BASICS OF WAVEFORM INTERPRETATION

Michael Haines, MPH, RRT-NPS, AE-C

Objectives

• Identify graphic display options provided by mechanical ventilators.

• Describe how to use graphics to more appropriately adjust the patient

ventilator interface.

Monitoring and analysis of graphic display of curves and loops during mechanical

ventilation has become a useful and popular way to determine not only how patient are being ventilated but also a way to assess problems occurring during ventilation.

Uses of Flow, Volume, and Pressure Graphic Display

• Confirm mode functions• Detect auto-PEEP• Determine P-V synchrony• Assess and adjust trigger levels• Measure the work of breathing• Adjust tidal volume and minimize overdistension • Assess the effect of bronchodilator administration• Detect equipment malfunctions• Determine appropriate PEEP level

Uses of Flow, Volume, and Pressure Graphic Display

• Evaluate adequacy of inspiratory time in pressure control ventilation

• Detect the presence and rate of continuous leaks• Assess inspiratory termination criteria during

Pressure Support Ventilation• Determine appropriate Rise Time

The graphic display of flow, pressure and volume is generally visualized in two

formats:

WaveformsLoops

Most Commonly used Waveforms

• Pressure vs. Time

• Flow vs. Time

• Volume vs. Time

Pressure vs. Time Curve

1 2 3 4 5 6

30

SecPawcmH2O

A BC

PIP

Baseline

Mean Airway Pressure

-10

Pressure-Time Curve

1 2 3 4 5 6

20

Sec

PawcmH2O

Pressure Ventilation

Expiration

Volume Ventilation

Pre

ssur

e

time

PIP:complianceresistancevolumeflowPEEP

PEEP

PEEP

PIP

Pplat

resistanceflow

compliancetidal volume

No active breathingTreats lung as single unit

end-inspiratoryalveolar pressure

Work to Trigger

1 2 3 4 5 6

30

Sec

PawcmH2O

-10

Adequate Flow During Volume-Control Ventilation

30

Time (s)

-10

1 2

awPcmH2O

Adequate flow

3

Inadequate Flow During Volume-Control Ventilation

30

Time (s)

-10

1 2

awPcmH2O

Adequate flow

Flow set too low3

Patient/Ventilator SynchronyVolume Ventilator Delivering a Preset Flow and Volume

Adequate Flow

1 2 3 4 5 6

-20

SecPawcmH2O

Patient/Ventilator SynchronyThe Patient Outbreathing the Set Flow

Air Starvation

1 2 3 4 5 6

-20

SecPawcmH2O

Plateau Time

Inadequate plateau time

-20

1 2 3 4 5 6

30

SECPawcmH2O

Adequate Plateau Time

-20

1 2 3 4 5 6

30

SECPawcmH2O

Plateau Time

Flow vs.Time Curve

1 2 3 4 5 6SEC

120

120EXH

INSP

V.

LPM

Inspiration

Flow vs.Time Curve

1 2 3 4 5 6SEC

120

120EXH

INSP

V.

LPM

Inspiration

Expiration

Flow vs.Time Curve

1 2 3 4 5 6SEC

120

120EXH

INSP

Inspiration

V.

LPM

Constant Flow Descending Ramp

Flow-Time Curve

1 2 3 4 5 6SEC

120

120EXH

INSP

Insp. Pause

Expiration

V.

LPM

Inspiratory TimeShort Normal Long

1 2 3 4 5 6

SEC

120

-120

V.

LPM

Expiratory Flow Rate and Changes in Expiratory Resistance

1 2 3 4 5 6

SEC

120

120

V.

LPM

Obstructed Lung

Delayed flow return

Combined Screens

1 2 3 4 5 6

20

Sec

PawcmH2O

V.

Volume Ventilation

Pressure-Time and Flow-Time Curves

1 2 3 4 5 6

20

Sec

PawcmH2O

Expiration

V.

Volume Ventilation

Pressure-Time and Flow-Time CurvesDifferent Inspiratory Flow Patterns

1 2 3 4 5 6

20

Sec

PawcmH2O

Expiration

V.

Volume Ventilation

Inspiration

20

Pressure-Time and Flow-Time Curves

1 2 3 4 5 6Sec

PawcmH2O

V.

Pressure Ventilation

Inspiratory Time

Volume Ventilation

Rise TimeInspiratoty Rise Time Percentage

Flow Acceleration Percentage

How quickly inspiratory flow accelerates to achieve set pressure.

Time

Minimal Pressure Overshoot

Pressure Relief

Slow rise Moderate rise Fast rise

P

V.

Flow Acceleration Percent Rise Time

Patient / Ventilator SynchronyVolume Ventilation Delivering a Preset Flow and Volume

Adequate Flow

1 2 3 4 5 6

30

-20

SecPawcmH2O

What options do we have?

Air Starvation

1 2 3 4 5 6

30

-20

SecPawcmH2O

Patient / Ventilator SynchronyThe Patient Is Outbreathing the Set Flow

We Can Switch to a Decelerating Flow Pattern: More Flow Up Front

1 2 3 4 5 6

SEC

120

-120

V.

LPM

If Peak Flow Remains the Same, I-Time Increases: Could Cause Asynchrony

LPM1 2 3 4 5 6

SEC

120

-120

V.

Changing Flow Waveform in Volume Ventilation: Effect on Inspiratory Time

1 2 3 4 5 6

SEC

120

-120

V.

LPM

Increased Peak Flow: Decreased Inspiratory Time

1 2 3 4 5 6

SEC

120

-120

V.

LPM

Note: There can still be pressure in the lung behind airways that are completely obstructed

Detecting Auto-PEEP

LPM

Zero flow at end exhalation indicates equilibration of lung and circuit pressure

1 2 3 4 5 6

SEC

120

-120

V.

Detecting Auto-PEEP

The transition from expiratory to inspiratory occurs without the expiratory flow returning to zero

1 2 3 4 5 6

SEC

120

120

V.

LPM

time

flow

inhalation

exhalation

0auto-PEEP

Flow Waveform

sensitivity-1 cm H2O

auto-PEEP10 cm H2O

trigger effort = 11 cm H2O

sensitivity-1 cm H2O

auto-PEEP3 cm H2O

trigger effort = 4 cm H2O

PEEP7 cm H2O

PEEP10 cm H2O

PEEP10 cm H2O

Auto-PEEP should be measured with set PEEP = 0

Volume vs.Time Curve

Inspiration

SEC

800 ml

2 3 4 5 61

VT

Volume vs.Time Curve

Expiration

SEC

800 ml

2 3 4 5 61

VT

Typical Volume Curve

1 2 3 4 5 6

SEC

1.2

-0.4

VT

Liters

I-TimeE-Time

A B

A = inspiratory volume

B = expiratory volume

Air Trapping or Leaks

1 2 3 4 5 6

SEC

1.2

-0.4

VT

Liters

A

A = exhalation that does not return to zero

Loops

• Pressure-Volume Loops

• Flow-Volume Loops

Pressure-Volume Loop

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

VT

Mandatory Breath

Inspiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

VT

Mandatory Breath

Expiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

Inspiration

VT Counterclockwise

Spontaneous Breath

Inspiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

VTClockwise

Spontaneous Breath

InspirationExpiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

VTClockwise

Work of Breathing

0 20 40 60-20-40-60

0.2

0.4

0.6

LITERS

PawcmH2O

VT

Assisted Breath

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

Assisted Breath

VT

Assisted Breath

Inspiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

Assisted Breath

VT

Assisted Breath

Inspiration

Expiration

0 20 40 602040-60

0.2

LITERS

0.4

0.6

PawcmH2O

Assisted Breath

VT Clockwise to Counterclockwise

Pressure-Volume Loop Changes

0 20 40 60-20-40-60

0.2

0.4

0.6

LITERS

PawcmH2O

VT

Changes in Compliances

Indicates a drop in compliance (higher pressure for the same volume)

0 20 40 602040-60

0.2

0.4

0.6

LITERS

PawcmH2O

VT

Lung Overdistension

Overdistension

B

A

0 20 40 60-20-40-60

0.2

0.4

0.6

LITERS

PawcmH2O

C

A = inspiratory pressure

B = upper inflection point

C = lower inflection point

VT

Pressure – Volume Loops

Pressure – Volume Loops

Pressure – Volume Loops

Flow -Volume Loops Volume Control

Flow

Volume

Tidal Volume

Inspiration

Expiration

Flow -Volume Loops Volume Control

Flow

Volume

Peak Expiratory FlowPeak Inspiratory FlowTidal Volume

Inspiration

Expiration

ETT or Circuit Leaks

Obstructive Pattern

Bronchodilator Response

2

1

1

2

3

3

VLPS

.

BEFORE

VLPS

.

Bronchodilator Response

2

1

1

2

3

3

VLPS

.

BEFORE AFTERWorse

2

1

1

2

3

3

VLPS

.

Bronchodilator Response

2

1

1

2

3

3

VLPS

.VT

INSP

EXH

BEFORE AFTERWorse Better

2

1

1

2

3

3

VLPS

.

2

1

1

2

3

3

VLPS

.

What Mode is This?

What Mode is This?

What Mode is This?

What Mode is This?

Remember Waveforms and loops are graphical representation of the data collected by

the ventilator. Typical Tracings Pressure-time, Flow-time, Volume -time Loops Pressure-Volume Flow-Volume

Assessment of pressure, flow and volume waveforms is a key aspect in the management of the mechanically ventilated patient.

The End!