Anaesthesia (SA11) Flashcards
What are the 3 main routes anaesthesia drugs are administered?
- Intravenous
- Intramuscular
- Inhalation
Why is it important to monitor anaesthetised patients closely and understand how to identify changes that may occur?
- Many homeostatic systems are suppressed
- Prevent long-term effects of anaesthesia
What is the autonomic nervous system?
- Regulates involuntary processes
- Comprised of sympathetic and parasympathetic
- Many processes suppressed under GA
- Expect return to normal function quickly in healthy patients
What does the sympathetic nervous system do?
- Dilates pupils
- Inhibits salivation
- Accelerates heart beat
- Dilates bronchi
- Inhibits digestion
- Stimulates glucose release in liver
- Stimulates epinephrine + norepinephrine release in kidneys
- Inhibit peristalsis and secretions in intestines
- Relaxes bladder
What does the parasympathetic nervous system do?
- Constricts pupils
- Stimulates salivation
- Slows heart beat
- Constricts bronchi
- Stimulates digestion
- Stimulates bile release in liver
- Stimulates peristalsis and secretion in intestines
- Contracts bladder
What is high vagal tone?
Parasympathetic nervous system more dominant, seen in brachycephalics
What order will inspired air pass through the respiratory system?
- Nasal cavity
- Pharynx
- Larynx
- Trachea
- Bronchi
- Bronchioles
- Alveoli
What controls the respiratory cycle?
- Chemoreceptors detect changes in O2 and CO2
- High CO2 lowers blood pH = more acidic
- Disrupting acid base balance
- Medulla changes depth and rate to expel CO2
- pH will return to normal
Where are chemoreceptors found?
- Walls of aorta and carotid artery
- In the medulla
What prevents over inflation of the lungs?
- Hering-Bruer reflex
- Stretch receptors in walls of bronchi + bronchioles
- Monitor stretching during inspiration
- Send impulses via vagus nerve to brain
- inspiratory centre prevents further inspiration
Gaseous exchange
- Only occurs in alveoli
- One cell thick walls + lots of capillaries
- Gases move by diffusion, high to low concentration
- O2, air into blood. CO2, blood into air
- Water vapour lost in process
- Inhalation anaesthesia reliant on effectiveness of this to be administered and excreted from the body
How can tissue hypoxia be prevented?
- Organ damage can occur even if tissues are deprived of oxygen for short periods
- Critical oxygen tension level must be provided for metabolic consumption
- 2-7mls/kg/minute for cats and dogs
- Not recommended
Acid base balance and respiration
- CO2 high, carbonic acid released
- Hydrogen ions released, decrease in pH
- Respiratory acidocis
- CO2 low, hydrogen ions lost, increases pH
- Respiratory alkalosis
- Even slight pH variations cause severe effects
How may respiratory disorder affect anaesthesia?
- Impaired diffusion of inhaled gases
- Risk of hypoxia from impaired gaseous exchange
Hypoxaemia
- Low level of oxygen in blood
Hypoxia
- Low oxygen in certain area
- E.G. tissue hypoxia
What is critical oxygen tension level?
- Oxygen level required for metabolic consumption to prevent tissue hypoxia
- 2-7ml/kg/minute for dogs and cats
What is respiratory acidosis?
- High levels of CO2
- High levels of carbonic acid
What is respiratory alkalosis?
- High levels of O2
- Low levels of CO2
What is hypercapnia?
Raised ETCO2
What is hypocapnia?
Low ETCO2
What is the normal CO2 level in patients?
35 - 45 mmHg
What causes hypercapnia?
Hypoventilation
What causes hypocapnia?
Hyperventilation
What is cardiac output?
Amount of blood heart pumps each minute
What is the cardiac output formula?
Heart rate x stroke volume
What is the blood pressure formula?
Cardiac output x systemic vascular resistance
What is systemic vascular resistance?
Degree of vasocontriction/dilation
How does the body control blood pressure?
- Baroreceptors (stretch receptors)
- In aortic arch, left + right atria, left ventricle, pulmonary circulation
- Medulla detects changes and corrects BP
- Alteration in HR or vasoconstriction/dilation of blood vessels
Why is blood pressure monitoring during GA important?
- Low BP reduces tissue and organ perfusion
- Can cause long term damage
- Anaesthesia often causes hypotension
What does the renal system control?
- Blood pressure
- Blood volume
- Electrolyte balance
Why is good renal function important in GA?
- Blood supply to kidneys may reduce during GA
- Kidneys need to excrete some anaesthetic drugs
- NSAIDs can reduce renal blood flow
- Good to check renal function before GA
- Monitor and maintain BP helps ensure adequate renal perfusion
Albumin and anaesthesia
- Hepatic system produced albumin
- Many anaesthetics and analgesics transported in blood bound to albumin
- If low albumin, higher levels of drugs ‘free’ in blood
- If low albumin, lower drug doses required
Liver and anaesthesia
- Anaesthetics can affect blood supply to liver and impair functions
- Liver metabolises many anaesthetic agents
- Liver disease can cause prolonged recovery times
- Fat accumulating in liver affects funtion
- Diabetes mellitus, chushings, hypothyroidism
The brain and anaesthesia
- Blood brain barrier selectively permeable
- Water, CO2, O2, lipophilic molecules pass easily
- Anaesthetic agents must be lipophilic to act on brain
Shock and anaesthesia
- Cardiac output too low, low blood flow to tissues
- Leads to cell damage, organ damage and death
- Haemorrhage and dehydration common causes
- Initially pale MMs, increased HR, low BP, reduced urine output
- Must be identified quickly and treated aggressively
- Will impact excretion of drugs, can further complicate condition
Sight hounds and anaesthetics
- Very sensitive to some drugs
- Barbiturate induction agents (Thiopentone)
- Redistributed around body, including in fat
- Recovery delayed due to low body fat
- Reduced specific liver enzymes needed to metabolise certain drugs
- Further slowing recovery
Bracycephalics and anaesthesia
- Bracys of any species at risk of airway obstruction
- Extra airway management care, close monitoring after pre-med
- Many are hypoxic at point of anaesthesia
- May require oxygen pre and post surgery
Collies and anaesthetics
- Some have gene mutation = sensitive to some drugs
- Gene affects blood brain barrier, higher levels can reach the brain
- Extra care taken with doses of certain sedatives and opioids
- Methadone and buprenorphine
What is the herring breuer reflex do?
- Avoid over inflation of lungs
What is anaesthesia
A controlled temporary loss of sensation or awareness for medical purposes
What are the 2 types of anaesthesia?
- General
- Local
What is general anaesthetic?
Reversible immobile state that induces amnesia
What is local anaesthetic?
- Application of anaesthetic to specific area of the body
- Usually with sedation or GA in animals
- Including local in protocols reduces dose of GA drugs, increasing safety
- Significant for analgesia
Why is anaesthesia needed?
- Welfare reasons
- Legal obligations
- Facilitate surgery - immobilise and muscle relaxation
- To control disease - seizures
- Euthanasia purposes
What is the order of signs in local anaesthetic overdose?
GI signs
Nervous system signs
Cardiac signs
Local anaesthetic safety
- Toxicity occurs with large doses
- Cats more susceptible to toxicity than dogs
- May cause seizures, CNS depression, hypotension, bradycardia and cardiac arrest
- Patients can injure selves due to lack of sensation on recovery
- Care taken in dental blocks to avoid tongue, patients may chew own tongue otherwise
What are the 3 local anaesthetic routes?
- Topical
- Infiltration
- Regional
What is infiltration local anaesthesia?
- Low concentrations anaesthetic agents injected intradermally, subcutaneously or intramuscularly
- Loss of sensation superficial and localised
- Mostly used around surgical sites prior to surgery
- Must not be administered IV, aspirate before injecting to check for blood
Why is adrenaline added to local anaesthetic agent?
- Adrenaline vasoconstricts
- Not carried away in blood
- Increases duration of local anaesthetic action
What are other specific local anaesthetic blocks used in practice, other than intradermally?
- Intra-testicular
- Splash blocks
What is regional anaesthesia?
- Nerve supply to region of body is blocked
- Reduces depth of GA needed, increases safety
- Nerve must be easily located and accessible
- Can use nerve stimulator or ultrasound to locate
Brachial plexus block
- Analgesia distal to elbow
- Good for distal forelimb procedures
- Toe amp, carpal surgery; arthrodesis, fracture repair
Femoral and sciatic nerve block
- Analgesia distal to mid shaft of femur
- Good for stifle and hock surgeries
- Cruciate ligament surgery, meniscus surgery, luxating patella, hock arthrodesis, fracture repairs
Lumbosacral epidural
- Analgesia caudal to thoracolumbar junction
- Good for hind quarter othopaedic surgery or perineal area
- Caesarean section
- Can use morphine only for abdominal surgery analgesia
- Epidural catheters can be placed to continue analgesia into recovery
Intercostal nerve block
- Analgesia post throacic surgery or trauma
- Blocks easily placed prior to closing thoracic cavity as easy nerve visualisation
Maxillary and mandibular blocks
- Different areas of jaw blocked depending on placement
- Useful for dental procedures or jaw surgery
Intra-articular blocks
- Local injected into joint
- Often used post joint surgery or analgesia for arthoscopy
What is a wound catheter?
- Placed at end of surgery, allows local infused into area at regular intervals following surgery
- Care of patient interference
What is meant by ‘balanced anaesthesia’
- Synergistic drugs in combination to reduce doses and improve safety
- Triad of anaesthesia; unconsciousness, analgesia, muscle relaxation
Who can administer anaesthesia?
- Induction of specific quantity can be administered by vet, RVN, SVN under supervision
- Incremental induction only by vet
- Maintaining anaesthesia responsibility of vet
- Monitoring + moving dials suitably trained person
Tachycardia
Increased heart rate
Bradycardia
Decreased heart rate
Bradypnoea
Decreased respiratory rate
Tachypnoea
Increased respiratory rate
Apnoea
Lack of breathing
Hypotension
Decreased blood pressure
Hypertension
Increased blood pressure
Hypovolaemia
Decreased blood volume
Dog heart rate
70 - 140 bpm
Cat heart rate
100 - 200 bpm
Dog respiratory rate
10 - 30 bpm
Cat respiratory rate
20 - 30 bpm
Tidal volume
Volume of gas exhaled in one breath
- BW x 10 / 15mls
- <10kg = 15mls / kg
- >10kg = 10mls / kg
Minute volume
Volume of gas exhaled in one minute
- Tidal volume x respiratory rate
Residual volume
Volume of air left in lung after forced respiration
- Prevents collapse of respiratory collapse
Atelectasis
Collapsed lung or lobe
- Alveoli deflate and become filled with fluid
Risks of atelectasis
- Associated with trauma
- Penetrating injury or RTA
- Known risk of anaesthesia
- Specific anaesthetic risks
- Pattern of respiration change
- Altered gaseous exchange
- Longer surgery
- Use of mechanical ventilators
- Extended period of lying on one side
Dead space
- Air that does not reach alveoli so is not involved in gaseous exchange
- More risk in smaller patients with lower tidal volume
- More dead space, lower alveolar ventilation, increased CO2 levels
Anatomical and mechanical dead space
- Anatomical; air in upper airway, trachea and bronchi to avoid collapse
- Can’t control or alter this
- Mechanical; gas in breathing equipment from end of patient airway to where ‘to and fro’ movement of breaths occur
Minimising mechanical dead space
- Cut ET tubes to size
- Avoid extra lengths of tubing or connectors
- Heat moisture exchangers + capnography will contribute to this
- Correct fresh gas flow rates
Cardiac arrhythmia
Abnormal heart rhythm
Sinus arrhythmia
Heart rate increase and slow with respiration
Normal in healthy dogs
Pulse deficit
Heart rate and pulse rate don’t match
- Usually lower pulse rate
- Indicative of sever cardiac problems
Vagal tone
Activity of vagus nerve affecting heart rate and vasoconstriction/dilation
- Increased vagal tone = lower heart rate
- Increased vagal tone common in brachys
- IV prone to very low heart rate under GA
Syncope
Fainting due to sudden drop in heart rate and blood pressure
- Vagal response
- Common in boxers
Inhalation agents
Produce anaesthesia by inhalation
- Liquids or gases
Volatile anaesthetic agents (VAA)
Liquids at room temperature
- Require conversion to vapour
- Isoflorane
Anaesthetic sparing
Using local and analgesia to reduce required anaesthetic depth for surgical procedures
Second gas effect
Use of nitrous oxide gas as well as volatile agent
- Increases uptake rate of volatile agent
Minimum alveolar concentration
- MAC
- Min concentration of inhaled anaesthetic when 50% of patients will not respond to stimulus
- Indicates if need higher or lower concentrations
Emetic drug
- Causes vomiting
Anti-emetic drug
- Prevents vomiting
Analeptic drug
- Central nervous system stimulant
Analgesic drug
Pain relief
Ecbolic drug
- Initiates uterine contractions
Hypnotic drug
- Sleep inducing
Amnesic drug
- Partial or full memory loss
Muscle relaxant drug
- Decreases muscle tone
Ataracric drug
- Calms or tranquilises
Anti-sialagogue drug
- Stops salivation
Spasmolytic drug
- Relieves spasms or slows gut contractions
Neuroleptanalgesia/anaesthesia
- Opioid and sedative (should be ACP)
- Produces profound sedation and analgesia
Dissociative drug
- Produces detached states
Narcotic drug
- Dulls senses
- Numbs pain
- Induces deep sleep
What is premedication?
- Part of all anaesthetic protocols
- Differ dependent on patient and surgery
- Contribute to triad of anaesthesia
- Can produce very different effects
- Important to know and understand effects to properly monitor
What are the aims of premedication?
- Calm patient
- Reduce stress
- Reduce anaesthetic drugs needed
- Contribute to balanced anaesthesia
- Provide analgesia
- Smooth recovery
- Reduce side effects from anaesthetic drugs
- Reduce autonomic side effects
Why is it important for premedications to calm the patient and reduce stress?
- Reduce struggle so reduce adrenaline
- Can effect heart
- Some anaesthetic drugs sensitise heart to adrenaline and lead to higher risk of arrhythmias
Why is it important for premedications to reduce autonomic side effects?
- Parasympathetic effects like salivation or bradycardia
- Some agents are added to prevent these
Why is it important for premedications to reduce side effects from other anaesthetic medications?
- Nausea is common side effect
- Some premeds have anti-emetic properties
What factors need to be considered when selecting premedication protocols for patients?
- Species and breed; considerations, licenses
- Temperament; route of administration
- Underlying disease/clinical history/current medication/previous reactions
- Age; geriatric = disease more likely
- Lab results; organ dysfunction, excretion route
- Type of surgery; how painful? muscle relaxation
- Duration of surgery; how long need to last for
What types of drugs are used in premedications?
Sedation
Analgesia
Anti-muscarinics
What factors may change route of induction?
- Medication
- Temperament
- Speed of onset needed
Must give sufficient time to see full benefits
What are the main groups of sedative agents?
- Phenothiazines
- Benzodiazepnes
- Alpha 2 agents
- Dissociatives
What is the only licensed veterinary phenothiazine?
Acepromazine (ACP)
- Widely used for premed
- Synergistic with opioids
- Lasts 6 hours; will smooth recovery
- Slower onset
- Can be given IV, IM and SC
- More effective if animals is relaxed
- Not reliable for aggressive animals and cats
- Neuroplept effect in higher doses - heavy sedation
What are the physiological effects of acepromazine?
- Prevent adrenaline induced arrhythmias
- Anti-emetic effect
- Antihistamine
- Vasodilation leading to hypotension
- Vasodilation + hypothalamus effects = hypothermia
- Boxers have known sensitivity; may collapse
- Decreased seizure threshold
- Decreased PCV due to splenic sequestrum; avoid in anaemic patients.
What are the benzodiazepines that are widely used in veterinary medicine?
- Diazepam; licensed vet product (Ziapam)
- Midazolam; unlicensed (Hypnovel)
Benzodiazepines for premedication use
- Not reliable sedation in healthy animals
- May cause excitement
- More reliable sedation in sick or very young
- Anxiolytic effect
- Often used in combination with other agents
- Can cause appetite stimulation in some animals
Physiological effects of benzodiazepines
- Minimal effects on cardiovascular; good for critically ill
- Muscle relaxation; useful when using ketamine
- Anticonvulsant effects; useful if increased risk of seizures
What are the widely used licensed veterinary alpha-2 agonists?
- Medetomidine (domitor, sedator, dorbene)
- Xylazine (rompun, chanazine, nerfasin)
- Dexmedetomidine, contains one isomer of medatomidine (dexmedased, dexdomitor)
Alpha-2 agonists as premedications
- Can be antagonised using atipamezole
- Reliable, profound sedation
- Synergistic with opioids, allowing lower doses for higher sedation levels
- Profound drug sparing effects, 50% in maintenance anaesthetic agents
- Marked cardiovascular effects
- Typically used in healthy patients
Physiological effects of alpha-2 agonists
- Reduces hepatic blood flow, don’t use in hepatic disease
- Can cause vomiting, more likely with xylazine
- Don’t use in occular, increased intercranial pressure or some GI surgeries.
- Xylazine can cause cardiac arrhythmias
- Cyanosis may be seen due to peripheral blood pooling
- Ecbolic effect; don’t use in pregnant animals
- Vasoconstriction; less risk of hypothermia
- Good muscle relaxation
- Short duration analgesia (45-60 minutes)
- Increases urine production
- Suppresses insulin secretion; hypergyclaemia
What is the dissociative drug used in veterinary anaesthesia?
- Ketamine
- Used for premed, induction or analgesia
- Schedule 2 controlled drug, record + lock away
- Good sedation when combined with other agents
- Excellent somatic analgesia
- Good drug sparing properties
- Often combined with benzodiazepines in cats for profound sedation
- Produces superficial sleep with amnesic properties
Physiological effects of dissociative ketamine
- If used alone, can induce seizures, especially in dogs
- Eyes remain open and position may not alter
- Eye position unreliable indicator, use corneal lube
- Stimulates cardiovascular; increase CA + BP
- Increases myocardial oxygen demand but direct myocardial depressant
- Increases intercranial pressure
- If used alone, increases muscle tone and rigidity
- Hallucinogenic effects in recovery, need dark, quiet environment
Most anti-cholinergics are …
Anti-muscarinics
What are anti-cholinergics used in practice?
- Atropine and glycopyrrolate
- Atropine is more commonly used as cheaper, licensed and will reduce more secretions
- Not routinely used in premeds, can be added for specific reasons
Physiological effects of anti-cholinergics
- Reduce respiratory secretions + salivation
- Useful for some throat surgeries or ETT blocked
- Increase HR in high vagal tone or if bradycardia is effect from other drug (opioids)
- Common in crash kit, used in emergencies
- Prevent bradycardia in brachycephalics
- Don’t treat bradycardia from alpha-2 agonsits
- Pupil dilation in eye surgery
Opioids used in practice
- Common in premed
- Classified by receptor act on; mu, kappa, delta
- and whether effect is full, partial or antagonistic
- Full mu agonists; schedule 2; methadone, fentanyl, morphine - most effective analgesia
- Partial agonists; schedule 3; buprenorphine - not as effective
- Mixed agonist/antagonist; kappa agonist/mu antagonist - butorphanol; schedule 4; some analgesia, good sedation
What side effects should be monitored for after premedication?
- Hypothermia is high risk after ACP
- Bradycardia and hypotension
- Airway patency; especially in bracycephalics
- High risk require more intense monitoring
- ECG, pulseoximetry or oxygen therapy
How can we ensure premedications are used safely?
- Weigh all patients for accurate doses
- Give at correct time
- Record drugs + times given on forms
- Calm, quiet environment for max effect
- pre-existing conditions and medications noted clearly on forms + considered
- Record baseline observations before giving any medications
What is apneustic breathing?
Expiratory pause is normal
Apneustic is an inspiratory pause
What are the 3 different induction techniques?
- Inhalation
- IV
- IM
What factors may influence the choice of anaesthetic agent?
- Species
- Temperament
- Protocol and drugs
- Age of patient
What are the methods of inhalant induction?
- Chamber
- Mask
What are the disadvantages of inhalant induction?
- Stressful
- Restraint difficult when masking
- Health and safety with leakage
- May breath hold or salivate excessively
What must injectable anaesthetic agents be to work?
Lipophilic so they are able to cross the blood brain barrier
What is alfaxalone
Steroid anaesthetic agent
What species is alfaxalone licensed for?
Cats
Dogs
Rabbits
How can alfaxalone be administered?
Licensed IV
Can give IM
Physiological effects of alfaxalone
- Minimal effects on blood pressure
- Possible apnoea on induction
- Can maintain anaesthesia incremental or CRI
- Excretion via hepatic route
- Minimal effects on neonates and rapid maternal recovery in caesareans
- Safely used on consecutive days
What is propofol?
Phenolic compound
What is thiopentone?
Barbiturate anaesthetic compound
What are the contraindications for Thipentone?
- Sight hounds as less fat
- Splenectomy as enlarged spleen
- Skin sloughing if out of IV due to alkaline
What is ketamine?
Dissociative
What are the 4 main ways to maintain anaesthesia?
- Inhalant gas
- Total intravenous anaesthesia (TIVA)
- Partial intravenous anaesthesia (PIVA)
- Injectable agents intramuscularly
What are the advantages of inhalant anaesthesia?
- Usually protected airway, oxygen easily supplied
- Depth easy to control
- Recovery not dependent on drug metabolism as mainly excreted via exhalation
- Rapid recovery times
What are the disadvantages of inhalant anaesthesia?
- Anaesthetic machine and trained staff required
- Scavenging system required
- Atmospheric pollution during recovery
- Inhalant gasses are greenhouse gasses so contribute to damage of ozone layer
Volatile agents
- Liquid at room temperature, vaporised
- Isoflurane and Sevoflurane
- Carried in another gas; oxygen, nitrous oxide or medical air - vapour at room temp so in gas canisters
Delivery of inhalant agents
- Via breathing system
- ## Passes through lungs, alveolar membrane and into blood stream
MAC
- Minimum alveolar concentration of inhaled anaesthetic
- Potency of agent
- Lower MAC = Less concentration of agent required
- Impacted by many factors; premeds used
What is the MAC value of Halothane?
0.75
What is the MAC value of Isoflurane?
1.15
What is the MAC value of Sevoflurane?
2.05
What is the MAC value of Desflurane?
5-10
Solubility in blood of inhalant agents
- Determines speed of induction and recovery
- Greater solubility, slower onset of action
- Newer agents less soluble for quicker induction and recovery
Another word for solubility of drugs?
Blood gas partition co-efficient
Factors effecting gaseous recovery
- Concentration of anaesthetic gas
- Alveolar ventilation, quality breathing
- Agent solubility in blood
- Cardiac output
Nitrous oxide
- Carrier gas, vapor at room temp
- Supplied in blue canisters
- Used along side oxygen
- Must not exceed 70% of fresh gas flow
- Less soluble, wears off very quickly
- Effective analgesic
- Patient must receive 100% oxygen for 5-10 minutes after turning off to avoid diffusion hypoxia.
What is diffusion hypoxia?
Contraindications for using nitrous oxide
- Will diffuse into and expand gas filled cavities
- Can expand ET tube cuff
- Avoid in patients with cardiovascular or respiratory disease
- Not used in rabbits as gassy hind gut fermenters
Second gas effect
- Using nitrous oxide to speed up induction
- Nitrous has low solubility so moves rapidly from lungs to bloodstream
- Will take volatile agent during rapid diffusion
- Will speed induction
TIVA
- Injectable IV agents incrementally or constant rate
- Can only be preservative free propofol
- Can only be 30 minutes for cats
- Propofol or alfaxan
Disadvantages of TIVA
- Some drugs may accumulate and prolong recovery
- Difficult to maintain constant depth via incremental
- CRI requires syringe driver for accuracy
- -
PIVA
- Combining inhalant and IV agent for maintenance
- Provides very balanced anaesthesia
- Protocols may be complicated so experiences anaesthetist required
Patient positioning under anaesthetc
- Patients in dorsal recumbency may have impaired respiratory function due to pressure on diaphragm
- Plastic troughs should not be too tight as may interfere with thoracic movement
- Tying forelimbs tightly may interfere with thoracic movement
- Patency of airway to ET tube may be compromised especially in ventral recumbency
What is pain?
- Sensory and emotional experience
- Associated with actual or potential tissue damage
- Caused by noxious stimuli
What is a noxious stimulus?
- Damaging to tissues
- Detected by nociceptors
- Then activates nociceptive pathways
- Transmitted by nerves to spine and up to brain
- Thermal, mechanical or chemical
- Usually results in pain
Nociception is
Perception of pain
Why is pain detrimental?
- Causes fear, anxiety and distress
- Delays wound healing
- Predisposes to intestinal ileus
- Impairs respiration affecting acid base balance
- Wound interference and self trauma
- Prolongs recovery
- Reduces food intake
- Affects cardiovascular function due to stimulation of sympathetic nervous system = increased HR and cause vasoconstriction
What are the 3 pain catagories?
- Physiological pain
- Inflammatory pain
- Neuropathic pain
What is physiological pain?
- Early warning device
- Alert to possibility of tissue damage
- Pain stops when stimulus stops
How can muscle relaxation be achieved?
- Selection of premed agents
- GA; high concentrations needed, not advised
- Regional anaesthesia/analgesia
- Neuromuscular blocking agents (NMBA)
What are some indications for use of neuromuscular blocking agents?
- Ocular surgery; prevent downward rotation of eye or unpredictable movements
- ## Facilitate IPPV; prevent natural override of ventilation
What are the 2 types of neuromuscular blocking agents?
DEPOLARISING
- Suxamethonium
- Can’t be reversed
NON-DEPOLARISING
- Atracurium, Pancuronium, Necuromium
- Can be topped up without prolonged effect
- Can be reversed
What are the functions of anaesthetic breathing systems?
- Transfer gases from GA machine to patient
- Remove C02 exhaled by the patient
- Deliver artificial breaths (IPPV)
- Measure airway pressure, gas volumes and composition
- Scavenge waste gasses
Rebreathing
- Inhalation of previously breathed gases that have taken part in gaseous exchange
Reservoir bag
- Open ended or closed bag attached to the breathing system
What is IPPV?
Intermittent positive pressure ventilation
Limbs of circuit
- Tube of breathing system where gases are carried
Uni-directional valves
- One way valves
- Ensure gas only flows in one direction
APL valve
- Adjustable pressure limiting valve (Pop off)
- Controls amount of gas contained within bag and how much escapes through scavenging
- Usually a plastic disc on spring
- Depressed when set pressure is exerted to allow gasses to escape
- Safety system but pressure needed quite high so patient probably have suffered some barotrauma
- Valve can close and open to control pressure
Fresh gas inlet
- Point where gas enters breathing system from common gas outlet on machine
Coaxial system
- Inspiratory and expiratory tubes within one another
Parallel system
- Inspiratory and exspiratory limbs run side by side
Lung compliance
- How well lungs stretch to accommodate a change in volume in relation to pressure applied
Circuit resistance
- Pressure drop when breathing through a tube
- Requires more effort
- Is increased the longer the tube is
- Is increased the more turbulent the airflow
- Smooth bore tubes lower resistance
- Can be bought for most circuits
- Smaller patients respiratory function will be effected more by higher resistance due to small tidal volume
Mapleson classification
- Way of classifying non rebreathing circuits
Advantages of non-rebreathing systems
- Inhalant gas can be adjusted rapidly
- Minimal circuit resistance
- Easy to use
- Cheap to purchase
- Safe and optimal use of N20
Disadvantages of non-rebreathing systems
- High gas flow rates
- Increase environmental contamination
- Higher gas and volatile agent costs
- Potential for rebreathing is tachypnoea leads to insufficient flow rate
- Heat loss through respiratory tract
What measures can be put in place to ensure rebreathing does not occur while using non-rebreathing circuits
- Calculate fresh gas flow rate for every patient
- Adjust fresh gas flow if RR increases during GA
- Calculate with correct circuit factor
- Consider use of capnography
Calculating fresh gas flow rates
- Tidal volume = 10-15ml/kg
- 10ml for over 10kg
- 15ml for under 10 kg
- Deep chested may require 12-15mls/kg
- Minute volume = Tidal volume x RR
- Fresh gas flow = MV x circuit factor
What is the maximum ratio for combination nitrous oxide and oxygen for fresh gas fow?
2:1
Considerations when selecting a circuit
- Patient weight (choose for lean weight)
- Will IPPV be needed?
- Drag on patient from circuit? (pull on tube)
- Gas flow rate - Higher - hypothermia and costs
Ayres T-Piece
What are rebreathing systems?
- Allows gasses previously exhaled to be reused
- Use C02 absorbent
What are the 2 main rebreathing systems?
- Circle
- To and fro
What are the advantages of rebreathing systems?
- Low gas flow rates; reduce cost and environmental contamination
- Reduced heat loss by reusing warm air
What are the disadvantages of rebreathing systems?
- Altering concentration of VA takes time unless system completely refilled
- C02 absorbent and valves increase resistance
- Requires understanding of use
- Maintenance; C02 absorbent changing
- Expensive to purchase
What is the most common rebreathing system in veterinary industry?
- Circle
- Models vary; different canisters and tubes
What size patient can use a circle system?
- Most suitable for over 10kg
- Smaller models available
What valves do circle systems contain?
- Unidirectional valves
- (Reubens valves)
What are the advantages of the circle system?
- Reduce fresh gas flow/VA costs
- Less environmental contamination
- Reduced heat loss from patient
- Inspired gases moistened
- Ideal for IPPV
What are the disadvantages of the circle system?
- Canisters need refilling; fiddly, hard to clean
- Canisters can be source of leak; inspect regularly
- Cost of C02 absorbent
- Knowledge + time needed for maintenance
- Will harbour moisture; needs to dry
- C02 absorbent creates resistance; not suitable for smaller patients
- Expensive to purchase
- May exacerbate hyperthermia in large patients
- Not advised to use nitrous oxide
Why shouldn’t nitrous oxide be used with circle circuits?
- Unless experienced anaesthetist with appropriate monitoring equipment
- Nitrous will accumulate in system
- Leads to higher concentrations over time
How does a circle work?
- At start, period of denitrogenation must take place
- May be operated as closed/semi closed
- Contains one way valves to ensure direction
- Patient breaths in from inspiratory limb
- One-way valves control exhaled gases
- Gases flow into reservoir bag
- Then through soda lime canister
- Soda lime converts CO2 to O2 to be reused
- Levels of VA remain same as exhaled from body
- Small amounts of fresh gas added to system
What is denitrogenation?
- Room air contains high levels of nitrogen
- First anaesthetised, exhale nitrogen
- If builds up in system, gas mixture hypoxaemic
- So higher gas flow used for first 15-20 minutes
- Usually 100ml/kg/min (Open, semi-open)
- Then gas flow dropped as semi-closed or closed
What does a ‘closed system’ mean in reference to circle circuits?
- Supply metabolic oxygen requirement only
- Operate with valve closed
Why are ‘closed systems’ in reference to circle circuits not recommended?
- Metabolic oxygen consumption varies (2-10ml/kg)
- Flowmeters don’t allow accurate delivery of low flow rates
- Vaporisers not calibrated for very low flow rates
- Accurate monitoring to ensure no CO2 rebreathing
- Reliant on function of CO2 absorbent
What is a semi-closed system in reference to circle circuits?
- Operate with valve semi-open
- 1 litre/min fresh gas flow after denitrogenation
- Lower FGF rates may be used where adequate CO2 level monitoring in place
Why should patients be intubated during anaesthesia?
- Protect airway
- Maintain patent airway
- Prevent soft tissue obstruction
- Prevent secretion obstruction
Why are some patients not intubated?
- Depends on size of patient
- <2kg difficult to tube
- Small diameter tube cause more resistance
- Some species can not easily be intubated
What are the type of endotracheal tubes?
- Red rubber (Magill)
- Polyvinyl chloride (PVC)
- Silicone
- Cuffed
- Uncuffed
- Re-enforced
What are the 2 main types of cuffs?
HIGH PRESSURE, LOW VOLUME
- More secure protection of airway
- More risk of pressure
LOW PRESSURE, HIGH VOLUME
- Pressure over larger area
- Airway protection not as secure
What tubes are usually chosen for dogs?
- Cuffed tubes
Why are cuffed tubes not ideal for use in cats?
- Very prone to tracheal necrosis
- Cuffed only used in high aspiration risk or during ventilation
- Great care must be taken with cuffed tubes
What is an alternative to a cuffed tube when preventing aspiration?
- Throat packs
- Useful in dentals
What are the advantages and disadvantages of red rubber (Magill) tubes?
ADVANTAGES
- Re-useable
- Wide range of sizes
- Can be autoclaved
- Easy to intubate as pre-moulded
DISADVANTAGE
- Expensive, especially larger sizes
- Perish with time
- Kink easily
- Cannot see internal contamination
- Cuff valve not self sealing
- Only low volume, high pressure
What are the advantages and disadvantages of of PVC tubes?
ADVANTAGES
- Cheap
- Designed to be disposable, can reuse
- Malleable when warm
- Fairly kink resistance
- Both low vol, high press + high vol, low press
- Easier to see internal contamination
- Cuffs valved
- Some have murphy’s eye
DISADVANTAGES
- Designed to be disposable
- Cannot be repaired
- Limited sizes - very large unavailable
- Cannot be autoclaved
- Connectors can loosen when tube warms
What are the advantages and disadvantages of silicone tubes?
ADVANTAGES
- Can be repaired
- Can be autoclaved
- Malleable when warm
- Fairly kink resistant
- Cuffs are valved
- Wide range of sizes
- Can just see internal contamination
- Some have Murphy’s eye
DISADVANTAGES
- Expensive
- May require stylet for intubation as no moulded curve
What are the advantages and disadvantages of armoured tubes?
ADVANTAGES
- Same as PVC, usually PVC material
- Internal wire coil to prevent kinking
DISADVANTAGES
- Very expensive
- If bitten down on will permanently occlude
- Do not spring back due to wire content
Bonus endotracheal tube facts
- Some have radiopaque line
- Tube size is internal diameter in mm
- Most tubes have length markings
What should be considered when choosing sizes of endotracheal tubes?
- Should be comfortable fit to avoid excessive cuff inflation
- Tube not too long; bronchial intubation may occur, increased dead space
- Tubes should not be too short, easily dislodged
- Measure from front of incisors to thoracic inlet
- Always get more than one tube size
- Brachys have very narrow trachea
What checks should be done on endotracheal tubes before intubating patients?
- Visually inspect for damage
- Check lumen clear, pass tube brush, never blow
- Check cuffs work, inflate, squeeze, fully deflate
What are some aids to intubation?
- Laryngoscope
- Stylets (can use urinary catheter)
- Silicone based sprays for lubrication
What are the types of laryngoscope?
- Miller blade (curved)
- Macintosh (straight blade)
- Other specialist blades available for rabbits etc.
How are stylets used to intubate patients?
- Stylet placed and threaded over the top
- Stylet can make tube rigid
- Urinary catheter can also be used for this
How does lube help with intubating?
- Sterile lube
- Silicone based sprays
- Reduce trauma
- If water-based can dry out, become sticky and block tube
What is the correct technique when intubating patients?
- Ideally positioned in sternal
- Head and neck in straight line
- Cats should have larynx sprayed with local to prevent larangeal spasm - wait 45 seconds
- Pull out tongue
- Depress just in front of epiglottis with laryngoscope
- Pass tube through glottis, into trachea and insert to correct level
- Secure in place
- Attach to breathing circuit
- Give gentle manual breath to listen for leak
- Slowly inflate cuff until no leak heard
How to confirm correct placement of endotracheal tube once placed?
- Ausculate thorax sides to ensure no bronchial intubation
- Watch for condensation inside tube
- Check patient chest and bag movement
- Monitor capnography for CO2 presence
How can the cuff be checked to ensure the correct amount of air has inflated it?
- Care to only inflate enough to prevent leakage
- Checked by listening when patient is given a manual breath
- More accurate to check pressure cuff is exerting on tissues
- Special cuff inflator to show pressure
- Some have automatic shut off
- Ideal pressure should be 20-30cmH2O
What are the different ways gas can be supplied?
- Cylinder
- Piped gas system
- Oxygen generator
How are piped gas systems run?
- Large cylinders outside of building
- Small cylinders can attach directly onto anaesthetic machines
How is the pressure of oxygen controlled on cylinders?
- Gas runs through pressure reducing valve (regulator)
- Reduces pressure to safe level
- Prevents surges
- Produces consistent pressure
- Built into machines that use small cylinders
- Located at source cylinder in piped systems
What is the colour code for oxygen cylinders?
- Black cylinder
- White shoulders
What is the colour code for nitrous oxide cylinders?
- Blue
How are cylinder sizes identified?
- Letters
- Later in alphabet, larger the cylinder
- Portable cylinders often E or F
- Piped gas systems J or similar
