3. Objectives
• Identify graphic display options
provided by mechanical ventilators.
• Describe how to use graphics to
more appropriately adjust the patient
ventilator interface.
4. Introduction
Monitoring and analysis of graphic display of
curves and loops during mechanical
ventilation has become a useful way to
determine not only how patient are being
ventilated but also a way to assess problems
occurring during ventilation.
5. Uses of Flow, Volume, and Pressure
Graphic Display
• Confirm mode functions
• Detect auto-PEEP
• Determine patient-ventilator 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
6. 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
32. Air Starvation
1 2 3 4 5 6
30
-20
SecPaw
cmH2O
Patient -Ventilator Synchrony
The Patient Is Outbreathing the Set Flow
33. If Peak Flow Remains the Same, I-Time
Increases: Could Cause Asynchrony
LPM
1 2 3 4 5 6
SEC
120
-120
V
.
34. Changing Flow Waveform in Volume
Ventilation: Effect on Inspiratory Time
1 2 3 4 5 6
SEC
120
-120
V
.
LPM
35. Increased Peak Flow: Decreased
Inspiratory Time
1 2 3 4 5 6
SEC
120
-120
V
.
LPM
36. 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
.
37. 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
55. 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
Paw
cmH2O
VT
56. Overdistension
B
A
0 20 40 60-20-40-60
0.2
0.4
0.6
LITERS
Paw
cmH2O
C
A = inspiratory pressure
B = upper inflection point
C = lower inflection point
VT
66. Remember!
Waveforms and loops are graphical representation of the data
generated 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 critical tool in the management of
the mechanically ventilated patient.
Notes de l'éditeur
A good way to identify an adequate plateau time is to observe the pressure-time curve. This slide shows an inadequate plateau time; no plateau has occurred. This could lead to an inaccurate estimation of plateau pressure.
Here, we see that a plateau has occurred, as evidenced by the flattening of the pressure curve at the arrow.
This slide depicts a long expiratory phase. Note the low expiratory flow rate and extended exhalation phase. This could be caused by a number of clinical situations: bronchospasm, COPD, expiratory filter contamination, secretions or water in the tubing. Watching for changes in expiratory flow helps judge the efficacy of any intervention.
A higher expiratory flow rate and a decreased expiratory time denote a lower expiratory resistance. A decrease in expiratory resistance may also be observed after the patient receives a bronchodilator (e.g. MDI or aerosolized neb tx). Monitoring the duration of the therapy’s effect can help determine the indicated frequency of therapy.
Exhalation, seen here in yellow, occurs when the tidal volume has been delivered or the high pressure limit has been reached. Flow ceases and exhalation begins. This is a passive process caused by the elastic recoil of the lung.
Let’s take a look at the pressure curve resulting from a volume-based breath with a decelerating flow pattern. As you can see in green here, pressure increases more rapidly from the PEEP level when the decelerating flow curve is used. This pressure curve is starting to look more like a pressure-based breath.
In pressure-based ventilation, once the Pinsp has been reached, the pressure then remains constant for the Tinsp set on the ventilator. Flow decelerates towards end inspiration and then remains at or near zero base line until the set inspiratory time is met. Note the pressure curves are similar. The difference is in the ventilators response to changes in resistance, compliance, or patient demand.
Did anybody say rise time or flow acceleration percent? Literature suggests that inappropriate flow rate, too high or too low at any time during the inspiratory phase in PCV or PSV, may result in increased inspiratory muscle effort or work of breathing. It can also increase the likelihood of patient discomfort and patient/ventilator asynchrony. FAP allows the clinician to sculpt the shape of the rise to pressure to meet patient demand or comfort. Slow, moderate, and aggressive rise to pressure curves are shown. What is happening at the two arrows?
On the pressure-time curve, it is a minimal pressure overshoot caused by an aggressive rise to pressure. On the flow curve, it is a pressure relief that occurs with an active exhalation valve. What happens when there isn’t an active exhalation valve?
All right, now let’s get on with the fun stuff: detecting abnormalities on waveforms. When your patient begins to fight the ventilator and becomes asynchronous, your job as a clinician is to determine why. We all know that many things can cause the patient to become out of synch. It could be caused by pain, frustration from trying to communicate, or their spouse may have just told them they are filing for divorce. But on a more serious note, patient ventilator dysychrony can be caused when the patient outstrips the peak flow set on the ventilator. Let’s take a look at this.
This is a normal pressure curve in volume ventilation with an adequate setting for peak flow.
What we see here is a patient’s inspiratory flow demand greater than the peak flow set on the ventilator, which can lead to patient/ventilator dysynchrony. What are we going to do to amend this situation?
However, remember what we talked about earlier. If the peak flow is left at the same setting when we switch to a decelerating flow pattern, the inspiratory time will increase. A decreased expiratory time may have the potential to cause patient/ventilator dysynchrony in and of itself. Perhaps a different mode of ventilatory support may be more appropriate, such as PCV or PS.
As we discussed earlier, both square and decelerating flow patterns are commonly used in clinical practice. We will not debate the clinical application of these flow patterns, but will point out the impact of changing from a square to decelerating flow curve in volume ventilation without changing the set peak flow. Note the increase in inspiratory time with the decelerating flow pattern.
If the goal is to maintain a similar inspiratory time, this can be accomplished by increasing peak flow to approximate the same inspiratory time with the decelerating flow pattern that existed with the square wave pattern. This could decrease the potential for developing Auto-PEEP.
We can look at our flow-time curve. If there is zero flow at the end of exhalation, it would indicate an equilibration of the lung and circuit pressure.
Note: There can still be pressure in the lungs behind airways that are completely obstructed.
On the other hand, if the transition from exhalation to inspiration occurs without the expiratory flow returning to zero, you have Auto-PEEP present.
The volume-time curve shows the gradual changes in the volume that is delivered during inspiration. Volume is typically measured in milliliters. Pictured in green is the inspiratory phase, in which volume increases continuously until the set tidal volume is achieved or the high pressure alarm limit has been reached, or I-Time has expired.
During expiration, seen in yellow here, the transferred volume decreases, again due to the passive recoil of the lung. Generally, what goes in comes out, unless you have a leak in the patient circuit or the patient, or gas is trapped in the lung.
Using both the volume- and flow-time curves provides insight to set the appropriate PIP and I-Time in PCV. For example, the physician orders PCV and tells you that he wants a VT of 500 cc. PCV is initiated with a PIP of 20 cm, resulting in a VT of 450 cc. Before increasing the inspiratory pressure to obtain additional VT, maximize inspiratory time. As shown here at the arrow, inspiratory flow does not return to zero before cycling into expiration. This could result in a lesser delivered volume.
Pictured in blue here is potentially lost VT. Increasing inspiratory time to allow the flow to return to baseline may increase VT without increasing PIP.
We have reviewed the normal components of the three standard time curves: Flow, Pressure, and Volume. Now, let’s investigate the normal components of the pressure-volume loop. Instead of plotting one parameter against time, the pressure-volume loop plots the interaction between pressure, on the horizontal axis, and volume, on the vertical axis.
On a ventilator-initiated mandatory breath, or VIM, the movement of the PV Loop is counterclockwise, starting with inspiration, shown here in green. During inspiration, the lung begins to fill and normally there is a simultaneous increase in both pressure and volume.
When the inspiration criteria are met, exhalation begins as pictured in yellow here. Normally, this curve resembles a football.
During a spontaneous, non-pressure-supported breath, the rotation is clockwise; inspiration and then expiration.
During a spontaneous, non-pressure-supported breath, the rotation is clockwise; inspiration and then expiration.
The arrow indicates patient work of breathing. Inspiratory effort to the left of the vertical axis translates into increased inspiratory workload for the patient. This is commonly addressed by instituting flow triggering.
When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath.
When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath.
When a patient-initiated mandatory (PIM) breath is triggered, you will initially see a clockwise rotation like a spontaneous breath; then the ventilator takes over and delivers the mandatory breath. At the point marked with the white arrow, it changes to the classic counterclockwise rotation seen with a VIM breath.
The pressure-volume loop changes, flattening out and moving to the right. What could cause this to happen?
Did anybody say decrease in compliance? The difference between the white arrow and the red arrow represents a change in compliance as indicated by an increase in pressure without a corresponding increase in tidal volume.
Overdistention is caused by a combination of PEEP and too much volume or pressure. A is the peak inspiratory pressure; B is the upper inflection point; C is the lower inflection point. The lower inflection point identifies the level of PEEP where the lung is more compliant. This is also referred to as critical opening pressure. The upper inflection point indicates where the lung becomes less compliant and illustrates where overdistension starts to occur. Decreasing the volume or pressure may help avoid barotrauma in this situation.
This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.
This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.
This example shows before and after flow-volume loops that indicate a response to bronchodilators. The loop at the far left (before) is the control. Compare the three peak expiratory flow rates and the lower half of each loop. In the center loop, the relatively low expiratory flow rate (A) and the scalloped shape (B) near end exhalation indicates a negative response to treatment. At the far right, the higher expiratory flow rate and the flatter shape near end exhalation indicate a positive response.