Flow Sensors used in Respiratory gas monitoring(RGM)

What is a Flow sensor?

Flow Sensor is a medical accessory used to  accurately measure the flow of gases  in Ventilators and Anesthesia workstations also known as Respiratory gas monitors(RGM). The flow sensor in a RGM measures the flow rate and derives to give Inspired and Expired volume of the patient during ventilation.

 Why are flow sensor used in Respiratory gas monitoring?

Ventilators and Anesthesia machines are used to support breathing in patients who have lost the ability to breathe on their own. The process of Ventilation  requires certain volume of Air and Oxygen mixture (other gases in Anesthesia workstation) to be supplied to patient based on their lung/clinical condition. This requires feeding the patient lungs with precise gas mixture at optimum flow rate.

Flow sensors are used in RGMs to measure the flow rate and volume of the supplied gas to the patient, and maybe placed both inside and outside the RGMs.
The Flow sensors placed inside the RGMs before inspiration measure the individual gases so that the gas mixture is delivered in accordance to the prescription.
Some Flow sensors are placed in close proximity to the patient to measure inhaled and exhaled gases. Some Flow sensors are placed only near the Expiratory valve to measure the expired gas mixture.
The ventilator and Anesthesia flow sensors are delicate and are a critical component to make breath measurements. For accuracy of measurement it is mandatory to calibrate the flow sensor when using for first time and periodically as recommended by the manufacturer.

Different types of flow sensor used in RGM?

Based on Technology the flow sensor can be differentiated as below

Hot wire Anemometry Technology

Differential pressure Technology 

Ultrasonic flow sensor

Pneumotachograph 

Hot wire Anemometry (HWA)

Hot wire Anemometry is based on heat transfer phenomenon.

HWA uses a fine electrical wire which is heated to an ambient temperature, the electrical resistance of which changes to change in temperature. As gas flows the heated wire tends to cool and thereby  reducing the resistance of the wire proportionally. Therefore monitoring the resistance of heated  wire with respect to  resistance value of ambient gas temperature the flow parameter is derived by 
Hot wire anemometer used in various medical Ventilators and Anesthesia workstations.

HWA can be classified by two types

• Constant Current anemometer
• Constant temperature anemometer

Constant Current anemometer:

The Wheatstone bridge along with anemometer(is a device used for measuring direction of wind and its speed)  is kept in the flowing gas stream to measure the flow rate. A constant current is passed through the sensing wire. The voltage across the bridge circuit is kept constant i.e without varying. Due to the gas flow in the anemometer, the heat transfers from sensor wire to flowing gas so the temperature of the heated wire is reduced causing change in resistance of sensing wire. This change in resistance is proportional to the flow.

 Constant temperature anemometer(CTA)

The Wheatstone bridge along with anemometer is kept in the flowing gas stream to measure the flow rate,In this CTA resistance of sensing wire kept constant. When the gas flows through the hot wire anemometer the temperature and resistance of the conducting electrical sensing wire changes at the same time voltmeter will help to adjust the change in resistance as a result temperature of the hot wire is maintained at a constant level.

 

Fig :1 Hot wire Anemometry Flow Sensors used in Drager

Fig :2 Cross sectional view of Hot wire Anemometry flow sensor

                           

Fig :3  Front view of the flow sensor

Few RGMs that use this model:
Drager :Babylog, Evita 
Care fusion: Avea, Bear1000, Bear Cub
Covidien : New port e360
Philips: Respronics V200
SLE: SLE 5000

Advantages:
High Sensitivity 
Good Accuracy
Fast response time
Low Flow resistance
 
Disadvantages:
Heating Elements are Fragile in nature
Cannot differentiate bi directional flow
Costing
 

Differential Pressure flow sensor

Differential pressure Sensors use Bernoulli’s equation to measure the flow of fluid in a pipe. Differential pressure Sensors introduce a restriction in the pipe that creates a pressure drop across the Sensor. When the flow increases, more pressure drop is created.

They are of two types

• Variable orifice flow sensor
• Fixed orifice Flow sensor

Variable Orifice:

The Variable orifice method has some valuable characteristics compare to others.
It has linearity, good dynamic response and low cost.The problem of non-linearity and decreased sensitivity at low flow rate is partially compensated by the modification done by introducing a flapper orifice plate between two pressure measuring points. This serves as a variable orifice that increase in size with larger flows. Because flow calibration is different with a variable orifice each sensor has a unique calibration table stored with the ventilator electronic memory.

Fig 4: 3D Cross sectional view of Differential Pressure Flow sensor

              

Fig 5: Front view of the Differential Pressure Flow sensor

Few RGMs that use this model: 
Hamilton Medical Model : Hamilton C1/C2 
Carefusion Model:Vela

Advantages :
Bi-directionality
Linear Responsiveness
At low flow rate high accuracy
Single use flow sensor are cheap

Disadvantages:
Increased Flow resistance
Presence of condensation decreases the response
Low response time compare to hot wire anemometry
Reusable Flow sensor are expensive
 
Fixed Orifice

A flow restrictor plate with small Orifice is placed in the pipeline where the gas flows. A pressure drop is created by the restriction when gas flows in either directions. The sensor output is usually measured by a differential pressure.
 The disadvantage in the fixed orifice technique is that it has non linear relationship between the differential pressure and the low gas flow rate.
Fixed orifice flow sensor require separate flow sensor for adult and paediatric patients. To reduce this problem variable orifice technique introduced . It uses a single flow sensor for both adult and paediatric patient as well as high and low flow rates.

 

 

Fig 6: Cross Sectional view of the Fixed Orifice Flow sensor

Few RGMs that use this model:
GE Versamed
Flight 60

Advantages : 
Bidirectionality
Single use flow sensor are cheap
Long Life

Disadvantages: 
Increased Flow resistance
Presence of condensation decreases the response
Low response time compare to hot wire anemometry

Ultrasonic flow sensors

They are based on the transmission of ultrasonic signal between a pair of transducers, analyses the changes in transit time caused by the velocity
of the intervening medium, in this case the patient gas mixture.
These ultrasonic sensors are based on the principle that the velocity of sound travelling through a medium is increased or decreased by movement of the gas mixture.The Transducers Emitter,Receiver are made of Piezo ceramic materials

 

Fig 7: Cross Sectional view of the Ultrasonic Flow sensor

Few RGMs that use this model:
Maquet Servo-I,
Many spirometry machines

Advantages:
Short response time
Negligible resistance
Low volume measurements

Disadvantages:
Not so accurate as the other sensors
Considerable difference between inspiratory
and expiratory tidal volume estimation.
 

Pneumotachographs:

The working principle of pneumotachographs is based on the placement of a resistance into a pipeline, in which the fluid is running full, the pressure drop across the resistance is linearly related to the flow-rate , according to the Hagen-Poiseuille law, under laminar regime. Therefore, the measurement chain needs a differential pressure sensor to measure the differential pressure. They are very much similar to differential pressure sensor, except for the resistance. The resistance introduced in the sensors may be in the form fine capillary tubes or wire mesh.

Few RGMs that use this model:
eVent Medical Inspiration Series Ventilator 
F7300000, F7200000, F7100000
Siemens Servo Ventilator 900

Advantages:
linear response and bidirectionality capable
good accuracy combined with sensitivity
Less response time

Disadvantages:
High resistance increases with sensitivity
presence of condensation influences the response
 

Applicable standards for Flow sensors

 1.EN ISO 13485:2016

Medical Devices -- Quality Management Systems - Requirements for Regulatory Purpose.

2.ISO 5356-1:2015

Anaesthestic and respiratory equipment

3.EN ISO-14971:2012

Medical devices -- Application of risk management to medical devices

4.EN1041:2008+A1:2013

Information supplied by the manufacturer of medical devices

5.EN ISO-15223-1:2016

Medical devices -- Symbols to be used with medical device labels, labeling and Information to be supplied

6.ISO 80601-2-12:2011

Medical electrical equipment — Part 2-12: Particular requirements for basic safety and essential performance of critical care ventilators

7.ISO 10993-1:2018

Biological evaluation of medical devices

Conclusion:

Mechanical ventilation and breathing monitoring is important practice in Intensive care unit as well as critical care unit so flow sensor plays major part for accurate and continuous monitoring of gas movement.