Transcript Ventilator Modes of Operation

ENTC 4350
Modern Ventilators
Modern Ventillators

Ventilation assistance is provided
under either of two conditions:
(1) breathing initiated by a timing
mechanism or
(2) patient-initiated breathing.

Automatically timed breathing is usually
provided for patients who cannot breathe
on their own.
• It provides inspiration and expiration at fixed
rates and durations except for periodic sigh; a
sigh is a rest period for the patient.

Patient-initiated breathing may be given
to one who has difficulty breathing due to
high airway resistance.
• The patient’s effort to inhale triggers the
respirator unit to deliver air at the positive
pressure prescribed.
Ventilator Modes of Operation

The following definitions are commonly used to
describe respirator/ventilator operation:
•
CMV
•
CPAP
Continuous mandatory ventilation: Once
initiated by either the ventilator operator or
the patient, the breath is driven to the patient.
Continuous positive airway pressure: Breaths
are spontaneous, unless the operator
intervenes. The spontaneous breaths are
determined entirely by patient effort.
However, the air/oxygen mixture is set by the
ventilator.

SIMV
Synchronized intermittent mandatory
ventilation: These breaths are initiated by
either the machine, the operator, or the
patient. The breaths may be either
spontaneous or mandatory. That is, if the
patient does not breathe within a preset time
period, the ventilator will deliver a breath.

PEEP
Positive end-expiratory pressure: The
pressure maintained by the ventilator that the
patient must exhale against.
• Apena
• Sigh
• Nebulizer
The patient has stopped
breathing.
A breath delivered by the
ventilator that differs in duration
and pressure from a nominal
breath.
A device for producing a fine
spray of liquid or medication into
the patient’s air.

The block diagram shows the external
flexible tubing and the ventilator unit.

Air from the ventilator during patient
inspiration passes through a bacterial
filter and humidifier.
• A nebulizer may spray medication into the air.
This air then forces valve 1 up to close off the
spirometer and deliver air to the patient.

After the inspiration air is turned off by
the ventilator, valve 1 drops and the
patient exhales into the bellows, which
has its outlet valve held closed
pneumatically by the ventilator unit.
• During the subsequent patient inspiration
cycle, that valve will open, causing the
bellows to fall and empty.

During patient expiration, the direction of
the air in the pneumatic system is
determined by the main solenoid, which
is switched appropriately by the system
electronics.
• Room air is drawn from the air inlet filter by
the main compressor and is directed through
the main solenoid to hold closed the upper
outlet valve of the bellows located inside the
unit.

Next, the weight of the bellows causes
the bottom bellows chamber outlet valve
to open, as the main solenoid directs air
to close the inlet bellows chamber valve.
• The weight of the falling bellows draws
oxygen-enriched air into it in preparation for
the patient-inspiration part of the cycle.

The oxygen content of the air flowing
into the bellows is controlled by a
percentage control valve, which
regulates the resistance to room air and
oxygen appropriately.

At the end of patient expiration, the
system electronics trip the main
solenoid, thereby initiating the patientinspiration part of he cycle.

During patient inspiration, the
compressor draws room air through an
air filter and then through the main
solenoid.
• It forces the bottom inlet valve of the internal
bellows chamber open and forces the bottom
bellows chamber outlet valve closed.

The high pressure in the bellows
chamber compresses the bellows,
forcing open the upper outlet valve set
free by the main solenoid.
• This allows the oxygen-enriched air to pass
through the main bacteria filter into the
external tubes and then to the patient lungs.

A sensitivity control monitors the
negative pressure necessary to initiate
inspiration when the respirator is used in
the patient-initiated breathing mode
called the assist mode.
• A nebulizer compressor may draw air from the
bellows and force it through an aspirator to
mix medication into the patient-inspired air.
• When inspiration is complete, the main
solenoid switches the direction of the
pneumatic air to repeat the expiration cycle,
and so on.

The black bag at the end
of the tubing simulates a
compliant lung.
•
This respirator may be
operated using compressed
air from the hospital air
supply.

In that mode, the ventilator can be
removed from its internal compressor,
thereby decreasing it in size; turning off
the compressor also reduces problems
in instrument noise control.

To aid patient respiration, hospital air
and oxygen enter the pneumatic
compartment, where it is filtered.
• A check valve reduces the pressure to a
nominal 10 psi.

A proportional solenoid valve assembly
allows the air/oxygen mix to be
controlled by the system electronics.
• A check valve directs air to the patient
during the inspiration cycle.

During the subsequent expiration
phase, the system electronics opens the
check valve, CV5, to provide a vent for
the patient exhalation air.
• In this case, the pneumatically operated
valves of older ventilators have been replaced
by valves controlled with microprocessorbased electronics.

A small positive-pressure ventilator is
illustrated with both front and back
views.
PNEUMOTACHOGRAPH AIRFLOW
MEASUREMENT

Patient airflow may be measured by
changes in resistance of a thermistor in
the airstream due to the cooling effect of
flowing air.
• But it must be calibrated to compensate for
changing ambient temperature.

To eliminate this
disadvantage, a
strain-gauge wire
mesh is often used.

The airflow in either direction puts a
strain on the screen and changes the
resistance of its strain gauge.

The strain gauge is a component of a
Wheatstone bridge.

Here the change in resistance, DR, is
proportional to the airflow, F, past the
wire mesh.
F  kD R
V A  V BB
V B  V BB
R
R  DR  R
R
Voltage divider rule
2R
V AB  V A  V A
R
R 



V BB
2R 
 R  DR  R
Getting a common denominator
V AB
V AB
V AB
R
2R  DR R 
 2R


V BB
2R  DR 2R 
 2 R 2 R  DR
2
2

2R
2 R  RDR


2
2
 4 R  2 RDR
4
R
 2 RDR

DR



V BB
 4R  2DR 

V BB


If R » DR
V AB
 DR 

V BB
 4R 
For the diff amplifier with a gain of AD ,
VF 
 A D V BB D R
4R

 A D V BB F
4 kR
for D R 
F
k
Integrator Circuit
For the integrator circuit,
V out  
1
C
V out  

t
iC dt  
t0
1
RC

t
t0
1
C

t
v in
t0
 A D V BB D R
4R
dt
R
dt
Note that the term inside the integral is a constant,
V out 
A D V BB D R
2
4R C

t
t0
dt 
A D V BB D R
2
4R C
t  kt
Pneumotachograph Volume
Measurement

A volume exhaled by a patient is
measured with the pneumotachograph
by first closing and then opening the
reset switch.
• This sets the initial charge on the capacitor to
zero and fixes Vout at zero.

The patient is then asked
to exhale through the
pneumotach mouthpiece.
•
The resulting change in DR
creates a voltage VF as a
function of time in proportion
to flow.

The air volume expired by the patient,
beginning at t = 0, when the reset is
activated, equals the area under the flow
vs. time curve.
• Mathematically, this area is computed by
integration.

The output voltage, Vout ,is proportional
to the volume of air expired from time t =
0 to the time, t, desired.
• The flow, F, is a function of time that may
increase, decrease, or stay constant, so long
as it goes in one direction.
Vout  
1
RC

t
t0
 A D V BB F
4 kR
dt
V out 

A D V BB
4 kRR 1C

t
F dt
t0
t
F dt  Total lung volum e  VOL
t0
V out 
A D V BB
VOL  k  VOL
4 kRR 1C
This implies that Vout is proportional to the total volume
of air that is passed through the pneumotach for the
time of observation.
THE PLETHYSMOGRAPH

The pneumotachograph can be used to
measure the rate of airflow during
respiration and the vital air capacity of
the lung VC.
• It cannot, however, measure the total lung
capacity, TLC.

The reason for this is that the
pneumotachograph can only measure
the amount of air a person can exchange
in respiration, and cannot detect the
residual volume of air, RV, left in the lung
after a forced exhaling.
• To measure the TLC, a body plethysmograph
may be used.

The plethysmograph
consists of an airtight
chamber the patient
can enter and sit in.

The principle of operation of the
plethysmograph depends directly on the gas
law for an ideal gas of volume V and pressure
P, namely Boyle’s Law
PV  k 1T
• where k1 is a constant and T is the absolute
temperature (K). In the chamber, the temperature
remains constant.

To measure TLC, the patient enters the
chamber.
• The door is sealed, and the valve on the
mouthpiece is closed.

Since the patient cannot breathe with the
valve closed, the air pressure in the
mouthpiece equals that in the lung, PT.
• That is, when the flow of air is zero, the
pressure drop from mouthpiece to lung is also
zero.

With the valve closed, a formula for the
thoracic volume is derived as follows.
• The gas equation for constant temperature
holds inside the lung
dP
dV
 
P
V

Using the previous equation, the TLC
can be written as:
dTLC
dPT

TLC
PT

Boyle’s Law also holds in the chamber,
so that
dVol C

Vol C
dPC
PC
• Here VOLC is the chamber volume and PC is
the chamber pressure.

Because the chamber is closed, any
increase in the thoracic volume
introduced by breathing motions causes
a decrease in the chamber volume of air.
• That is
dVol
C
 dTLC

Combining the equations
dTLC

TLC
dPT
PT
dVol C
Vol C

dPC
PC
• with the previous fact:
dVol
C
 dTLC

This yields:
dVol C  
TLC
PT
Vol C
PC
dPT  
dPC   dTLC 
Vol C
PC
dPC
TLC
PT
dPT

During the test PT = PC approximately,
since the changes in pressure induced
by breathing motions are small when the
patient is resting.
• Thus,
TLC  Vol C
dPC
dPT

This equation gives the means for measuring
the lung volume, TLC, by the following steps:
1. Close the mouthpiece valve on the patient
sealed in the chamber.
2. Ask the patient to make breathing motions.
3. Read the change in pressure dPT on meter 1.
4. Read the change in pressure dPC in the chamber
on meter 2.
5. Since the chamber volume VOLC is a known
specification of the plethysmograph, use the
result in steps 3 and 4 to compute TLC.