Anaesthesia Flashcards
Define anaesthesia
Controlled depression of the CNS so as to produce a lack of awareness of painful inputs (nociception)
What area of the brain do anaesthetic agents ideally work on and why?
Minimal depression of hindbrain functions (preventing depression of cardiovascular centres) while having depressive effect on forebrain
Why is unconsciousness required?
To allow pain free surgery (no nociception)
Define nociception
The response to a stimulus that may be painful
Define pain
The conscious awareness of nociceptive input
What are the desired effects of anaesthesia?
- Hypnosis
- Anti-nociception (analgesia)
- Muscle relaxation
- Areflexia
- Maintenance of oxygen delivery
What are the potential risks of anaesthesia?
- Direct toxicity (reaction to drug)
- Indirect toxicity (e.g. halothane causing liver damage killing patients a few days later)
- Accidents
What are the features of successful anaesthesia?
- No CV depression
- No reduction in oxygen tissue delivery
- No cardiac dysrhythmias
- No cerebral hypoxia
- Survival
How is anaesthetic risk classified?
- American Society of Anesthesiologists (ASA) Physical Status Classification
- Categories I to VI
- VI not currently used (donor status)
Describe category I anaesthetic risk physical status
Normal healthy patient, no discernible disease e.g. elective spay, castration, dentistry
Describe category II anaesthetic risk physical status
- Pre-existing disease
- No discernible systemic signs
- E.g. skin tumour, fracture without shock, uncomplicated hernia, localised infection, compensated cardiac disease
Describe category III anaesthetic risk physical status
- Pre-existing disease
- Mild systemic signs
- e.g. fever, dehydration, mild anaemia, mild cachexia, moderate hypovolaemia
Describe category IV anaesthetic risk physical status
- Pre-existing disease
- Severe systemic signs
- e.g. uraemia, toxaemia, severe dehydration and hypovolaemia, severe anaemia, cardiac decompensation, emaciation, high fever
Describe category V anaesthetic risk physical status
- Moribund patient
- Not expected to survive with or without intervention
- Extreme shock and dehydration, terminal malignancy or infection, severe trauma
How can anaesthetic risk be minimised?
- Evaluation and planning
- Support with oxygen at all times
- Fluids
- Warmth
- Monitoring during anaesthesia and recovery
- Anaesthetic record sheets
- Trained anaesthetist
Outline the normal process of anaesthesia
- History and examination
- Formulate anaesthetic plan (ideally multiple)
- Place cannula
- Induce anaesthesia with injectable agent via cannula
- Intubate, check ABC
- Connect to anaesthetic machine, supply volatile anaesthetic agent in oxygen
- Alter inspired concentration in response to physical signs
- Supply analgesia
List the 3 key components for a balanced anaesthetic
- Hypnosis
- Anti-nociception
- Muscle relaxation
- Aka Triad of Anaesthesia
What is hypnosis?
- Artificially induced “sleep”
- More like lack of awareness (cannot be woken)
What is anti-nociception?
- Blocking of reaction to painful stimulus
- Are not perceiving pain but nociception can still occur
Where can nociception be blocked?
- Transduction using NSAIDs
- Transmission suing local anaesthetics
- Spinal cord with opioids and alpha-2 agonists
- CNS with opioids
Why is muscle relaxation require in anaesthesia?
- Intubation
- To gain access to surgical site
How can muscle relaxation be induced?
- Using more of the same agent producing hypnosis (but will decrease all physiological functions)
- Centrally acting muscle relaxant e.g. diazepam, midazolam
- Using specific neuromuscular junction blocking agent e.g. curare
Describe the stress response in anaesthetised animals
- Anaesthesia increases cortisol
- Initial increase in BP and subsequent decrease as administer more or anaesthetic agent
- Patient becomes cold as stop the physiological processes that keep it warm
Why is balanced anaesthesia important?
- Improves success rate and recovery
- E.g. one agent may have all 3 effects of triad but decreased brainstem function also caused, may lead patient to crash
- By using combination of agents can have better control of all 3 parts of triad
Describe the traditional method of monitoring “depth of anaesthesia”
- 4 stages
- Look at pupil position
- Resp pattern
- Pulse rate
- Blood pressure
- No longer used
Describe the modern method of monitoring “depth of anaesthesia”
- 3 states: conscious, anaesthetised, dead
- Level of CNS depression can be altered and monitored
- Signs relating to muscle relaxation and physiology
- Eye position, 5 classifications (A to E)
Describe eye position A in anaesthetic monitoring
- Conscious
- Pupils responsive to light
Describe eye position B in anaesthetic monitoring
- Some CNS depression
- Eye rotates ventrally and medially
Describe eye position C in anaesthetic monitoring
- Operative levels of anaesthesia
- Eye rotated ventromedially
- Mostly see white of eye and a little of iris
Describe eye position D in anaesthetic monitoring
- Too much CNS depression
- Only whites of eye visible
Describe eye position E in anaesthetic monitoring
- Pupil fixed centrally and gaping
- Non-responsive to light
- May appear same as A but is emergency stage
Describe the signs of too much CNS depression in anaesthesia
- Resp rate down, heart rate down, blood pressure down
- Animal loses ability to maintain normal physiology
What is an important consideration when monitoring CNS depression in horses?
- Need to assess both eyes, as may be doing different things
- Eyes tend to be doing the same thing in small animals, cows, sheep, alpacas
Define local anaesthetic
Blocking of peripheral nerves
Define tranquilisation
Relief of anxiety
Describe sedation
Central depression, drowsiness, less aware of surroundings but may still be anxious
Define narcosis
Drug induced sleep produced by narcotics (opiates e.g. morphine, methadone)
Describe dissociative anaesthesia
Induced by drugs such as ketamine, dissociate the thalamo-cortical and limbic systems
Normal reflexes are still present but are unconscious (may appear awake)
What are potential sites of application of local anaesthetics?
- Topical
- Infiltrative
- Conductive
- Epidural
- Subarachnoid
Name the different types of cannula
- Over the needle
- Through the needle
- Peel away cannula
- Seldinger/over the wire cannula
Describe over the needle cannulas
- 24-10 gauge needle, 1.9-12.3cm long
- Relatively stiff material
- Central stylet, cannula placed over the top, insert whole set into vein, slide cannula off stylet into the vein
Describe through the needle cannulas
- Large bore insertion needle
- Cannula passed through the needle
- Used for central veins
- Not commonly used, mainly ICU settings
Describe peel away cannulas
- Place through an over the needle cannula
- Insert needle with peel away sheath, remove needle leaving just the sheath
- Insert cannula through the sheath
- Peel away sheath leaving just the cannula
Describe Seldinger/over the wire cannulas
- Start with clean site, place normal IV cannula into jugular vein
- Pass wire down cannula, into vein
- Remova cannula leaving the wire in the venous space
- Add dilator to open up tissues
- Once dilated, feed cannula over the wire and into the vein
- Secure in place, remove wire, left with cannula deep into vascular system
Describe the ideal biological properties of IV cannula materials
- NOn-irritant
- Non-carcinogenic
- Non-thrombogenic
- Non-toxic
- Resists microbial adhesion
Describe the ideal physical properties of IV cannula materials
- High tensile strength
- Resists compression
- Optimum flexibility
- Low friction coefficient
- Dimensional stability
- Tolerates physical sterilisation methods
- Ease of fabrication (cost)
- Non-permeable
- Radiopaque
Describe the ideal chemical properties of IV cannula materials
- Absence of leachable additives
- Stable during storage
- Stable on chemical sterilisation
- Stable on implantation (non-biodegradable)
- Permits adhesives in fabrication
- Accepts surface coatings
- Compatibility with chemical compounds and solvents
- MRI compatible
What is the most commonly used cannula material?
Teflon
Describe the importance of cannula tip shape
- Shape of tip can reduce endothelial trauma
- Rounded preferable to square cut or bevel ended
What size cannula is best for small patients?
22G for rabbits and small cats
What size cannula is best for most patients (esp. cephalic access)?
20G
What size cannula is best for dogs over 20kg?
18G, in very large dogs may use 16-14G
What is the importance of cannula size?
- Want biggest size possible
- Flow through catheter is dependent on internal diameter and inversely proportional to the length of the catheter
- Flow through catheter inversely proportional to viscosity of the fluids
- If halve the size of the cannula, reduce flow rate 16 times
Name the different designs of cannula
- Butterfly
- Vascular access ports
- Intraosseous
- Subcutaneous
Describe Butterfly cannulas
- Difficult to secure without damaging vessel
- Useful for sampling
- 27-14G, 1.6-3.2 cm long
- Metal cannula, wings, attached to extension set
- Useful for drianing pneumo, pyo and haemothorax in small animals
Describe vascular access port cannulas
- Implanted under skin attached to bone
- Directly into vein
- Good for repeat doses e.g. fluids for cats in renal failure, chemotherapy
List the different types of intraosseous catheter
- 16-20G bone marrow needle
- Cook intraosseous needle
- 20Gx3.75cm spinal needle (cats and puppies)
- 18-25G hypodermic needle for neonates nad birds
Describe the use of intraosseous catheters
- Good for patients with inaccessible peripheral vessels
- Patietns in circulatory collapse needing fluids, blood products or drugs
- Quick access via bone marrow sinusoids and medullary venous channels
- Rapid delivery of fluids to neonates, small animals and birds
- Local anaesthetic over bone
- Can administer everything except cytotoxic drugs
What sites are commonly used for intraosseous administration of drugs or fluids?
- Intertroachanteric fossa of femur
- Tibial tuberosity
- Greater trochanter of humerus
- Wing of ileum
Describe the use of Subcutaneous cannulas
- No longer used much as subcut fluids are minimally useful
- Can be implanted under skin
What is the function of multilumen cannulae?
- Can administer drugs and take blood from different attachments on a single cannula
- Outlets along different parts along cannula so drugs administered are not mixed with blood taken for sampling
List the common veins used for cannula placement
- Cephalic
- Saphenous
- Jugular
- Auricular
Describe placement of a cannula in the cephalic vein
- Most common for dogs and cats
- Also rabbits, horses and cows
- Accessory cephalic that joins 3rd of way between carpus and elbow, avoid this and use dorsal cephalic
- Start distally so can move proximally if goes wrong
Describe the placement of a saphenous cannula
- Requires more assistance, restrain in lateral
- Medial or lateral can be used
- Use vein on caudal aspect of leg as it ascends
- Medial easier for cat and non-fat dog (straighter)
- Easy to see
- Well tolerated long term
- Swelling common, not a problem if placed aseptically
Describe the placement of a jugular cannula
- Dogs, horses
- Right is straighter in dogs
- Useful for long term therapy and regular sampling
- Well tolerated
- Multilumen cannulae can be used (Seldinger technique to place cannula)
Describe the placement of an auricular cannula
- Animals with large floppy ears e.g. rabbits, dogs, ruminants
- Runs very lateral on the ear
- EMLA cream (lidocaine and prilocaine) can help reduce discomfort
Describe the preparation of a site for the placement of a cannula
- Clip (or cut hair with scissors, or part hair)
- Surgical scrub
- Alcohol
- Ensure sufficient contact time of disinfecting agents
- Dry hair prior to cannulation so tape will stick
Describe how to secure a cannula
Secure with tape underneath and on top, then use vet wrap or soft wrap on top
Describe the monitoring of cannulas
- Check regularly, flush every 6 hours with heparinised saline
- Normal cannulae can be maintained for up to 3 days (1 day more if looks very good)
- Swab ports prior to injection
- Replace giving sets, bungs, and T ports after 3 days even where are not changing cannula
- Must be unwrapped and inspected every day
What complications can arise from placing a cannula?
- Extravasation
- Thrombosis
- Thrombophlebitis
- Infection
- Emboli
- Exsanguination
- Haematoma/abscess following IV infusion
Describe thrombophlebitis
- Inflammation of vein
- Seen as swelling in neck and jaw of horse, remove cannula, pack site and use ice, raise head, administer systemic and topic anti-inflammatories, antibiotics following culture, heparin and vasodilators may also be used
Outline treatment following infection at site of cannula
- Remove
- Sample and do culture and sensitivity
- Antibiotics may be needed
How can anaesthetic agents be administered?
- IM
- IV
- IP
- Subcut (rare)
- Induction chamber/mask
- Immersion/contact (fish))
List the groups of injectable anaesthetic agents used in veterinary species
- Propofol
- Thiobarbiturates
- Oxybarbiturates
- Injectable steroid anaesthetics (saffan, no longer in UK)
- Dissociative agents
- Imidazole anaesthetics (etomidate, not licensed)
Name the thiobarbiturate used in veterinary species
Thiopentone sodium
Name the oxybarbiturate used in veterinary species
Pentobarbitone
Describe the anaesthetic induction in horses
- Usually IV
- In foals can use inhalants or nasotracheal intubation and gas
- Ketamine or guaifenesin common used
How is anaesthesia maintained in horses?
- Incremental “top ups” IV
- TIVA
- Inhalation agents
Outline the use of ketamine as an induction agent in horses
- Combined with other drugs
- Eyes remain open and central
- Less cumulative vs thiopentone
- Can be used as “top ups”
- Often used in combination with acepromazine, alpha2 agonists, BZDs or guaifnesin
Outline the use of guaifenesin as an induction agent in horses
- GGE
- Centrally acting muscle relaxant
- Infused immediately prior to induction until horse is ataxic, and as part of TIVA
- Not licensed
- Can be used alone in foals but only as a combination in adults
What alternatives to guaifenesin can be used in horses?
- BZDs
- Midazolam
- Diazepam (zolazepam)
Describe the induction of anaesthesia in ruminants
- Many procedures performed standing
- Induction usually IV (calves can be mask induced)
- Some cows may drink chloral hydrate
- Ketamine and alpha2 agonists mostly used
Describe the induction of anaesthesia in pigs
- Can be administered deep IM, IV, or mask
- Ketamine, alfazalone and propofol used
- Ketamine combinations
- Some risk of malignant hyperthermia
Describe the intubation of pigs
- Challenging as mouth does not open wide
- Trachea changes direction by 90degrees twice
- Smaller tubes required vs equivalent weight dog
Describe the principles of anaesthesia in exotic species
- Same general principles as for other species
- Few pieces of specialised equipment required
- Easily stressed
- Need to weigh accurately
- Have an increased surface area:bodyweight ratio so will lose heat quickly
- Risk of hyper or hypothermia
Describe the induction of anaesthesia in rabbits
- Higher risk
- IV, IM, SC, mask
- Pre-oxygenate
- Fentanyl/fluanisoe recipes or alfaxalone IV are licensed, ketamine +medetomidine and opioid (butorphanol or buprenorphine) IM or IV, or BZD+opioid followed 15 mins later by alfaxalone or propofol
Explain why there is a higher risk of anaesthesia in rabbits than other species
- Hypothermia
- Prolonged recovery
- Some protocols can lead to GI disturbances
- difficult intubation
- Respiratory obstruction leading to cyanosis
Describe the induction of anaesthesia in birds
- Inhalant e.g. isoflurane in oxygen
- Apnoea common so intubate (Cole tube)
- Air sac cannulation also possible
Outline the risks in anaesthesia of birds
- Hide illness well
- High metabolism so conditions change quickly
- Easily become hypoglycaemic so do not fast for too long
- Care with changing body position (need to do this slowly)
- Cannot ventilate when in sternal recumbency
Outline the principles of anaesthesia in reptiles
- Challenging
- Blood shunting can make gas induction/maintenance impossible
- Heart rate stays stable regardless of anaesthetic plane
- Respiration abolished at surgical planes so IPPV needed
- Chelonia esp difficult to induce with gas
- Venous access difficult
- Reptiles respond slowly to changes in gas concentration
- Use toe, tail pinch and tongue retraction to assess anaesthetic level
- Avoid sudden changes in body position
What are the methods administration of induction agents in reptiles?
- IV
- Intraosseous
- Gas (complicated due to blood shunting)
What drugs are used in the anaesthesia of reptiles?
- Popfol
- Alfaxalone
- Analgesia with butorphanol +/- NSAID (also buprenorphine in some)
Describe the anaesthesia of lizards
- Inhalant, IV (ventral coccygeal veins)
- Always intubate
- IPPV provided 1-2x/minute
Describe the anaesthesia of chelonians
- Fluid therapy beneficical (oral, IV, intracoelomic, epicoelomic, intraosseous)
- Anaesthesia via inhalant, injectable +inhalant, injectable “top ups”
- Oxygen advised
- Anaesthesia usually well tolerated
- Intubation easy, use T piece circuit to deliver O2 and iso/sevo, IPPV
Describe the anaesthesia of snakes
- Fluid therapy advised (SC, intracoelomic)
- Anaesthesia via injectables or mask with inhalant (better for very sick snakes)
- Can be intubated and anaesthesia maintained using inhalants
- IPPV 4x/minute
How is blood sampled from snakes?
Cardiocentesis or tail vein
Describe the anaesthesia of fish and amphibians
- MS-222 (tricaine methane sulphonate, 1:2500 dilution in sterile water)
- Some species may require injectable techniques
- Make solution from powder
- Effects are cumulative so remove from solution once anaesthetised
- Short procedures only require patient to be kept moist, longer procedures require oxygenated water to be passed over gills +/- anaesthetic solution
What drugs are used in the anaesthesia of amphibians?
- Phenoxyethanol
- Benzocaine (in acetone)
- Ketamine IM
- Bubble isoflurane or halothane through water
- Isoflurane in KY jelly
- Clove oil (eugenol)
What are the ideal properties of an injectable induction agent?
- Water soluble
- Long shelf life and stable when exposed to light
- Small volume required
- No local toxicity
- No effect on organ function
- Minimal individual variation
- Safe therapeutic ratio
- Onset within one vein to brain circulation time
- Short duration of action
- Non-toxic metabolites
- No histamine release
Describe the formulation of propofol
- Alkyl phenol, white emulsion, 10mg/ml
- Contains soyabean glycerol, egg lecithin, no preservative and NaOH so short shelf life once open
Describe the pharmacokinetics of propofol
- Rapid onset due to rapid uptake by CNS
- Short period of unconsciousness (5-8 min)
- Lipophilic (large VD)
- Rapid smooth emergence due to redistribution away from vessel rich areas and efficient metabolism
- Metabolites are inactive
- Repeat administration produces deeper plane of anaesthesia but also extends eradication phase
- Suitable for TIVA
Describe the clinical properties of propofol
- Respiratory depression (apnoea)
- Cardiovascular depression
- Rapid and smooth recovery
- Adequate muscle relaxation
- Good anticonvulsant
- Non irritant
- descreases intracranial pressure
- May cause paddling following administration
- Dose depends on premed
Outline the side effects and contraindications of propofol
- Avoid in shock or sepsis
- Decrease dose in old dogs and cats (give to effect)
- Pawing and sneezing in cats due to histamine release
- Can be diluted with 5% dextrose (not saline)
Describe the principles of using thiobarbiturate induction agents
- Give to effect
- Dose depends on premedication
- Slow injection in sick patients
- Used in horses when are moving dangerously on operating table
Describe thiobarbiturate induction agents
- Sodium salt powder
- Highly lipid soluble
- Very alkaline solution (pH 14)
- Difficult to source
- No longer licensed in dogs, cats and horses
- Mainly used in horses
Describe the side effects of anaesthetic induction with thiobarbiturates
- Rapid loss of consciousness
- Respiratory depression
- Cardiovascular depression
- Peripheral vasodilation
Describe the pharmacodynamics of thiobarbiturates
- Highly protein bound, displaced by other drugs e.g. flunixin, phenylbutazone
- Unionised fraction penetrates cells so pH can affect response
- Plasma protein concentration important
- Crosses placenta
- Metabolised in liver
- Prolonged recovery (hours) in sighthounds
Describe oxybarbiturate induction agents
- Similar to thiopentone in structure
- Slower onset of action vs thiopentone
- Plasma protein binding less, less lipid soluble
- Slow recovery, difficult to source
- CVS and respiratory depression, irritant, poor
- No longer available
Name the injectable steroid anaesthetics
- Saffan (no longer available in UK)
- Alfaxalone/alphadonolne
- Solubilised by Cremaphor
What are the contraindications for injectable steroid anaesthetics?
- Formulations with Cremaphor not used in dogs or cats as causes massive histamine release, anaphylaxis in some cats
- Avoided in atopic/airway cases
Describe the formulation of injectable steroid anaesthetics used in cats and dogs
- No Cremaphor, instead use 2-hydroxypropyl beta-cyclodextrin HPBCD
- ## Alfaxan
Describe the pharmacokinetics and dynamics of alfaxan
- Short plasma elimination half life, cleared from body quickly
- Can be given as repeated boluses or TIVA
- Premed prefereable: induction is smooth but recovery worse than propofol
Describe the mechanism of action of injectable steroid anaesthetics
Activates GABA inhibitory receptors
Describe the clinical properties of ketamine anaesthetic induction
- Rpaid induction
- Mixed respiratory effects
- Good analgesia
- Cardiovascular effects depend on dose but generally good at maintaining cardiac output
- Muscle tone increased
- Salivation and lacrimation increased
- Do not appear anaesthetised
Describe the chemical properties of ketamine
- Weak organic base pH 3.5
- Can be administered IV, IM, SV, IP, PO, epidural
Outline the useage of ketamine as an anaestehtic induction agent
- Can be combined with BZDs, alpha2 agonists, acepromazine and opioids
- Versatile induction agent
- Invariably needs to be combined with something
Outline the effects of ketamine in horses
- CVS and respiratory depression
- No analgesia
Given an example of a ketamine alternative
- Zoletil
- 250mg tiletamine and 250mg zolazepam as powder per vial
- Indicated for dog and cat general anaesthesia
Describe etomidate
- Not commonly used in UK
- Imidazole derivative
- Induction agent for trauma, C section, CSF, shock and neurosurgery
- HR, ABP, CO remain stable
- Myoclonus, phlebitis and pain may occur on injection
- Addisonian crises where there is no adrenal function
- Available as propylene glycol or emulsion formulations
Describe the side effects of propofol
- Rigidity, twitching
- apnoea
- Profound bradycardia
- Care in hypoproteinaemia
- Heinz body anaemia in cats
- Pain on injection
- Local reaction
Describe the contraindications for use of thiopentone induction agents
- Fractious animals without secure IV access as contact with skin causes sloughing
- Hypoproteinaemic patietns
- Worsen CSF, arrhythmias
- Liver disease (metabolised by liver)
- Renal disease (increases K, worsen arrhythmias)
- Anaemia, hypovolaemia
- Hypotheyroid, hypoadrenocortic patients (hypotension difficult to control)
Describe the side effects of steroid anaesthetic induction agents
- Poor recovery with alfaxalone
- Occasional apnoea seen
- If disturbed during recovery can cause excitement
Describe the side effects of ketamine induction use
- Stormy recovery if disturbed or not adequately premedicated
- Depth assessment different (eyes open)
- Corneal drying
- Vomiting common with A2A combinations, avoid in patients with GI obstruction
- Avoid in patients with increased intracranial pressure, ocular surgery, fever, hyperthyroid
- Pupils may be asymmetric, does not lead to ventromedial rotation of eye
