Chapter 13: The Presidency By, Bryce Heth Alli Peters Will Gilmore Jaryd Hiser 7 th hour 3/28/06.
Part III: Adjusting Flow-cycle Criteria in PSV When Using a Critical Care Ventilator for NPPV By:...
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Transcript of Part III: Adjusting Flow-cycle Criteria in PSV When Using a Critical Care Ventilator for NPPV By:...
Part III: Adjusting Flow-cycle Part III: Adjusting Flow-cycle Criteria in PSV When Using a Criteria in PSV When Using a Critical Care Ventilator for Critical Care Ventilator for NPPVNPPV
By: Susan P. Pilbeam, MS, RRT, FAARC
John D. Hiser, MEd, RRT, FAARC
Ray Ritz, BS, RRT, FAARC
American Association for Respiratory Care
December, 2006
Section ObjectivesSection Objectives
After reviewing this section, the participant will be able to:
Describe how the ventilator ends a breath in pressure support ventilation
Explain how the flow curve during a pressure support breath varies depending on the lung pathology
Recommend to the flow-cycle value in patients with COPD who are actively exhaling
Suggest a new flow-cycle value if a large leak is present during PSV
Problems with PSV During NPPVProblems with PSV During NPPV
Using critical care ventilators for NPPV can be problematic
Large leaks may prevent cycling in some ventilators Some will auto cycle the breath, some will not
Setting high flow-cycle criteria can reduce volume delivery
This section will look at how to set flow cycle percentage during pressure support with NPPV
Characteristics of a Pressure Characteristics of a Pressure Support BreathSupport Breath A pressure support breath is patient
triggered. (a patient’s inspiratory effort begins inspiratory gas flow)
It is pressure limited. The pressure level goes to the pressure value set by the operator during inspiration.
And, it is flow-cycled. The inspiratory flow ends when the ventilator detects inspiratory flow has dropped to a specific flow value.
Characteristics of a Pressure Characteristics of a Pressure Support BreathSupport Breath In this pressure-time
curve, arrow “A” marks the patient’s inspiratory effort
Arrow “B” marks the set pressure
Notice there is no arrow “C” to indicate the flow terminating criteria
We need to examine a flow-time curve to see how flow-cycling works
Pre
ssu
reA
B
Flow-cycling and PSVFlow-cycling and PSV
The flow time curve to the right illustrates flow cycling
Peak inspiratory flow is 100 L/min
The ventilator is set to flow-cycle the pressure support breath at 25%
Time
100
75
50
25
Flo
w i
n L
/min
Purpose of Flow-CyclingPurpose of Flow-Cycling
The purpose of flow-cycle is to end inspiration when the patient is about to stop inhaling
We know this because the flow drops off toward the end of inspiration
The flow at which a ventilator cycles into exhalation can be a fixed value or it may be a value selected by the operator.
Examples of Fixed Flow-CycleExamples of Fixed Flow-Cycle
For example, on some ventilators the flow cycle is set at a constant value of 25% of peak inspiratory flow
On at least one other ventilator the flow cycle is set at 5 L/min…not a percentage, but a specific flow value
Flow-cycling and PSVFlow-cycling and PSV
In newer ventilators this parameter is an adjustable control
The flow-cycling variable is adjustable anywhere from 1% to 80% of the measured peak inspiratory flow
The exact range depends on the ventilator
Names for Flow-Cycle Variable Names for Flow-Cycle Variable
The flow-cycling variable is given different names depending on the ventilator in use
Some example names are – Inspiratory cycle-off Inspiratory flow termination, Expiratory flow sensitivity, Inspiratory flow cycle %, E-cycle etc…
The name varies with each ventilator
Different Settings for Flow-CycleDifferent Settings for Flow-Cycle
Using the flow-cycle control allows the RT to vary the setting depending on the leak or on the patient’s desired inspiratory time.
100
75
50
25
Flow Cycle Percentage and Flow Cycle Percentage and Inspiratory Time in PSVInspiratory Time in PSV
100 l/min
40 l/min
25 l/min
Patient 1 Patient 2
Effects of Flow-Cycle %Effects of Flow-Cycle %
A lower percentage gives a longer breath
A higher percentage gives a shorter breath
Using a higher percentage can reduce the tidal volume delivery
How can the RT correct the volume delivery when the flow-cycle is set high?
Where Do We Set Flow-Cycle?Where Do We Set Flow-Cycle?
First, flow-cycle is set based on patient pathology
In patients who have increased airway resistance, we might want to use a higher flow cycle, such as 40%
In patients who have decreased compliance, we might want to use a lower flow cycle
The next few slides will examine why we do this
0
The Flow CurveThe Flow Curve
Inspiratory flow determined by set pressure and patient effort
100 L/min
25 cm H2O
The Flow CurveThe Flow Curve
Inspiratory flow determined by set pressure, rise time and patient effort
100 L/min
25 cm H2O
18
The Flow CurveThe Flow Curve
Inspiratory flow determined by set pressure, rise time and patient effort
100 L/min
25 cm H2O
19
% flow forcycling
Patients With Increase Airway Patients With Increase Airway Resistance and Reduced Resistance and Reduced ComplianceCompliance With high airway resistance and low
compliance – a long time constant
Longer, slower flow curve
25 cm H2O
0Flow
Where Do We Set Flow Where Do We Set Flow Termination?Termination? Longer, slower flow curve
Use a higher flow termination to allow for adequate exhalation time
25 cm H2O
15
40% ofPeak flow
PSV and COPDPSV and COPD
Patients with COPD commonly have a longer expiratory time due to their disease process
They also use their accessory muscles more than normal individuals
They tend to be active breathers
This creates another problem with PSV
Pressure-Time Curves in a Patient Pressure-Time Curves in a Patient With COPD on PSVWith COPD on PSV The pressure spike at the end of
inspiration is caused by the patient trying to actively exhale
Time
Pressurecm H2O
Pressure Spike
Normal Breath
One More Potential ProblemOne More Potential Problem
One more difficulty may occur when using NPPV with a critical care ventilator set for pressure support breaths--
When there is a large leak in the system, inspiratory time may be prolonged
If the leak is large, the flow might never decrease to the flow-cycle level that is set because gas keeps escaping and the ventilator continues the flow
For ExampleFor Example
If peak inspiratory flow is 100 L/min and the flow-cycle is set at 25%, PS inspiration will normally end when the inspiratory flow drops to 25 L/min
However, if there is a leak in the circuit large enough to allow 35 L/min to flow out of the circuit, then the ventilator will never see the flow drop to 25 L/min and inspiration is prolonged
Ventilator Response to the Ventilator Response to the Large Leak Problem in PSVLarge Leak Problem in PSV In this instance, time will end the breath
Most ventilators have an ‘I-time too long’ alarm to alert the clinician of this situation
The breath ends when the maximum time criteria is detected by the ventilator
To Solve the Problem When a Leak is To Solve the Problem When a Leak is PresentPresent
The RT can eliminate or minimize the leak or set a higher cycling percentage
Using the previous example…
If the RT sets the flow-cycle at 45% or 50%, then the ventilator will be able to detect the drop in flow and end the breath
Remember from the example the leak was 35 L/min (peak inspiratory flow 100 L/min)
Section SummarySection Summary
Let’s review what we learned in this section How the ventilator ends a breath in pressure
support ventilation? How the flow curve during a pressure support
breath varies depending on the lung pathology?
When to adjust the flow-cycle value in patients with COPD who are actively exhaling?
How to readjust the flow-cycle criteria if a large leak is present?