Anaesthetics: Pharmacology - General anaesthesia Flashcards
What are the 6 A’s of anaesthesia?
- Anaesthesia (loss of consciousness)
- Analgesia
- Amnesia
- Areflexia (loss of autonomic and sensory reflexes)
- Akinesia (skeletal muscle relaxation)
- Anxiolysis
Compare and contrast volatile vs inhaled anaesthetics in terms of vapour pressure, boiling point, and state at room temperature
Volatile: low vapour pressure, high boiling point, liquid at room temp
Inhaled: high vapour pressure, low boiling point, gas at room temp
What are the stages of anaesthesia (Guedel’s signs)?
- Analgesia: with amnesia in later part of stage 1
- Excitement: increased RR (also irregular), HR and BP; delirious, may vocalise but completely amnesic
- Surgical anaesthesia: begins with decreased RR and HR, eventuates in apnoea; four planes described based on changes in ocular movements, eye reflexes, and pupil size
- Medullary depression: severe CNS depression (including medullary vasomotor centre and brainstem respiratory centre)
What are the most reliable signs of reaching stage III of anaesthesia?
Loss of eyelash reflex
Establishment of regular respiratory pattern
Give five examples of volatile anaesthetics
Halothane
Enflurane
Isoflurane
Desflurane
Sevoflurane
Give two examples of gaseous inhaled anaesthetics
Nitrous oxide
Xenon
Describe the drugs typically used for monitor anaesthetic care
Midazolam for anxiolysis, amnesia and mild sedation
Followed by propofol for moderate-to-deep sedation
+/- potent opioid or ketamine for analgesia
Describe the principles of conscious sedation: what drugs are used, and what state is achieved?
Uses smaller doses of sedative medications (e.g. diazepam, midazolam, propofol)
Anxiolysis and analgesia with less alteration of consciousness
Maintains airway and responsiveness to verbal commands
What are the four factors influencing uptake and distribution of inhaled anaesthetics?
- Inspired concentration and ventilation
- Solubility
- Cardiac output (pulmonary blood flow)
- Alveolar-venous partial pressure difference
How does inspired drug concentration and ventilation influence uptake of inhaled anaesthetic? How can these factors be manipulated to increase uptake?
Higher inspired anaesthetic concentration = higher rate of transfer into blood (Fick’s law; steeper gradient between inspired and alveolar partial pressure)
Increasing partial pressure of inhaled anaesthetic increases uptake
Increasing ventilation (i.e. increased RR, TV) increases uptake
How does solubility of an inhaled anaesthetic agent affect its uptake?
Increased solubility increases the time required for F(A)/F(I) to reach equilibrium (therefore slower onset of induction)
How is solubility of an inhaled anaesthetic agent quantified?
Quantified by the blood:gas partition coefficient (measure of the relative affinity of an anaesthetic
for blood, compared to air)
Higher coefficient, higher solubility
What is F(A)/F(I) and what is its significance?
Alveolar concentration / inspired concentration
The faster it approaches 1 (i.e. inspired-to-alveolar equilibrium), the faster anaesthesia onset during inhaled induction
What is MAC?
Minimal alveolar concentration: anaesthetic concentration that produces immobility in 50% of patients exposed to a noxious stimulus
What is partial pressure? Give a worked example
Fraction of a gas mixture that a particular component comprises
E.g. mixture of 70% nitrous oxide, 29% oxygen, 1% isoflurane at barometric pressure (760mmHg) = 532mmHg nitrous oxide, 220mmHg oxygen, 7.6mmHg isoflurane
Give two examples of inhaled anaesthetics with low solubility and one with high solubility. What are their respective blood:gas partition coefficients?
Low solubility: desflurane, nitrous oxide (B:GPC < 0.5)
Higher solubility: halothane (B:GPC > 2)
How does cardiac output affect uptake of inhaled anaesthetic agents?
Increased CO increases pulmonary blood flow
Increased pulmonary blood flow increases blood volume exposed to gas and thereby slows rate of rise in F(A)/F(I), slowing induction
How does alveolar-venous partial pressure difference affect uptake of inhaled anaesthetic agents?
Partial pressure difference between alveolar and mixed venous blood is dependent on anaesthetic uptake by tissues (including nonneural)
Increased uptake by nonneural tissues increases the concentration gradient of returning venous blood
Increased concentration gradient increases amount of time to reach equilibrium with brain tissue
This is mainly influenced by highly perfused organs (brain, heart, liver, kidneys, splanchnic bed)
What three factors influence transfer of inhaled anaesthetic agent to various tissues?
- Solubility
- Concentration gradient between blood and respective tissue
- Tissue blood flow
What are the two main differences between factors affecting uptake and elimination?
Elimination follows some of the same principles in reverse as uptake, with two main differences:
- Transfer of anaesthetic from lungs to blood can be enhanced by increasing inspired concentration, but reverse process cannot be enhanced (concentration in lungs can’t be reduced below zero)
- Anaesthetic gas tension in different tissues may be variable depending on agent and duration of anaesthesia (whereas at induction, initial concentrations are zero)
How does solubility influence elimination of inhaled anaesthetic?
Decreased solubility increases speed of washout
How does duration of anaesthetic exposure influence speed of emergence?
Prolonged exposure causes accumulation of anaesthetic in muscle, skin and fat
Anaesthetic slowly eliminated from these tissues, so speed of emergence is slower
What determines time to recovery from inhaled anaesthetics?
Rate of elimination of anaesthetic from brain
What impact does ventilation have on recovery from inhaled anaesthetics?
Increased speed of recovery with hyperventilation
What impact does drug metabolism have on recovery from inhaled anaesthetics?
Elimination of modern inhaled agents is mainly by ventilation, and metabolism typically plays no significant role (but may play a role in toxicity)
In older inhaled agents (e.g. halothane), hepatic metabolism may play a role (particularly with prolonged duration of anaesthesia: significant hepatic metabolism during anaesthesia results in faster-than-expected washout)
What is minimum alveolar concentration?
Median alveolar concentration that results in immobility in 50% of patients when exposed to a noxious stimulus (e.g. surgical incision)
Measure of potency and represents ED50 on a quantal dose-response curve
At what MAC is immobility vs amnesia achieved?
Immobility at 1.0 MAC
Amnesia at 0.2-0.4 MAC
Three factors that decrease MAC
- Elderly
- Hypothermia
- Adjuvant agents (e.g. opioids, sympatholytics, sedative-hypnotics)
Two factors that increase MAC
- Pregnancy
- Chronic use of centrally active drugs
- EtOH abuse
What is the MAC of N2O and what is the significance of this?
MAC >100%
Means that even at 100% alveolar concentration, it does not produce full anaesthesia in 50% of patients and another agent must be used
List inhaled anaesthetic agents in order of degree of hepatic metabolism (from most to least)
HE SaID NO:
Halothane
Enflurane
Sevoflurane
Isoflurane
Desflurane
Nitrous oxide
Four CNS effects of inhaled anaesthetics
- Decreased cerebral metabolic rate
- Changes in cerebral blood flow:
- Decreased CMR causes decreased cerebral blood flow, but some volatile anaesthetics have a direct vasodilatory effect and increase cerebral blood flow
- At MAC 0.5, reduction in CMR > vasodilation, so cerebral blood flow decreases
- At MAC 1.5, vasodilation > reduction in CMR, so cerebral blood flow increases
- At MAC 1.0, vasodilation = reduction in CMR, so cerebral blood flow is unchanged - EEG changes
- Activation at lower doses, slowing at 1.0-1.5 MAC, suppression at 2.0-2.5 MAC
- Isolated epileptiform patterns at 1.0-2.0 MAC (frank seizure activity only seen with enflurane) - Sedation
What is the importance of changes in cerebral blood flow caused by inhaled anaesthetics?
Important to consider in patients with or at risk of raised ICP (at higher MAC, cerebral blood flow and therefore ICP is increased)
Effect is smallest with N2O
Four CVS effects of inhaled anaesthetics (including specific effect of halothane)
- Decreased MAP (proportional to alveolar concentration)
- Changes in HR:
- Reflex tachycardia: desflurane, isoflurane
- Bradycardia (via attenuation of baroreceptor response): halothane, enflurane, sevoflurane - Decreased myocardial O2 demand and increased coronary blood flow
- Halothane sensitises myocardium to circulating catecholamines
Which inhaled anaesthetic agents cause a decrease in MAP by decreasing CO, and which cause a decreased in MAP by reducing TPR?
Decreased CO: halothane, enflurane
Decreased TPR: isoflurane, desflurane, sevoflurane
What is the clinical relevance of choosing an inhaled anaesthetic agent that reduces MAP by reducing TPR vs CO?
Agents that reduced MAP by reducing TPR (e.g. isoflurane, desflurane, sevoflurane) are better for patients with impaired heart function, as they preserve CO whilst decreasing preload and afterload
What is the effect of N2O on CO?
Reduces CO but increased sympathetic stimulation, with net effect of preservation of CO
What is the clinical significance of halothane’s effect on myocardium?
Sensitises myocardium to circulating catecholamines, increasing the risk of ventricular arrhythmias if sympathomimetics given or in setting of increased endogenous catecholamines (e.g. stress, inadequate analgesia, phaeochromocytoma)
Five respiratory effects of inhaled anaesthetics
- Bronchodilation
- Airway irritation and bronchospasm
- Dose-dependant reduction in TV and increase in RR (with exception of N2O): produces overall decrease in ventilation and increase in PaCO2
- Increased apnoeic threshold and decreased response to hypoxia
- Decreased mucociliary response: mucus plugging, atelectasis, increased risk of hypoxaemia and LRTI
Which two inhaled anaesthetic agents are the drug of choice in airways disease?
Halothane
Sevoflurane