line - dexamethasone 4-8mg
Recovery:
3rd line - cyclizine 50mg TDS (antihistamine)
What conditions are important to screen for in pre-operative assessment?
CVS:
- MI or IHD - when was it?
- Hypertension
- Heart failure
Respiratory system:
- Asthma
- COPD
- Recent chest infections
- Sleep apnoea
MSK:
- Rheumatoid/osteo arthritis
- Neck problems
GI:
- Diabetes
- Reflux
- Liver/renal disease
Neuro:
- Epilepsy
Dental problems
If history of previous anaesthetics, what things should be asked about in pre-op assessment?
Any complications Difficulty intubating Post-operative nausea + vomiting Delayed recovery Malignant hyperthermia
What drugs should not be taken the morning of surgery?
Anticoagulants Aspirin Clopidogrel - stop 5-7 days before NSAIDs Diuretics Metformin - diabetic patients should be first on list Insulin
What drugs should be considered stopping weeks before surgery?
COCP + HRT - stop 4 weeks before, restart 2 weeks after
Ophthalmic drugs - anticholinesterases, beta-blockers, alpha-blockers
What is the physical status classification system for assessing fitness for surgery?
ASA Grading
- Grade 1 - A healthy patient with no systemic disease
- Grade 2 - Mild to moderate systemic disease with no functional limitation
- Grade 3 - Severe systemic disease imposing functional limitation on patient
- Grade 4 - Severe systemic disease which is a constant threat to life
- Grade 5 - Moribund patient who is not expected to survive with or without the operation
- Grade 6 - A brainstem-dead patient whose organs are being removed for donor purposes
Give examples of minor (grade 1) surgeries
Excision skin lesion
Cystoscopy
Drainage of an abscess
Give examples of intermediate (grade 2) surgeries
Inguinal hernia
Tonsillectomy
Give examples of major (grade 3) surgeries
Hysterectomy
Thyroidectomy
Give examples of major+ (grade 4) surgeries
Joint replacement
Thoracic operations
Radical neck dissection
According to the NCEPOD classification of surgery, what surgeries fall under ‘Immediate’?
Ruptured AAA
Fasciotomy
According to the NCEPOD classification of surgery, what surgeries fall under ‘Urgent’?
Bowel obstruction
Septic appendicitis
Bowel perforation
According to the NCEPOD classification of surgery, what surgeries fall under ‘Epedited’?
Ectopic pregnancy
Neck of femur fractures
Repair of tendon or nerve injuries
Excision of tumour with potential to bleed or obstruct
According to the NCEPOD classification of surgery, what surgeries fall under ‘Elective’?
Elective AAA repair
Lap chole
What length of time does water and food remain in the digestive system?
Water
- Half life 10-20 mins
- in 2 hours less than 1% remains in stomach
Food
- 50% of stomach contents emptying - 2.5-3 hours
- Total emptying of stomach - 4- 5 hours
- 50% emptying of small intestine - 2.5-3 hours
- Transit through the colon - 30-40 hours
What factors delay gastric emptying?
- Metabolic: Diabetes (in diabetics, with autonomic neuropathy can get delayed gastric emptying); End stage renal failure
- Anatomical – pyloric stenosis
- Mechanical – obesity, pregnancy (from 20 weeks they have higher risk of aspiration, the mechanical risk and high progesterone causes dilation of pyloric sphincter)
- Trauma – RTA, head injury
- Others - High fat content, anxiety, alcohol
What is the minimum fasting time for solid food and milk-containing drinks?
6 hours
What is the minimum fasting time for breast-fed infants?
4 hours
What is the minimum fasting time for alcohol?
At least 24 hours - delays gastric emptying
What is the minimum fasting time for clear fluids?
2 hours
How much clear fluid is allowed up to the time of surgery?
30ml
What is the process of general anaesthesia for LMA?
- Oxygenation
- Opioid - fentanyl/alfentanil
- Induction agent - propofol
- Inhalation agent
- Bag valve mask ventilation
- LMA insertion
What is the process of general anaesthesia for intubation?
- Oxygenation
- Opioid - fentanyl/alfentanil
- Induction agent - propofol
- Inhalation agent
- Bag valve mask ventilation (unless RSI)
- Muscle relaxant - non-depolarising (unless RSI)
- Endotracheal intubation
Should a patient fast if they are only having a spinal/epidural/nerve block?
yes because if the local anaesthetic fails part way through the surgery, the anaesthetist may have to use general anaesthetic or opioids, both of which carry risk of aspiration
What external anatomical features are important to consider in a pre-op airway assessment?
Poor dentition Small lower jaw Body habitus Beards Big tongue
What score is used to classify the difficulty of intubating a patient?
Mallampati Score - ask the patient to open mouth wide and stick their tongue out
Grade I - soft palate, uvula, fauces (the arched opening at the back of the mouth leading to the pharynx), pharyngeal pillars
Grade II - Soft palate, uvula, fauces
Grade III - Soft palate, base of uvula
Grade IV - Hard palate only
Grades 3 or 4 suggest difficulty intubating
What might cause obstruction to the airway for intubation?
Angioedema Epiglottitis Tumours Burns Dentures
What systems relevant to anaesthesia can rheumatoid arthritis affect? How are they affected?
Joints CVS - asymptomatic pericardial effusion Respiratory - pulmonary nodules and fibrosis Anaemia Renal impairment Peripheral neuropathy
What investigations are important to do pre-op for a patient with rheumatoid arthritis? What is each investigation hoping to exclude?
FBC - anaemia
U+Es - renal impairment
CXR - pulmonary nodules + fibrosis, pericardial effusion
ECG
Cervical spine X-ray - glottic stenosis + atlanto-axial subluxation
Echo (if indicated) - pericardial effusion
Pulmonary function test - pulmonary nodules + fibrosis
What systems relevant to anaesthesia can diabetes mellitus affect? How are they affected?
CVS - hypertension, silent angina/MI due to neuropathy
Respiratory - increased infections, thickening of soft tissues due to glycosylation
Renal failure
GI - delayed gastric emptying
What investigations are important to do pre-op for a patient with diabetes mellitus?
BM
Urine ketones and glucose
ECG
U+Es
Which lead is shown on an anaesthetic machine ECG and why?
Lead II i.e. the rhythm strip
It lies close to cardiac axis (overall direction of electrical conduction through the heart) so is best for detecting arrhythmias and looking at P and QRS complexes
It provides an image of the antero-lateral wall of heart (i.e. left ventricle supplied by LAD), which is where most ischaemia occurs in heart
What is the inheritance pattern of susceptibility to malignant hyperthermia from suxamethonium?
Autosomal dominant
If one parent has had it but other has not, child has 50% chance of having it
What type of muscle relaxant is suxamethonium?
Depolarising neuromuscular blocker
When are nasopharyngeal airways contraindicated?
Base of skull fractures
If a patient takes prednisolone and are about to undergo a surgery, what do they require?
Hydrocortisone supplementation
What reverses the action of benzodiazepines? How does it work?
Flumazenil - it competes at GABA binding sites
What are the 5 ways you know that the endotracheal tube has been correctly inserted?
- You see it go through vocal cords
- Mist
- Air over chest and none over abdomen
- CO2 expired air
- Chest rise and fall
