Ventilator Graphics

高雄長庚醫院

胸腔內科 呼吸治療科

吳沼漧醫師

Introduction

Understanding ventilator graphics is an

integral part of adequately treating

patients on mechanical ventilators.

Just as pulmonary functions tests are

used to better understand the lung

pathophysiology in nonmechanically

ventilated patients, ventilator graphics

are an important part of understanding

the pathophysiology in mechanically

ventilated patients.

Understanding airway pressures The respiratory system can be thought of as a mechanical

system consisting of resistive (airways +ET tube) and elastic

(lungs and chest wall) elements in series

Diaphragm

ET Tube

airways

Chest wall

PPL Pleural pressure

Paw Airway pressure

Palv Alveolar pressure

ET tube + Airways

(resistive element)

Resistive pressure varies with airflow

and the diameter of ETT and airways.

Flow resistance

The elastic pressure varies with volume and

stiffness of lungs and chest wall.

Pel = Volume x 1/Compliance

Paw = Flow X Resistance + Volume x 1/Compliance THUS

Lungs + Chest wall

(elastic element)

Airways + ET tube

(resistive element)

Lungs + Chest wall

(elastic element)

Understanding the basic

ventilator circuit diagram

ventilator

Diaphragm

Essentially the circuit diagram of a

mechanically ventilated patient can be

broken down into two parts…..

The ventilator makes up the first part

of the circuit. Its pump like action is

depicted simplistically as a piston

that moves in a reciprocating fashion

during the respiratory cycle.

The patient’s own respiratory system

makes up the 2nd part of the circuit.

The diaphragm is also shown as a

2nd piston; causing air to be drawn into

the lungs during contraction.

These two systems are connected by

an endotracheal tube which we can

consider as an extension of the

patient’s airways.

ET Tube

airways

Chest wall

Let us now understand how the respiratory systems’

inherent elastance and resistance to airflow

determines the pressures generated within a

mechanically ventilated system.

Ventilator

Diaphragm

RET tube

Rairways

Raw

Understanding basic respiratory mechanics

The total ‘airway’ resistance (Raw) in the mechanically ventilated patient

is equal to the sum of the resistances offered

by the endotracheal tube (R ET tube)

and the patient’s airways ( R airways)

The total ‘elastic’ resistance (Ers) offered by the

respiratory system is equal to the sum of

elastic resistances offered by the

Lung E lungs and the

chest wall E chest wall

Elungs

Echest wall

Thus to move air into the lungs at any given time (t),

the ventilator has to generate sufficient

pressure (Paw(t)) to overcome the combined

elastic (Pel (t)) and resistance (Pres(t)) properties

of the respiratory system

Ers

ET Tube

airways

Thus the equation of motion for the respiratory system

is

Paw (t) = Pres (t) + Pel (t)

ventilator

Diaphragm

Ppeak

Pres

RET tube

Rairways

Pres

Pplat

Understanding the pressure-time waveform

using a ‘square wave’ flow pattern

time

pre

ssure

The pressure-time waveform is a reflection

of the pressures generated within the

airways during each phase of the

ventilatory cycle.

At the beginning of the inspiratory cycle,

the ventilator has to generate a pressure Pres

to overcome the airway resistance.

Note: No volume is delivered at this time.

After this, the pressure rises in a linear fashion

to finally reach Ppeak. Again at end inspiration,

air flow is zero and the pressure drops by an

amount equal to Pres to reach the plateau

pressure Pplat. The pressure returns to

baseline during passive expiration.

Pres

Now let’s look at some different pressure-time

waveforms using a ‘square wave’ flow pattern

This is a normal pressure-time waveform

With normal peak pressures ( Ppeak) ;

plateau pressures (Pplat )and

airway resistance pressures (Pres)

time

pre

ssure

Pres

Pplat

Pres

Scenario # 1

Paw(peak) = Flow x Resistance + Volume x 1/ Compliance

time

flo

w

‘Square wave’

flow pattern

Paw(peak)

Waveform showing high airways resistance

This is an abnormal pressure-time waveform

time

pre

ssure

Ppeak

Pres

Pplat

Pres

Scenario # 2

The increase in the peak airway pressure is driven

entirely by an increase in the airways resistance

pressure. Note the normal plateau pressure.

e.g. ET tube

blockage

Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP

time

flo

w

‘Square wave’

flow pattern

Normal

Three areas of clinical

interest in ventilator graphics

Dynamic characteristics of the

respiratory system.

Static compliance of the respiratory

system.

Patient-ventilator interaction--

Asynchrony, synchrony.

Using Ventilator Graphics to

Identify Patient-Ventilator

Asynchrony

Patient-ventilator interaction can be

described as the relationship between 2

respiratory pumps: (1) the patient’s

pulmonary system, which is controlled by

the neuromuscular system and influenced

by the mechanical characteristics of the

lungs and thorax, and (2) the ventilator,

which is controlled by the ventilator

settings.

Clinical applications of ventilator

waveform analysis

• Ventilator waveforms can be very useful in many

different situations including:

– Diagnosing a ventilator that is ‘alarming’

– Detecting obstructive flow patterns on the ventilator

– Detecting air trapping and dynamic hyperinflation

– Detecting lung overdistention

– Detecting respiratory circuit secretion build-up

– Detecting patient-ventilator interactions

• Dyssynchrony

• Double triggering

• Wasted efforts

• Flow starvation

When the 2 pumps function in

synchrony, every phase of the breath is

perfectly matched.

Anything that upsets the harmony

between the 2 pumps results in

asynchrony and causes patient

discomfort and unnecessarily increases

work of breathing.

Deleterious Effects of Patient-

Ventilator Asynchrony Patient fights the ventilator

More sedation required

Higher work of breathing

Muscle damage

Ventilation-perfusion problems

Dynamic hyperinflation

Delayed or prolonged weaning

Longer stay

Higher costs

Factors That Affect Patient-Ventilator

Synchrony Ventilator Factors

Trigger variables

Sensitivity setting

Rise-time capability

Design, mode, and

settings of the flow

delivery system

Flow pattern selected

Design of the exhalation

valve

How PEEP is generated

by the software

Extraneous flow (eg, from

a nebulizer, oxygen)

Patient Factors

Sedation level: pain,

splinting

Inspiratory

effort/respiratory drive;

neural timing

Pathology of the

respiratory system or

abdomen; secretions

Intrinsic PEEP

Size and type of airway

Presence of leaks

Graphical displays

Graphical displays are common in the

ICU. The following are examples of

graphical displays:

Ventilator waveforms

Arterial waveforms

ECG waveforms

Venous waveforms (central venous

pressure)

Intracranial pressure waveforms

……………

Types of Ventilator

Waveforms Scalar:

Pressure – time

Volume – time

Flow – time

Loops:

Pressure – volume

Volume – Flow

Flow – pressure

Trends:

4 components of a breath

Dividing a mechanical breath into the

following 4 components helps to better

identify and correct asynchronies…

Trigger phase

Inspiratory phase

Cycle phase

Expiratory phase

Dyssynchrony = asynchrony

Definition: Uncoupling of the

mechanical delivered breath and neural

respiratory effort.

Dysynchrony can occur in each phase

of the breath

Trigger phase

Inspiratory phase

Cycle phase

Expiratory phase

Trigger Phase

The mechanical breath may be initiated

by the patient (patient trigger) or as a

function of time (time trigger).

Patient trigger-- The 2 common types of

triggering available are pressure and

flow.

Patient Triggering

Trigger phase: 4 components

Trigger pressure (TP)

Inspiratory trigger time (ITT)

Rise time to baseline pressure (RTBP)

Inspiratory delay time (IDT)

The trigger phase can be

divided into 4 components

Trigger pressure (TP) is the pressure that

must be attained by patient effort to trigger

the mechanical breath.

Inspiratory trigger time (ITT) is the time

elapsed from the patient’s effort to reach

the TP set on the mechanical pressure.

in patients with a low respiratory drive, this time

could be prolonged and vice versa.

Rise time to baseline

pressure (RTBP)

RTBP is the time elapsed from the

mechanical breath trigger to attain the

baseline pressure (the PEEP).

The patient does not receive any support until

the circuit is pressurized to the baseline

pressure, and considerable work could be

spent during this time if not appropriately set.

Inappropriate rise times can also affect the

time for the pressure to rise to the peak

airway pressure.

Inspiratory delay time (IDT)

Inspiratory delay time (IDT) is the total

time elapsed from the initial patient

effort to the pressurization of the circuit

to baseline pressure.

In other words, the IDT can be expressed

by the following equation: IDT = ITT +

RTBP.

Trigger-phase asynchrony

Trigger asynchrony can occur with any

mode of mechanical ventilation.

Common trigger problems include

autotriggering, missed triggering, and

double triggering.

Appropriate valve sensitivity settings

are required to avoid overtly sensitive

settings that can lead to autotriggering

and insensitive settings that can lead to

missed triggering.

Auto-triggering

Auto-triggering

Ventilator show “assisted mode”, but no

patient trigger waveform.

Double triggering

Double triggering is the delivery of 2

consecutive ventilator cycles separated

by a very short expiratory time, with the

first cycle being patient triggered.

Double triggering occurs when the

patient’s ventilatory demand is high and

the inspiratory time set on the ventilator

is too short.

Double triggering

Double triggering

Double triggering occurs more

commonly in patients whose

PaO2/FiO2 ratio is lower, such acute

lung injury or ARDS.

Management: Increasing the inspiratory

time or increasing the tidal volumes.

Volume mode changing to a variable flow

(eg, pressure-control ventilation) or a dual-

control mode.

Sedation adjustments

Triggering delay

Trigger delay indicated by a large

negative deflection in pressure

waveform prior to delivery of breath.

ineffective efforts

(Trigger failure)

Trigger failure patient factors

low respiratory drive: Alkaline pH

weak inspiratory muscles

partially blocked ETT or tracheostomy

dynamic hyperinflation resulting in intrinsic

PEEP, eg COPD

ventilator factors

high level of pressure support or high tidal

volume causing intrinsic PEEP

expiratory asynchrony with delayed opening

of exhalation valve

in-line nebulizers: flow >> pressure triggering

Air Trapping

Inspiration

Expiration

Normal Abnormal

Time (sec)

Flo

w (

L/m

in)

Air Trapping Auto-PEEP

}

※ Causes of air trapping or auto PEEP?

※ How to measure the magnitude of auto PEEP?

1T 2T 3T 1T 2T 3T

Trial ＆ error for

applied PEEP

If measured PEEPi is not available, trial and error can be used to provide applied PEEP.

One would add small increments (e.g., 2 cm H20) and watch the patient response. If successful, the patient effort to trigger the breaths should become less as the appropriate level of PEEP is provided.

Increasing dyspnea and other signs of increasing intrathoracic pressure (e.g., decreasing blood pressure, worsening of clinical signs of hyperinflation:↓BS, chest hyperresonant to percussion) suggest that excessive PEEP is being provided.

NAVA ( Neurally Adjusted

Ventilatory Assist ) in COPD

synchronization with NAVA is perfect.

Inspiratory Phase

Flow may be inadequate or excessive,

both of which may contribute to patient-

ventilator asynchrony.

inappropriate flow rates and patterns

may lead to a number of secondary

asynchronies.

For example, inappropriate flow may

reduce the expiratory time, leading to

auto-PEEP, which may lead to

ineffective triggering.

Flow can be delivered in the

following 3 forms Fixed flow (eg, CPPV, SIMS)

Variable flow (eg, PCPPV)

Combined fixed and variable flows (dual

modes, eg, volume-assured pressure

support and pressure augmentation)

Asynchrony in

inspiration phase

Inadequate Inspiratory Flow

Induce pt-ventilator dys-synchrony

Adequate Flow

Time (sec)

Inadequate Flow

Pa

w

(cm

H2O

) F

low

Flow starvation

The Stress Index

• In AC volume ventilation using a

constant flow waveform observe

the pressure time scalar.

• Normal, linear change in airway

pressure

Stress index =1

• Upward concavity indicates

decreased compliance and lung

overdistension

Stress index > 1

• Downward concavity indicates

increased compliance and

potential alveolar recruitment

Stress index < 1

flow

time

Paw

Note: Patient effort must be absent

STRESS INDEX (SI) and

prevention of VILI Atelectrauma : SI < 0.9

Barotrauma: SI > 1.1

The SI window of the SERVO-i : your

guide to lung protection

A peak ? B peak ? What’s the cycle-off in figure?

How to adjust rise-time or adjustable flow-cycle?

What’s the cycle-off criteria for PSV ?

Pressure

Flow

Volume

(L/min)

(cm H2O)

(ml)

Time (sec)

Normal inadequate rise time

A primary asynchrony may lead to

multiple secondary asynchronies.

Premature inspiratory-

expiratory cycling inspiratory muscle contraction

continues into mechanical expiratory

phase

associated with modes of ventilation

with short inspiratory time

in pressure support mode:

low levels of pressure support

short respiratory time constant (eg ARDS)

relatively high cycling off threshold

dynamic hyperinflation

Premature inspiratory-

expiratory cycling may result in a characteristic change in

flow waveform

Flow先高後低

Delayed opening of

exhalation valve mechanical inspiration continues into

neural expiration

may result in dynamic hyperinflation,

particularly in patients with COPD which

may increase trigger dyssynchrony

associated with long set inspiratory time

in pressure support mode:

long respiratory time constant (eg COPD)

high pressure support level

low cycling off threshold

The subject’s expiratory effort begins just

prior to the end of the mechanical inspiratory

time, which causes pressure spikes (arrows).

Also note the small inspiratory zero flow

plateau at the end of the inspiratory flow

waveform, which indicates that flow into the

lung has stopped just prior to mechanical

expiration.

Recognizing active expiration (pressure support)

Look at the flow-time

& pressure-time

waveform

Patient’s active expiration

during the inspiratory

phase causes a pressure spike.

Notice the high and variable

expiratory flow rates due to

varying expiratory muscle effort

This is a high drive state where increased sedation/paralysis and mode change may be appropriate for lung protection.

Pressure

Flow

Volume

(L/min)

(cm H2O)

(ml)

Time (sec)

BIPAP set too long Ti

Flow先低後高

Expiratory Phase--Shortened expiatory time

may lead to auto-PEEP

Inspiration

Expiration

Normal Abnormal

Time (sec)

Flo

w (

L/m

in)

Air Trapping Auto-PEEP

} 1T 2T 3T

1T 2T 3T

Understanding how flow rates affect I/E

ratios and the development of auto PEEP

Lluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1

Decreasing the flow rate

Increase the inspiratory time

and consequently decrease the

expiratory time

(decreased I/E ratio)

Thus allowing incomplete emptying

of the lung and the development

of air trapping and auto-PEEP

A Higher Expiratory Flow Rate and a Decreased Expiratory Time Denote a

Lower Expiratory Resistance

1 2 3 4 5 6

SEC

120

120

V .

LPM

Pressue-Volume loop-- overdistension

B

A

0 20 40 60 -20 -40 -60

0.2

0.4

0.6

LITERS

Paw

cmH2O

C

A = inspiratory pressure B = upper inflection point C = lower inflection point

VT

”beaking”

Airway Resistance

Increased expiratory resistance:

secretions, bronchospasms….

Increased inspiratory resistance: kinked

ET tube, patient biting tube….

Flow-Volume Loop

Normal airway obstruction

PIP vs Pplat

Normal High Raw

High Flow Low Compliance

Time (sec)

Paw

(c

m H

2O

)

PIP

PPlat

PIP

PIP PIP

PPlat PPlat

PPlat

Active exhalation and volume gain

Airway Secretions/ Water in the Circuit

Inspiration

Expiration

Volume (ml)

Flow (L/min)

Normal: Abnormal:

“saw-tooth” pattern, on the expiratory part. on the

inspiratory curve in severe case.

Flow (L/min)

Time (sec)

What happen to PSV ?

Time (sec)

Flow (L/m)

Pressure (cm H2O)

Volume (mL)

The most common

cause ?

Inspiration

Expiration

Time (sec)

Flo

w (

L/m

in)

Paw (cm H2O)

Paw

(c

m H

2O

)