Monitoring of gas delivery: wrights respirometer, pneumotachograph, ventilator alarms Flashcards
How are the volumes of inhaled and exhaled gases routinely monitored
Either a Wright’s respirometer or a pneumotachograph
Tidal volume
Definition, how best measured
= the volume of air breathed in or out in one breath.
Minute volume = volume of air breathed in or out in 1 min.
Both inspired and expired volumes should be monitored, to give more info about TV and presence of any leaks or losses in the breathing system.
If not able to monitor both, best to monitor expired tidal volume.
Note there is usually some difference between inspired and expired volumes during controlled ventilation.
Wright’s respirometer: uses, advantages
A device used to measure the tidal volume and minute volume continuously.
Advantages
* Small, portable (weights less than 150g)
* Does not require a power supply
* Offers minimal resistance when the patient is breathing
Wright’s respirometer: function
Rotation speed
- Basic design incorporates a light mica vane that rotates within a cylinder
- Allows gas to flow in one direction only
- Gas is directed through tangential vents in the cylinder wall, giving the gas a circular motion that causes the vane to rotate (uni-directional)
- The device is designed to rotate at a rate of 150 revolutions per L
- The vane’s rotation is geared to show an accurately calibrated flow rate on the display dial
- By averaging recorded volumes over time, flow rates can be calculated.
Wright’s respirometer: accuracy of the mechanical version
- Accurately reads the tidal volume and minute volume (+/- 5-10%) within the flow range of 4-24L/min
- Minimum flow rate of 2L / min is required for the respirometer to funciton accurately
Causes of inaccuracy
* Under-reads at low tidal volumes (due to friction) and at low flow rates
* Over-reads at high tidal volumes (due to momentum) and at high flow rates, such as teh hifh peak flow at the beginning of expiration
* Calibrated for normal tidal volumes and respiratory rates. Is not accurate if used to measure a continuous flow
* Water condensation from the expired gases can cause pointer to stick -> cannot rotate freely -> corrosion
Electrical version of Wright’s respirometer
Use electronics to detect the vane rotation
* Vane rotates alternately clockwise and anticlockwise as the patient breathes in and out - i.e. is bidirectional
* Movement is measured by infra-red beams positioned across the vane
* Reduces inaccuracies due to inertia and water condensation
* However requires a power supply, signal processing and a display unit
Other devices use a semi-conductive device that is sensitive to changes in magnetic field. TV and MV can be measured by converting these changes electronically.
Where is Wright’s respirometer positioned within the breathing system
Usually positioned on the expiratory side of the breathing system, which is at lower pressure than the inspiratory side.
This minimizes the loss of gas volume due to leaks and expansion of the tubing.
Can Wrights respirometer be used in paediatrics
Yes: a paediatric version exists which can accurately measure TV between 15 and 200ml
Pneumotachograph
What is, basic principle, factors affecting accuracy
A device used for measuring gas flow, from which gas volume can be calculated. Types:
* Fleish pneumotachograph
* Lilly pneumotachograph
* Combined pneumotachograph and Pitot tube
Basic mechanism
* Tube through which a laminar flow of gas passes over a small and fixed resistance
* Changes in pressure across the resistance are detected by two sensitive pressure transducers located on either side of the resistor
* Pressure change is v small, but is linearly proportional, over a certain range, to the flow of gas passing over the resistor -> accurate measurement of gas flow can be achieved
Capable of measuring bi-directional flows, which allows it to be used to measure flows during both inspiration and expiration.
Gas volume can be calculated from the flow to measure the tidal volume
Timed over 1 minute -> summated to calculated minute volume
Causes of inaccuracy
* Water vapour condensation at the resistor -> turbulent flow affecting accuracy. Can be controlled by heating the resistor
* Accumulation of secretions -> corrosion, sticking (similar to Wrights respirometer)
Fleisch pneumotachograph
Mechanism, advantages/ disadvantages
- Bundle of small capillary tubes arranged in parallel to the direction of the flow, provide the small and fixed resistance
- These tubes ensure laminar flow through the sensor body, thereby producing a nearly linear relationship between flow and differential pressure.
Advantages/ disadvantages.
* Excellent for short term monitoring
* Easily contaminated by sputum and water condensate
* Not suited for continuous respiratory monitoring due to relatively large surface area or dead space
* Heavy, costly, difficult to clean.
Lilly pneumotachograph
- Flow head uses a metal mesh as the flow restrictor. Some designs use three screens as the flow restrictor to create a linear flow/ differential pressure relationship
Similar problems to the Fleisch in monitoring continuous proximal airway flows
* Not suited for continuous respiratory monitoring due to relatively large surface area or dead space
* Heavy, costly, difficult to clean.
* Easily contaminated by sputum/ water condensate
Combined pneumotachograph and Pitot tube
Function, nature of relationship with flow, advantages (2)
Bi-directional device
* Two pressure ports connected to pressure transducers facing in opposite directions within the gas flow
* Pressure difference between the ports is proportional to the square of the flow rate
* Microprocessors can be programmed to store the parameters of these flow sensors, and to compensate for the non-linear pressure-flow relationship
* Readings can be affected by density and viscoscity of gas. To compensate for this, continuous gas composition analysis via a sampling tube can be used.
Advantages
* Allows more accurate measurement, even in neonates and infants
* Allows the calculation and measurement of compliance, airway pressures and resistance, gas flow and pressure/volume and flow/volume loops
Factors that might affect flow reading on a pneumotachograph
- Location
- Gas composition
- Gas temp
- Humidity
- Apparatus dead space
- Operating range of flow sensor: should be designed to function accurately with wide range of TVs, I:E ratios, frequencies and flow ranges
- Inter-sensor variability: individual sensors can have different performances. There should be no need for individual device calibration of the flow/pressure characteristics.
Image shows pneumotachograph as part of an anaesthetic breathing system
How does location of pneumotachograph affect flow reading
Where should it be placed
- Should be placed ** between the breathing system Y-piece and the tracheal tube**
- Flow measurements at patient end can be substantially different from those measured inside or at the ventilator end: can be a substantial difference between what is delivered to the patient and what ventilator reports as delivered, due to compression of gas.
- This wasted portion of tidal volume does not ventilate the patient, remains within the breathing system, and causes elongation and distension of the breathing system tubing
How do gas composition and temperature affect the flow reading on a pneumotachograph
Gas composition
* Nominal values of inspired and expired gas composition need to be known & sensors calibrated accordingly
* Not properly compensating for gas concentrations may result in significant change in reported flow value
Gas temp
* Sensors software usually provides default values for a typical patient
* If inspired gas mixture is heated in the ventilator breathing system, greater accuracy can be achieved by entering the set temp of the heater as the inspiratory temp
Pressure sensitive alarm
How activated, type of patient
- Monitors the pressure in the breathing system through a side port, and compares this to pre-set low and high pressure alarm limits, within a certain period of time .
- Activation occurs when inspiratory airway pressure exceeds max pressure level, or fails to reach minimum pre-set level
- Applicable to ventilated patients.
