Misc Flashcards
What is the difference between an inotrope and an ionotrope?
An inotropic agent is a substance that alters the strength of contraction of the heart muscle e.g. digoxin increases inotropy, B-blockers decrease it
An ionotrope is a type of cell surface receptor, which opens/closes ion channels on the cell membrane in response to the binding of a specific ligand. This can lead to depolarisation or hyperpolarisation of the cell, depending on the specific ligand/ion channel combination e.g. ACh binds to ACh receptor, Na moves into cell, depolarisation occurs
What effect do B-blockers have on the hypoglycaemic response, and why? Where is this important?
Hypoglycaemia induces an acute stress/sympathetic response: tachycardia, tremors, sweating.
B-blockers suppress sympathetic responses by preventing norepinephrine from acting at B-receptors at effector sites (where post-ganglionic neurons synapse with viscera)
The exception to the rule is sweat glands, where the post-ganglionic neuron signals to the glands using ACh.
Hence, B-blockers “mask” all of the signs of hypoglycaemia except for sweating.
This is important in diabetics on treatment who may unknowingly become hypoglycaemic - these patients must be educated and check glucose levels when they experience sweating alone
What is Hofmann elimination? Where is this important?
Hofmann elimination is the spontaneous breakdown of molecules when subjected to normal body pH and temperature.
This is the major way that atracurium and cis-atracurium are broken down in the human body, although ester hydrolysis also plays a (small) role
These molecules are metabolised to laudanosine, a molecule which has a moderate CNS stimulant effect and lowers the seizure threshold, which is excreted by the kidneys
Which factors affect Hofmann elimination and how?
Do affect:
- pH: increased pH = increased elimination
- Temperature: decreased temp = decreased elimination
Don’t affect:
- Age
- Obesity
- Renal function
- Liver function
- ?Plasma esterase activity
What is the Fink effect/diffusion hypoxia?
Fink effect/diffusion anoxia/diffusion hypoxia/third gas effect:
When soluble gases (e.g. N2O) are breathed in in large quantities, they can be dissolved in body fluids rapidly. This can lead to a temporary increase in the concentration of O2 and CO2 in the alveolus, increasing their respective partial pressures (when inhaling N2O) - this is the 2nd gas effect, the 3rd gas effect is the opposite of this:
When this gas is exhaled at the termination of anaesthesia, the converse effect occurs, where large quantities of N2O enter the alveolus from the bloodstream, resulting in a temporary decrease in O2 and CO2 by dilution in N2O. This can lead to a decrease in their partial pressures which can lead to hypoxaemia –> hypoxia (low O2) + suppression of respiratory drive (low CO2)
How is diffusion hypoxia averted in anaesthetic care of a patient?
Increasing ventilator FiO2 when recovering from N2O anaesthesia (immediately after turning off N2O)
What is Henry’s law? Where does this apply in anaesthetics?
The amount of gas dissolved in a liquid is proportional to its partial pressure in its gaseous phase
Applications:
- Volatile anaesthetics: PP of AA in blood is proportional to the PP of AA in the alveoli, so by increasing inspired amount of AA, can increase amount in blood and increase effect
- Hyperbaric O2 therapy: can increase O2 delivery to tissues independent of other factors (e.g. Hb) by increasing pressure to 3 atm and FiO2 to 100%
What is the partial pressure of a gas?
The pressure which a gas in a mixture would exert if it occupied the volume of the mixture by itself
Which factors affect the partial pressure of a gas in blood?
- The saturated vapour pressure of the specific gas
- The concentration of the gas in the alveoli
- The ambient temperature
- NOT the ambient pressure –> vaporiser settings don’t need to be altered at different altitudes
What is saturated vapour pressure?
The pressure at which a gas becomes so saturated that it cannot hold any more particles, and it condenses back into liquid form. The saturation vapour pressure is thus the measurement of the vapour pressure when the vapour is in equilibrium with the liquid phase
This is unique to the substance, and is dependent on temperature (increased temp = increased SVP)
What is vapour pressure?
The pressure of a vapour above the surface of a liquid
What is the effect does vapour pressure have on a gas/liquid?
The equilibrium vapour pressure of a substance is an indication of its evaporation rate
A substance with a high vapour pressure at normal temperatures = a volatile –> evaporate easily
What is equilibrium vapour pressure?
The pressure exerted by a vapour when in equilibrium with it’s condensed phases at a given temperature in a closed system
What is Graham’s law of diffusion?
The rate of diffusion of a gas is inversely proportional to the square root of its molecular weight, thus, the larger the molecule, the slower it will diffuse across a membrane
What is the anaesthetic/clinical application of Graham’s law?
The second gas effect:
The effective concentration of a gas in the alveoli, by rapid uptake of smaller molecules, thus concentrating larger molecules left behind in the alveoli. This facilitates uptake of the second, larger, molecule. Diffusion hypoxia is the reverse effect
Gas induction:
If N2O is added to the inhaled volatile, the alveolar concentration of the volatile is increased (and thus its PP), thereby accelerating its uptake
