Mechanical ventilation is the process of using a machine to assist with or replace spontaneous breathing. There are many different types of ventilators, but they all work by using positive pressure to move air into the lungs.
Modern ventilators have a built-in interface that displays different waveforms and graphics on a monitor. This allows practitioners to visualize a real-time display of a patient’s ventilatory status.
In this article, we will break down the basics of ventilator waveforms and graphics. We’ll take a look that the most common types, what they represent, and how they can be used to troubleshoot problems with the ventilator.
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Basics of Ventilator Waveforms
Three basic variables determine the appearance of ventilator waveforms:
The volume of air delivered by the ventilator depends on the amount of flow and the patient’s inspiratory time. The flow is determined by the pressure difference between the ventilator and the patient’s lungs. Therefore, the higher the pressure gradient, the higher the flow and the faster the lungs fill with air.
The pressure needed to inflate a patient’s lungs depends on the patient’s lung compliance and resistance to airflow.
Lung compliance is a measurement of the distensibility of the lungs and chest wall. The higher the compliance, the more compliant (or “stretchy”) the lungs and chest wall are. This means that the lungs can inflate with less pressure.
Airway resistance is a measurement of the opposition to airflow. The higher the resistance, the more difficult it is for air to flow into the lungs. These three variables are what determine the shape of the waveforms seen on the monitor.
Types of Waveforms
There are two primary types of waveforms used during mechanical ventilation:
Scalar waveforms display pressure, flow, and volume graphed relative to time. In other words, they are representations of specific respiratory variables over time.
Scalars produce six basic shapes during mechanical ventilation:
- Rectangular (also called square wave or constant waveform)
- Descending ramp (also called decelerating ramp)
- Ascending ramp (also called accelerating ramp)
- Sinusoidal (often called sine wave)
- Rising exponential
- Decaying exponential
The ventilator mode and characteristics of a patient’s respiratory mechanics determine the appearance of each scalar waveform.
The pressure waveforms are usually displayed as rectangular or rising exponential. The volume waveforms are usually displayed as ascending ramp or sinusoidal.
On the other hand, the flow waveforms can be displayed in various forms. For example, they may appear as rectangular, ascending ramp, descending ramp, sinusoidal, or decaying exponential.
Types of Scalar Waveforms
There are three primary types of scalar graphics, which include:
- Volume-time scalar
- Flow-time scalar
- Pressure-time scalar
The volume, flow, and pressure variables are plotted on the vertical y-axis against time, which is plotted on the horizontal x-axis.
Note: Flow and pressure are measured values, while the volume must be calculated for each breath. Therefore, a scalar waveform represents an entire breathing cycle (i.e., from inspiration to the end of expiration).
The volume-time scalar is a ventilator graphic that represents the volume of gas delivered to the lungs by the machine over time.
This graphic shows the volume of air on inspiration and expiration. The upward slope represents the inspiratory volume, while the downward slope represents the expiratory volume.
The inspiratory and expiratory volumes should appear similar on the display. However, the second scalar shows a sudden drop in volume, which may occur when an air leak is present.
This type of scalar waveform is also useful in evaluating a patient’s spontaneous breath and how adjustments to the ventilator settings may affect their tidal volume.
The flow-time scalar is a ventilator graphic that represents gas flow between the ventilator and the patient over time.
The first graphic (scalar a) represents a square waveform pattern of a patient in a volume-controlled mode. The second graphic in scalar a represents a descending pattern of a patient in a pressure-controlled mode.
The bottom graphic (scalar b) shows examples of flow waveform abnormalities that represent an obstruction or changes in airway resistance.
The inspiratory flow is represented on the top portion of the graph, while the expiratory flow is on the bottom portion. The shape of the inspiratory part of the curve depends on the ventilator mode that is being used.
Example: In pressure-targeted modes, the flow is variable, while the PIP inspiratory time are set. At the beginning of inspiration, the flow is delivered at a high rate but then begins to taper off. This results in the curve having a descending shape. Volume-controlled modes may result in a constant flow or square shape because the patient’s tidal volume, inspiratory time, and flow are all preset.
The shape of the expiratory portion of the curve helps assess the patient’s lung compliance and airway resistance. For example, if an obstruction is present, the scalar will show a decreased peak expiratory flow and a prolonged expiratory, which is displayed on the curve as it takes longer to return to zero.
The pressure-time scalar is a ventilator graphic that represents the patient’s airway pressure over a period of time.
The first waveform in the top graphic (scalar a) represents a controlled breath. The second waveform shows a volume-controlled breath. Scalar a also shows the patient’s peak inspiratory pressure (PIP) and positive end-expiratory pressure (PEEP).
The bottom graphic (scalar b) displays a graphical representation of plateau pressure. This is the pressure measured during a pause at the end of inspiration. In other words, it’s the pressure needed to keep the lungs inflated in the absence of airflow.
In a pressure-controlled mode, the pressure level is preset and constantly delivered, resulting in a square-shaped scalar. In a volume-controlled mode, the volume is preset, and the pressure gradually increases, resulting in an ascending scalar.
Loop waveforms display a graph of two different variables that are plotted on x and y coordinates. In other words, loop graphics display either pressure or flow plotted against volume.
Each loop waveform displays an inspiratory and expiratory curve that actually forms a “loop” when graphed together. This explains how this waveform got its name.
The types of loop graphics include:
- Flow-volume loop
- Pressure-volume loop
The flow-volume loop is a ventilator graphic that represents how air flows in and out of the lungs during a breathing cycle. It’s also a common measurement used during pulmonary function testing (PFT) to determine if a patient has an obstructive or restrictive lung disease.
A typical flow-volume loop graphic during mechanical ventilation displays inspiration on the top and expiration on the bottom. As the patient exhales, the line returns to the baseline, forming a complete loop that represents an entire breathing cycle.
If the patient has an obstructive disease, their peak expiratory flow will be decreased. This results in a scooped-out appearance of the expiratory limb, as seen in the second graphic (loop b).
This graphic also displays a representation of air trapping, which occurs when air remains in the lungs due to an incomplete exhalation. This can be seen on the loop where the expiratory limb does not return to the baseline.
The pressure-volume loop is a ventilator graphic that represents the pressure in the lungs compared to the volume.
The lower inflection point (LIP) occurs due to the opening of collapsed alveoli, resulting in a sharp increase in volume. The upper inflection point (UIP) occurs near the end of inspiration when more pressure leads to only a minimal increase in volume.
The second graphic (loop b) displays how overdistension and hysteresis appear on a pressure-volume loop. Overdistention occurs when the lungs receive too much volume or pressure and can result in injury. Hysteresis refers to lung tissue that behaves differently on inspiration and expiration.
In other words, it takes more energy for the lungs to inflate than it does to deflate. Therefore, hysteresis on a pressure-volume loop refers to the space between the inspiratory and expiratory limbs. When the patient’s lung compliance or airway resistance changes, so will the hysteresis and, thus, the appearance of the loop.
Note: A pressure-volume loop under normal conditions should resemble the shape of a football. A curve with a flat appearance indicates decreased lung compliance. A steep curve, on the other hand, indicates increased lung compliance. A wide curve indicates increased airway resistance, whereas the opposite is true if the loop appears more narrow.
Now that you know the basics, continue reading through the practice questions below to learn more about ventilator graphics and waveforms.
Ventilator Waveform Practice Questions:
1. What are the uses of flow, volume, and pressure graphic displays?
To detect Auto-PEEP, determine patient-ventilator synchrony, measure work of breathing, adjust tidal volume and minimize overdistention, asses the effect of bronchodilator administration, determine the appropriate PEEP level, evaluate the adequacy of inspiratory time in pressure control ventilation, detect the presence and rate of continuous leaks, and determine the appropriate rise time.
2. What is the key to selecting a flow pattern?
Select the one that will best ventilate the patient, low peak airway pressure, low mean airway pressure, and IE ratio of 1:2 or less.
3. What is the square wave?
It utilizes a high-pressure source (from the machine), the flow peaks and stays constant, uninfluenced by changes in resistance and compliance.
4. When is the square wave used?
It is used with patients with non-compliant (stiff) lungs and increased respiratory rates. It decreases inspiratory time and has better air distribution/gas exchange.
5. What is a caution of the square wave?
It could increase peak airway pressure and the mean airway pressure. It pushes too quickly. You should use the lowest possible pressure.
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It is the same pattern, breath after breath but the flow is not constant. It is most like normal breathing. The sine wave uses decreased airway resistance by decreasing flow; decreases peak airway pressures.
7. What is a caution of the sine wave?
It may result in a decrease in mean airway pressure (MAP).
8. What are scalars?
Plots of pressure, flow, or volume against time.
9. What are loops?
Plots of pressure, flow, or time against each other. Time is not graphed.
10. What are the types of volume control flow delivery waveforms?
Square, ascending, descending, and sine.
11. What are the types of pressure control flow delivery waveforms?
Descending and decay.
12. What do square waveforms represent?
A constant or set parameter.
13. What do ramp waveforms represent?
Parameters that vary with changes in lung characteristics.
14. When are sine waves seen?
Spontaneous, unsupported breathing.
15. What breath types does the pressure-time curve identify?
Ventilator-initiated, patient-initiated, pressure control, and spontaneous.
16. How do you identify a ventilator-initiated mandatory breath?
A pressure rise without a pressure deflection below the baseline.
17. How do you identify a patient-initiated breath?
A pressure deflection below baseline right before a rise in pressure.
18. How do you identify spontaneous breaths?
Pressures above and below the baseline.
19. How do you identify pressure support breaths?
A rise to a plateau and a display varying inspiratory times.
20. How do you identify pressure control breaths?
A rise to a plateau and display constant inspiratory times.
21. What does a pressure waveform detect?
Auto-PEEP, airway obstruction, bronchodilator response, respiratory mechanics, active exhalation, PIP, Pplat, triggering effort, and asynchrony.
22. What is seen on a pressure-time curve?
Baseline pressure, MAP, PAP, inspiration, and expiration.
23. PEEP is set to no more than what percentage of auto-PEEP?
24. What is asynchrony?
It is known as “flow starvation”. The inspiratory portion of the pressure waveform shows a dip due to inadequate flow.
25. What happens to PIP and Pplat if the resistance increases?
The PIP will increase while the Pplat stays the same.
26. What happens to the waveform, PIP, and Pplat when compliance decreases?
The waveform size increases while the difference in PIP and Pplat remain the same.
27. How do you identify a leak on a pressure-time curve?
The baseline pressure dips downward and the low-PEEP alarm will go off.
28. What are the three basic shapes of waveforms?
Square, ramp, and sine.
29. What are the three types of waveforms?
Pressure, volume, and flow.
30. What does a volume waveform detect?
Air trapping, airway obstruction, bronchodilator response, active exhalation, breath type (Pressure vs. Volume), inspiratory flow, asynchrony, and the triggering effort.
31. When inspiratory flow takes longer to return to baseline, what does this indicate on a flow waveform?
32. When expiratory flow doesn’t return to baseline, what does this indicate on a flow waveform?
33. In order to assess improvement after a breathing treatment, you should see what?
You should see an improved PEF and a shorter expiratory time.
34. What does a pressure-volume loop assess?
Lung Overdistension, airway obstruction, bronchodilator response, respiratory mechanics (C/Raw), WOB, flow starvation, leaks, and the triggering effort.
35. On a pressure-volume loop, describe if inspiration and expiration is upward or downward?
Inspiration = upward; Expiration = downward.
36. On a pressure-volume loop, what does beaking suggest?
37. What does fishtail indicate?
Negative pressure (flow or pressure trigger).
38. An increase in airway resistance causes the pressure-volume loop to do what?
It causes it to widen.
39. What does a break in the loop indicate?
That a leak is present.
40. What does a shift upward indicate on a pressure-volume loop?
41. What does a shift downward indicate on a pressure-volume loop?
42. What can flow-volume loops detect?
Air trapping, airway obstruction, airway resistance, bronchodilator response, inspiratory/expiratory flow, flow starvation, leaks, water or secretion accumulation, and asynchrony.
43. What indicates a leak on a flow-volume loop?
The expiratory part of the loop does not return to the starting point.
44. Which flow pattern decreases the risk of barotrauma in PCV?
45. What does a pressure loop indicate?
46. What are the four stages of a mechanical breath?
Beginning of Inspiration ( triggering parameter), Inspiration, End of inspiration / Beginning of expiration (cycling parameter), and Expiration.
47. What is the units of measure for a pressure-time curve?
48. What is the baseline variable for a pressure-time waveform?
5 cm H20
49. What does the vertical and horizontal axis represent for a pressure-time waveform?
Vertical = pressure; horizontal = time.
50. What is the airway pressure on a graph?
It is the area under and to the left of the PIP.
51. What may a pressure-time curve be used to determine?
Identify the type of breath during MV, assessing the work to trigger a breath, breath timing (inspiration and expiration), adequacy of inspiration, the adequacy of inspiratory plateau or static pressure, the adequacy of the peak flow rate, and the adequacy of the rise time setting.
52. What do you do if the deflection if greater than normal?
Decrease the sensitivity to make it easier to trigger.
53. What does it mean if you have a lag in the pressure rise?
It means that there is too low of a flow setting.
54. How can you tell if the flow is set too high?
A steep rise and higher than normal peak pressure value.
55. How can we go about assessing the adequacy of the plateau pressure?
During pressure support or pressure control ventilation failure to attain plateau could indicate a leak or the inability to deliver the required flow. During the determination of static compliance or airway resistance, a stable plateau pressure is required to make these measurements accurate. If the patient makes an inspiratory effort or coughs or fights during inspiration pause then the reading will be inaccurate.
56. Which waveform is most likely to determine a leak in the system?
Flow-time waveform for the rate of continuous leaks. Volume-time waveform for leaks in the patient circuits.
57. What is the units of measure for flow waveforms?
Liters per unit or liters per second.
58. Describe the flow-time waveform:
On the vertical axis, it shows inspiratory and expiratory flow. On the horizontal axis, it shows time. It shows volume moved per unit of time and provides a picture of the flow variable during inspiration and expiration.
59. When is inspiratory time for flow time waveform?
From the beginning of inspiration to the beginning of expiration.
60. When is the expiratory time for flow-time waveform?
From the beginning of expiration to the beginning of inspiration.
61. Where is the majority of inspiration taking place in a flow pattern?
Above the horizontal axis.
62. Where is the majority of expiration taking place in a flow pattern?
Below the horizontal axis.
63. What is the highest flow rate measured during inspiration?
Peak inspiratory flow.
64. What is the highest flow rate measured during expiration?
Peak expiratory flow.
65. What may a flow-time curve be used to determine?
To verify waveform shapes, type of breathing, the presence of Auto-PEEP, patients response to bronchodilators, adequacy of inspiratory time in pressure control ventilation, and the presence and rate of continuous leaks.
66. What are the four types of inspiratory flow patterns?
Square/constant flow waveform (CFW); Decelerating /Descending Ramp flow waveform (DRFW); Accelerating; and Sine.
67. Why are square wave and decelerating patterns the most commonly used?
For their initially high inspiratory flow, they provide better patient-ventilator synchrony.
68. Which type of inspiratory flow pattern is most commonly used in the clinical setting?
Square and decelerating.
69. Describe the descending ramp flow pattern:
The set peak flow is delivered at the beginning of a breath, then it decreases in a linear fashion until the volume is delivered. It may produce lower peak pressures (usually desired outcome). It may increase inspiratory time significantly (may lead to Auto-PEEP). It is the most popular waveform choice thought to improve the distribution of ventilation.
70. Describe the square wave flow pattern:
A set peak flow is delivered at beginning of a breath. The flow is constant throughout the entire inspiratory phase. It may produce higher peak pressures and may decrease the inspiratory time significantly.
71. What is the square waveform used to calculate?
It is used to accurately calculate the airway resistance on some ventilators.
72. Which waveform is most likely to determine a sensitivity setting problem?
Pressure time waveform.
73. Which waveform is most likely to determine the presence of Auto-PEEP?
Flow time waveform.
74. Which waveform is most likely to determine the beneficial effects of a bronchodilator treatment?
Flow time waveform.
75. Which waveform is most likely to show the presence of PEEP?
Pressure time waveform.
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76. Which waveform is most likely to show a square wave or descending wave pattern?
Flow time waveform.
77. Which waveform is most likely to show the presence of air trapping?
78. Which waveform is most likely to show a plateau/static pressure reading?
Pressure time waveform.
79. How can the flow waveform access for Auto-PEEP?
The flow waveform can indicate the presence of Auto-PEEP but cannot measure the amount of Auto-PEEP.
80. What is the units of measure for volume time waveform?
Liter or milliliters
81. How can you detect the presence of air trapping and patient circuit leaks on a waveform?
Air trapping or leaks in the patient circuit can be suspected if the expiratory waveform does not return to baseline. Usually, volume in should equals volume out, thus the expiratory volume waveform does not return to baseline. If the expiratory volume waveform does not return to baseline this indicates a loss of exhaled volume. A leak should show a consistent loss of volume on the expiratory waveform.
82. What will you see on the waveform during a circuit leak?
The flow waveform will show reduced expiratory flows since less volume is delivered. The volume waveform does not return to the baseline.
83. Traditionally, you will see what 3 different waveforms on the ventilator screen?
1) Pressure over Time, (2) Volume over Time, and (3) Flow over Time.
84. What are the 4 types of Scalars?
Decelerating, Square, Sine, and Ascending.
85. How can we fix auto-PEEP?
(1) Increase flow rate to decrease inspiratory time. (2) Bronchodilator therapy, suction the airway. (3) Increase PEEP level to auto-PEEP reading if auto-PEEP cannot be eliminated through other means. This maneuver will decrease WOB by increasing the sensitivity to trigger the machine on. (4) A change in flow pattern may also decrease auto-PEEP. A square waveform may decrease auto-PEEP in comparison to a decelerating waveform. Decelerating waveforms are commonly used because they allow for a lower PIP. A longer e-time may be needed if a decelerating flow pattern has been decided is best for the patient.
86. What is Dyssynchrony?
When patients and ventilators don’t work together, this causes some problems. Patients have to work harder to breathe, they consume more oxygen, they become anxious, they increase minute ventilation, and it puts stress on their heart.
87. How can you correct insufficient flow?
Decrease i-time or increase peak flow. Also, a change in mode can help.
88. What can cause oscillations on exhalation?
1) It could simply be the tubing laying on the patient picking up motion from the heart rate. (2) It could be secretions in the airways. (3) It could be condensation in the tubing. (4) Secretions in the vent tubing.
89. How can flow/volume loops demonstrate that a leak is present?
The flow/volume loop demonstrates the absence of volume returning to baseline, and thus, indicates a leak.
90. How can pressure/volume loops demonstrate that a leak is present?
In the pressure/volume loop, it also demonstrates a leak by the volume not returning to zero in a given breath.
Doctors and respiratory therapists use ventilator waveforms and graphics to quickly learn more about a patient’s condition. They help determine how well or poorly a patient is interacting with the machine.
For example, patient-ventilator asynchrony describes a mismatch of the timing and gas delivery between a patient and the mechanical ventilator. This can lead to a number of complications, such as an increased work of breathing, auto-PEEP, V/Q mismatch, and ventilator-induced lung injuries.
Therefore, it’s essential for medical professionals to quickly and easily interpret ventilator graphics to provide the best possible care for their patients. Thanks for reading, and, as always, breathe easy, my friend.
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
- Chang, David. Clinical Application of Mechanical Ventilation. 4th ed., Cengage Learning, 2013.
- Pilbeam’s Mechanical Ventilation: Physiological and Clinical Applications.
- Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
- “Understanding Ventilator Waveforms—and How to Use Them in… : Nursing2020 Critical Care.” LWW, Jan. 2009, journals.lww.com/nursingcriticalcare/fulltext/2009/01000/understanding_ventilator_waveforms_and_how_to_use.11.aspx.
- Emrath, E. (2020). The Basics of Ventilator Waveforms. Current Pediatrics Reports, 9(1), 11-19. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7782574/.