Physics of Ventilation

National Chairperson (Ex)

Intensive Care Chapter I A P

Founder Chairman.....

National conference on pediatric critical

care

Professor of pediatrics ( Hon )

JNMC:Wardha

Nagpur : INDIA

Dr Deopujari

The evolution of Ventilator

Three problems of ventilation

Applied Physics

• Compliance

• Static compliance

• Dynamic compliance

• Resistance

• Work of breathing

Flow

Pressure

Volume

Surface area

What can we manipulate ?

Evolution of Ventilators………………

What can we manipulate ?

1)Minute ventilation

2)Pressure gradient

3)Surface are

4)Pulmonary vasculature

5)Solubility

How can we do this ?

Compliance

Compliance is a quotient between volume

and corresponding pressure change.

C =

V

P

Dynamic compliance

Static compliance

The compliance of any structure is the ease is

with which the structure distends ( C= ∆ V / ∆ P)

C

O

M

P

L

I

A

N

C

E

C.C.P.

C.O.P.

V O L U M E

PRESSURE

c.c.p. = critical closing pressure c.o.p. = critical opening pressure

PEEP

Pressure

Volume

Optimal Benefit Of PEEP

EX. INS.

Peak pressure

Plateau pressure Pressure

Inspiratory pause

Flow

Time

Cst = Tidal volume.

Cc

Cst : static compliance

Cc : circuit compliance

end expiratory pressure (Pend-ex)

Pend-ex = Peep + P peep

Resistance

Compliance

Static compliance

Static pressure – end expiratory pressure

Flow …………………….

Flow indicates circulation of an element

per unit of time through a given place

Flow….

Linear

Turbulent

FLOW

Palv

Pmo

FLOW THROUGH A PIPE LIKE

STRUCTURE REQUIRES A

DRIVING PRESSURE (Palv-

Pmo) TO OVERCOME THE

FRICTIONAL RESISTANCE

FLOW DEPENDS ON

Pressure Difference and

Resistance V=P/R

Pressure …………….

Represents the force that a volume of gas exercises upon a unit

area.

Kinetic theory states that gas molecules bomb the walls of a

container .

If volume remains constant , pressure varies in direct proportion

to Temperature.

If temperature remains constant , pressure varies in inverse

relation to Volume .

Increase in pressure decreases volume

Resistance ……………

Resistance is defined as the relationship between pressure (p) and flow ( 0 )

Therefore important when there is air flow.

R = -----------

Resistance depends upon

Diameter of airway

Length of airway

Viscosity and density of air

Resistance during inspiration

Resistance during expiration

P

O

And how can we do this ?

• Conditional variables

• Control variables

• Phase variables

Trigger variables

Limit variables

Cycle variables

Base line

Mechanical Ventilation

Concept

Conditional variables alone or in combination are

analyzed by Ventilator’s control logic. The state of this variable determines as to which of

two types of breath would be delivered.

SIMV is based on timing window and accordingly the

ventilator delivers …………………………….

a pressure triggered ( patient ) or time triggered

( machine) breath.

In this situation the patient effort and time are

conditional variables for determining triggering.

CONDITIONAL

VARIABLE

Conditional variables

• Pressure

• Volume

• Flow

• Time

Control variable manipulates the conditional to cause inspiration.

Pressure , volume , flow and time are control variables.

The behavior of control Variable remains constant in spite of changed

Ventilatory load.

CONRTOL VARIALBE

Control variable manipulates the conditionals to cause inspiration.

PRESSURE , VOLUME , FLOW and time are control variables.

The behavior of control Variable remains constant in spite of changed

Ventilatory load.

CONRTOL VARIALBE

Pressure

Rectangular Exponential

Control variable manipulates the conditional to cause inspiration.

Pressure , VOLUME , FLOW and time are control variables.

The behavior of control Variable remains constant in spite of changed

Ventilatory load.

Volume

Ramp Sinusoidal

CONRTOL VARIALBE

Control variable manipulates the conditional to cause inspiration.

PRESSURE , VOLUME , FLOW and time are control variables.

The behavior of control Variable remains constant in spite of changed

Ventilatory load.

Flow

sinusoidal rectangle ramp

ramp exponential

CONRTOL VARIALBE

During pressure support ventilation

though one selects a level of support

the Inspiration continues till

predetermined flow rate or termination

criteria is reached.

During PSV patient determines the

Rate ……………..

Inspiratory time…

Flow……………..

LIMIT

VARIBALE

Peak pressure

Plateau pressure Resistance

Compliance

End of inspiration

Volume

Flow

Pressure

Volume control

End of insp. flow

Volume

Flow

Pressure

Pressure control

What does not change …….

1)Volume curve

2)Exp. Flow curve

End of insp. flow

Volume

Flow

Pressure

Pressure control

Peak p. Plateau p. Resist.

Compliance

End of insp. flow

Volume

Flow

Pressure

Volume control

Volume limited and pressure limited ventilation ………….

Volume limited pressure limited

Advantages

Tidal Volume guaranteed

Precise control of Inspiratory flow

Easy detection of changed

respiratory impedance

Precise control pf pressure

Decelerating flow reported to

improve distribution of ventilation

decrease dead space ventilation

decrease PIP

match Inspiratory flow

Disadvantages

PIP vary

Inspiratory flow may not match the

patients needs

Variable tidal volume

Changes in impedance not easily

detected

Modes of ventilation

or moods of ventilation

P

R

E

S

S

U

R

e

F

L

O

w

V

O

L

U

M

e

A B C

A normal lung B decreased compliance C increased resistance

Controlled Ventilation…………….

SIMV

Patient triggered ventilation

Synchronized to patient breath

if the threshold is met .

Patient controlled variables

Respiratory rate

Inspiratory time

Clinician controlled variables

PIP if pressure limited

Tidal volume if volume cycled

Inspiratory time if time cycled

Flow

SIMV rate

Flow cycling …….

Insp. Terminated at % of peak

flow rather than time…………

Synchronizes expiratory and

Insp. flow thus total synchrony

achieved.

When SIMV is used, the patient receives three different types of breath:

The controlled (Mandatory) breath.

Assisted (synchronized) breaths.

Spontaneous breaths, which can be pressure supported.

A B C

A .Controlled and time triggered

B .Spontaneous

C .Synchronized and assisted

Flow

PaW

Volume

Trigger

Spontaneous

breath Assisted breath

Back up ventilation period

Controlled breath

Spontaneous

volume

Trigger

Spontaneous

breath

Pressure

Volume

Pressure support

Pressure support ventilation is a spontaneous mode

of ventilation.

Inspiratory effort is assisted by the ventilator at an

airway pressure that remains constant during the

phase of inspiration.

Inspiration is terminated when the peak Inspiratory

flow reaches a preset level. (usually 25%)

Patient determines………………

• Rate

• Inspiratory time

• Airflow

PRVC

A control mode, which

delivers a set tidal volume with

each breath at the lowest

possible peak pressure.

Delivers the breath with a

decelerating flow pattern that is

thought to be less injurious to

the lung……

Volume Support

Equivalent to pressure support

set a “goal” tidal volume

the machine watches the delivered

volumes and adjusts the pressure

support to meet desired “goal” within limits set by you.

Airway Pressure Release Ventilation

Can be thought of as giving a patient

two different levels of CPAP

Set “high” and “low” pressures with

release time.

Length of time at “high” pressure

generally greater than length of time at

“low” pressure.

By “releasing” to lower pressure, lung

volume is allowed to decrease to FRC

Certain other issues

Fixed insp. Time

Termination sens. off Termination sens. on

Flow

Pressure

Inspiratory cycle off…………

Proper Inspiratory cycle termination

avoids lung hyperinflation and

Increased work of breathing

40 %

10 %

10 %

5%

1%

PEEP PIP

Vt.

COMPLIANCE LINE

Pressure limited…

Over distension

PEEP PIP

Vt.

COMPLIANCE LINE

Pressure limited…

PEEP PIP

Vt.

COMPLIANCE LINE

Volume limited…

Pressure – Volume loops

Low

compliance

A

B C

D

Volume

Pressure in CM

B C

Volume

Pressure in CM A

D

Normal resistance

Increased resistance

volume

Pressure

Triggering ……………………

Triggering ……………………

Neuro – Ventilatory coupling ………….

Central Nervous System

Phrenic Nerve

Diaphragmatic contraction

Chest Wall and Lung expansion

Air way pressure , flow and volume

NAVA Ventilator

Current

Technology

Ideal technology

Neurally Adjusted

Ventilatory Assist

EA di

Waveform

Tidal volume ÷ P plat – PEEP Tidal volume ÷ PIP – PEEP PIP – P plat ÷ Flow rate PIP- P2 ÷ Flow rate

Static compliance

Dynamic

characteristics Maximum resistance

index

Minimum resistance

index

Elastic and resistive

property of respiratory

System

Time constant

×

Resistive property

Conditional variables

Control variables

Pressure : Volume : Flow

Phase Variables

Trigger : Limit : cycle : Base line

You can comfortably SLEEP even in difficult situation

if you know your physiology well……………

Thanks

SLEEP