Physics MCQ's 4 Flashcards
Infra red measures which of the following gas and vapours and why?
Nitrous oxide Halothane Carbon dioxide Oxygen Isoflourane
All except oxygen because only inter-atomic bonds of dissimilar atoms of molecules absorb infra-red light. Therefore oxygen does not, neither does helium or nitrogen.
What is paramagnetism?
Is attraction to a magnetic field and is due to the presence of unpaired electrons in the outer shell.
What is a paramagnetic analyser used for and how does it work?
Newer analysers have two chambers separated by
a sensitive pressure transducer. The sample gas is
delivered to one chamber and room air is delivered
to the reference chamber.
An electromagnet is rapidly switched on and off creating a changing magnetic field
to which the sample gas is subjected. The magnetic
field causes the oxygen molecules to be attracted and
agitated.
This results in changes in pressure on either
side of the pressure transducer. The pressure difference
across the transducer is proportional to the oxygen
partial pressure difference between the sample gas and
the reference gas (room air, containing 21% oxygen).
What gas fills the two glass spheres of the paramagnetic analysers?
Does water vapour affect O2 measurement in the paramagnetic analyser?
Nitrogen- which is then displaced out of the magnetic field when surrounded by oxygen.
Yes.
Movement of the dumb bell in the null deflection analyser?
An opposing magnet field prevents movement of the dumbell.
How is the rotation of the dumbbell in the paramagnetic analyser balanced?
Balanced by torsion of the suspending filament.
Regarding end tidal pCO2:
- How will infra-red measurement be affected by NO2 presence?
CO2 and NO2 absorbed at a similar wavelengths and therefore in the presence of both gases mixing causes ‘collision broadening’.
Ie. Broadening the absorption spectrum for both. Over estimation of both.
Regarding end tidal pCO2:
- How will mass spectometry measurement be affected by NO2 presence?
The molecular mass of CO2 and NO2 are identical - 44.
Therefore mass spec cannot separated the gases easily.
Regarding end tidal pCO2:
The presence of water in mass spec may under/ over estimate?
Because mass of water is 18, should not interfere.
Regarding end tidal pCO2:
During IPPV will be more accurate estimate of arterial PaCO2 if PEEP is applied?
False, PEEP increases the alveolar dead space which only reduced the alveolar pCO2.
How is the Fick principle used to measure cardiac output?
It is calculated by dividing the oxygen consumption, in ml/min (250 normally)
by A-V difference, in ml per litre of blood (50ml.L).
Which of these could lead to a severely damped radial artery trace?
- Malfunction flushing system
- Bubble in connecting tube
- More than one stopcock included in connecting tube
- Use of 20G cannula
- length exceeding 120cm
Only first two.
Ideally the tubing from transducer to cannula should be short and stiff l, but a 120cm will not significantly affect quality of reading.
20G is normally used.
Using the Fick Principle to measure cardiac output, using oxygen as the indicator gas, is essential to know?
- Oxygen uptake
- Arterial oxygen content
- Respiratory quotient
- Mixed venous oxygen content
- Arterial carbon dioxide content
Oxygen uptake, arterial oxygen content, mixed venous oxygen content.
The Fick Principle states that the amount of a substance passing per unit time in a flow is equal to the product of the input and output concentration difference of the substance and the flow rate.
Equations for calculation:
- Pressure =
Work/ volume
Force =
Mass x acceleration
