FRCA- Clinical Measurement MCQ: Part 2 Flashcards
What voltage do nerve stimulators require? What current do they commonly produce?
Nerve stimulators must produce voltages high enough to overcome skin resistance and may reach 150 V (or more), typically square wave signals are used, pulse currents need to be high enough to produce a twitch so currents of 20-80 mA are common.
What must remain constant to produce twitches with a nerve stimulator?
At a constant voltage an increase in resistance will reduce the current flow and typically constant current stimulation is used to produce reproducible twitch strengths.
What frequency, and length of time is required to produce a train-of-four?
It consists of four supramaximal twitches at 0.5 sec intervals (2 Hz).
Why is the facial nerve used for monitoring?
The facial nerve is often used for monitoring because it is convenient.
Its response to muscle relaxants is similar to that of the diaphragm although it tends to be obscured by direct muscle stimulation.
How can you use this to indicate a patient is adequately reversed for extubation?
Even if you get all four twitches there may still be significant fade, indicating inadequate reversal.
TOF should be >0.7 adequate reversal.
Recent evidence suggest that 0.9 is required.
What is the Double Burst Stimulation?
DBS consists fo two bursts of tetanus at 50 Hz.
Each impulse is therefore separated by 20 msecs (50 impulses in 1 second).
The two bursts are 750 msecs apart.
The DBS ratio has similar properties to the TOF ratio but tactile evaluation of the DBS ratio has been shown to be more accurate than that for the TOF ratio.
Profound block is best assessed by PTC.
In a rotameter, what Laws become relevant at low flow and high flow?
Low flow - With a longer and narrower constriction flow is a function of the viscosity of the gas (Poiseuille’s law).
High flow - When the constriction is shorter and wider (at high flow), flow depends on the density of the gas (Graham’s law).
How does a hyperbaric chamber affect a flowmeter reading?
In a hyperbaric chamber, a flowmeter will deliver less gas than the setting indicate.
Flowmeters are calibrated at atmospheric pressure. Pressure changes will affect both the viscosity and the density of a gas and so influence the accuracy of the indicated flow rate.
What is the accuracy of a flowmeter?
accurate to within 2%
What is a Renault’s Hygrometer used for?
The Regnault’s hygrometer = RELATIVE humidity
and relies on air bubbled through ether that is contained within a silver-coated glass tube.
How is absolute humidity calculated?
Absolute humidity is the MASS of water vapour in unit volume of air at specified pressure & temperature or the mass concentration (kg/m3).
How is relative humidity calculated?
Relative humidity is the partial pressure of water vapour divided by the saturated vapour pressure of water at specified temperature, i.e. VP/SVP.
Is absolute humidity dependant on temperature?
No, it is independent of temperature.
