Part III: Adjusting Flow-cycle Criteria in PSV When Using a Critical Care Ventilator for NPPV

1. Part III: Adjusting Flow-cycle Criteria in PSV When Using a Critical Care Ventilator for NPPV 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

2. Section 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

3. Problems 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

4. Characteristics of a Pressure Support 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.

5. Characteristics of a Pressure Support Breath B • 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 Pressure A

6. 100 75 50 25 Flow-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% Flow in L/min Time

7. Purpose 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.

8. Examples 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

9. Flow-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

10. 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

11. 100 75 50 25 Different 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.

12. Flow Cycle Percentage and Inspiratory Time in PSV 100 l/min 40 l/min 25 l/min Patient 1 Patient 2

13. 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?

14. 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

15. 25 cm H2O 100 L/min The Flow Curve • Inspiratory flow determined by set pressure and patient effort 0

16. 25 cm H2O 100 L/min 18 The Flow Curve • Inspiratory flow determined by set pressure, rise time and patient effort

17. 25 cm H2O 100 L/min 19 % flow for cycling The Flow Curve • Inspiratory flow determined by set pressure, rise time and patient effort

18. 25 cm H2O 0 Flow Patients With Increase Airway Resistance and Reduced Compliance • With high airway resistance and low compliance – a long time constant • Longer, slower flow curve

19. 25 cm H2O 15 40% of Peak flow Where Do We Set Flow Termination? • Longer, slower flow curve • Use a higher flow termination to allow for adequate exhalation time

20. PSV 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

21. Normal Breath Pressure Spike Pressure cm H2O Pressure-Time Curves in a Patient With COPD on PSV • The pressure spike at the end of inspiration is caused by the patient trying to actively exhale Time

22. One 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

23. For 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

24. Ventilator Response to the Large 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

25. To Solve the Problem When a Leak is Present • 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)

26. Section 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?