1. Ventilator Waveforms: Basic Interpretation and Analysis
Vivek Iyer MD, MPH
Steven Holets, RRT CCRA
Edited for ATS by:
Cameron Dezfulian, MD and Nitin Seam, MD

2. Outline of this presentation
Goal:
To provide an introduction to the concept of ventilator waveform analysis in an interactive fashion.
Content:
Outline of types of ventilatory waveforms.
Introduction to respiratory mechanics and the ‘Equation Of Motion’ for the respiratory system
Development of the concept of ventilator waveforms
Illustrations and videos of waveforms to illustrate their practical applications and usefulness.

3. Types of Ventilator Waveforms: Scalars and Loops
Scalars are waveform representations of pressure, flow or volume on the y axis vs time on the x axis
Flow vs time
scalar
Inspiratory
arm
Expiratory
arm
Pressure vs time
scalar
Volume vs time
scalar

4. Types of Ventilator Waveforms: Scalars and Loops
Loops are representations of pressure vs volume or flow vs volume
Expiratory
arm
Pressure Vs volume
loop
volume
pressure
Inspiratory
arm
Flow Vs volume
loop
Expiratory
arm
flow
volume

5. Understanding the flow-time waveform
Inspiration
There are two commonly used types of flow patterns available on most ventilators
The ‘square wave’ or ‘constant flow’ pattern
The ‘decelerating wave’ or ‘ramp type’ pattern
Volume preset mode: determined by ventilator setting
Pressure preset mode: reflects respiratory mechanics and patient effort.
Expiration
If muscles inactive: determined by elastic recoil of the respiratory system and resistance of intubated airways.
If muscles active: waveform influenced by patient effort in addition to above.

6. The ‘square wave’ flow pattern
The inspiratory flow rate
remains constant over
the entire inspiration.
Inspiratory
arm
flow
The expiratory flow is
determined by the
elastic recoil of the
respiratory system and
resistance of intubated
airways
time
Expiratory
arm
Inspiratory time = Tidal volume
Flow rate

7. The ‘decelerating ramp’ flow pattern
The inspiratory flow rate
decelerates as a function
of time to reach zero flow
at end inspiration
time
flow
Inspiratory
arm
For a given tidal volume,
the inspiratory time is
longer in this type of flow
pattern as compared to
the square wave pattern
Expiratory
arm
Inspiratory time = Tidal volume
Flow rate

8. Now let us try to understand the following in the next few slides
Airway pressures
A basic ventilator circuit diagram
The equation of motion for the respiratory system
The pressure-time waveform

9. 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
THUS
Paw = Flow X Resistance + Volume x 1/Compliance
Diaphragm
ET Tube
airways
Chest wall
PPL
Pleural pressure
Paw
Airway pressure
Palv
Alveolar pressure
ET tube + Airways
(resistive element)
Lungs + Chest wall
(elastic element)
Airways + ET tube
(resistive element)
Lungs + Chest wall
(elastic element)
Resistive pressure varies with airflow
and the diameter of ETT and airways.
The elastic pressure varies with volume and
stiffness of lungs and chest wall.
Pel = Volume x 1/Compliance
Flow resistance

10. Understanding the basic ventilator circuit diagram
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.
ET Tube
These two systems are connected by
an endotracheal tube which we can
consider as an extension of the
patient’s airways.
airways
Diaphragm
Chest wall

11. Understanding basic respiratory mechanics
Thus the equation of motion for the respiratory system is
Ventilator
Paw (t) =
Pres (t) +
Pel (t)
Elungs
RET tube
ET Tube
Raw
Ers
airways
Rairways
Echest wall
Let us now understand how the respiratory systems’
inherent elastance and resistance to airflow
determines the pressures generated within a
mechanically ventilated system.
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
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
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)
Diaphragm

12. Understanding the pressure-time waveform
using a ‘square wave’ flow pattern
Ppeak
Pres
pressure
ventilator
Pplat
Pres
RET tube
time
Pres
Rairways
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.
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.
The pressure-time waveform is a reflection
of the pressures generated within the
airways during each phase of the
ventilatory cycle.
Diaphragm

13. Now let’s look at some different pressure-time waveforms using a ‘square wave’ flow pattern
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance
Scenario # 1
Paw(peak)
pressure
Pres
Pplat
Pres
time
flow
‘Square wave’
flow pattern
time
This is a normal pressure-time waveform
With normal peak pressures ( Ppeak) ;
plateau pressures (Pplat )and
airway resistance pressures (Pres)

14. Waveform showing high airways resistance
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP
Scenario # 2
Normal
Ppeak
pressure
Pres
e.g. ET tube
blockage
Pplat
Pres
time
flow
‘Square wave’
flow pattern
time
The increase in the peak airway pressure is driven
entirely by an increase in the airways resistance
pressure. Note the normal plateau pressure.
This is an abnormal pressure-time waveform 14

15. Waveform showing increased airways resistance
‘Square
wave’ flow
pattern
Ppeak
Pplat
Pres

16. Waveform showing high inspiratory flow rates
Paw(peak) = Flow x Resistance + Volume x 1/compliance + PEEP
Scenario # 3
Paw(peak)
Normal
pressure
Pres
e.g. high flow
rates
Pplat
Pres
time
flow
‘Square wave’
flow pattern
time
Normal (low)
flow rate
The increase in the peak airway pressure is caused
by high inspiratory flow rate and airways resistance.
Note the shortened inspiratory time and high flow
This is an abnormal pressure-time waveform

17. Waveform showing decreased lung compliance
Paw(peak) = Flow x Resistance + Volume x 1/ Compliance + PEEP
Scenario # 4
Paw(peak)
Normal
pressure
e.g. ARDS
Pres
Pplat
Pres
time
flow
‘Square wave’
flow pattern
time
The increase in the peak airway pressure is driven
by the decrease in the lung compliance.
Increased airways resistance is often
also a part of this scenario.
This is an abnormal pressure-time waveform

18. Waveform showing decreased lung compliance
‘Square
wave’ flow
pattern
Ppeak
Pplat
Pres

19. Now lets look at pressure-time tracings using a ‘decelerating ramp’ flow pattern
High initial pressure spike reflecting airways resistance and flow settings.
As flow declines pressure drops eventually reflecting plateau pressure
40 cmH20
time
Low compliance, pressure rises until end of flow. High end inspiratory peak and plateau pressures
60 cmH20
time
Normal resistance and compliance, initial pressure rise ( flow setting and ETT resistance) then slope flattens as flow and volume delivery ends.
20 cmH20
time

20. Now let us try to understand the practical aspects of ventilator waveform analysis in an interactive fashion.

21. 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

22. Some ventilators with waveform displays
Puritan Bennett 840
Puritan Bennett 7200
Dräger Evita XL
Siemens Servo 300A
Bear 1000 series
Respironics Esprit

23. Waveform selection on different ventilators
PB 840
Ventilator
Select different
waveforms
Size adjustment
Time scale
Push to start waveforms

24. Waveform selection on different ventilators
Respironics
Espirit
ventilator
Push to select waveforms

25. Waveform selection on different ventilators
Switch between
waveforms
Respironics
Espirit
ventilator
Press to adjust size
Switch between
loops and scalars

26. Variables that govern how a ventilator functions and interacts with the patient
Control variable
‘The Mode of Ventilation’
Pressure, flow, or volume
controlled
Limit Variable
Volume, pressure or flow
can be set to be constant
or reach a maximum
Triggering variable
pressure, flow or volume
sensing that initiates
the vent cycle
Cycle variable
Pressure, volume, flow,
or time that ends the
inspiratory phase

27. So what waveforms should I be observing and analyzing?
HINT
Look at the waveforms that are ‘dynamic’ for the current ventilator settings

28. Mode of ventilation -> useful waveforms
Independent variables
Dependent variables
Waveforms that will be useful
Waveforms that normally remain unchanged
Volume Control/ Assist-Control
Tidal volume, RR, Flow rate, PEEP, I/E ratio
Paw
Pressure-time:
Changes in Pip, Pplat
Flow-time (expiratory): Changes in compliance
Pressure-volume loop: Overdistension, optimal PEEP
Volume-time
Flow time (inspiratory)
Flow-volume loop
Pressure Control
Paw, Inspiratory time (RR), PEEP and I/E ratio
Vt, flow
Volume-time and flow-time: Changes in Vt and compliance
Pressure-time
Pressure support/
CPAP
PS and PEEP
Vt,and RR, flow, I/E Ratio
Volume- time
Flow- time
(for Vt and VE)
Vt=tidal volume; RR=respiratory rate; Paw=airway pressure; PEEP= positive end expiratory pressure;
I/E ratio= inspiratory/expiratory time; VE= minute ventilation; Pip = Peak inspiratory pressure; Pplat = Plateau pressure

29. Waveforms to observe during volume assist/control ventilation
Pressure-time waveform:
Is ‘dynamic’ and is affected by patient effort and changes in respiratory system compliance and resistance.
Flow-time waveform:
The inspiratory arm of the flow time waveform is ‘static’ and reflects what you set for flows.
But the expiratory flow-time waveform is ‘dynamic’ and reflects the elastic recoil pressure of respiratory system and patient effort

30. Waveforms to observe during pressure targeted (PS or PCV) ventilation
Pressure-time waveform:
Is ‘static’ and reflects the pressure targets you selected for ventilation
Flow-time and volume-time waveform:
Are ‘dynamic’ and reflect the patient’s intrinsic respiratory effort and changes in compliance and resistance of the respiratory system
PS= pressure support ventilation; PCV= pressure control ventilation

31. Waveforms to observe during pressure targeted ventilation: PCV***
Pressure-time waveform usually will not change
Flow-time and volume-time waveform will be affected by changes in compliance, resistance and the patient’s respiratory muscle strength (independent variables)

32. Now let us begin riding the ‘waves’ by looking at a few ventilator waveforms!

33. Basic ventilator waveforms
Mode of ventilation: Assist/control volume – square wave flow
Airway pressures: dependent on lung compliance, tidal volume and flow (dependent variable)
Tidal volumes, respiratory rate: ventilator controlled
Flow pattern: ventilator controlled (square wave pattern)
Inspiratory time: ventilator controlled (flow setting)
Waveforms shown: flow-time and pressure-time

34. Square wave volume assist/control mode
Any abnormalities? : No
PEARL: always look at both inspiratory and expiratory arms of the flow-time waveform. Make it a habit!

35. Basic ventilator waveforms
Mode of ventilation: Assist/control volume – decelerating flow pattern
Airway pressures: dependent on lung compliance, tidal volume and flow (dependent variable)
Tidal volumes: ventilator controlled
Respiratory rate: ventilator controlled minimum
Flow pattern: ventilator controlled (decelerating wave pattern)
Inspiratory time: ventilator controlled
Waveforms shown: flow-time and pressure-time

36. Decelerating flow volume assist/control mode
Any abnormalities? : No
PEARL: At similar flow rates, the inspiratory time is shorter (and peak pressures higher) for the square wave flow as compared to the decelerating flow pattern.

37. Basic ventilator waveforms
Mode of ventilation: CPAP + PS
Airway pressures: limited by ventilator setting
Flow rate: determined by patient effort and rise time setting.
Inspiratory time: determined by patient effort and ventilator cycle criteria (E-sens/E-trigger) setting.
Respiratory rate: patient controlled
Waveforms shown: flow-time and volume-time

38. CPAP with Pressure Support
Any abnormalities?: No
PEARL: notice how each breath differs in flow rate and tidal volume.

39. Basic ventilator waveforms
Mode of ventilation: pressure control ventilation (PCV)
Airway pressures: ventilator controlled
Respiratory rate: ventilator controlled
Tidal volume: dependent variable (lung compliance)
Flow waveform: ventilator controlled (decelerating in this instance)
Flow rate: dependent variable (varies with changes in resistance, compliance and Pmus)
Waveforms shown: flow-time and pressure-time

40. Pressure Assist/Control – Decelerating Flow
Any abnormalities? : No
PEARL: Tidal volumes and flow rates are determined by respiratory system mechanics. Increasing inspiratory time after flow ends will only decrease expiratory time, without any increase in tidal volume.

41. Let us now shift gears and see how waveforms can help us recognize some common ventilator related problems!

43. Let us briefly revisit the flow-time waveform
As previously noted, the flow-time waveform has both an inspiratory and an expiratory arm.
The shape of the expiratory arm is determined by:
the elastic recoil of the lungs
the airways resistance
and any respiratory muscle effort made by the patient during expiration (due to patient-ventilator interaction/dys=synchrony)
It should always be looked at as part of any waveform analysis and can be diagnostic of various conditions like COPD, auto-PEEP, wasted efforts, overdistention etc.

44. Recognizing Lung Overdistension

45. Recognizing lung overdistension
Suspect this when:
There are high peak and plateau
Pressures…
Accompanied by high expiratory
flow rates
PEARL: Think of
low lung compliance (e.g. ARDS),
excessive tidal volumes,
right mainstem intubation etc

46. 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
Paw
Note: Patient effort must be absent
time

47. The pressure-volume loop can tell us a lot about lung physiology!
Compliance (C)
is markedly reduced in the
injured lung on the right as
compared to the normal lung
on the left
Normal
lung
Upper inflection point (UIP)
above this pressure,
additional alveolar recruitment
requires disproportionate
increases in applied
airway pressure
ARDS
Lower inflection point (LIP)
Can be thought of as the
minimum baseline
pressure (PEEP)
needed for optimal
alveolar recruitment

48. Observe a pressure-volume loop illustrating the concept of overdistension
Peak
inspiratory
pressure
Upper
inflection
point
The LIP purportedly shown here really represents attainment of the set inspiratory flow, not a true LIP. I’ve done enough of these VP curves on test lungs (and patients) to recognize that the flow rate has not been set sufficiently low to remove the flow artifact. I think you should emphasize that flow must be set very low in order for the pressure in this graph to meaningfully reflect a VP curve.
Added comment. Seeing that LIP on inspiratory limb has fallen out of favor should we delete?
Note: During normal ventilation the LIP cannot be assessed due to the effect of the inspiratory flow which shifts the curve to the right 48

49. Recognizing Auto-PEEP

50. Detecting Auto-PEEP Recognize Auto-PEEP when Expiratory flow continues
and fails to return to
the baseline prior to the new
inspiratory cycle

51. The development of auto- PEEP over several breaths in a simulation
Notice how the expiratory flow fails
to return to the baseline indicating
air trapping (AutoPEEP)
Also notice how air trapping causes
an increase in airway pressure
due to increasing end expiratory
pressure and end inspiratory
lung volume. 51

52. Development of auto-PEEP
Notice how the expiratory
flow fails to return to
the baseline causing
progressive air trapping
Click here to watch video
Also notice how the
progressive air trapping
causes a gradual
increase in airway
pressures because of
decreasing compliance

53. Understanding how flow rates affect I/E ratios and the development of auto PEEP
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
Lluis Blanch MD, PhD et al: Respiratory Care Jan 2005 Vol 50 No 1

54. Understanding how inspiratory time affect I/E ratios and the development of auto-PEEP
In a similar fashion, an increase in inspiratory time can also cause a decrease in the I: E ratio and favor the development of auto-PEEP by not allowing enough time for complete lung emptying between breaths.
Watch in the next video how auto-PEEP develops in a patient on Pressure control ventilation at a RR of 20, just by increasing the inspiratory time from 0.85 sec to 1.0 sec (no auto-PEEP develops) and then to 1.5 sec (development of auto PEEP)

55. Ventilator settings before and after the development of auto-PEEP
Mode of ventilation: PCV ( pressure control ventilation)
Waveforms depicted: flow-time and pressure-time
Inspiratory pressure: 15cm H2O with PEEP of 5 cm H2O
Respiratory rate: 20 bpm
Ventilator
settings
Initial
Subsequently
Final settings
Inspiratory time
0.85 sec
1.0 sec
1.5 sec
Expiratory time
2.15 sec
2.0 sec
I : E ratio
1 : 2.5
1: 2
1: 1
Auto PEEP No
Yes

56. Development of auto-PEEP with inadequate expiratory time
Click here to watch video

57. Recognizing Expiratory Flow Limitation (e.g. COPD, asthma)

58. Recognizing prolonged expiration (air trapping)
Recognize
airway obstruction
when
Expiratory flow quickly tapers off
and then enters a prolonged
low-flow state without returning to
baseline (auto- PEEP)
This is classic for the flow
limitation and decreased lung
elastance characteristic of COPD
or status asthmaticus

60. Recognizing: Wasted efforts Double triggering Flow starvation Active expiration

61. Recognizing ineffective/wasted patient effort
Patient inspiratory effort
fails to trigger vent
resulting in a wasted effort
Results in fatigue, tachycardia,
increased metabolic needs,
fever etc
Causes: High AutoPEEP,
respiratory muscle weakness,
inappropriate sensitivity settings

62. Recognizing double triggering
High peak airway
pressures and
double the inspiratory
volume
Continued patient inspiratory effort
through the end of a delivered
breath causes the ventilator to trigger again
and deliver a 2nd breath immediately after
the first breath.
This results in high lung volumes and pressures.
Causes: patient flow or volume demand exceeds ventilator settings
Consider: Increasing tidal volume, switching modes i.e. pressure support, increasing sedation or neuromuscular paralysis as appropriate

63. Another example of double triggering

64. Recognizing flow starvation
Look at the pressure-time
waveform
If you see this kind of
scooping or distortion instead
of a smooth rise in the
pressure curve….
Diagnose flow starvation
in the setting of patient
discomfort, fatigue,
dyspnea, etc on the vent

65. Recognizing active expiration (pressure support)
Look at the flow-time
& pressure-time
waveform
Notice the high and variable
expiratory flow rates due to
varying expiratory muscle effort
The patient’s active expiratory efforts
during the inspiratory
phase causes a pressure spike.
PEARL: This is a high drive state where increased sedation/paralysis and mode change may be appropriate for lung protection.

67. Recognizing Airway Secretions & Ventilator Auto-Cycling

68. Recognizing airway or tubing secretions
Normal flow-volume
loop
Flow volume loop
showing a ‘saw tooth’
pattern typical of
retained secretions

69. Characteristic scalars due to secretion build up in the tubing circuit

70. Recognizing ventilator auto-cycling
Think about auto-cycling when the respiratory rate increases suddenly without any patient input and if the exhaled tidal volume and minute ventilation suddenly decrease.
Typically occurs because of a leak anywhere in the system starting from the ventilator right up to the patients lungs
e.g. leaks in the circuit, ET tube cuff leak, lungs (pneumothorax)
May also result from condensate in the circuit
The exhaled tidal volume will be lower than the set parameters and this may set off a ventilator alarm for low exhaled tidal volume, low minute ventilation, circuit disconnect or rapid respiratory rate.

71. Waveform video showing the ventilator ‘auto cycling’
Click here to watch video

72. Take home points
Ventilator waveform analysis is an integral component in the management of a mechanically ventilated patient.
Develop a habit of looking at the right waveform for the given mode of patient ventilation.
Always look at the inspiratory and expiratory components of the flow-time waveform.
Don’t hesitate to change the scale or speed of the waveform to aid in your interpretation.

73. Additional links Follow these links for more waveform videos:
Excessive airway secretions:
Flow starvation