Purpose of anaesthesia Flashcards
What are ‘triad’ aims of anaesthetics
- Unconsciousness
- Muscle relaxation
- The inhibition of pain
What is meant by analgesia in the unconscious patient?
- conscious perception of pain is normally associated with the awake state
HOWEVER
- noxious stimuli can still evoke physiological responses in the anaesthetized patient → drugs that decrease these responses are considered to be providing analgesia
Do volatile agents have analgesic properties?
- contribute to analgesia while a patient is anaesthetised
- do not provide any pain relief post-op
*large doses of volatile agents are needed to inhibit sympathetic response
How to quantify a potency of a volatile agent?
By Minimum Alveolar Concentration (MAC) value
MAC = concentration of the vapour in the lungs that is needed to prevent 50% of patients moving when subjected to a standard surgical incision
What do volatile agents do?
- produce unconsciousness
in adequate doses:
- partial muscle relaxants → by suppression of spinal reflexes
- inhibition of movement to pain
What are the effects of IV anaesthetics? (2)
- unconsciousness
- amnesia
Do IV anaesthetics provide pain relief?
They are purely hypnotic agents → no analgesic properties
* with exception to Ketamine
Can IV anaesthetics inhibit movements in response to the surgery?
No, as they lack the ability to suppress spinal reflexes
Do benzodiazepines used in the theatre produce unconsciousness and amnesia, on their own?
Not on their own, in the doses used in the theatre
They are amnesic when combined with opiates and volatile/IV anaesthetics (synergistic effect)
Can benzodiazepines inhibit movements?
They contribute to muscle relaxation but do not completely inhibit movements (in standard doses)
Are benzodiazepines analgesics?
No
Mode of action of muscle relaxants
Muscle relaxants
(aka neuromuscular blockers)
MoA: they block transmission at the neuromuscular junction
Do muscle relaxants have anaesthetic or analgesic action?
No
*use of muscle relaxants without adequate anaesthesia → awareness while being paralysed
Can opioids produce unconsciousness and amnesia?
No
The aim of opioids use (in terms of surgery)
Analgesia during and after surgery
Can opioids produce a muscle relaxant effect?
- do not produce muscle relaxation
BUT
- they blunt perception of painful stimuli → inhibition of movements to pain during the surgery
*some opioids in very high doses may produce rigidity
What does regional anaesthesia do?
Produces immobility, muscle relaxation and analgesia within a distribution of the block
What’s multimodal analgesia?
Balanced analgesia requires the use of combinations of analgesic agents and techniques
This is also termed multimodal analgesia
Advantages of balanced analgesia
- reduce the amounts of each drug required while still being clinically effective
- this may reduce some of the unwanted side-effects of the individual drugs
Example of multimodial / balanced anaelgesia
What are the responsibilities of anaesthetist?
- providing balanced anaesthesia (during surgery)
Peri-operative period:
- Reduction of the surgical stress response
- Effective postoperative analgesia
- Prevention of postoperative nausea and vomiting
- Fluid homeostasis and management of bleeding
- Temperature maintenance with prevention of hypothermia
- Care of patient positioning and protection of pressure areas and eyes
What are physiological responses for the painful stimuli and tissue injury during the peri-operative period?
- Sympathetic nervous system mediated physiological changes
- Neuro-humoral changes
- Other hormonal and metabolic changes
What do physiological responses to a painful stimuli/tissue injury (in a peri-operative period) may lead to?
- Unwanted cardiovascular changes
- Fluid retention
- Electrolyte disturbances
- Metabolic changes with a period of catabolism and hyperglycaemia
- A systemic inflammatory response
- Hypercoagulability
The physiological responses in a patient to surgical insult cause raised or increased (8)
- Adrenocorticotrophic hormone
- Growth hormone
- Vasopressin
- Prolactin
- Insulin resistance
- Carbon dioxide (CO2) production
- Oxygen consumption
- Circulating catecholamine levels
Aim of regional anaesthesia
- prevents the transmission of nociceptive (painful) signals to the central nervous system, thus reducing the neuroendocrine stress response
- this occurs for the duration of the block
Does general anaesthesia stop physiological responses to pain/tissue injury (e.g. surgery)?
- General anaesthesia does not abolish these responses completely
- Patients still demonstrate a tachycardia under anaesthesia in response to stimulating procedures if inadequate analgesia has been given
What physiological responses to pain can high-dose opioids block?
The opioid, e.g. remifentanil or large doses of fentanyl ⇒ prevents the hypertension and tachycardia and helps to maintain cardiovascular stability
What do opioid drugs act on?
Receptors in CNS, both:
- dorsal horn of the spinal cord
- brain
How does Paracetamol work?
- act via inhibition of COX 3 ⇒ reduce prostaglandin synthesis
- a weak anti-inflammatory
How does Clonidine work?
- alpha-adrenoreceptor agonist
- analgesic, sedative and anti-hypertensive actions
How does Ketamine work?
- acts in the brain and spinal cord (central action)
- local anaesthetic properties
- used as adjunct to some regional blocks
What does Tramadol act on?
- opiate effects
- also acts on descending inhibitory pathways → to reduce upward pain transmission
What do intrathecal local anaesthetics do?
block conduction in spinal nerves
How do local anaesthetics work?
Block sodium channels in a nerve cell membranes → signal conduction is reduced or prevented
The pain ladder
