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?