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Pressure
Waveform
Registered Respiratory Therapist
Lead Therapist at St. Joseph Regional Medical Center
Certified Asthma Educator
NRP Instructor
25+ years in Respiratory Care
I want to
be like Ted
Understand the importance of monitoring and interpreting ventilator graphics.
Learn the basic shapes used for waveforms
Identify the different types of waveforms and loops available on most ventilators.
Learn to use the graphics to identify patient/ventilator problems, and make the appropriate adjustments.
Allow fine tuning of ventilator to decrease WOB, optimize ventilation, and maximize patient comfort.
Ventilator graphics reflect the patient-ventilator system and their interaction together.
The graphics display will have different appearances and configurations, depending on the make and model of ventilator.
SCALERS
Plot pressure, flow, or volume against time.
Time is on the x-axis.
LOOPS
Combine pressure or flow, with volume. There is no time component.
Generally, The ascending and descending ramps can be considered the same as exponential ramps.
Just remember the three shapes: Square, Ramp, and Sine waves.
SQUARE
Square shapes usually represent a fixed, constant, or set parameter.
RAMPS
Ramps represent a variable. They can be accelerating or decelerating.
SINE
Sine waves are seen with spontaneous, unassisted breathing
Pressure
Flow
Volume
Pressure
The pressure waveform displays the amount of pressure generated with each breath.
The Peak Inspiratory Pressure is the highest amount of pressure generated during inhalation. The PIP on this image is about 22 cmH20.
Flow
The flow waveform displays the flowrate associated with each breath. Inspiration is displayed above baseline. Expiration is below the baseline.
In this example, the breath begins at a fast flowrate and then decreases during inspiration.
Volume
The volume waveorm displays the amount of volume inhaled (delivered to the patient), and the amount of volume exhaled (returned to the ventilator).
This patient’s tidal volume is between 350 ml and 400 ml.
Here a a short YouTube video that shows the types of waveforms in realtime
Is it a volume or pressue mode?
Here is an easy way to tell what type of mode (or type of breath) being delivered, just by looking at the graphics.
Look at the pressure waveform and remember the letter “P”. With Pressure breaths…The Pressure waveform…will have a Plateau
For modes such as SIMV/PS, the graphics can show both volume breaths and pressure breaths.
The Pressure waveform can be used to assess:
Setting an inspiratory pause time or (insp. hold maneuver), will create a plateau on the waveform.
The plateau allows for easy visualization of Peak inspiratory pressure (PIP), Plateau pressure (Pplat), and airway resistance (Raw).
Pressing the inspiratory hold button on the ventilator stops air flow.
Since the plateau pressure does not include airway resistance, it is essentially a reflection of pressure measured at the alveolar level.
The baseline for the pressure waveform will be higher when PEEP is added.
When there are patient triggered breaths, there will be a negative deflection at the beginning of the waveform.
While performing an expiratory hold maneuver, trapped air will cause the waveform to rise above the baseline.
Auto-PEEP should be < 5cm H2O.
Increased airway resistance (Raw) will cause the PIP to increase while Pplat remains normal.
Decreased lung compliance will cause the entire waveform to increase in size. The difference between PIP and Pplat will remain normal.
What are we seeing?
Increased airway resistance (Raw)
Flow
Waveform
The flow waveform can be used to assess:
A newer feature does allow you to select the desired flow pattern
This example illustrates how the decelerating flow pattern may be preferred over the constant flow pattern.
The same tidal volume can be delivered to the patient, but with a lower peak pressure.
In patients with severe airway obstruction, this can become a problem in flow-cycled modes, such as Pressure Support.
If the expiratory portion of the waveform doesn’t return to baseline before the beginning of the next breath, there could be air trapping present (emphysema, improperly set I:E ratio).
To assess response to bronchodilator therapy, you should see an increase in peak expiratory flow rate and shorter expiratory times.
In Pressure modes that are time-cycled (Pressure Control), the flow waveform should return to baseline.
In Pressure modes that are flow-cycled (Pressure Support), the flow waveform does not return to baseline.
This called a "zero-flow" state
Question:
What problem are we seeing on this image?
Answer:
Air-trapping or auto-peep
Question:
What changes should we see on the flow waveform after administering a bronchodialator?
Answer:
Improved PEF and shorter E-time
Volume
Waveform
The volume waveform can be used to assess:
If the exhalation side of the volume waveform does not return to baseline it could be from a leak, air-trapping or an improperly set inspiratory time.
If there is an increase in airway resistance, it may take longer for the exhaled volume to return to baseline.
This is can be seen with a damp or blocked expiratory valve or filter.
Question:
The volume waveform is most commonly used to assess which two situations?
Answer:
Air trapping and leaks
Question:
Is this a volume mode or pressure mode of ventilation?
The P/V loop can be used to assess:
Volume is plotted on the y-axis, Pressure on the x-axis.
Inspiratory curve is upward, Expiratory curve is downward.
Spontaneous breaths go clockwise and positive pressure breaths go counterclockwise.
The bottom of the loop will be at the set PEEP level. It will be at 0 if there’s no PEEP set.
If an imaginary line is drawn down the middle of the loop, the area to the right represents inspiratory resistance and the area to the left represents expiratory resistance.
(Cdyn)
volume/pressure
If PEEP is added, the loop will begin at the set PEEP level.
The top part of the P/V loop represents dynamic compliance (Cdyn).
Pressure modes deliver a set “constant” pressure during inspiration, creating a plateau on the pressure waveform.
This will also create a plateau on the P/V loop.
When lung overdistention occurs, pressure continues to increase with little or no change in volume. This can create a “bird beak”.
If the patient is triggering the breath, you will see a “crossover” or “tail”, at the beginning of the loop.
As WOB increases, the tail will become larger.
Decreased Compliance
Increased Compliance
Increased Compliance
Decreased Compliance
The loop does not completly return to baseline.
Point of alveolar opening
Some lung protection strategies for treating ARDS, suggest setting PEEP just above the lower inflection point to hold the alveoli open.
What are we seeing?
Decreased lung compliance
The F/V loop can be used to assess:
Air trapping
Airway Obstruction
Airway Resistance
Bronchodilator Response
Insp/Exp Flow
Leaks
Water or Secretion accumulation
Flow Starvation
Asynchrony
Flow is plotted on the y axis and volume on the x axis
Flow-Volume loop
Flow volume loops used for ventilator graphics are usually shown upside down.
Inspiration is above the horizontal line and expiration is below
The shape of the inspiratory portion of the curve will match the flow waveform.
The shape of the exp flow curve represents passive exhalation.
Looks circular with spontaneous breaths
The shape of the inspiratory portion of the curve will match the flow waveform.
Airway Obstruction
With conditions that cause airway obstruction (asthma, COPD), you may see a lower peak expiratory flow (PEF).
You may also see “scooping” on the expiratory portion of the loop. Scooping because visible due to airway obstruction.
This is how a Flow-volume loop appears on a Pulmonary Function Test
Airway Obstruction
The F-V loop usually appears “upside down” on most ventilators.
Air trapping or leaks
If there is air-trapping, or a leak, the loop will not return to the starting point
Water or Secretions in circuit
If there is a collection of water in the ventilator circuit or a build up of secretions, you may see a jagged, “sawtoothed” pattern.
What 2 things are we seeing?
Scooping and Decreased PEF
What are we seeing?
Leak or air trapping
The inspiratory portion of the pressure waveform shows a “dip”.
Because of an inadequate flowrate , the patient's inspiratory effort causes a drop in tge pressure being measured. This is also know as "Flow mismatching"
Some ventilators may have an adaptive flow system that will automatically increase flow to try and meet the patient’s demand.
Notice that on the next breath, the flowrate has been increased.
Any “dips” or irregular shaped loops can be an indication of asynchrony.
Causes: (Flow, Rate, or Triggering)
Air hunger (flow starvation)
Neurological Injury
Improperly set sensitivity
How to identify it on the graphics:
Pressure waveform: Patient tries to inhale/exhale in the middle of the waveform, causing a dip in the pressure
Flow waveform: Patient tries to inhale/exhale in the middle of the waveform, causing erratic flows/dips in the waveform
Pressure/Volume loop: Patient makes effort to breath causing dips on either inspiratory or expiratory side.
Flow/Volume loop: Patient makes effort to breath causing dips in loop on either inspiratory or expiratory side.
How to fix it:
Try increasing the flow rate, decreasing the I-time, or increasing the set rate to “capture the patient” to better meet the patient’s needs.
Change the mode - sometimes changing from partial to full support will solve the problem.
If neurological, may need paralytic or sedative.
Adjust sensitivity
Causes:
Insufficient expiratory time
Early collapse of unstable alveoli/airways during exhalation
How to Identify it on the graphics:
Pressure waveform: While performing an expiratory hold, the waveform rises above baseline.
Flow waveform: The expiratory flow wave doesn’t return to baseline before the next breath begins.
Volume waveform: The expiratory portion doesn’t return to baseline.
Flow/Volume Loop: The loop doesn’t return completely to baseline
Pressure/Volume Loop: The loop doesn’t return completely to baseline
How to Fix:
Give a bronchodilator treatment, adjust I-time, increase flow, adjust PEEP.
Causes:
Bronchospasm
ETT problems (too small, kinked, obstructed)
High flow rate
Secretion build-up
Damp or blocked expiratory valve/filter
Water in the HME
How to identify it on the graphics:
Pressure waveform: PIP increases, but the plateau stays the same
Flow waveform: It takes longer for the exp side to reach baseline
Volume waveform: It takes longer for the exp curve to reach the baseline
Pressure/Volume loop: The loop will be wider. Bulge to the left = exp resistancw, bulge to right = insp resistance.
Flow/Volume loop: decreased exp flow, scooping on the exp curve
How to fix:
Give a bronchodilator treatment, suction patient, drain water, change HME, change ETT, add a bite block, reduce flowrate, change exp filter.
Decreased compliance
Causes:
ARDS
Atelectasis
Abdominal distension
CHF
Consolidation
Fibrosis
Overdistention
Pneumothorax
Pleural effusion
How to identify it on the graphics:
Pressure wave: PIP and plateau both increase
Pressure/Volume loop: lays more horizontal
Increased compliance
Causes:
Emphysema
Surfactant Therapy
How to identify it on the graphics:
Pressure wave: PIP and plateau both decrease
Pressure/Volume loop: Stands more vertical (upright)
Causes:
Expiratory leak: ETT cuff leak, circuit connections, chest tubes, BP fistula, NG tube in trachea
Inspiratory leak: Loose connections, faulty flow sensor, ventilator malfunction
How to identify it on the graphics:
Volume waveform: Expiratory side of wave doesn’t return to baseline
Flow waveform: PEF decreased
Pressure/Volume loop: Exp side doesn’t return to the baseline
Flow/Volume loop: Exp side doesn’t return to baseline
How to fix it:
Check possible causes listed above
Do a leak test and make sure all connections are tight
Rise Time
Insp Cycle
Off %
Rise Time
The inspiratory rise time is the time it takes to reach full inspiratory flow, or pressure, at the start of each breath.
The rise time can be expressed as a percentage of the breath cycle time (%) or in seconds (s).
If rise time is set too fast, you can get an overshoot in the pressure wave, creating a pressure “spike”.
If this occurs, you need to increase the rise time. This will make the flow valve open more slowly.
1. Here we can see the “spike” on our graphics.
2. The rise time is set at .05 seconds
4. The waveform will return to normal
3. Increasing the rise time will open the
valve slower
If rise time is too slow, the pressure waveform will become more slanted, when it should look more square. This may affect Vt delivery and may not meet the patient’s inspiratory demands.
If this occurs, you will need to decrease the rise time to open the valve faster.
Also know as:
Inspiratory cycle threshold
Inspiratory flow termination,
Expiratory flow sensitivity,
Inspiratory flow cycle %,
E-cycle, etc…
In flow-cycled modes, like Pressure Support & Volume Support, the inspiratory cycle off % determines when the ventilator cycles from inspiration to expiration.
In flow-cycled modes, like Pressure Support & Volume Support, the inspiratory cycle off % determines when the ventilator cycles from inspiration to expiration.
100%
50%
Peak Insp Flow
(PIF)
30%
60%
10%
Setting the percentage lower, will cycle inspiration off later.
This may make the breath too long. If so, you may see an expiratory spike.
Setting the percentage higher, will cycle inspiration off sooner.
This could make the breath too small, effecting tidal volume
What are we seeing here?
Rise time is too slow
Rapid Interpretation of Ventilator Waveforms, Waugh, Harwood, and Deshpande
Ventilator Waveform Analysis, Pearson
Anatomy of Servo-i Graphics, Maquet, inc.
Golden Moments in Mechanical Ventilation, Maquet, inc
Waugh, Jonathan B. Rapid Interpretation of Ventilator Waveforms. Pearson Prentice Hall, 2007.
Bill Pruitt, “Ventilator Graphics Made Easy / RT.” RT: For Decision Makers in Respiratory Care, www.rtmagazine.com/2007/02/ventilator-graphics-made-easy/.
American Thoracic Society - Ventilator Waveform Interpretation and Analysis, www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/mechanical-ventilation/ventilator-waveform-analysis.php.
“Ventilator Graphics Made Easy.” AARC, www.aarc.org/webcasts/ventilator-graphics-made-easy/.