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
