Gas Flowmeters
&
Mechanical Ventilation
© D. J. McMahon 2014
rev 141114
Some Basics Before We Ventilate:
Fluid: any substance that assumes the shape of its container
- Liquid:
a fluid whose shape, but not volume, can be changed at room temperature
- Gas:
a fluid that is compressible at room temperature
Bernoulli’s Principle:
As the speed of a moving fluid increases,
the pressure within the fluid decreases.
http://howthingsfly.si.edu/media/bernoulli-principle
Poiseuille's Law (pwas-WEE):
defines the flow (Q) of fluid passing through a tube in terms of:
> the fluid's viscosity (η)
> the tube's radius
(r)
> the tube’s length
(L)
> the pressure difference along the tube (ΔP)
π r4 ΔP
Q = -----------8ηL
:
π r ΔP
Q = -----------8ηL
4
Fluid flow in a tube is directly proportional to the fourth power
of the radius.
So, doubling the radius of the tube increases the fluid
flow by a factor of 16.
Dalton’s Law of Partial Pressures
In Gas Mixtures
The total pressure of a mixture of gases is equal to the
sum of the pressures of all of its constituent gases:
PressureTotal = Pressure 1 + Pressure 2 ... Pressure n
Example: Exhaled gases of 5% CO2, 16% O2, and 79% N2.
If the current ambient pressure is 740mmHg,
CO2 =
5% of 740: .05 x 740 =
O2 = 16% of 740:
37 mmHg
.16 x 740 =
118 mmHg
N2 = 79% of 740: .79 x 740 =
585 mmHg
Total:
740 mmHg
Flowmeters for Respiratory Gases
The Many Types of Flow Meters:
- Venturi: Uses pressure and tube diameters
- Pitot Tube: Uses static pressure and impact pressure
- Rotameter: Uses a variable area tube with a bobbin
- Turbine: Uses a vane whose rotations translate to flow
- Electromagnetic: Uses emf developed by flowing fluid
- Ultrasound: Uses Doppler Shift
- Thermal: Uses two sensors which differ in temperature
- Hot Wire: Uses two heated wires whose current relates to flow
Venturi Type: uses Bernoulli’s Principle
Flow is directly proportional to the pressure difference P1-P2.
Advantages: no moving parts.
Pitot Tube: also Bernoulli’s Principle
Differential pressure (and therefore the flow) relates to the
total pressure Pt and static pressure Ps.
Rotameter - Gas flow pushes a bobbin up within a vertical tube.
Gas flow is indicated by the gradations on the tube.
Commonly used for medical gases and vacuum,
for O2, ventilators, and anesthesia machines.
ball
bobbin
Turbine:
Rotating vane is connected to a mechanical or electronic
interface to indicate flow.
Turbine -
Wright Spirometers -
early turbine type
Hot Wire:
Two filaments are in the gas flow stream, heated by a constant
current source. As gas flow causes the temperature of the
wires to change, the current is changed to maintain a stable
temperature. That current is directly proportional to flow.
Respiration
Respiration vs Ventilation –
Respiration refers to the entire system of O2 delivery and CO2 elimination.
- i.e. upper airway, lungs, vascular system, and metabolism at the cells
Ventilation refers to the movement of airway gases.
- only includes the mechanics of inspiration and expiration
Respiration:
> Supply of oxygen and removal of carbon dioxide from the body
> Conduction Section: upper airway to the bronchioles
> Respiratory Section: bronchioles, alveolar ducts, alveolar sacs
in the adult lungs:
-- 300 million alveoli
-- 70 sq meters of surface area
-- total lung capacity: 4 to 9 liters (male),
2.5 to 7 liters (female)
Inspiration is created first by the diaphragm, then by the
intercostal muscles, then by accessory muscles (neck & back).
The diaphragm is the
primary breathing muscle
Intercostal muscles
Movement of gas thru the airway:
7500 ml/min
=
5250 ml/min
+
2250 ml/min
Lung Capacities:
Lung Capacities:
Ventilation Parameters:
Total Lung Capacity
(TLC)
Inspiratory Reserve Volume
(IRV)
Tidal Volume
(VT)
Expiratory Reserve Volume
(ERV)
Reserve Volume
(RV)
Inspiratory Capacity
(IC)
Functional Residual Capacity
(FRC)
Vital Capacity
(VC)
Basic artificial ventilation:
Respirators or Resuscitators
> Essentials: mask, breathing valve, self-refilling bag
> Provide positive pressure to the airway
- positive pressure is essential to respiration
> Classic examples:
- Ambu Bag:
- Laerdal Bag:
Respirators or Resuscitators
Detail:
Mechanical ventilators:
> Originally used negative pressure on the thorax to create
respiration
- eg the “Iron Lung” of the 50’s and the Drinker machines
> Now all use positive pressure by forcing gas into the airway
> Assisted Ventilation:
- ventilator starts only when the patient makes an
inspiratory effort
> Mandatory Ventilation:
- ventilator controls every aspect of respiration
Ventilator Terminology:
Lung Compliance
Tidal Volume
Minute Volume
I:E Ratio
Intermittent Mandatory Ventilation (IMV)
Synchronized Intermittent Mandatory Ventilation
(Fractional) Inspired Oxygen Percentage (FiO2)
Peak Airway Pressure (during inspiration)
Positive End Expiratory Pressure (PEEP)
Continuous Positive Airway Pressure (CPAP)
(SIMV)
Continuous positive airway pressure (CPAP) is the use of
continuous positive pressure to maintain a continuous level
of positive airway pressure in a spontaneously breathing
patient. CPAP and PEEP are the same, but “PEEP valves”
are used to increase oxygen uptake.
Relationships of ventilation variables:
Modes of Mechanical Ventilators:
Controlled Mandatory Ventilation (CV)
Intermittant Mandatory Ventilation (IMV)
Continuous Mandatory Ventilation (CMV)
Synchronized Intermittent Mandatory Ventilation (SIMV)
Continuous Positive Airway Pressure Ventilation (CPAP)
Enhancements to ventilation:
Positive End-Expiratory Pressure (PEEP)
Apnea Detection
Pressure Support
Pressures during a respiratory cycle:
Volumes during an assisted respiratory cycle:
Pressure – Volume – Flow Diagrams:
> Show changes in Pressure, Volume, or Flow vs Time.
> Very useful to see how a ventilator performs.
Flow-Volume Loops:
> Used by respiratory therapists and anesthesiologists to
monitor and optimize ventilation in critical care patients or
during surgery.
Pressure-Volume-Flow
Diagram
Flow-Volume Loop (read ccw):
Humidification:
> artificial ventilation removes moisture from the airway
and can be very detrimental to the airway and alveoli
> water vapor can be replaced by a heated water source
placed in the ventilation circuit,
> or by a disposable which prevents water vapor from
leaving the airway (esp used in anesthesiology)
The evolution of ventilators:
The polio epidemic of the 1950’s created the development of
better mechanical ventilators beyond the “Iron Lung”
“March of Dimes”
poster from the ’50s
Associated Press / May 29, 2008
MEMPHIS, Tenn.—A woman who defied medical odds and spent
nearly 60 years in an iron lung after being diagnosed with polio as a
child, died Wednesday after a power failure shut down the machine
that kept her breathing, her family said.
Dianne Odell, 61, had been confined to the 7-foot-long metal tube
since she was stricken by polio at 3 years old.
Family members were unable to get an emergency generator working
after a power failure knocked out electricity to the Odell family's
residence near Jackson, about 80 miles northeast of Memphis,
brother-in-law Will Beyer said.
Bird ventilators: ’70’s & ’80’s
Dr. Forrest Bird
Bennett Assister-Controller: 1970’s
Puritan-Bennett model MA-1
Puritan-Bennett model 7200
Puritan-Bennett model 7200
Current ventilators :
> Separated pneumatics and electronics modules
> Pneumatics:
filters
O2 / air mixer
reservoir or compressor
O2 sensor
safety valves
> Electronics:
Microprocessor and firmware controls user settings:
rate, tidal volume, flow form, Fi02, PEEP, display, alarm
settings, etc.
Block diagram of typical ventilator
Maquet Servo-i
Nellcor- Puritan-Bennett
840
Drager
Evita
Hamilton
S1
Carefusion (Pulmonetics)
Avea
Philips (Respironics)
Trilogy
Impact Instrumentation
Eagle series
Pulmonetics LTV 1200
Allied Healthcare MCV100 AC/battery driven for mass casualty scenarios
High Frequency Ventilators:
> use low-amplitude pulses of gas at 5 to 20 Hz
> supply tidal volumes without high inspiratory pressures
> eg Drager’s “Babylog 8000”
High Frequency Oscillatory Ventilator
(HFOV)
Carefusion 3100A High Frequency Oscillatory Ventilator (HFOV)
CPAP systems for sleep apnea:
Variant on CPAP –
BiPAP:
Bi-level Positive Airway Pressure:
Provides two levels of pressure:
Inspiratory Positive Airway Pressure (IPAP) and
Expiratory Positive Airway Pressure (EPAP) for easier exhalation.
Maintenance of ventilators –
> Ventilators are life-support devices
> Manufacturer’s service school is best training
> Typical full PM can be several hours
> Involves replacement of expendables like filters, seals,
O-rings, O2 sensors, etc; function checks, alarm
checks.
Test Equipment for Ventilators:
Respironics “Certifier Plus” --
BC Biomedical PFC-3000
Fluke VT-Mobile
Fluke VT-305
Test lungs - simulate varied lung conditions
Major Players in Ventilators:
> Nellcor Puritan-Bennett
> Maquet (Siemens)
> Carefusion
> Philips-Respironics
> Drager
Service training for ventilators:
Manufacturers:
Drager
Phillips / Siemens
Nellcor – Puritan Bennett
Independent service school:
South Pacific Biomedical:
http://www.southpb.com/fr_index.htm?/schedule.htm