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 @ Murdoch Children’s Research Institute, 2017
After a generation of surfactant, steroids, machines and ‘gentleness’ are preterm respiratory outcomes better? Doyle L et al NEJM 2017 2
Conventional Ventilation in 2018 is confusing CMV/IMV SIMV AC/SIPPV/PTV PSV Is the discussion CPAP regarding the mode to choose PC-APRV the right question? MMV VTV 3
What does a Ventilator do? Gives a Pressure Via a Flow of gas Modulates (Limits) the pressure against Time or Flow Terminate Limit PIP 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 Initiate? May try to adapt the inflation pressure to maintain a constant tidal volume Ti All Neonatal Ventilators are TCPL +/- VCPL PEEP PEEP 4
What do the lungs do? 1. Oxygenation a) F IO2 b) Adequate recruitment - PEEP , PIP , Ti 2. Ventilation = CO 2 clearance a) Alveolar Minute Ventilation = Rate x ( V T – V D ) V T influenced by: - D P (PIP – PEEP) - Flow and R RS - Ti - Volume State of the lung (PEEP and C RS ) 3. Diffusion 4. Perfusion 5
The lung is a mechanical system that requires motion to work The natural state of the lung is deflation (elastic system) Lung inflation requires generating enough pressure (and energy) to overcome the forces E Force M R EQUATION OF MOTION Force = (E x distance) + (R x speed) + (M x acceleration) 6
Principles of Lung Mechanics Applying the equation of motion to achieve gas exchange PRESSU RE STAT! R RS C RS = D V/ D P 7
Inflating the Lung Generating a tidal volume - Compliance Compliance describes the ability of the lung to move when a pressure is applied to it C RS = D V/ D P Volume Surfactant PEEP PIP 8
Inflating the Lung Takes time - Resistance Compliance, C RS Resistance, R RS ΔV L ΔP L P Active Inspiration AO Inspiratory Flow = constant Higher R RS = Greater Pressure to move the lung for any given period of time Resistance, R RS ΔV L ΔP L Passive Expiration P AO = 0 Inspiratory Flow Compliance, C RS 9
Ti and Te need to allow the lung to fully inflate and deflate Time Constant: t (tau) = C RS x R RS Describes the slope of the exponential D V curve Salazar and Knowles J Appl Physio 1964 10
Time constants Normal lung: τ 3 mL/cm H 2 O x 0.04 cm H 2 O/mL/sec 0.12 sec (insp and exp) Parenchymal disease: τ 0.5 mL/cm H 2 O x 0.04 cm H 2 O/mL/sec 0.02 sec (insp and exp) Airway disease: τ 2 mL/cm H 2 O x 0.1 cm H 2 O/mL/sec 0.2 sec (exp > insp) Time Constant: t (tau) = C RS x R RS 11
The ventilator tells you how to inflate and deflate the lung 12
Understanding Time Constants at the bedside All about the FLOW wave form 13
Time is also important in expiration Auto-PEEP 14
Not all babies in the NICU are the same HMD Pulmonary Hypoplasia Evolving BPD MAS Gas Trapping Atelectasis Hypoplasia Mixed Disease 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 15
Disease state, PEEP and Compliance Safe Zone Deflation Limb = Stable Zone of Overdistension = High homogenous EEV, Low C RS homogenous EEV, maximal C RS P max Sp O2 100 V T RIP MV HF Region of optimal V Tao volume V L (%) Tc CO2 50 Optimal P aw range P final 0 Safe Zone Safe Zone Inflation Limb = Increasing P initial (heterogeneous) EEV, Improving C RS 0 25 50 75 100 Zone of Atelectasis = Low heterogeneous EEV, Low C RS P aw (%) Dargaville Int Care Med 2010, Tingay Crit Care Med 2013 16
Optimising Volume Targeted Ventilation 1. Correct V T for Min Vent, V Alv and V D Correct PEEP (volume state) 2. Correct Ti and Te to to optimise C RS allow V T to be achieved Klingenberg et al Cochrane Database 2017, Keszler Arch Dis Child Fetal Neo 2018 17
Physiological Rationale for triggering mechanical inflations 6540 inflations (n=10), 42% triggered Triggered PIP 12.9 (4.9) cm H 2 O vs 17.0 (3.3) cm H 2 O McCallion et al ADC F&N 2008; 93: F36-9 18
Rheotrauma – impact of flow (volume change over time) In low compliance states low flow (2-4 L/min) more protective than high flow (8-10 L/min) Pre-surfactant Post-Surfactant 30 b 200 Wave Power (W/s 2 ) a a b a = inflation 15 Flow (L/min) a 100 b = deflation Red = forward 0 Blue = backward 0 b -15 -100 a b 0.4s -30 Adapted from J Pillow Smolich Tingay Pilot Data 19
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 CO 2 clearance? 20
Initial Ventilatory Approach for the Neonatal Lung Aim for NIV as soon as practical Where possible target V T not PIP and synchronise What is the pathophysiological process? Ventilation Modality* Approach Disease State? Mechanical State? Regional? Ti & Te CMV/ C RS , R RS N Atelectasis HFOV Short HVS Homogeneous Cautious CMV/ Normal Lung C RS N, R RS N Short HVS+P HFOV C RS N , R RS Gas Trapping CMV Normal NVS C RS , R RS N Hypoplasia Heterogeneous HFJV/ Long LVS HFOV Normal N or In complex mixed regional lung pathophysiology HFJV/HFO - Long LVS+P the correct approach is usually dictated by the V/CMV current primary problem – strategy needs frequent re-evaluation. Short N or LVS HFJV P = positioning, VS = Volume Strategy; H = high, N = normal, L=low 21
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