Volume and Flow Measurement

Question 1

What is a Benedict Roth spirometer

Answer 1

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.

Question 2

What is a Vitalograph

Answer 2

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.

Question 3

Summarise the principle of action of a dry gas meter

Answer 3

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.

Question 4

How does a Wright Respirometer work

Answer 4

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

Question 5

What are the disadvantages of a Wright Respirometer

Answer 5

1. Measures tidal volume NOT continuous flow

2. No electrical output for analysis and recording

Question 6

What is an electronic volume monitor

Answer 6

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.

Question 7

What are the pro’s and con’s of the electronic volume monitor

Answer 7

Pros

1. More accurate measurements as drag on gears is eliminated
2. Allows continuous flow monitoring
3. Electrical analysis

Cons

1. Less portable
2. More expensive
3. Requires power supply

Question 8

How can red cell volume and plasma volume be measured.

How is blood volume calculated?

Answer 8

Using dilution techniques with radioactive labels

Concentration of labelled cells = Dose labelled cells
_______________
Red cell volume

Red cell volume = Dose labelled cells
_______________
Concentration of labelled cells

Same using labelled Albumin to calculate plasma volume
__________________________________________

If haematocrit is known

Blood volume = Red cell volume
____________
Haematocrit

Blood volume = Plasma volume
______________
1 - Haematocrit

Question 9

Under which circumstances are dilution techniques to calculate Red cell volume, plasma volume and blood volume inaccurate?

Answer 9

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.

Question 10

What is a rotameter and how does it work

Answer 10

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.

Question 11

When is sticking of the boobin to the walls more likely to occur

Answer 11

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

Question 12

What is the nature of the valve at the base of a variable orifice rotameter

Answer 12

Needle valve with a spindle attached to a control knob which screws into the inlet to turn off the gas inlet.

Question 13

How is leakage from around the spindle of the needle valve of a variable orifice rotameter prevented

Answer 13

Gland and gland nut

Question 14

What is present to prevent particulate matter from entering the variable orifice rotameter

Answer 14

there is a filter made from sintered metal at the base of the tube near the needle valve

Question 15

Why are flowmeters not affected by addition by the anaesthetist of apparatus that increases downstream pressure (e.g. vaporizers, breathing circuit valves)

Answer 15

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.

Question 16

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?

Answer 16

A leak in the central rotameter will lead to more O2 leak out than N2O potentially resulting in hypoxic gas delivered to patients.

1. Re-arrange the order of flow meters so that O2 is closest to patient (not acceptable as universal order)
2. Create an O2 specific channel to prevent the affect of leakage from the other rotameters.

Question 17

How can a safety mechanism preventing less than 25% O2 solution being delivered to the patient?

Answer 17

Chain that links the O2 and N2O controls. With this system

1. Opening N2O controls also opens O2

Question 18

Apart from the variable orifice flowmeter, what instrument uses a similar variable orifice principle. Explain how this device works

Answer 18

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

Question 19

What is an alternative to the Wright peak flowmeter?

Answer 19

Peak flowmeter - has a cylindrical shape and works on a similar principle. No spring or ratchet required.

Question 20

What is a pneumotachograph and how does this work.

Answer 20

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

Question 21

What affects the accuracy of a pneumotachograph and why? What are soutions to the problem

Answer 21

1. 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.
2. Water condensation –> wetting gauze and changing resistance

Solution:
Heating element to maintain constant temperature and prevent H2O condensation.

Question 22

How can the pneumotachograph also be used to measure gas volumes?

Answer 22

Integrating the flow - time curve generated by the device electronically

Question 23

What are Pitot Tubes?

Answer 23

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.

Question 24

What is the electronic mass flowmeter and how does it work?

Answer 24

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 .

Question 25

How is the infusion rate estimated by counting the drops through a drip chamber

Answer 25

Count drops per minute
Divide by ‘drops/ml’ illustrated on set (either 20 or 60)
Multiple by 60 minutes / 1 hour (Dimensional correcction)

Left with ml/hour.

Question 26

What are the forces acting on a droplet in a drip chamber

Answer 26

Gravity –> downward

Surface tension –> upward

Question 27

On what factors does the final volume of the drop (prior to falling) depend?

Answer 27

1. Density of liquid (depends on temp)
2. Surface tension of liquid (depends on temp)
3. Size and shape of the tube
4. Rate at which liquid flows through the tube.

Question 28

What is a drip coounter

Answer 28

Beam of infrared radiation produced by a LED passes through the chamber to strike a photodetector on the other side. Interruptions in light intensity –> drip count.

Question 29

What are more commonly used infusion controllers and how do these work?

Answer 29

Volumetric pump
A disposable cassette is loaded into a motorized holder containing multiple pistons and valves which control the infusion rate and avoid the problems assocaited with drip counters.

Syringe drivers
Motorized holder that compresses syringes at the appropriate rate to accurately control infusion of small volumes of fluid.

Question 30

What are target controlled infusion (TCI) systems?

Answer 30

1. Microprocessor-based infusion devices
2. Based on a 3 - compartment Pharmacokinetic model which describes the distribution and elimination of the drug.
3. Allows the appropriate target plasma or target organ concentration of propofol to be set and rapidly achieved by automated bolus/stops and maintenance infusion rates for any specific patient or level of surgical stimulation.

Question 31

What are some of the practical problems associated with TCI systems

Answer 31

1. High pressures! –> caution with drip size, position and site to avoid extravasation.

Question 32

Describe the Fick principle with regard to the calculation of blood flow to the lungs with regard to O2 uptake

Answer 32

1. Patient breaths 100% O2 via a Benedict Roth spirometer –> Oxygen uptake is plotted and the gradient shows that it is 250 ml of O2 per minute
2. CvO2 = 150 ml O2 / L blood
3. CaO2 = 200 ml O2 /L blood
   Difference: 50 ml O2 is added to each Litre of blood per minute.

250 ml O2 / minute divided by 50 ml O2 / L blood per minute = 5 L of blood

Question 33

Describe the application of the Fick Principle to calculate blood flow to the lungs using CO2 elimination

Answer 33

Draw diagram with Y connector into ‘rebreathing valve and circuit’ into flow sensor into CO2 detector into patient.

Intermittent rebreathing for 50 seconds is used as a non-invasive way to calculate cardiac output.
Venous etCO2 during normal breathing is unchanged during rebreathing for 50 seconds. So the two equations can be equated.

FICK’S PRINCIPLE
Blood flow lungs = Rate uptake/excretion substance x
__________________________
a - v [x] difference of substance x

Blood flow to lungs = Rate CO2 elimination
_______________
a - v [CO2]

So a - v CO2 can be measured

But how is CO2 elimination calculated?
During rebreathing arterial CO2 rises BUT venous CO2 takes 50 s to change.

CO = Vn - Vr
______
ar - an

Vn is the rate of elimination CO2 = Flow rate x etCO2 present in expiratory flow during normal breathing

Vr is the rate of elimination CO2 = Flow rate x etCO2 present in expiratory flow during rebreathing

ar is etCO2 during normal breathing (approximates PaCO2)

an is etCO2 during rebreathing (approximates PaCO2)