Ventilator Waveforms and Graphics Vector

Ventilator Waveforms and Graphics: An Overview (2024)

by | Updated: Apr 9, 2024

Mechanical ventilation involves using a machine to assist with or replace spontaneous breathing. The process involves the use of positive pressure to facilitate air movement into the lungs.

Modern ventilators are equipped with interfaces that present various waveforms and graphics on a screen, providing healthcare professionals with a dynamic view of the patient’s respiratory condition.

This article breaks down the fundamentals of ventilator waveforms and graphics, exploring the most common types and their significance in clinical practice.

What are Ventilator Waveforms and Graphics?

Ventilator waveforms and graphics are visual representations displayed on the machine, showing real-time data of a patient’s respiratory status. These include pressure, volume, and flow waveforms, providing critical insights into the effectiveness of ventilation and patient-ventilator synchrony while identifying the need for adjustments to the ventilator settings.

Basics of Ventilator Waveforms

The basic variables that determine the appearance of ventilator waveforms include:

  1. Volume
  2. Flow
  3. Pressure

The volume of air delivered by the ventilator is influenced by the flow rate and the duration of the patient’s inspiratory time.

Flow, in turn, is governed by the pressure differential between the ventilator and the patient’s lungs. Consequently, a greater pressure gradient results in a higher flow, facilitating quicker lung inflation.

The pressure necessary to expand a patient’s lungs is contingent upon the lung compliance and the resistance to airflow:

  • Lung Compliance refers to the ease with which the lungs and chest wall expand. Higher compliance indicates more “stretchy” lungs, allowing for easier inflation with less pressure.
  • Airway Resistance measures the opposition to airflow through the respiratory passages. Increased resistance implies greater difficulty for air to enter the lungs.

Note: These variables collectively influence the characteristics of the waveforms displayed on the monitor, providing essential insights into the patient’s respiratory mechanics.

Primary Types of Ventilator Waveforms

Mechanical ventilation utilizes two primary types of waveforms:

  1. Scalars
  2. Loops

Scalars

Scalar waveforms graph pressure, flow, and volume against time, providing a temporal representation of these key respiratory variables.

Types of Scalar Ventilator Waveforms Illustration

During mechanical ventilation, scalar waveforms can manifest in one of six basic configurations:

  1. Rectangular (or Square Wave/Constant Waveform): Represents a constant value over time.
  2. Descending Ramp (or Decelerating Ramp): Characterized by a gradual decrease in value over time.
  3. Ascending Ramp (or Accelerating Ramp): Features a gradual increase in value over time.
  4. Sinusoidal (Sine Wave): Mimics the shape of a sine wave, indicating a smooth, repetitive oscillation.
  5. Rising Exponential: Shows a sharp increase followed by a gradual plateau.
  6. Decaying Exponential: Displays a sharp initial value that decreases exponentially.

The specific mode of ventilation and the patient’s respiratory mechanics dictate the shape of each scalar waveform.

For instance:

  • Pressure waveforms often adopt rectangular or rising exponential shapes, indicating the pattern of applied pressure.
  • Volume waveforms typically appear as ascending ramps or sinusoidal shapes, reflecting changes in lung volume.
  • Flow waveforms vary more widely, possibly appearing as rectangular, ascending or descending ramps, sinusoidal, or decaying exponential shapes, each illustrating different flow characteristics through the respiratory cycle.

Types of Scalar Waveforms

Scalar waveforms in mechanical ventilation are categorized into three main types, each representing a different aspect of the respiratory cycle:

  1. Volume-time scalar
  2. Flow-time scalar
  3. Pressure-time scalar

In these graphics, the respective variable (i.e., volume, flow, or pressure) is plotted along the vertical y-axis, while time is represented on the horizontal x-axis.

It’s important to note that while flow and pressure values are directly measured, the volume is derived from these measurements for each breath.

Thus, a scalar waveform provides a comprehensive view of an entire breathing cycle, from the beginning of inspiration to the end of expiration.

Volume-Time Scalar

Volume-time scalar ventilator waveform graphic illustration

The volume-time scalar is a ventilator graphic that illustrates the volume of gas administered to the patient’s lungs over time.

It displays the volume of air during both the inspiratory and expiratory phases: the upward slope signifies the volume inhaled during inspiration, and the downward slope indicates the volume exhaled during expiration.

Ideally, the inspiratory and expiratory volumes displayed should be similar, suggesting a balanced respiratory cycle. However, a notable deviation, such as a sudden drop in volume depicted in the scalar, may indicate the presence of an air leak.

Furthermore, this scalar waveform is instrumental in assessing the effectiveness of a patient’s spontaneous breathing efforts. It also aids in understanding the impact of ventilator setting adjustments on the patient’s tidal volume, providing valuable insights for optimizing respiratory support.

Flow-Time Scalar

Flow-time scalar ventilator waveform graphic illustration

The flow-time scalar is a ventilator graphic that depicts the gas flow from the ventilator to the patient over a period of time.

Scalar a illustrates two distinct patterns:

  • The first pattern displays a square waveform, characteristic of a patient in a volume-controlled ventilation mode, indicating a consistent flow throughout inspiration.
  • The second pattern shows a descending waveform typical of a patient in a pressure-controlled mode, where the flow decreases as lung pressure reaches the set target.

Scalar b highlights flow waveform anomalies suggestive of airway obstruction or variations in airway resistance, demonstrating how abnormalities can manifest in flow dynamics.

Inspiratory flow is depicted in the upper part of the graph, with expiratory flow at the bottom. The inspiratory curve’s shape varies based on the ventilation mode:

In pressure-targeted modes, the flow initially spikes to rapidly meet the pressure target, then tapers off, creating a descending pattern. This variable flow reflects the ventilator’s adjustment to maintain the preset peak inspiratory pressure (PIP) over the set inspiratory time.

Volume-controlled modes typically produce a constant or square waveform, as the tidal volume, inspiratory time, and flow rate are all predefined, ensuring a steady flow rate during inspiration.

The expiratory curve portion is crucial for evaluating lung compliance and airway resistance. For instance, an obstruction might be inferred from a reduced peak expiratory flow rate and an extended expiratory phase, evident from a prolonged time for the curve to return to baseline.

Pressure-Time Scalar

Pressure-time scalar ventilator waveform graphic illustration

The pressure-time scalar is a ventilator graphic that displays the variations in a patient’s airway pressure over time during mechanical ventilation.

Scalar a illustrates two key patterns:

  • The first waveform represents a controlled mechanical breath, indicating a uniform application of pressure throughout the inspiratory phase.
  • The second waveform exemplifies a volume-controlled breath, highlighting the patient’s peak inspiratory pressure (PIP) and the positive end-expiratory pressure (PEEP) settings.

Scalar b offers a visual representation of plateau pressure, measured during an inspiratory pause. This pressure, observed in the absence of airflow, reflects the amount needed to maintain lung inflation at the end of inspiration.

The mode of ventilation influences the scalar’s shape:

  • In pressure-controlled ventilation, the pressure is set to a predetermined level, leading to a consistently square-shaped waveform, as the pressure remains constant throughout the inspiratory phase.
  • In volume-controlled ventilation, a predetermined volume is delivered, causing the pressure to rise progressively until the set volume is achieved. This results in an ascending waveform, where the slope indicates the increase in pressure required to deliver the preset volume.

Loops

Loop waveforms present the relationship between two different respiratory variables plotted on an x-y coordinate system, offering a dynamic view of the mechanical ventilation process.

Specifically, loop graphics illustrate either pressure or flow against volume, providing insights into the interaction between these variables during the respiratory cycle.

Each loop waveform consists of an inspiratory and an expiratory curve. When these curves are graphed together, they form a distinctive “loop” shape, which is the basis for the waveform’s name.

The primary types of loop graphics are:

  1. Flow-volume loop
  2. Pressure-volume loop

Note: These loops are invaluable for diagnosing and monitoring changes in respiratory mechanics, allowing clinicians and respiratory therapists to visually assess and adjust the ventilator settings to optimize patient care.

Flow-Volume Loop

The flow-volume loop is a graphical representation used in both mechanical ventilation and pulmonary function testing (PFT) to visualize air movement into and out of the lungs throughout a respiratory cycle.

This tool is crucial for diagnosing obstructive or restrictive lung diseases.

Flow-volume loop ventilator waveform graphic illustration

In a typical flow-volume loop:

  • Inspiration is depicted on the upper part of the graph, showing air being drawn into the lungs.
  • Expiration is represented on the lower part, with the curve returning to the baseline to complete the loop, symbolizing a full breath cycle.

During mechanical ventilation, the shape of the loop provides insights into pulmonary mechanics.

For example, in patients with obstructive lung disease, the peak expiratory flow is reduced, leading to a characteristic “scooped-out” appearance of the expiratory limb (as illustrated in the second graphic, loop b).

Additionally, the graphic can indicate air trapping, a condition where air remains in the lungs due to incomplete exhalation. This phenomenon is identified when the expiratory limb fails to return to the baseline, suggesting residual volume in the lungs post-exhalation.

Pressure-Volume Loop

The pressure-volume loop is a ventilator graphic illustrating the relationship between lung pressure and volume during a respiratory cycle.

Pressure-volume loop ventilator waveform graphic illustration
  • Loop a outlines a standard pressure-volume loop trajectory, ascending counterclockwise to form a complete loop. It highlights critical inflection points that indicate rapid changes in the curve’s slope. The lower inflection point (LIP) signifies the opening of collapsed alveoli, marked by a notable volume increase. Conversely, the upper inflection point (UIP) is observed near the end of inspiration, where additional pressure yields only marginal volume increases.
  • Loop b illustrates the phenomena of overdistension and hysteresis within the pressure-volume loop. Overdistension, a risk of lung injury, occurs when the lungs are subjected to excessive volume or pressure. Hysteresis, on the other hand, describes the differential lung tissue behavior during inflation and deflation, highlighting that inflating the lungs requires more energy than deflating them. This difference manifests as a gap between the inspiratory and expiratory limbs of the loop, reflecting changes in lung compliance or airway resistance.

Note: Under normal circumstances, a pressure-volume loop resembles a football shape. A flattened curve suggests reduced lung compliance, while a steeper curve indicates enhanced compliance. A broader curve signals increased airway resistance, and a narrower loop suggests lower resistance.

Free Access
25+ RRT Cheat Sheets and Quizzes

Get instant access to 25+ premium quizzes, mini-courses, and downloadable cheat sheets for FREE.

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 to minimize overdistension, assess 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 best ventilates the patient with low peak airway pressure, low mean airway pressure, and an I:E ratio of 1:2 or less.

3. What is the square wave?
It utilizes a high-pressure source from the machine, where the flow peaks and remains constant, uninfluenced by changes in resistance and compliance.

4. When is the square wave used?
It is used with patients who have non-compliant (stiff) lungs and increased respiratory rates to decrease inspiratory time and improve air distribution/gas exchange.

5. What is a caution of the square wave?
It could increase peak airway pressure and mean airway pressure since it delivers air too quickly. The lowest possible pressure should be used.

6. What is the sine wave?
It repeats the same pattern breath after breath, but the flow is not constant, mimicking normal breathing. The sine wave decreases airway resistance by reducing flow and 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?
Scalars are plots of pressure, flow, or volume against time.

9. What are loops?
Loops are plots of pressure or flow against volume. Time is not graphed in loops.

10. What are the types of volume control flow delivery waveforms?
The types include square, ascending, descending, and sine waveforms.

11. What are the types of pressure control flow delivery waveforms?
Descending and decaying

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?
In spontaneous, unsupported breathing.

15. What breath types does the pressure-time curve identify?
Ventilator-initiated, patient-initiated, pressure control, and spontaneous breaths.

16. How do you identify a ventilator-initiated mandatory breath?
By a pressure rise without a preceding pressure deflection below the baseline.

17. How do you identify a patient-initiated breath?
By a pressure deflection below the baseline immediately before a rise in pressure.

18. How do you identify spontaneous breaths?
By pressures that fluctuate above and below the baseline.

19. How do you identify pressure support breaths?
By a rise to a plateau with varying inspiratory times.

20. How do you identify pressure control breaths?
By a rise to a plateau with 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. How do you identify a leak on a pressure-time curve?
The baseline pressure dips downward, and the low-PEEP alarm will sound.

23. What is seen on a pressure-time curve?
Baseline pressure, mean airway pressure (MAP), peak airway pressure (PAP), inspiration, and expiration phases.

24, PEEP is set to no more than what percentage of auto-PEEP?
80%

25. What is asynchrony?
Known as “flow starvation,” asynchrony occurs when the inspiratory portion of the pressure waveform dips due to inadequate flow.

26. What happens to PIP and Pplat if resistance increases?
PIP increases, while Pplat remains unchanged.

27. What happens to the waveform, PIP, and Pplat when compliance decreases?
The waveform size increases, while the difference between PIP and Pplat remains unchanged.

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 triggering effort.

31. When inspiratory flow takes longer to return to baseline, what does this indicate on a flow waveform?
Airway obstruction

32. When expiratory flow doesn’t return to baseline, what does this indicate on a flow waveform?
Air trapping

33. What does a pressure-volume loop assess?
Lung overdistension, airway obstruction, bronchodilator response, respiratory mechanics (compliance/resistance), work of breathing (WOB), flow starvation, leaks, and triggering effort.

34. On a pressure-volume loop, describe if inspiration and expiration is upward or downward?
Inspiration is upward; expiration is downward.

35. On a pressure-volume loop, what does beaking suggest?
Overdistention

36. What does a fishtail indicate?
Negative pressure (flow or pressure trigger).

37. An increase in airway resistance causes the pressure-volume loop to do what?
Widen

38. What does a break in the loop indicate?
A leak is present

39. What does a shift upward indicate on a pressure-volume loop?
Increased compliance

40. What does a shift downward indicate on a pressure-volume loop?
Decreased compliance

41. 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.

42. What indicates a leak on a flow-volume loop?
The expiratory part of the loop does not return to the starting point.

43. Which flow pattern decreases the risk of barotrauma in PCV?
Ascending ramp

44. What does a pressure loop indicate?
Compliance

45. 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.

46. What is the unit of measure for a pressure-time curve?
cmH2O

47. What is the baseline variable for a pressure-time waveform?
5 cmH2O

48. What do the vertical and horizontal axes represent for a pressure-time waveform?
The vertical axis represents pressure; the horizontal axis represents time.

49. What is the airway pressure on a graph?
It is the area under and to the left of the PIP.

50. What may a pressure-time curve be used to determine?
Identifying the type of breath during mechanical ventilation (MV), assessing the effort required to trigger a breath, timing of breaths (inspiration and expiration), adequacy of inspiration, adequacy of inspiratory plateau or static pressure, adequacy of the peak flow rate, and adequacy of the rise time setting.

51. What do you do if the deflection is greater than normal?
Decrease the sensitivity to make it easier for the patient to trigger a breath.

52. What does it mean if there is a lag in the pressure rise?
It indicates that the flow setting is too low.

53. How can you tell if the flow is set too high?
By observing a steep rise and a higher than normal peak pressure value.

54. How can we go about assessing the adequacy of the plateau pressure?
During pressure support or pressure control ventilation, failure to attain plateau pressure could indicate a leak or the inability to deliver the required flow. For accurate measurements of static compliance or airway resistance, a stable plateau pressure is necessary. Inaccurate readings may occur if the patient makes an inspiratory effort, coughs, or resists during the inspiratory pause.

55. Which waveform is most likely to determine a leak in the system?
The flow-time waveform for the rate of continuous leaks and the volume-time waveform for leaks in the patient circuits.

56. What is the unit of measure for flow waveforms?
Liters per minute or liters per second

57. Describe the flow-time waveform:
The vertical axis represents inspiratory and expiratory flow, while the horizontal axis represents time. It shows the volume moved per unit of time and provides a visual representation of the flow variable during both inspiration and expiration.

58. When is the inspiratory time for the flow-time waveform?
From the beginning of inspiration to the beginning of expiration.

59. When is the expiratory time for the flow-time waveform?
From the beginning of expiration to the beginning of inspiration.

60. Where does the majority of inspiration take place in a flow pattern?
Above the horizontal axis

61. Where does the majority of expiration take place in a flow pattern?
Below the horizontal axis

62. What may a flow-time curve be used to determine?
Verifying waveform shapes, type of breathing, presence of Auto-PEEP, patient’s response to bronchodilators, adequacy of inspiratory time in pressure control ventilation, and the presence and rate of continuous leaks.

63. What are the four types of inspiratory flow patterns?
Square/constant flow waveform (CFW), decelerating/descending ramp flow waveform (DRFW), accelerating, and sine.

64. Why are square wave and decelerating patterns the most commonly used?
They provide an initially high inspiratory flow, improving patient-ventilator synchrony.

65. Which type of inspiratory flow pattern is most commonly used in the clinical setting?
Square and decelerating patterns

66. Describe the descending ramp flow pattern:
A set peak flow is delivered at the beginning of a breath and then decreases in a linear fashion until the desired volume is delivered. It may produce lower peak pressures (a usually desired outcome) and may significantly increase inspiratory time (which could lead to Auto-PEEP). It is a popular waveform choice thought to improve ventilation distribution.

67. Describe the square wave flow pattern:
A set peak flow is delivered at the beginning of a breath, and the flow remains constant throughout the entire inspiratory phase. It may produce higher peak pressures and may significantly decrease inspiratory time.

68. What is the square waveform used to calculate?
It is used to accurately calculate airway resistance on some ventilators.

69. Which waveform is most likely to determine a sensitivity setting problem?
Pressure-time waveform

70. Which waveform is most likely to determine the presence of Auto-PEEP?
Flow-time waveform

71. Which waveform is most likely to determine the beneficial effects of bronchodilator treatment?
Flow-time waveform

72. Which waveform is most likely to show the presence of PEEP?
Pressure-time waveform

73. Which waveform is most likely to show a square wave or descending wave pattern?
Flow-time waveform

74. Which waveform is most likely to show the presence of air trapping?
Volume-time waveform

75. Which waveform is most likely to show a plateau/static pressure reading?
Pressure-time waveform

76. How can the flow waveform indicate the presence of Auto-PEEP?
The flow waveform can suggest the presence of Auto-PEEP by showing that expiratory flow does not return to zero before the next breath begins, but it cannot measure the exact amount of Auto-PEEP.

77. What are the units of measure for a volume-time waveform?
Liters or milliliters

78. 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, indicating a loss of exhaled volume. This is because the expiratory volume should equal the inspiratory volume; a discrepancy suggests a leak, showing a consistent loss of volume on the expiratory waveform.

79. What will you see on the waveform during a circuit leak?
During a circuit leak, the flow waveform will show reduced expiratory flows due to less volume being delivered, and the volume waveform will not return to baseline.

80. Which three different waveforms are seen on the ventilator screen?
1) Pressure over Time, 2) Volume over Time, and 3) Flow over Time.

81. What are the four types of Scalars?
Decelerating, Square, Sine, and Ascending.

82. How can you correct insufficient flow?
To correct insufficient flow, decrease inspiratory time (i-time) or increase peak flow. Changing the mode of ventilation can also help.

83. What can cause oscillations on exhalation?
Oscillations on exhalation could be due to the tubing lying on the patient and picking up motion from the heart rate, secretions in the airways, or condensation in the tubing.

84. How can flow/volume loops demonstrate that a leak is present?
Flow/volume loops can demonstrate a leak by showing an absence of volume returning to baseline, indicating a leak.

85. How can pressure/volume loops demonstrate that a leak is present?
Pressure/volume loops indicate a leak when the volume does not return to zero in a given breath, suggesting a loss of volume.

Final Thoughts

Understanding ventilator waveforms and graphics is essential for managing mechanically ventilated patients.

These visual tools not only provide real-time insights into the patient’s respiratory status but also offer a means to diagnose conditions, optimize ventilator settings, and improve patient outcomes.

Through the detailed examination of volume-time, flow-time, and pressure-time scalars, along with loop waveforms, healthcare professionals can make informed decisions regarding patient care.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

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.

References

  • 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.
  • Emrath E. The Basics of Ventilator Waveforms. Curr Pediatr Rep. 2021;9(1):11-19. doi: 10.1007/s40124-020-00235-4. Epub 2021.

Recommended Reading