Clinical Measurement Flashcards
What is “Current”
- Defined as the flow of charge through a medium → typically carried by moving electrons
in a conductor (Eg. wire) - SI unit is “Ampere” (A) → measured as the charge flowing through a point in a circuit
per unit time → Coulombs/second - This can be – (i) Unidirectional (DC) or (ii) Bidirectional (AC)
What is Potential difference
Potential difference:
- Defined as the difference in electric potential energy per unit charge between 2 points →
when applied across a conductor (Eg. wire), it produces an electrical current!
- SI unit is “Volts” (V) → measured as the work done by an external force to move a unit
of charge against an electric field → Joules/coulomb
Resistance: definition, units, what its proportional/inversly proportional to and how its measured in series or parallel.
Resistance:
- Defined as the opposition to the passage of an electrical current within a circuit → as per
Ohm’s Law, where V = IR such that R = V/I (assuming a DC circuit)
- SI unit is “Ohm” (Ω) → measured as potential difference required to generate a unit of
current → Volts/ampere
- The resistance of an object (assuming uniform cross-section) is:
o (i) proportional to resistivity (ρ) and length (l)
R =Lρ/A
o (ii) inversely proportional to its cross-sectional area (A)
- Resistance in series (R = R1 + R2 + R3 +…) vs. parallel (1/R = 1/R1 + 1/R2 + 1/R3 +…)
What is impedance
in “Ohm” (Ω)
- It is the sum of – (i) resistance (resistor), (ii) capacitive reactance (capacitor), and (iii)
inductive reactance (inductor or coils)
- As frequency of AC current ↑, this has effects on impedance of certain components of
the circuit:
o Capacitor – Impedance ↓ exponentially
o Resistor – Impedance remains unchanged o Inductor – Impedance ↑ linearly
Capacitance: define, Unit, what its dependent on?
- Define: Ability of an object to store charge for electrical energy potential
○ C = Q/V- SI is “Farad”: 1F is ability to store 1 C of charge for applied potential difference of 1V
- Dependent on:
○ Size of conducting plates
○ Separation of conducting plates
○ Material used - Energy stored by a capacity is E = 1⁄2 CV2
So an increase in potential difference means there must be an increase in energy to store the same amount of charge.
What is Boyles Law
Boyles Law:
At a constant temperature, volume of a given mass varies inversely proportional to pressure
- V α 1/P → so PV = K
(where K is rate constant)
Or P1V1 = P2V2
What is Charles Law
At constant pressure, the volume of a given mass is directly proportional to its absolute temperature.
- V α T → so V/T = K
“If you heat Charles up he gets bigger, if you cool him down he gets colder”
What is the 3rd Gas Law / Gay-Lussac Law
At constant volume, absolute pressure of given mass varies directly with temperate-
- P α T → so P/T = K
Heat it up, pressure increases (in fixed volume)
What is the equipment used in an invasive arterial monitor
A catheter (inserted via needle, wire via seldinger technique).
non-compressible tubing
counter pressure fluid
Transducer
Monitor
What are the Principles regarding invasive arterial blood pressure
Pressure wave is transmitted via a fluid column to a transducer
Change in pressure are converted by a Wheatstone bridge transducer to changes in resistance
The change in current is displayed as a graph
The sensor is calibrated to a known range of pressures. Allowing the graph to be displayed as pressure change over time.
What are the practical advantages and disadvantages of invasive arterial monitoring
Advantages:
- “Gold standard”
- continuous blood pressure over time. Can allow for PPV
- availability to sample upstream blood
- the wave form can be a source of information.
- adjustable transducer height allowing for different pressure readings at different heights
Disadvantages:
- Requires arterial puncture
- Single use
- requires re-zeroing and re-levelling
- transducer height can vary and give unreliable readings
- requires training
- requires monitoring to be attached
- more expensive
What are the sources of error for invasive arterial pressure monitoring
Position/level of transducer (malposition/mislevelling)
Dampening: effected by:
- length of tubing
- air-bubbles
- clots
- pressure in counter pressure fluid bag
position of catheter in vascular tree
Transducer malfunction or miscalibration
inaccurate ‘zero’ reading
How does a Transducer work
Transducer changes one form of energy into another
When a Piezoresistive strain gauge sensor (which nis a thin semi-conductive membrane) is deformed by change in pressure, there is a corresponding change in resistance to the circuit.
Wheatstone bridge is a circuit with four resistors. R1, R2, R3 are constant and R4 is a semiconductive membrane. between them is a galvanometer that measures current. If R1/R2 = R3/R4 then there is no current through the galvanometer. Therefore pressure which corresponds to resistance in R4 also corresponds to current.
R2 is variable which allows the current to be zeroed
