Physiological based management of hypoxaemic respiratory failure - - PowerPoint PPT Presentation

@ Murdoch Children’s Research Institute, 2017

Physiological based management of hypoxaemic respiratory failure

David Tingay

• 1. Neonatal Research, Murdoch Children’s Research

Institute, Melbourne

• 2. Neonatology, Royal Children’s Hospital
• 3. Dept of Paediatrics, University of Melbourne

2

After a generation of surfactant, steroids, machines and ‘gentleness’ are preterm respiratory outcomes better?

Doyle L et al NEJM 2017

3

Conventional Ventilation in 2018 is confusing

SIMV AC/SIPPV/PTV PSV VTV MMV CPAP PC-APRV CMV/IMV

Is the discussion regarding the mode to choose the right question?

4

What does a Ventilator do?

• Gives a Pressure
• Via a Flow of gas
• Modulates (Limits) the pressure against Time or Flow
• To generate an inflation of the lung actively and deflation passively
• May try to synchronise the start and/or end of this process with the

spontaneous breathing pattern

• May try to adapt the inflation pressure to maintain a constant tidal

volume

• All Neonatal Ventilators are TCPL +/- VCPL

PEEP PEEP PIP Ti Initiate? Terminate Limit

5

What do the lungs do?

• 1. Oxygenation

a) FIO2 b) Adequate recruitment

• PEEP

, PIP , Ti

• 2. Ventilation = CO2 clearance

a) Alveolar Minute Ventilation = Rate x (VT – VD) VT influenced by:

• DP (PIP – PEEP)
• Flow and RRS
• Ti
• Volume State of the lung (PEEP and CRS)
• 3. Diffusion
• 4. Perfusion

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The lung is a mechanical system that requires motion to work

EQUATION OF MOTION Force = (E x distance) + (R x speed) + (M x acceleration)

M E R Force

The natural state of the lung is deflation (elastic system) Lung inflation requires generating enough pressure (and energy) to overcome the forces

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Principles of Lung Mechanics Applying the equation of motion to achieve gas exchange

RRS CRS = DV/DP

PRESSU RE STAT!

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Inflating the Lung Generating a tidal volume - Compliance

• Compliance describes the ability of the lung to move when a pressure is applied to it
• CRS = DV/DP

PEEP PIP Volume Surfactant

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Inflating the Lung Takes time - Resistance

Compliance, CRS Resistance, RRS Inspiratory Flow = constant ΔVL ΔPL P

AO

Active Inspiration

Compliance, CRS Resistance, RRS Inspiratory Flow ΔVL ΔPL P

AO = 0

Passive Expiration Higher RRS = Greater Pressure to move the lung for any given period of time

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Ti and Te need to allow the lung to fully inflate and deflate

Salazar and Knowles J Appl Physio 1964

Time Constant: t (tau) = CRS x RRS Describes the slope of the exponential DV curve

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Time constants

Normal lung:

τ  3 mL/cm H2O x 0.04 cm H2O/mL/sec

 0.12 sec (insp and exp) Parenchymal disease:

τ  0.5 mL/cm H2O x 0.04 cm H2O/mL/sec

 0.02 sec (insp and exp) Airway disease:

τ  2 mL/cm H2O x 0.1 cm H2O/mL/sec

 0.2 sec (exp > insp)

Time Constant: t (tau) = CRS x RRS

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The ventilator tells you how to inflate and deflate the lung

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Understanding Time Constants at the bedside

• All about the FLOW wave form

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Time is also important in expiration Auto-PEEP

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Not all babies in the NICU are the same

Different manifestations of disease have different mechanical properties of the lung There can not be a single mode or single (or tight) range of ventilator settings that are always correct

HMD Pulmonary Hypoplasia Evolving BPD MAS Atelectasis Hypoplasia Gas Trapping Mixed Disease

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Disease state, PEEP and Compliance

Dargaville Int Care Med 2010, Tingay Crit Care Med 2013

Zone of Overdistension = High homogenous EEV, Low CRS

25 50 75 100 50 100

Paw (%) VL (%)

Pmax Pfinal

SpO2

Region of optimal volume

TcCO2 MVHF VTao VTRIP

Optimal Paw range Pinitial

Safe Zone

Zone of Atelectasis = Low heterogeneous EEV, Low CRS Safe Zone Inflation Limb = Increasing (heterogeneous) EEV, Improving CRS Safe Zone Deflation Limb = Stable homogenous EEV, maximal CRS

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Optimising Volume Targeted Ventilation

• 1. Correct VT for Min Vent, V

Alv and VD

Correct PEEP (volume state) to optimise CRS

• 2. Correct Ti and Te to

allow VT to be achieved

Klingenberg et al Cochrane Database 2017, Keszler Arch Dis Child Fetal Neo 2018

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Physiological Rationale for triggering mechanical inflations

McCallion et al ADC F&N 2008; 93: F36-9

6540 inflations (n=10), 42% triggered Triggered PIP 12.9 (4.9) cm H2O vs 17.0 (3.3) cm H2O

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Rheotrauma – impact of flow (volume change over time)

Adapted from J Pillow

• 100

100 200

Wave Power (W/s2)

a a b b 0.4s

• 30
• 15

15 30

Flow (L/min)

a b a b Pre-surfactant Post-Surfactant a = inflation b = deflation Red = forward Blue = backward Smolich Tingay Pilot Data

In low compliance states low flow (2-4 L/min) more protective than high flow (8-10 L/min)

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Ventilator settings in the NICU 5 fundamental questions

1. What is the right modality for the pathophysiology? 2. What PEEP is appropriate for the lung disease and the desired lung volume? 3. What insp time is appropriate for the time constant of the lung, ± the baby’s respiratory pattern? 4. What PIP is needed to produce an appropriate tidal volume? 5. What rate is needed to produce adequate minute ventilation, and therefore CO2 clearance?

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Initial Ventilatory Approach for the Neonatal Lung

• Aim for NIV as soon as practical
• What is the pathophysiological process?

Homogeneous Atelectasis Normal Lung Gas Trapping In complex mixed regional lung pathophysiology the correct approach is usually dictated by the current primary problem – strategy needs frequent re-evaluation. CRS, RRSN CRS N, RRS N CRS N, RRS

P = positioning, VS = Volume Strategy; H = high, N = normal, L=low

HVS Disease State? Mechanical State? Regional? Cautious HVS+P LVS N or LVS+P Ventilation Approach Ti & Te Short Short Normal Long Normal

• Long

Modality* CMV/ HFOV CMV/ HFOV CMV HFJV/ HFOV HFJV/HFO V/CMV NVS Hypoplasia CRS, RRS  N

Where possible target VT not PIP and synchronise

N or LVS Short HFJV Heterogeneous