1. Patient-ventilator interaction: insight from waveforms
Dr Tang Kam Shing
AC, ICU, TMH
29 April 2009

2. Waveforms on ventilators
Most modern ventilators have a graphical display on the panel of various mechanical parameters
Mostly ignored in daily practices in ICU
They actually give a lot of information regarding:
The mechanical properties of the respiratory system
The interaction between the patient and the ventilator

3. Waveforms on ventilators
However, there are several important points you have to bear in mind before you try to interpret waveforms
Whether the patient is paralyzed or actively breathing
The waveform analysis for both groups completely different
The mode of the ventilator is in
PCV or VCV (constant flow or decelerating flow)

4. Passive patients
Either paralysed or passive patients with no significant triggering or ventilatory efforts
General principle:
the controlled variable waveform usually showed not much information (because it is solely controlled by ventilator in passive patients)
The dependent variables waveforms showed much more information
For VCV, look at the pressure time and flow time waveform
For PCV, look at the flow time and volume time waveforms

5. Passive or paralyzed patient
Volume control mode
Constant flow

6. Constant flow (VC)
Idealized waveforms from constant flow volume control on left side
Real life tracing from ventilator

7. Pressure-time waveform
At the beginning of flow, an almost vertical pressure increase occurs
because of the frictional forces generated by gas flow
which is necessary to overcome the resistance of the airways and the ET tube
The curve shape then changes to a linear increase and follows a given slope to its maximum value
normally linear
Depends on the respiratory system compliance alone

8. Pressure-time in VCV Upper tracing measures at ventilator
Lower tracing meaures at ET tube tip
1st vertical part increased due to resistance of ET tube
2nd part same due to identical compliance of respiratory system
resistive
compliance

9. Resistance
problem
Compliance
problem

10. Signs of over-stretching or decreasing compliance

11. End-inspiratory pause hold
P2 is the static pressure of the respiratory system
Which in the absence of flow equals the alveolar pressure
reflects the elastic retraction of the entire respiratory system
pressure drop from PIP to P1 represents the pressure required to move the inspiratory flow along the airways
Representing the pressure dissipated by the flow-dependent resistances

12. End-inspiratory pause hold
slow post-occlusion decay from P1 to P2 depends on the viscoelastic properties of the system and on the pendulum-like movement of the air (pendelluft)
elastic rearrangement of lung volume
Allows the different pressures in alveoli at different time constants to equalize
Due to the inhomogeneity of the lung parenchyma
rapid zones (low time constant) emptied into slow zones (high time constant)

13. End-inspiratory pause hold
Cstat = VT/P2
Cdyn = VT/PIP
static compliance of the respiratory system mirrors the elastic features of the respiratory system
dynamic compliance also includes the resistive (flow-dependent) component of the airways and the ET tube

14. Passive or paralyzed patient
Pressure control mode

15. Pressure control ventilation
Salient points
Decelerating flow
Decelerating lung expansion
Constant pressure
Whether end-inspiratory pause occur depend on inspiatory time and mechanical properties of respiratory system

16. PIP and plateau pressures in PCV
Flow stopped
Plateau pressure

17. Resistance and compliance in PCV
Increasing resistance
Reducing compliance

18. Rate of pressurization in PCV
Increasing rate of pressurization

19. Inspiratory artifacts common in daily practices
Spike due to pressurization of circuit especially for low pressure rise time

20. Expiratory artifacts common in obstructive lungs
COPD
Spike from recoil of
ventilator circiut
Normal

21. Visualization of dynamic hyperinflation: auto PEEP

22. Measurement of auto-PEEP: Not so simple

24. Spontaneously breathing patients
A bit more complicated

25. Patient-ventilator interaction
In passive patients
Basically only the ventilator and passive mechanical properties of respiratory system involved
Any abnormal pattern is due to the ventilator or the respiratory system
In actively breathing patients
4 different systems interacting: ventilator, respiratory system, respiratory pump and central ventilatory drive
Patterns can be due to either one alone or in combinations

26. Now consider this…
There are two pumps with two control acting on the same respiratory system
Ventilator that is controlled by computer programs that is set by doctors (that commonly do not know what to do)
Neuro respiratory centre controlling the respiratory muscles with its own “desirable setting” (that commonly do not agree with doctors)

27. Patient-ventilator asynchrony
Simply means the two pumps are not agreeing with each other
A lot of times working against each other
Ineffective ventilation
Increase work of breathing prolonged weaning
I am simple minded, so in essence there are two ways to due with this problem
Stop the neuro drive if you believe the neuro drive is not appropriate by sedation +/- paralysis, may be needed in initial management of status asthmaticus
Change your ventilator setting to agree as much as possible with neuro ventilatory drive (best to do but not easy)
Change neuro drive to agree with your setting (impossible)

28. Why border?

29. Types of asynchrony
Essentially 3 phases of mechanical ventilation can show asynchrony
Trigger asynchrony
Flow asynchrony
Termination (cycling) asynchrony

30. Trigger asynchrony The ventilator failed to detect a patient trigger
Inspiratory effort completely wasted
The start of inspiration is slower or later than neural inspiration
Resulting in wasted efforts in the initial part of spontaneous breaths
Double triggering
Auto triggering

31. Failed triggering in COPD

32. PEEP = 0
PEEP = 10
Failed trigger and PEEP setting

33. Double triggering from too short inspiratory phase

34. Double triggering

35. Double triggering from far too low flow and volume setting

36. Auto-triggering from leak

37. Other causes of auto-triggering
Too sensitive trigger setting
more prevalent in patients who had acquired valve disease and had more dynamic circulatory characteristics
Larger heart size
larger cardiac output
higher ventricular filling pressure
lower respiratory-system resistance).
result in larger cardiogenic pressure oscillations, which when transmitted to the airway, causing auto-triggering.

38. Pressure or flow trigger??
2 common types of trigger mechanism available on commercial ventilators:
pressure trigger and flow trigger
initial clinical studies indicated that flow-triggering offered some advantage in reducing trigger asynchrony
recent advances in the development of pressure transducers have resulted in nearly equivalent or comparable results

39. Flow asynchrony
Occurs whenever the ventilator flow does not match the patient flow requirement
a common problem, and the flow setting may be the most frequently incorrectly-set ventilator parameter
divided into 2 sections, based on the 2 general methods for delivering gas:
VCV with a fixed flow pattern
PCV with variable flow

40. Flow asynchrony in VCV
Remember the waveform of passive VCV breath with constant flow
The shaded area represent the work done by the respiratory pump of the patient

41. Remember “dished out appearance of VCV waveform

43. Flow asynchrony in PCV or PSV
In essence should not be inadequate flow as pressure is fixed with flow as required by patient
Still possible with too slow pressure rise time
In initial phase of respiration
Also result in a “dished out appearance in pressure waveform

44. Flow starvation from slow rise time

45. Too fast rise time flow asynchrony
Pressure-time
Flow-time

46. Cycling asynchrony Basically two possibilities
Neuro expiration earlier than mechanical expiration, too late termination of support
Neuro expiration later than mechanical expiration, too early termination of support

47. Too late termination of support
Problem is we usually do not have EEG of expiratory muscles
Can we detect this problem with waveforms?

48. Too long inspiratory time

49. Premature termination of support: too early cycling

50. Too early cycling (cycling asynchrony)
Notch in expiratory flow

51. Conclusion
Waveforms a good supplement to clinical examination as asynchrony can be subtle
Always ask yourself 2 questions:
Active or passive
Mode of ventilation
Familiarize with the appearance and usually it is spot diagnosis

52. Any questions? Otherwise go home and relax!
The End
Any questions?
Otherwise go home and relax!