Inhalational anaesthetic agents Flashcards
(39 cards)
What is the difference between a vapour and a gas
A vapour is a substance that can exist as a liquid at room temperature (i.e. room temperature is below its critical temperature). A substance whose critical temperature is below room temperature cannot exist as a liquid at room temperature regardless of the amount of pressure exerted and is considered a gas.
What does the dial indicator of ‘1%’ mean
The vapour occupies 1% of the outflow from the vaporizer
Define MAC
This is the mean alveolar concentration of the volatile agent required to prevent 50% of patients moving when subjected to a standard midline incision
What determines potency: partial pressure of VA administered or concentration of VA delivered?
Partial pressure of volatile agent delivered
Why is partial pressure of VA delivered independent of atmospheric pressure
If 1% is dialled into the vaporizer then 1% of isoflurane is delivered at 20ºC and 100kPa (SVP of isoflurane is 32kPa)
Inside the vaporizer:
32kPa/100kPa x 100/1 = 32%
1/100 x 100kPa/1 = 1kPa
If 1% is dialled into the vaporizer then 0.5% of isoflurane is delivered at 20ºC and 200kPa (SVP of isoflurane is 32kPa)
Inside the vaporizer:
32kPa/200kPa x 100/1 = 16%
0.5/100 x 200kPa/1 = 1kPa
As vaporizers are calibrated in % and because one atmosphere is approximately 100kPa, there is a simple relationship between % and partial pressure of the agent: 1% = 1kPa. How is it possible for N2O to have a MAC of 103 (above 101.325 =atm pressure)
This means that the MAC of N2O cannot be achieved at atmospheric pressure.
The MAC values are quoted in terms of % concentration but their potency is actually determined by PARTIAL PRESSURE rather than % concentration.
Partial pressure is independent of atmospheric pressure. In a hyperbaric chamber a MAC of 103kPa can be obtained because the atmospheric pressure is increased e.g. 150 kPa
103kPa/150kPa = 69%
Is a concentration of 69% of N2O is delivered to a patient at Pb of 150 –> MAC of 103 kPa is obtained.
How do vaporizer adjust for changes in altitude?
They have mechanisms which ensure the delivery of a set partial pressure of anaesthetic agent (not concentration as displayed on the dials. The percentage of agent delivered will vary with altitude but the Partial pressure and the MAC remain unchanged
How are MAC values calculated
In volunteers just breathing anaesthetic agent n oxygen. (No other gases present)
How does the MAC change when N2O is added?
The MACs are additive e.g. half MAC N2O + half MAC Isoflurane = 1 MAC. (same calculation is releavant for sevo and iso together but this is NOT done in practice
What factors reduce MAC
IV anaesthetics used by infusion N2O Adjuvant medications (benzodiazepines, opioids, a2-adrenergic agonists) Acute alcohol intoxication Chronic amphetamine use (depleted CATS)
Reduced GCS
Hypothermia
Hypothyroidsim
Increasing age
What factors increase MAC
Chronic alcohol dependence
Exogenous catecholamine use and stimulus
Anxiety and stress
Hyperthyroidism
Hyperthermia
Young age
What effects the speed of onset of anaesthesia when using a volatile agent for induction?
How quickly FA/Fi approaches 1
How rapidly the alveolar concentration equals the inspired concentration of volatile anaesthetic
What is overpressure with regard to inhalation induction
Overpressure involves setting the initial concentration on the vaporizer above that needed for maintenance during induction and then, over 5 minutes or so, reducing it towards a maintenance value of approximately 1 MAC.
Incremental increases over a minute or so will minimize airway irritation (also airway irritation much less with sevoflurane)
Why do we use an intravenous induction in adults?
To avoid the unpleasantness of breathing a volatile –> do not delay initiating the volatile
What are the main reasons why N2O is used?
It is used as a carrier gas as it reduces the MAC required for a volatile agent and has useful analgaesic action
Explain the second gas effect
N2O has a very low B:G partition co-efficient (0.47)
Nitrogen has a relatively high B:G part. coeff. (0.015)
The B:G partition co-efficient for N2O is 30 x GREATER than that of Nitrogen.
Rate of diffusion = ∆P x SA x Solubility / √MW x L
∆P = partial pressure gradient SA = Surface Area for gas exchange Solubility = B:G partition co-efficient MW = Molecular weight L = Thickness of the diffusion barrier
In comparing N2 to N2O in lungs –> since N2O will replace N2 the ∆P is the same (constant), SA, L - constant (same lungs).
The variables are therefore MW and Solubility of each gas
For N2:
MW = 28 g/mol
B:G = 0.015
Relative Rate of diffusion = 0.015/√28 = 0.0028
For N2O
MW = 44 g/mol
B:G = 0.47
Relative Rate of diffusion = 0.0708 –>THIS IS 25 TIMES FASTER THAN N2.
Define the concentration effect and draw the FA/Fi graph for all the volatile agents and N2O to illustrate this effect
The concentration effect is an observed phenomenon that describes the disproportionate
rate of rise of the alveolar fraction compared with the inspired fraction when high
concentrations of N2O are inspired.
What implications does the difference in B:G coefficients for N2O and N2 have for anaesthetists
- Second gas effect
- Diffusion hypoxia
N2O diffuses 25 x faster than N2. When an anaesthetic agent is co-administered with N2O the effect of rapid diffusion of N2O out of the alveoli with relative slow diffusion of Nitrogen into the alveoli leads to a pressure gradient between the alveoli and the adjacent respiratory bronciles and other airways bringing more alveolar gas into the alveoli –> the effect is concentration of both the N2O and the volatile agent and increased FA of both and the increase concentration of the latter is the outcome of the second gas effect.
Diffusion hypoxia occurs because, during emergence in a patient in whom N2O was administered N2O leaving the blood and moving into the alveoli occurs at a faster rate than Nitrogen which is leaving the alveoli and entering the blood –> this has the effect of diluting the gases in the alveoli.
What effects do all volatile agents have on RR and Vt
Increase RR and decrease VT
As tidal ventilation decreases dead space remains unchanged and hence alveolar ventilation decreases and may become inadequate with high concentrations of volatile.
What effect does N2O have on minute ventilation
Minimal
Briefly describe the technique for inhaled induction with sevoflurane
Minute 1: incremental dial up of sevo to 8 % starting with 0% but arriving at 8% by one minute
When patient is almost asleep (at about 2 mins) dial down to 3% to MAINTAIN TIDAL VENTILATION. If 8 % is continued –> Vt could drop to 40 ml and RR up to 22 but this means VA = 0
Which volatile agents cause decreased SVR and reduced myocardial contractility
All volatile agents
Isoflurane
- drops SVR slightly more than sevoflurane + accompanied by a reflex tachycardia
- Also has been associated with coronary steal
Sevoflurane
- less pronounced drop in SVR, BP and HR better preserved
What effect does N2O have on the CVS
Minimal reduction in cardiac contractility - usually offset by increased SNS activity
What are the implications related to the vasodilation of coronary vessels caused by volatile anaesthetic agents
Volatile anaesthetic associated coronary vasodilation may reduce perfusion in coronary vessels distant to stenosed regions of these vessels precipitating ischaemia.
