Volume and Flow Measurement Flashcards
What is a Benedict Roth spirometer
A light bell moves with the patient’s breathing. This movement is recorded by a pen attached to the moving bell via a connecting wire over two pulleys. The bell is sealed to prevent leakage of gas via a water seal. The water seal is low volume to reduce amount of gas lost by dissolving into the liquid.
What is a Vitalograph
Bellows are used to measure gas volume
The top plate of bellows is pivoted and its motion transferred to a scriber which records volume changes on a chart. The chart is driven by a motor at a set rate to allow for Volume - Time graph plots.
Summarise the principle of action of a dry gas meter
Two compartments A and B. Gas into B via inlet –> compresses bellows and simultaneously empties compartment A. The bellows move a lever transmitted to a gauge to measure the volume. The bellows also move a switch which now closes compartment B and opens compartment A. Inflowing gas no fills compartment A and the process recurs reverting back to compartment B. The volumes are recorded via gears to a gauge.
How does a Wright Respirometer work
The continuous rotation of a vane is monitored as it is moved by a continuous flow of gas through slits which elicit circular motion of the gas. This movement is transmitted via a set of gears to a gauge. The vane does not rotate when the flow is reversed
What are the disadvantages of a Wright Respirometer
- Measures tidal volume NOT continuous flow
2. No electrical output for analysis and recording
What is an electronic volume monitor
Vane mounted on jewelled bearings within the airflow. To create a spiral movement of air (opposed to circular in Wright Respirometer) there are six angled vanes on either side of the vane. The vane moves either clockwise or anticlockwise as the patient breathes in and out. The vanes movement is monitored by two infrared beams. An electronic processor analyses the signal to indicate tidal volume and minute volume.
What are the pro’s and con’s of the electronic volume monitor
Pros
- More accurate measurements as drag on gears is eliminated
- Allows continuous flow monitoring
- Electrical analysis
Cons
- Less portable
- More expensive
- Requires power supply
How can red cell volume and plasma volume be measured.
How is blood volume calculated?
Using dilution techniques with radioactive labels
Concentration of labelled cells = Dose labelled cells
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Red cell volume
Red cell volume = Dose labelled cells
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Concentration of labelled cells
Same using labelled Albumin to calculate plasma volume
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If haematocrit is known
Blood volume = Red cell volume
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Haematocrit
Blood volume = Plasma volume
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1 - Haematocrit
Under which circumstances are dilution techniques to calculate Red cell volume, plasma volume and blood volume inaccurate?
Shocked patients –> pooling and stasis (inadequate mixing)
Polycythaemia –> sample Hct might differ from whole body Hct. Therefore: Plasma and RCV would have to be measured to calculate blood volume as HCT inaccurate.
What is a rotameter and how does it work
Variable orifice flowmeter is a bobbin supported in the middle of a tapered glass/plastic tube by the gas flow and as the flow increases, the bobbin rises in the tube and the clearance around the bobbin in the tube increases. There is a variable orifice around the bobbin which depends on gas flow.
When is sticking of the boobin to the walls more likely to occur
When the bobbin is near the bottom of the tube
When there is an accumulation of electrostatic charge and no conductive strip or coating on the inner surface of the tube to disperse these charges
What is the nature of the valve at the base of a variable orifice rotameter
Needle valve with a spindle attached to a control knob which screws into the inlet to turn off the gas inlet.
How is leakage from around the spindle of the needle valve of a variable orifice rotameter prevented
Gland and gland nut
What is present to prevent particulate matter from entering the variable orifice rotameter
there is a filter made from sintered metal at the base of the tube near the needle valve
Why are flowmeters not affected by addition by the anaesthetist of apparatus that increases downstream pressure (e.g. vaporizers, breathing circuit valves)
The needle valve mechanism and wall gas supply pressures allow for gas flows around the needle valve to approach the speed of sound and achieves a constant value. Consequently the flow is only affected by the area of the channel and unaffected by small changes in up stream pressure.
What is the problem related to a leak in the central variable orifice flowmeter with the universally accepted layout of the rotameters? What are the solutions?
A leak in the central rotameter will lead to more O2 leak out than N2O potentially resulting in hypoxic gas delivered to patients.
- Re-arrange the order of flow meters so that O2 is closest to patient (not acceptable as universal order)
- Create an O2 specific channel to prevent the affect of leakage from the other rotameters.
How can a safety mechanism preventing less than 25% O2 solution being delivered to the patient?
Chain that links the O2 and N2O controls. With this system
1. Opening N2O controls also opens O2
Apart from the variable orifice flowmeter, what instrument uses a similar variable orifice principle. Explain how this device works
The Wright peak flowmeter.
Patient maximally exhales into the device
Airflow pushes a movable vane which rotates and opens a circular slot around the base of the instrument and so allows the gas to escape.
rotation is opposed by a coiled spring and a pointer mounted on an axis of the vane registers registers its movement on a calibrated dial.
When the maximum flow is reached a ratchet prevents the vane from returning to the start and the peak flow is recorded.
A zero button reset the device
What is an alternative to the Wright peak flowmeter?
Peak flowmeter - has a cylindrical shape and works on a similar principle. No spring or ratchet required.
What is a pneumotachograph and how does this work.
It is a fixed resistance, variable pressure, flow sensor.
Gauze screen with large enough diameter to maintain laminar flow. Gauze screen acts as constant resistance to flow. With air flow there is a small drop in pressure across the gauze screen. P change measured by a transducer which converts the pressure change to an electrical signal which can be displayed and recorded. Laminar flow allows for the Hagan-Poiseuille equation to be used to calculate the flow.
Considering the HP formula, changes in viscosity due to water condensation on gauze and addition of anaesthetic vapurs alter viscosity and may alter the accuracy of this device
What affects the accuracy of a pneumotachograph and why? What are soutions to the problem
- Laminar flow depends on fluid viscosity - changes in the composition or temperature of the gas affect its accuracy e.g. the addition of anaesthetic gases can affect calibration.
- Water condensation –> wetting gauze and changing resistance
Solution:
Heating element to maintain constant temperature and prevent H2O condensation.
How can the pneumotachograph also be used to measure gas volumes?
Integrating the flow - time curve generated by the device electronically
What are Pitot Tubes?
This is an alternative form of pneumotachograph.
Two tubes facing opposite directions within the breathing system are connected to a pressure transducer outside of the breathing system.
The kinetic energy in the breathing system (proportional to the square of fluid velocity) is converted to pressure within the Pitot tube (proportional to potential energy). The pressure differential is converted to an electronic signal similar to the gauze pneumotachograph and displayed and recorded.
What is the electronic mass flowmeter and how does it work?
Electric current flows into a thermistor. The resistance of a thermistor varies with temperature. The current heats the thermistor and the gas flowing being measured carries the heat away. To maintain the thermistor at a constant temperature, more current must be applied. The current required is a measure of the gas flow.
However, the heat dissipation is also a product of the temperature of the gas being measured. A second thermistor is therefore used prior to the gas flow thermistor to measure the temperature of the gas .
