Injectable and Inhalational Anaesthetic Agents Flashcards
Injectable agent use
Injectable agents are commonly used to induce and sometimes maintain general anaesthesia.
Indications for use of injectables
Induction agent for inhalational anaesthetics
To supplement inhalational agents
Sole agent for short surgical procedures, clinical examination, radiography etc.
To suppress CNS stimulation in certain conditions eg. some poisonings, tetanus, seizures etc.
Injectable routes of administration
Intravenous
Intramuscular
Subcutaneous
Intraperitoneal
What is induction?
Induction is the process where the animal leaves the normal conscious state and enters into the anaesthetised state.
I/v route is the most common route used for induction of anaesthesia
The route of drug administration affects…
a) The speed of onset of anaesthesia
b) Duration of the anaesthetic
c) Timing of the peak effect of the drug
How are injectable anaesthetics metabolised
Metabolised by the liver where they are converted from lipid to water soluble forms.
This allows them to be excreted in bile and urine via the kidneys.
Induction agents
Propofol
Alfaxalone
Barbiturates
Ketamine
Propofol - Route and characteristics
Propofol is a water soluble PHENOL anaesthetic
Route - IV only
Characteristics -
- Rapid loss of consciousness
- Rapid recovery – metabolised by liver
- Useful in total intravenous anaesthesia (TIVA)
Propofol - Advantages and disadvantages
ADV-
• rapid metabolism and recovery
• Free from hangover if single dose given
• Non-cumulative therefore can be used as part of TIVA protocol
• Non-irritant peri-vascularly
DIS
• May cause APNOEA if injected too fast
• May cause hypotension and occasional bradycardia
• May cause twitching, spontaneous muscle activity and excitable recoveries
Alfaxalone - Route and characteristics
This is a STEROID anaesthetic
Route- IV (dogs, cats and rabbits)
Alfaxalone - Advantages and disadvantages
ADV- • Rapid smooth induction • Minimal cardiorespiratory depression • No pain on injection/ no perivascular tissue irritation • Rapidly eliminated from the body
DIS-
• Adequate premed required for smooth induction
• Must be kept quiet during the recovery period to avoid over excitement (drug is rapidly eliminated from system)
Barbiturates - Advantages and disadvantages
Barbiturates as anaesthetic agents in modern practice has now been superceded by agents such as propofol and alfaxalone.
ADV-
rapid induction of unconsciousness combined with good muscle relaxation
DIS-
no analgesia is provided , drugs generally have myocardial and respiratory depressant effects
Ketamine - Advantages and disadvantages
Ketamine is a ‘dissociative anaesthetic’.
Route- IV, IM, SC, MMs
ADV- Analgesic properties (may be used with lignocaine & morphine via MLK infusion)
can be used alone in cats
DIS-
Hard to monitor
The onset of anaesthesia tends to be SLOW
Poor muscle relaxation
injection can be painful
TIVA
Total Intravenous Anaesthesia
Alternative to gaseous anaesthetics
Ideally utilising constant rate infusion & syringe driver
useful where inhalational delivery would compromise access for surgery
Barbiturate anaesthetics
Highly lipid soluble, (enter the brain tissue rapidly - onset of anaesthesia is fast 15-30 seconds)
Barbiturate levels in muscle and fat rise more slowly
Gradually blood levels fall as drug enters muscle/fat
When blood level falls drug leaves brain tissue into plasma and is redistributed
Animal shows signs of recovery but drug is still present in muscle/fat and is slowly released and metabolised by the liver and excreted in urine over time
Not suitable choice for animals with renal hepatic or cardiovascular disease
Increased potency and prolonged recovery in thin animals due to lack of body fat to redistribute drug
Not suitable for shocked patients due to reduced blood flow to fat and increased cardiac and cerebral blood flow
Signs of dissociative anaesthesia
Eyes remain open
Pupils dilated
Protective airway reflexes maintained
Cranial nerve reflexes less depressed compared to other agents
Heart rate increases – BP usually maintained
Breathing moderately reduced
Salivation increases
Neuroleptanalgesics
Tranquilliser combined with analgesic e.g. benzodiazepine and ketamine or alpha2-agonist and ketamine
Ketamine mixtures
ACP
Diazepam
Xylazine
Medetomidine
Opioid mixtures
Often combined with neurolepts eg. ACP but beware of hypoventilation and arrhythmmias
I/M administration
Can only be used for certain drugs
Dose required is usually much greater than the iv dose
Whole calculated dose must be given
Takes longer to onset of effect than if given iv
May be painful
May be associated with prolonged recovery
Hypnorm and ketamine mixtures are commonly given i/m
Carrier gases
Inhaled anaesthetics are transported in carrier gases
OXYGEN
NITROUS OXIDE –see later
Use of O2
Uses; 100% pure O2 is given to animals with
Anaemia
Pulmonary pathology
Hypoventilation
Animals recovering from N2O anaesthesia/supplied during recovery if needed
It may be used pre and post operatively routinely *
Nitrous Oxide Advantages
Odourless
Combined with oxygen as a carrier gas
Analgesic properties (at concentrations of over 20%) and non-irritant
Used in conjunction with other anaesthetic agents
Usually given at 66% delivered
Has an anaesthetic sparing effect
second gas effect – it enhances the uptake of other inhalational agents
Nitrous Oxide Disadvantages
not use more than 80% N2O as this will lower oxygen below normal levels
diffuses into gas-filled spaces & accumulates – can compromise certain patients.
Should be avoided in cases with pathological abdominal or thoracic distension Eg. pneumothorax or GDV
Nitrous oxide- diffusion hypoxia
N2O has low solubility in blood
At the end of anaesthesia when administration of N2O ceases, the N2O present in blood diffuses back into the alveoli and ‘dilutes’ the alveolar air so less O2 is available.
If the animal is breathing room air hypoxia may occur.
ALL PATIENTS SHOULD RECEIVE 5-10 MINS OF PURE OXYGEN ONCE THE N2O IS SWITCHED OFF
Inhalational anaesthetics advantages
Depth of anaesthesia readily altered
Concentration of anaesthetic in blood or brain may be altered rapidly by changing vaporiser setting
Elimination of inhaled agents occurs mainly via the lungs therefore advantage if liver/kidney patient
Allows constant delivery of oxygen alongside agent
Most patients are intubated therefore IPPV may be performed and anaesthetic is ‘safer’
Inhalational anaesthetics advantages
Need for anaesthetic machine and range of equipment – cost
Hazards, personnel health risk, explosion risk
Knowledge required of circuits and machine
Induction and recovery possibly delayed due to lung pathology
MAC
Minimum Alveolar Concentration
Measure of anaesthetic potency
Quantity of anaesthetic required to immobilize 50% of patients
Agents with lower MAC value are most potent.
High blood gas solubility
Agents with a high solubility take longer to work and recovery may be longer as they can build a reservoir in plasma eg. halothane
Low blood gas solubility
Agents with lower blood gas solubility tend to have more rapid induction and recovery eg. sevoflurane.
Inhalational agent examples
Halothane
Isoflurane
Sevoflurane
Desflurane
Halothane
Formerly the most commonly used volatile anaesthetic in practice
Sweet smelling, clear liquid at room temperature
Non flammable and non explosive
Decomposes in UV light
Contains antioxidant
Fast acting anaesthetic
Halothane- physiological effects
Potent anaesthetic
Modest muscle relaxation
Poor analgesic
Dose-dependent depression of CVS
Predisposes heart to arrhythmias – sensitises the heart to catecholamines
Vasodilatation (brain, uterus, skin)
Decreases respiratory rate
Isoflurane
CURRENT ‘POPULAR’ VOLATILE AGENT IN VETERINARY PRACTICE
Liquid without additives or preservatives
Pungent smell
Greater margin of safety than halothane
Rapid induction
Rapid recovery
Is less potent therefore higher inspired concentrations needed to maintain anaesthesia
Isoflurane properties
Good muscle relaxation
Good analgesic
Rapid recovery
Transient excitability esp following painful surgery
Hypotension
Does NOT sensitise the heart to arrhthymias
POTENT RESPIRATORY DEPRESSANT
POORLY TOLERATED FOR MASK INDUCTION – pungent smell and irritant to airways.
Sevoflurane
INCREASING POPULARITY IN VETERINARY PRACTICE
Rapid induction and rapid recovery
when compared to isoflurane as has lower blood;gas solubility
Pleasant non-irritant odour
Well tolerated for mask induction
UNSTABLE IN PRESENCE OF SODA LIME
Sevoflurane physiological effects
Similar cardio-pulmonary effects as iso
Hypotension (dose dependent)
Does NOT sensitise the heart to catecholamines
RESPIRATORY DEPRESSION (DOSE DEPENDENT)
Very little liver metabolism – liver friendly
Desflurane
Relatively ‘new’ agent
Lowest blood;gas solubility allowing rapid alterations in anaesthetic depth and fast recovery
Irritant to respiratory tract in high concentrations (but well tolerated in mask induction)
Rapidly excreted via lungs
Requires a special temperature controlled pressured vapouriser - expensive
