Anaesthesia Flashcards
Why do you carry out a pre-anaesthetic assessment?
- to see if you think the animal will survive the anaesthetic
- to see if it is worth doing the procedure
- to establish the suitability of the patient to undergo sedation/ anaesthesia
- to detect any deviation from normal which may influence or be influenced by anaesthesia
- helps us to select appropriate anaesthetic and sedation agents
- helps us come to a prognosis
Questions to ask when taking a history for pre-anaesthetic assessment
- need consent for the procedure
- need contact details of the owner
- when was the last time the animal ate and drank
- has the animal had any previous anaesthetics - if so was there any complications
- is the animal on any medication - even herbal ones can have an effect, if so ask for the dose, frequency and last administration
- Any current or new symptoms or problems noticed by owener
Questions to ask when taking a history for pre-anaesthetic assessment - cardiorespiratory, renal and neurological
cardiovascular:
- any exercise intollerance
- any syncope - passing out
- any recent weight gain
- any lethargy
Respiratory:
- nasal or ocular discharge
- any coughing or sneezing
- any excessive panting or exercise intollerance
Neurological: - senile dogs need lower anaesthetic drug doses as can increase severity of senility and dementia
- any seizures
- any behavioural changes
Renal:
- any excessive drinking
- any excessive urination
What are the ASA grades?
- normal haelthy patient
- a patient with mild systemic disease
- a patient with severe systemic disease
- a patient with severe systemic disease that is a constant threat to life
- A moribund patient who is not expected to survive without the operation
E = emergency
What are the advantages and disadvantages of pre-anaesthetic bloods
Advantages:
- increases the information you have on he patient and allows you to better individualise care and anaesthetic protocol
- gives owner reassurance
- can pick up on disease - most commonly liver and kidney
- increases accuracy at predicting potential complications
- to provide evidence in the case of a court case
Disadvantages:
- cost to owner
- can cause animal unnecessary stress
- can find abnormal results which can cause unnecessary worry
- if already pre-meded can alter blood results - always screen before pre-med
What is the protocol for feeding before and after an anaesthetic
- Withholding food for 24-48 hours before an op increass risk of regurgitation
- best practice give a light wet meal 8 hours before surgery
- feed as soon as possible after op to reduce nausea
What should be looked for on a pre-anaesthetic physical exam
General demeanor:
- body condition
- hydration status
- looking alert and responsive
- quiet animals will need lower pre-med doses
CVS:
- CRT - prolonged is significant but animals with a problem can still have a problem
- check pulses
- check heart rate
- check mucous membrane colour
Resp:
- discharges
- increased respiratory effort
- auscultation - abnormal respiratory noise
Temperature - don’t bother if animal is stressed or difficult unless you suspect something
- pyrexia
- hypothermia
Hepatic and renal::
- urinalysis
- jaundice
- bloods to check liver and kidney enzymes
- if liver enzymes are abnormal best way to check liver health and function is a bile acid stim test
- mouth
- integument
What are the 2 types of muscle relaxant?
What are other muscle relaxants
- neuromuscular blocking agents - act peripherally, capable of paralysing all striated muscle
- centrally acting muscle relaxants - weaken postural muscles and act on the interneuronal relays in the spine
- botulinum toxin interferes with ACh release acts on the neuromuscular junction
- dantroline interferes with excitation and contraction coupling so muscles can’t contract when they recieve an impulse
Why are muscle relaxants used?
- can aid surgical access
- for assistance with intubation - cats and pigs
- as part of a balanced anaesthesia technique - e.g. to compliment other drugs like ketamine which increase muscle tone
Disadvantages to muscle relaxants
- can paralyse respiratory musculature rendering patient unable to breath
- makes it more difficult to judge anaesthetic depth - sometimes reflexes are prevented too
How is anaesthetic depth monitored
- heart rate
- arterial blood pressure
- salivation/lacrimation/defecation/urination
- sweating - horses
- anal tone
What is the order of muscle paralysis in a dog
- facial expression - more resistant in horses and cattle
- tail
- distal limbs and neck
- proximal limbs
- throat
- abdominal wall
- intercostals
- diaphragm
recovery is usually in the reverse order
How do peripherally acting neuromuscular blockers work
- compete with acetylcholine at the post-synaptic nicotinic ACh receptors so block normal neuromuscular transmission
What are the 2 types of peripherally acting neuromuscular blockers
non-depolarising nm blocking agents
e.g amino-steroids - vercuronium, rocuronium
benzylisoquinoliniums - atracurium
block post-synaptic NaCh receptors and prevent ACh activating them - must block at least 75% to have an effect
depolarising agents
e.g. succinylcholine
firstly stimulates the post synaptic NACh receptors then as membrane becomes refractory blocks them - only needs to act on 5-20% of receptors for effect
How do you assess the degree of block
- jaw tone
- eye position
- presence of reflexes
- any signs of movement
How are neuromuscular blocks reversed
- anti-cholinesterase drugs - reduce break down of ACh if a significant amount is available in the synaptic cleft, increases amount of ACh in nmjs
- reversible agents = edrophonium and neostigmine
irreversible agents not used in practise
be aware that you see an increase in both nicotinic and muscarinic receptors - don’t want muscarinic effects
What are the muscarinic receptor effects
parasympathetic effects:
- bradycardia
- bronchoconstriction
- salivation
- defeacation and urination
- miosis
require anticholinergics to prevent these effects
What are anticholinergics
examples: atropine - don’t use in horses and rabbits
glycopyrrolate
coupling with anticholinesterases:
- neostigmine and glycopyrrolate
- endrophonium and atropine - endrophonium has fewer muscarinic effects so may not need an anticholinergic
When should you reverse neuromuscular blockers
when signs of recovery are seen e.g. twitches, animal fighting to breath on it’s own
don’t reverse unless you are sure it can move and breath for itself
be aware if you reverse too early excess amounts of ACh can desensitise the nmj deepening the block
What is the difference between sedation and premedication
- sedation more commonly done in large animals
- sedation is more dangerous as it’s not monitored as well
- sedation often to calm an animal enough to carry out a procedure while pre-medication is to calm down and reduce the dose of anaesthetic drug needed
- pre-medication doesn’t have the same significant sedation effect
Why do we sedate and pre-med
- ease of handling
- to calm and animal and reduce stress
- to reduce the amount of anaesthetic agent needed
- to reduce muscle tone
- to provide analgesia
- to reduce seizures
Briefly describe the effects of ACP
- Acepromazine - a phenothiazinine
- reduces spontaneous motor activity, good for seizures
- onset: 30 mins
- duration: 4-8 hours
- metabolised by liver - be aware in liver disease
- Note: dose on body surface area not weight - larger the animal the less you need
Positives:
- Anti-arrhythmic – reduces chance of getting arrhythmias under GA but won’t have an effect on already established murmurs
- muscle relaxation - good to use in combination with ketamine
· Weak anti-histamine – good for mast cell tumour removal as might help reduce degranulation but don’t use in reactive skin tests as can interfere with results
· Antispasmodic
· Anti-emetic – reduce chances of vomiting esp with an opioid
· Works really well in Labradors
· Research shown giving ACP before a pre-med in horses increases chance of surviving op
Contraindications:
- side effects: causes vasodilation by blocking alpha 1 adrenoreceptors - beware of heat loss, provides no analgesia and provides unreliable sedation - easily aroused
- don’t use an adrenaline reversal - drops cardiac output dramatically
· Don’t give with a respiratory depressant drug as although fine on its own it will potentiate the effects of the other drug
· Will cause muscle relaxation – be aware in brachiocephalic dogs as it can relax the muscles around the pharynx which can occlude airways – can be used but in low doses, perfectly safe
· In boxes ACP can cause syncope due to excessive vasodilation causing bradycardia and respiratory depression - be aware you need a much lower dose
· Will reduce PCV as much as 50% because a lot of RBCs congregate in the spleen and due to the vasodilation creating more room for the same number of RBCs, not pathological doesn’t cause a problem
What should an ideal sedation/pre-medication drug be?
· Be easy to administer via several routes
· Have no side effects
· Have a rapid onset and reasonable duration of action
· Be of a sensible volume to inject
· Be able to be used in all species safely
. Be readily reversible
Briefly describe the effects of Alpha 2 agonists
- different ones licensed for different species
- have a variable onset and duration
Advantages:
- provide reliable sedation
- provide good analgesia
- have a synergistic effect on other drugs so need less anaesthetic agent
- provides muscle relaxation so can be used with ketamine to counteract it’s muscle tone effects
- Are reversible – atipamezole can reverse all alpha 2 effects, however side effects include tachycardia, muscle tremors, hypertension, over-alertness, panting and vomiting – don’t give IV give (more likely to get side effects) IM unless an emergency.
Disadvantages:
- cardiovascular effects - causes vasoconstriction and bradycardia
- don’t give in young animals can’t compensate CV system well
- large animals less stable with alpha 2s
- muscle relaxation - care in brachyocephalics
· Diuresis – block secretion and responsiveness to ADH, issue in animals unable to urinate, sometimes empty bladder before revival as full bladder can cause an animal to come round too soon which can be more detrimental (horses, blocked cats), will mess up urine specific gravity
· Increases uterine tone so avoid in pregnant animals
· Reduced GI secretions, blood flow and motility so care in horses – more likely to have ileum
· Hyperglycaemia – reduces secretion and response to insulin – take note on biochem and bloods
· Increased risk of gastro-oesophageal reflux
· Respiratory effects: take deeper breaths, won’t breath then will take a few deep breaths – normal for an animal on alpha 2s
- increased tidal volume
- Reduced resp rate
- Overall minute ventilation doesn’t change
- Can cause reduced cell counts and total proteins – care when analysing bloods
- Can cause mydriasis and reduced aqueous humour production
Briefly describe the effects of benzodiazepines
- none licensed for veterinary use
- most common drugs - diazepam and midazolam
- commonly used for induction rather than sedation
- Tends to be used as a co-induction agent with ketamine and propofol to reduce dose and get good muscle relaxation
Advantages:
- minimal CVS and resp depression
- anticonvulsant
- works well in very young or very old animals
- good muscle relaxation
disadvantages:
- may cause excitement rather than sedation
- do not give with liver disease
- tend to heighten any trait - aggression, excitement
- never give to cats go crazy - sympathetically dominated
-
What alpha 2 agonists can be used in small animals
medetomidine - domitor
dexmedetomidine
What alpha 2 agonists can be used in horses
xylazine
romifidine
detomidine
What alpha 2 agonists can be used in large animals
xylazine
detomidine
Briefly describe the effects of opioids
- butorphenol, methodone, buprenorphine
- butorphanol best for sedation but no analgesia - good for quick non-invasive procedures e.g. x-rays
- effects e.g. onset and duration are species, dose and drug dependant
Advantages:
- Causes minimal CVS depression – decrease in heart rate but not significant except pethidine which increases heart rate
Disadvantages:
- horses tend to get excited
- not reliable in cats
- Can cause respiratory depression in some species – don’t give in combination with another respiratory depressant
Briefly describe the effects of ketamine
· More often used as an induction agent – main in horses, not used on own in small animals
· Used in cats for triple combination – metatomidine, opioid and ketamine
Advantages
· Can be used to enhance sedation of other drugs
Gives good analgesia – good in low doses ·
Disadvantages:
- Increases muscle tone – use with a drug that causes muscle relaxation – benzodiazepine or alpha 2
Briefly describe the effects of propofol and alfaxalone
- used a lot in small animals
- quick onset
- short duration (15 mins)
- can be titrated to effect
- top ups used to enhance sedation of other drugs
Why is the patient monitored under anaesthetic?
safety of patient safety of personnel to maintain the correct depth of anaesthesia to maintain organ function legal implications
What is monitored during anaesthetic?
cardiovascular system
respiratory system
the CNS
and temperature
How is the cardiovascular system monitored during GA
monitoring tissue oxygen delivery through looking at peripheral perfusion, oxygenation, Arterial blood pressure and cardiac output
Basic indicators include: auscultation for heart rate, palpation of pulse (rate, rhythm, strength, defects), mucous membrane colour (dehydration, perfusion, blood oxygenation) and capillary refill time
Advanced indicators - ECG, urinary output, central venous pressure, cardiac output
How do you calculate cardiac output
heart rate x Stroke volume
What are the normal heart rates under GA
Dogs - over 60 bpm - smaller dogs may be higher
Cats - over 80 bpm
Horses - over 25 bpm
Cows - over 50 bpm
younger animals will be higher
What is the arterial blood pressure and how is it calculated
it is the pressure exerted by the circulating blood on the walls of the blood vessels
calculated by cardiac output x systemic vascular resistance
What should systolic and diastolic output be?
systolic - 120mmhg
diastolic - 60-80 mmhg
what is hypotension
abnormally low blood pressure, can lead to damage and reduced perfusion to vital organs
What can cause hypotension
- bradycardia
- drugs
- vasodilation
- inadequate stroke volume
- too deep anaesthesia
- incorrect body position
- acid-base disturbances
- hypovolaemia
- intermittent positive pressure ventilation
What is hypovolemia
a decreased volume of blood circulating the body
How is the respiratory system monitored during GA
- assess breathing by looking for bag movement, chest movement and by auscultation
- asses if the patient is breathing effectively - assess rate, rhythm, tidal volume, look at mucous membrane colour (purple/cyanotic means poor oxygen delivery to the peripheral tissues while brick red indicates hypercapnia or endotoxaemia)
How is total lung ventilation calculated
dead space ventilation (doesn’t participate in gas exchange) + alveolar ventilation
what is the best indicator of alveolar ventilation
partial pressure of carbon dioxide should be 35-45mmHg if it's high indicates hypoventilation if its low indicates hyperventilation carbon dioxide levels should not exceed 60 mmHg - this is hypercapnia
What can cause ventilatory failure
- drugs
- neuromuscular disease
- pain
- trauma
- a shunt
- hyperthermia
- hypoventilation
What monitoring aids can be used to assess the respiratory system
- pulse oximetry - measures oxygen saturation of haemoglobin
- capnograph - measures % carbon dioxide over inspiration and expiration
- respiratory or blood gas analysis
- spirometry - measures the volume of air breathed out in 1 second versus the total volume of air you breath out
During GA why is the CNS depressed
- to prevent movement
- to avoid consciousness
- to relax muscles
- so there is no awareness
- so there is no pain
- to allow stable haemodynamic parameters
What do the eyes say about the depth of anaesthesia
- when light they are central
- when adequate anaesthesia is reached they rotate down
What part of the brain controls temperature
the hypothalamus
what reflexes can you assess to check anaesthesia depth
- palpebral
- corneal in exotics in emergencies
- jaw tone
- muscle twitches
- withdraw limb reflex
What percentage water makes up an adult and neonate animal
- 60-70% in an adult
- 80% in a neonate
How is total body water calculated
- 60% of bodyweight e.g. 500kg horse - 60% of 500 = 300 so the total body water in a 500kg horse is 300 litres
of the body water in an animal how much is intracellular and how much is extracellular
of the 60% body water 40% is intracellular and 20% is extracellular
of the extracellular fluid in an animal how much is interstitial and how much is intravascular
of the 20% of extracellular fluid, 15% is interstitial and 5% is intravascular plasma
What is the total blood volume in a horse, dog, cat, rabbit, sheep and cow
in dogs and horses total blood volume is 8-9% of body mass
in the cat, rabbit, sheep and cow the total blood volume is 6-7% of body mass
What are the rules of fluid therapy
replace intravascular volume first
then you can worry about other deficits
then worry about ongoing losses
followed by ongoing maintenance requirements
What are the normal water gains in the body
- voluntary oral intake - majority
- metabolic water production - only small amounts
what are the normal water losses in the body
- sensible losses - these are regulatable = urine
- insensible losses - these are non- regulatable= faeces from the GI tract, sweat, moisture from the respiratory tract and obligatory minimal mandatory urine excretion
How much water is lost through sensible and insensible losses per day ?
25 ml/kg/ day each so a total of 50ml/kg/day
What are the daily water requirements for a neonate and an adult
for adults:
50ml/kg/day (often quoted as a range between 40-60) so an hourly requirement of 2-2.5ml/kg
for neonates:
100ml/kg/day or 5ml/kg/hr
what is anaesthetic/surgical maintenace of fluids
1-2 x maintenance
Name the different types of water loss
- pure water loss - dehydration
- water and electrolyte loss - diuresis, diarrhoea, vomiting
- water, electrolyte and protein loss - transudates, exudates and effusions , severe enteritis, protein losing enteropathy or nephropathy
- blood - haemorrhage
What is the definition of dehydration
- deficit of water so all compartments are affected including the intravascular one
for this reason animals will always be hypovolaemic if its dehydrated
What is the definition of hypovolaemia
- deficit of circulating blood volume so affects liquid in the intravascular space
- an animal can be hypovolaemic without being dehydrated e.g. in the case of acute haemorrhage only the intravascular compartment is affected so there isn’t much relative water loss
What are crystalloids
can be isotonic - similar sodium concentration to plasma / ECF
can be physiologically hypotonic wich means it contains low sodium but includes glucose which makes the solution isotonic still so RBCs don’t swell and burst
can be hypertonic where there is a higher concentration of sodium than in the plasma / ECF
watery crystalloides disperse rapidly throughout all fluid compartments while high sodium crystalloides distribute throughout the extravascular space - about a quarter remains intravascularly
What is an isotonic solution
has the same osmotic pressure as the blood so there is no net movement of water
What is a hypertonic solution
has higher osmotic pressure than the blood meaning it encourages fluid to be drawn out of cells into the blood
What is a hypotonic solution
has a lower osmotic pressure than the blood so causes water to be pushed out of the blood into the cells
Name some crystalloides that can be used in practice
isotonic solutions include 0.9% saline and hartmans
physiologically hypotonic solutions include 5% dextrose and 4% dextrose with 1/5th 0.18 saline
hypertonic solutions include - anything above 0.9% saline - 7.2% commonly used
What are colloides
contain large chunky molecules e.g. proteins, sugars and starches
remain well in the intravascular compartment encouraging water from the cells to be drawn into the blood
also known as plasma volume expanders
not easily filtered out by capillaries
Name some colloides used in practice
- plasma and albumin are natural colloides
- synthetic colloides include gelatins, dextrans and hydroxyethyl starches
Other than colloides and crystalloides what other fluids are available
blood and blood products such as whole blood, packed red cells, platelet rich plasma or fresh frozen plasma
oxygen carrying solutions such as haemoglobin and oxyglobin
special fluids such as sodium bicarbonate, high potassium fluids, mineral and vitamin rich fluids such as calcium and magnesium, paraenteral nutrition solutions including glucose, lipids and amino acids
What fluid should be used for pure water loss?
low sodium crystalloid - physiologically hypotonic
What fluid should be used for blood loss?
colloids, blood or haemoglobin or high sodium crystalloids
What fluid should be used for proteinaceous loss?
colloides
What fluid should be used for general ECF loss?
high sodium crystalloides like hartmans and normal saline
colloides
What is the aim of a maintenance fluid
to provide and distribute water between all compartments
What is the aim of a replacement fluid
aim to restore intravascular volume and replace salt and water
What are the clinical signs of intravascular hypovolaemia
- tachycardia
- weak peripheral pulses (decrease in arterial blood pressure)
- cool extremities
- change in mucous membrane colour and CRT
- tachypnoea
- reduced urine output (increased urine specific gravity )
- altered mentation (depression, inactivity, recumbency)
What are the clinical signs of dehydration - this can include hypovolaemia
- tachycardia
- weak peripheral pulses (decrease in arterial blood pressure)
- cool extremities
- change in mucous membrane colour and CRT also become tacky
- tachypnoea
- sunken eyes
- reduced skin tugor
- reduced urine output (increased urine specific gravity )
- altered mentation (depression, inactivity, recumbency)
Describe the key features of each percentage dehydration
less than 5% - no changes seen
7% - tacky or dry mucous membranes, may see decrease in skin turgor
10% - dry mucous membranes, marked decrease in skin turgor
12% - dry mucous membranes, marked decrease in skin turgor, altered mentation, becoming moribund
15% - all of the above and dying
Why are PCV and TP sometimes unreliable for assessing fluid deficit
because changes do not occur immediately can take a few hours to alter after an acute change e.g. haemorrhage
What are the different routes of fluid administration
enteral - best way if the gut is working sufficiently via voluntary intake or for large volumes orogastric tube
paraenteral - IV, intra-osseus, subcut (can have poor absorption) or intraperitoneal - care!
what should you monitor with a dehydrated / hypovolaemic patient
- heart rate
- arterial blood pressure
- CRT and mm colour, tackiness
- tmperature
- urine output and specific gravity
- changes in PCV/ TP
- Lactate levels
- central venous pressure
What is the bolus technique for fluid therapy
10-20ml/kg of crystalloid IV within 15-30 mins then reassess
or 2.5-5ml/kg of colloides IV within 15-30 mins
How do you work out drips per second
drops/ml (usually on the giving set e.g. 20 or 60) x rate x bodyweight / 3600
gives rate in ml/kg/hr x by 24 to get per day
Which drugs provide analgesia
alpha 2 agonists opioids ketamine nitrous oxide all local anaesthetics
What drugs provide reliable sedation or unconsciousness
ACP - unreliable sedation alpha 2 agonsits - sedation opioids - sedation barbituates ketamine propofol inhalation agents
what drugs provide muscle relaxation
ACP Benzodiazepines alpha 2 agonists barbituates propofol inhalation agents
What inhalation agents are available n practice
- halofluorane - no longer used except in some equine practices - no longer sold
- isoflurane - licensed for use in most species
- sevoflurane - licensed in dogs only
- nitrous oxide uncommon - causes foetal abnormalities, health hazard
What are the advantages and disadvantages of inhalation agents for anaesthesia
pros:
- allows oxygen delivery with agent
- allows ventilation
- easy to alter level of anaesthesia
- running costs low once set up
- easy to reverse - just breathed out and effects wear off
cons:
- high initial cost to set up
- not portable
- ET tube needed
What are the advantages and disadvantages of TIVA (total IV anaesthesia)
pros:
- easy to do in field
- minimal equipment
- cheap to set up
cons:
- harder to control level of anaesthesia
- can’t be reversed once administered
- oxygen delivery must be separate
- drugs can be expensive
What is the minimal alveolar concentration
concentration at which 50% of patients fail to respond to a noxious stimulus
it is affected by species, changes in body temperature, drugs that act on the CNS
it is lower in pregnant, young or old animals as well as if under hypotension or hypoxia
what is the correct minimal alveolar concentration during stable anaesthesia
1.2-1.5x MAC
side effects get considerably worse after 2.5 x MAC
Describe the general features of isofluorane
- gives greatest degree of respiratory depression of all inhalation agents
- causes potent vasodilation - heat loss
- pungent - animals averse to the smell - not nice for inhalation induction
- cheap
- licensed in all species
- no analgesia
Describe the general features of sevofluorane
- low solubility - quick induction and recovery
- non-pungent - good for inhalation induction
- less respiratory depression
- no analgesia
- only licensed in dogs
- expensive
- causes vasodilation
Describe the general features of Haloflurane
- no longer sold - some equine vets prefer it for use in GA
- no analgesia
- most soluble so longer recovery
- causes myocardial depression and sensitisation to catecholamines
Describe the general features of Desflurane
- requires a special vaporiser
- no analgesia
- extremely pungent - can’t be used for inhalation induction
- quickest recovery times - not always a good thing
Describe the general features of nitrous oxide
- has the lowest solubility
- expensive
- teratogenic- need an active scavenging system, health and safety risk
- needs to be restricted to sub anaesthetic doses to provide anaesthesia as MAC is 100% and would kill an animal
What are the 2 methods of TIVA
Bolus
continuous infusion
what are the advantages and disadvantages to continuous infusuion anaesthetic
pros:
- have an element of control over the level of anaesthesia
- doesn’t need repeated doses and catheter handling
- should result in quicker recovery
cons:
- takes more time to set up
- costs more in equipment and often drugs as you make up more than you need
- can easily result in overdose if surgeon isn’t being observant
What agents can be used for TIVA
All have cumulative effects so can only be used for short procedures
- Propofol
- ketamine
- Alfaxalone
- Thiopentone
often used to facilitate IV GA as muscle relaxants:
Alpha 2 agonists
GGE
Bensodiaepines
Describe the general features of Alfaxalone
- neurosteroid
- non irritant extravascularly
- causes CV and resp depression but not as pornounced as propofol
- metabolised rapidly
- short duration of action
- no analgesia
Describe the general features of propofol
- non irritant extravascularly
- causes potent CV and resp depression
- no analgesia
- never use in cats - results in heinz body formation
Describe the general features of Ketamine
- NMDA antagonist
- in healthy animals doesn’t cause CV probs
- requires heavy sedation to prevent excitement
- causes increased tone of muscles - must be used with a relaxant
- excellent analgesia
Describe the general features of thiopentone
- barbituate
- no longer available in the UK
- extravascular irritant
- causes potent resp and CV depression
- mainly an induction agent - used with GGE in horses
What is shock
a failure of the microcirculation to deliver adequate oxygen and metabolic substrates to the cells and to remove their waste
results in altered cell metabolism, cell death and ultimately organ dysfunction or failure
it is an imbalance between oxygen supply and demand and itself triggers an inflammatory response which can directly or indirectly lead to SIRS and multiple organ dysfunction syndrome
What are the types of shock
- hypovolaemic shock
- distributive shock
- obstructive shock
- cardiogenic shock
- metabolic shock
Describe the general features of hypovolaemic shock
- results from haemorrhage, trauma or severe fluid loss
Describe the general features of distributive shock
- can be high resistance caused by neurogenic problems or excessive vasopressor therapy
- can be low resistance caused by anaphylaxis or SIRS
- this is when there is poor distribution of the available blood volume leading to tissue hypoxia
- distributive shock is also septic shock from the immune response to bacterial fragments in the blood resulting in the release of inflammatory mediators causing vasodilation, vascular leakage and coagulopathies
Describe the general features of Obstructive shock
- can be a result of a pneumothorax , cardiac tamponade (compression of heart due to accumulation of fluid in pericardial sac) or embolus (micro or thrombo)
Describe the general features of cardiogenic shock
- due to heart failure, congenital heart disease or dysrhythmias
Describe the general features of metabolic shock
- caused by anaemia, hypoglycaemia, toxicity, acute adrenal insufficiency
What is septicaemia, sepsis syndrome and septic shock
speticaemia is evidence of bacteria or bacterial endotoxins in the blood
sepsis syndrome is when evidence of the bacterial infection has resulted in SIRS
Septic shock is sepsis syndrome which is unresponsive to treatment
What are the 3 phases of distributive shock
- hyperdynamic phase - brick red MM, short CRT, bounding pulse, raised cardiac output and tachycardia
- hypodynamic phase - pale or congested MM, increased CRT, weak pulses, decreased CO, cold extremities
- refractory phase - same as hypodynamic but unresponsive to treatment
What are the cardiovascular consequences of distributive (septic) shock
hypovolaemia vascular leakage coagulopathies altered rheology (blood viscosity) vasoplegia - low systemic vascular resistance
How do you treat shock
- provide oxygen/ oxygen rich environment
- give vasoactive agents
- fluid therapy - isotonic crystalloid 40-100ml/kg bolus
- colloides 10-20 ml/kg bolus over 15 mins
- hypertonic saline - 4ml/kg bolus over 15 mins
What are the advantages and disadvantages of injectable induction agents
pros:
- minimal equipment needed
- rapid, smooth induction
- no environmental pollution
cons:
- inreased stress due to restraint to gain IV access
- IV access very difficult in some animals
- once given retrieval is impossible
- need to know patient weight
- CV and resp depression
What are the advantages and disadvantages of mask induction
pros:
- no IV access required
- easy to bring round from anaesthetic
cons:
- prolonged
- more equipment needed
- risk or air pollution
- may increase stress due to smell, airway irritation and restraint
What are the advantages and disadvantages of chamber induction
pros:
- no IV access required
- less stressful than restraining to gain IV access
- easy to bring round from anaesthetic
cons:
- prolonged
- more equipment needed
- risk or air pollution
- may increase stress due to smell, airway irritation and entrapment
what are the main sites of action for induction agents
- GABA receptors
- glyciene receptors
- potassium channels
- NaCh receptors
- NMDA receptors
What are the pros and cons of giving induction agents slowly and to effect
pros:
- allows you to use reduced amounts
- less CV and resp depression
cons:
- may become excited
- slow
- requires an easily handleable patient
What are the pros and cons of giving induction agents as a rapid bolus
pros:
- rapid
- less likely to become excited
- safer handling
cons:
- more pronounced CV and resp depression
- higher doses administered
What injectable induction agents are available
- barbituates - pentobarbital and thiopental
- neurosteroids - alfaxalone
- phenols - propofol
- caboxylated imidazoles - etomidate
- phencyclidine derivatives - ketamine and tiletamine
Describe the general features of thiopental
- no veterinary licensed product in the UK
- ultra short acting 5-15 minutes
- comes as a powder must be made into a suspension with water
- high pH stings on injection IV only
- lipid soluble - fast acting
- no analgesia
- causes bronchoconstriction
- causes vasodilation
- is a negative inotrope so causes myocardial depression
- is an arrhythmogenic - sensitisation to catecholamines
Describe the general features of alfaxalone
- few allergic reactions
- licensed in cats and dogs
- fairly neutral pH so no pain on injection
- acts on GABA receptors
- causes tachycardia
- causes vasodilation
- no analgesia
- can give IV or IM
- Good muscle relaxation - can cause in-coordination on recovery
- rapidly metabolised
Describe the general features of propofol
- cerebroprotectant
- highly lipid soluble
- neutral pH - no pain on injection
- licensed in cats and dogs
- causes hypotension
- is a mild negative inotrope
- causes vasodilation
- decreases baroreceptor sensitivity so no reflex tachycardia
- respiratory depression and apnoea
- rapidly metabolised
- don’t use in cats - heinz body formation
- no analgesia
Describe the general features of ketamine
- licensed in cats, dogs and horses - main induction agent in the horse
- low pH - irritant
- NMDA receptor antagonist
- slow onset - 1-2 mins
- dissociates mind from body
- good analgesia
- minimm resp depression
- increases intracranial pressure
- increases muscle tone
- animals can get noise hypersensitivity
- painful IM - IV preferred
- cranial nerve reflexes can remain active
- not good with renal disease as excreted unchanged
Describe the general features of tiletamine
- related to ketamine but with a longer duration
- used in combination with zolazepam
- licensed for IM in small animals but low pH so stings
- acts on NMDA receptors
- used for darting in zoo animals - also etorphine is used but very dangerous
Describe the general features of etomidate
- not lisenced for veterinary used but used in referrals for high risk patients
- acts on GABA receptors
- no analgesia
- poor muscle relaxation
- minimal cardiovascular depression
- cerebroprotective
- can suppress adrenocortical function
What should the tidal volume of a breathing circuit be
10mg/kg
What would your approach be to intra-operative hypotension in the horse
- check anaesthesia depth
- give fluid therapy
- is hypovolaemia the cause
- are CV deppressive drugs the cause
- could administer cardioactive drugs e.g. positive inotropes, vasoconstrictors`
- try to tilt the horse to aid circulation and resp
What are the consequences of intra-operative hypothermia in a horse?
- cardiovascular changes - bradycardia, hypotension, hypocoagulable blood, increased PCV
- Resp changes - hypoventilation, carbon dioxide retention, reduced tissue perfusion
- CNS depression
- slow recovery
- delayed wound healing
What can an oesophageal stethoscope measure
heart and resp rate
What is the best way to measure cardiac function
arterial blood pressure - invasive or non-invasive technique
what are the advantages and disadvantages of the doppler method of measuring arterial blood pressure
advantages
- simple technique
- non-invasive
- inexpensive equipment
- good for small animals
disadvantages
- hair and fat can interfere
- gives intermittent measurements
- only gives reading for systolic arterial pressure
what are the advantages and disadvantages of the oscillometric technique of measuring arterial blood pressure
advantages
- simple, non-invasive
- measures both systolic and diastolic atrial pressure
- continuous - reading every 1-10 mins
- also measures mean atrial pressure and heart rate
disadvantages
- more expensive
- doesn’t work well if arrhythmias are present
What is direct arterial blood pressure monitoring
h
