Inhalational anaesthetics Flashcards
Define MAC
The minimum alveolar concentration of an inhalational anaesthetic that prevents reaction to a standard surgical stimulus in 50% of subjects at 1 atmosphere (equivalent to ED50)
A measure of inhalational potency
List the factors that increase MAC (8)
- Infancy
- Hyperthermia
- Hyperthyroid
- Catecholamines/Sympathomimetics
- Chronic opioid use
- Chronic EtOH
- Acute amphetamine use
- Hypernatraemia
List the factors that decrease MAC (11)
- Increasing age
- Pregnancy
- Hypotension
- Hypothermia
- Hypothyroid
- a2-agonists
- Sedatives
- Acute opioid use
- Acute EtOH
- Chronic amphetamine use
- Lithium
List the factors that effect delivery of inhaled anaesthetic from machine to alveolus
Inspired partial pressure - faster rise in alveolar partial P, faster induction
Alveolar ventilation (lung wash in)
FRC - large FRC -> dilutes agent -> dec onset (Va:FRC ratio)
Characteristics of the anaesthetic circuit - Fresh gas flow, solubility of the agent in circuit components, volume of the circuit
List the factors that effect uptake of inhaled anaesthetics from alveoli to blood
Blood:gas partition coefficient (solubility)
Cardiac output (greater effect in highly soluble gases)
Alveolar to venous partial pressure difference (tissue uptake)
List the factors that effect transfer of inhaled anaesthetics from arterial blood to brain/tissue
Tissue:blood partition coefficient (measure of fat solubility)
Tissue blood flow
Arterial to tissue partial pressure difference
Define critical temperature
The temperature above which a gas cannot be liquefied regardless of the pressure applied
Define pseudo-critical temperature
The critical temperature of a mixture of gases
For Entonox, it is the temperature at which Oxygen and Nitrous Oxide laminate
Define a vapour
A substance in its gaseous phase at a temperature below its critical temperature
Define a saturated vapour
A vapour which is in equilibrium with its own liquid
As many molecules returning to the liquid as there are escaping
Define saturated vapour pressure
The pressure exerted by the vapour which is in equilibrium with its own liquid
Dependent on temperature - the higher the temperature, the more molecules enter the vapour phase, the higher the SVP
Define boiling point
The temperature at which the vapour pressure of a liquid equals the environmental pressure surrounding the liquid
For all of the commonly used volatiles the boiling point is between 48-58’ (except Desflurane - 23.5)
What is Boyle’s law?
P.V = k at a constant temperature (pressure is inversely proportional to volume)
What is Charle’s law?
V = k.T at a constant pressure (Volume is directly proportional to temp)
What is the third gas law (Gay-Lussac’s law)?
P = k.T at a constant V (Pressure is directly proportional to temperature)
What is Dalton’s law?
In a mixture of gases, the pressure exerted by each gas is the same as that which would be exerted if that gas occupied the container alone. The total pressure of the mixture is the sum of the partial pressures of all the gases present
What is the ideal gas equation?
P.V = n.R.T (n = number of molecules of a gas, R = the universal gas constant)
What is Henry’s law?
At constant temperature the amount of gas dissolved in liquid is directly proportional to its partial pressure
What is the Meyer-Overton hypothesis?
A proposed mechanism of action of general anaesthetics. Relates MAC (potency) and oil/gas partition coefficient (lipid solubility) of various inhalational agents. The more lipid soluble, the more potent
What is the concentration effect?
The rise in PA of a gas (eg N2O) is disproportionately rapid when it is administered in high concentrations. Due to the solubility of N2O being higher than N2. The N2O diffuses out >> N2, dec volume of alveolus -> remaining gas is in higher conc
What is the second gas effect?
The speed of onset of inhalational anaesthetics is increased when they are admininstered with N2O as the carrier gas. Inc uptake of N2O (1st gas) -> inc rate of rise of PA of concurrently administered gas
Outline the process of elimination of inhaled anaesthetics
Elimination is via the lungs. In general the factors that affect uptake also apply to elimination, except elimination also depends on length of administration and the blood:gas solubility of the inhaled anaesthetic - as tissue acts as a reservoir
Outline the CNS effects of inhaled anaesthetics
- Dose dependent inc cerebral blood flow due to inc cerebral vasodilation and dec cerebral vascular resistance (MAC >0.6)
- Inc ICP proportional to inc CBF
- Dec cerebral activity and therefore CMRO2 (except enflurane)
- Uncouples CBF and CMRO2
- Enflurane can cause fast frequency, high voltage EEG changes resembling seizures
Outline the CVS effects of inhaled anaesthetics
- Dec MAP and CO - Iso, Des and Sevo dec SVR with compensatory inc HR (Tachycardia with Sevo only seen at MAC >1.5)
- Halothane - Catecholamine sensitisation and arrythmias
- Isoflurane - Coronary steal -> arteriolar dilation that causes diversion of blood, diseased vessels unable to dilate -> myocardial ischaemia
- Halo, Enf, Iso - QT prolongation
Outline the respiratory effects of inhaled anaesthetics
- Dose dependent resp depression - inc in RR 2’ to CNS stimulation coupled to dec Vt with overall reduction in MV -> dec alveolar ventilation -> inc PaCO2
- Blunt hypercarbic response (more profound in Des and Sevo)
- Blunt ventilatory response to hypoxia normally mediated by carotid bodies
- Dec ability to clear secretions
- Inc risk of laryngospasm
- Dec FRC by 15-20% mediated by dec insp mm tone and upward diaphragm shift
- Dec mucociliary escalator activity
- Suppression hypoxic vasoconstriction response
- Bronchodilation
