1. Pumping up the Pressure
Dr Steve Pincus July 2018

2. Terminology Mode -3 variables
Control variable –volume &flow or pressure
What triggers the breath-time, flow, pressure
What limits it-time, pressure
What ends it-time, volume
Breath sequence
Continuous mandatory
Intermittent mandatory
Spontaneous
Type of control
Fixed, variable, adaptive, intuitive

3. Lung pressure response
Optimal compliance = part of loop where compliance if linear =alveoli open and gradually distend as pressure rises
Lower inflection point= critical opening pressure ie the initial “flat” rise in pressure represents the pressure required to recruit closed alveoli
La Place’s law =it takes more P to re-inflate a collapsed alveolus than distend a deflated one
Upper infection point= a feature of the expiratory limb , the flat upper portion represents the deflation of the over distended lung units where the pressure drops significantly without drop in volume
The problem is that the lung is heterogeneous and respond differently

4. CPAP Continuous Positive Airway Pressure
Application of continuous pressure throughout the respiratory cycle
CPAP does not = PEEP CPAP is voluntary breathing mode ,PEEP occurs in mandatory ventilation and has no negative deflection their physiological effects area largely the same

5. Physiology of CPAP WOB Lung volume Gas exchange
increases distension
Decreases airway closure & increases recruitment
Gas exchange
Increase lung volume increases gas exchange surface ( O2 > C02)
Increase alveolar volume reduces interstitial fluid which decreases diffusion distance= dewaters the lung
WOB
Positive assist for inspiration
Antagonises intrinsic PEEP
Intrinsic PEEP respiratory muscles must overcome the + P to create a –P addition of extrinsic PEEP can return the system to balance ( may also result in gas trapping)

6. Physiology of CPAP- cardiovascular
Decreases preload
Decreased LV compliance=dilation of RV septum bulges to left
Increased pulmonary vascular resistance
Decreases Afterload = decreased transmural pressure & P gradient intra to extra thoracic
LV stroke volume reduced by decreased RV output LV workload decreased by decreased SV and Dec Afterload
The effect on VR is compounded by hypovolaemia
In normal pt CPAP decreases Cardiac output in CCF may increase by L shift on frank starling curve

7. Pressure Support

8. IPS or IPAP Inspiratory pressure support
Patient triggered assist for inspiration
Increased tidal volume, decreased WOB
Need to set
pressure support
Trigger (flow patient generated to trigger IPS
Rate, volume , I:E ratio are all set by patient
Can set an escape or backup rate
Flow must be faster than the patients own max inspiratory flow or P will drop ( also have to allow for leaks depending on interface)

9. Physiology of IPS Increased Tidal volume, decrease WOB
Negative effects of CPAP are largely increased by addition of IPS
As the pressure is not sustained many of the benefits of CPAP are not greatly improved by adding IPS
CPAP for CCF & severe pneumonia
CPAP + IPS for everything else

10. SIMV- Synchronised Intermittent Mandatory Ventilation
Volume or pressure controlled
Triggered (time or flow)
Time cycled
Patient can influence the triggering of both mandatory breaths and spontaneous breaths
Mandatory breaths= assist control breath
True spontaneous breaths may have an IPS added

11. Mandatory breath
Triggered Mandatory breath
Spontaneous breaths

12. PCV Pressure controlled ventilation
Time or flow triggered (SIMV)
Time cycled
Set
peak inspiratory pressure
Inspiratory time
Rate
Trigger
+/- IPS for true spontaneous breaths

13. PCV SIMV T insp critical
Tinsp too Short flow >0 at end insp
Tinsp too long flow =0 before end insp
This can be both beneficial and harmful
Volume is variable depends on set variables & patient variables
Patient can both increase and decrease volume of mandatory breaths
Patients receive and “assist” for both triggered and spontaneous breaths

14. PCV Flow is dynamic & decelerates as approaches set Pinsp
Volume variable
Good for patients where you are happy to let them “take what they want”
May reduce sedation needs
Decreases WOB for spontaneous and triggered breaths
Dynamic flow good for variable compliance

15. Adaptive Pressure Control - APC
Aims to achieve a desired volume with the lowest possible pressure
Flow dynamic depends on set volume, Tinsp & lung compliance
Volume controlled
Time triggered &/or flow triggered
Time cycled
Pressure regulated=set max Pinsp

16. Where to next
APC
ASV
APRV

17. APC
Set Tv
Pinsp max
IPS for true spontaneous breaths
Flow Trigger for triggered & spontaneous breaths
Rate & Tinsp
Ventilated “explores/learns” compliance & varies pressure to achieve targeted volume
Still has problems of all VCV –fixed volume
Autoflow is~=APC

18. the current and next breath /
Calculated
Underestimated
compliance
Test breath
On target
Ventilator calculates Pinsp required to achieve target Volume
Target volume
This process is continuous & the ventilator will attempt to adapt each time
Compliance varies.
Volume is still set so if set V = patient desired volume can still get dyssynchrony
If patient breathes with or against ventilator compliance varies this will affect both
the current and next breath /

19. Adaptive Support Ventilation
Volume regulated pressure controlled ventilation
Aims to maintain minimum minute ventilation whilst avoiding
Rapid shallow breathing or excessive dead space ventilation
Excessive PEEP
Aims to recreate- tidal volume & rate
Least WOB
Least ventilator applied pressure

20. ASV does not make clinical decisions Operator specifies
Patient height & gender
% minute volume= for high ventilatory demand patients increase e.g. patient with temp > 38.5C % MinVol 120%
Trigger for assist and triggered breaths
Expiratory trigger sensitivity, PEEP, FiO2, ramp as required

21. Once started the ventilator calculates breathing pattern, tidal volume and rate, that are compatible with a lung protective strategy
ASV pressure limit is 10 mmHg below P high
Low Vt = 2x dead space= 4.4*IBW
High rate limit is calculated to prevent gas trapping and AutoPEEP

22. This process is continuous based on the last 5 breaths
Initial Mode PCV SIMV
If patient breathes – triggers a breath which is Pressure supported and flow cycled
If patient does not breathe-PC and time cycled
Ventilator supports/delivers 3 breaths and compares the F and Vt to the target
If Vt < target Pinsp is
If F < target Rate is
This process is continuous based on the last 5 breaths
Really it is what we do without having to continuously fiddle with the ventilator

23. Airway Pressure Release Ventilation APRV
Effectively Bilevel PEEP (inverse ratio PCV)
Expiration occurs during low PEEP
Inspiration occurs during transition from low to high
Low PEEP short to avoid de-recruitment usually set to be when expiratory flow falls to ~75-50% of maximum
(I:E ratios 6-12:1 Tlow ~0.25-1second)
Patient can breath during either phase
At high PEEP the patient can still breath but does from a full lung so compliance maybe very low

24. Low PEEP is usually set to 0, however as T low is vey short there is gas trapping
and intrinsic PEEP that prevent/reduced expiratory lung collapse
Alveolar recruitment occurs during the sustained high PEEP
Tuning
Increase Thigh & Phigh and decrease T low increases oxygenation
Increase Tlow & Phigh increases Tv and decreases CO2
Decreasing sedation and adding an assist for spont breaths can increase
minute volume and decrease CO2
Cardiovascular effects
Same as CPAP