Anaesthetics - pharmacology and principles Flashcards
What is the triad of anaesthesia?
Analgesia
Relaxation
Hypnosis
What is used for analgesia?
Local anaesthetics
Opiates
General anaesthetic agents
What is used for hypnosis?
General anaesthetic agents
Opiates
What is used for relaxation?
Muscle relaxants
Local anaesthetic
General anaesthetic agents
What are features of balanced anaesthesia?
Different drugs doing different jobs
Doses titrated seperately and more accurately
Avoid overdosage
Flexibility
What are problems of balanced anaesthesia?
Polypharmacy - drug interactions
Muscle relaxation - airway control required
Relaxation without hypnosis = awareness
How do general anaesthetic agents work?
Open chloride channels leading to hyperpolarised neurones which are less likely to fire
How are cerebral functions lost in general anaesthesia?
From top down
Most complex processes interrupted first - loss of conscious early
Reflexes spared
How must patients be managed during general anaesthesia?
Airway management - maintenance and protection
Impairment of resp function and control of breathing
Cardiovascular impact
Care of the unconscious patient
What are features of IV anaesthesia?
Rapid onset - 1 brain-arm circulation time
Rapid recovery - drug is removed from circulation
How is the drug from IV anaesthesia stored within the body?
Rapidly picked up by viscera, slowly picked up by fat and muscle and then slowly metabolised
What type of drugs are used as inhalational anaesthetics?
Halogenated hydrocarbons
How are inhalational anaesthetic drugs taken up by the body?
Goes along concentration gradient from lungs - blood - brain
Crosses alveolar membrane easily
Arterial concentration = close to alveolar pressure
What is Minimum Alveolar Concentration?
Concentration of drug required in alveoli to produce anaesthesia with a specific agent
What can be interpreted from the minimum alveolar concentration?
Measure of potency of anaesthetic
What are features of inhalation anaesthesia?
Slow induction
Very flexible in maintenance
For awakening - washout to reverse conc gradient to drive anaesthetic agent out
What is the most common sequence of general anaesthesia?
IV induction followed by inhalation maintenance
How does general anaesthetic affect the cardiovascular system?
Depress the CVS centre in the brain - reduce sympathetic outflow
Vasodilation
Venodilation
How do general anaesthetics affect the respiratory system?
Respiratory depressant - reduce hypoxic and hypercarbic drive
Decreased tidal volume and increased rate
Paralysed cilia
Reduced lung volume and VQ mismatch - may be prolonged effect
What are indications for muscle relaxants?
Ventilation and intubation
When immobility is essential
Body cavity surgery
What are problems with muscle relaxants?
Awareness - no way to know if patient is still aware
Incomplete reversal can lead to airway obstruction and ventilatory insufficiency in immediate post op
Apnoea
Why do we give intraoperative analgesia?
Prevent arousal from pain
Opiates contribute to hypnotic effect of GA
Suppression of reflex responses to pain
What are features of regional anaesthetic agents?
Intense/complete analgesia
No direct hypnotic effects
What are features of local anaesthesia and regional anaesthesia?
Patient retains awareness and consciousness
Lack of global effects of GA
Derangement of CVS physiology proportional to size of area affected
Sparing of resp function
What is the main limiting factor in local anaesthetics?
Toxicity
What causes toxicity of local anaesthetic agents?
High plasma levels - IV injection with absorption>metabolism
How can we reduce toxicity?
Reduce absorption - vasoconstrictors reduce blood flow therefore reducing absorption
What does toxicity depend on?
Dose used
Rate of absorption - site dependent
Patient weight
The drug used
What are clinical features of local anaesthetic toxicity?
Circumoral and lingual numbness and tingling Light-headedness Tinnitus/visual disturbances Muscular twitching Drowsiness CVS depression Convulsions Coma Cardiorespiratory arrest
What is differential blockade in local anaesthetic?
Different nerve types take up different amounts of drug meaning some nerves are affected more than others
What nerves take up the least local anaesthetic?
Motor
Which nerves are easily blocked due to local anaesthetic?
Pain fibres
