Question 4

A 47 year old man has severe ARDS following a perioperative aspiration. He is endotracheally intubated and ventilated in SIMV mode with PEEP 5 cm H2O and an FiO2 0.4 resulting in a PaO2 of 45 mm Hg (6 kPa). On the chest X-Ray, the endotracheal tube is properly positioned in the trachea. The only abnormality on the chest X-ray was bilateral diffuse alveolar infiltrates.

List the steps you could take to improve his oxygenation. Include a brief comment on the rationale for each step.

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College Answer

Basic Measures
°     Increase FiO2): Improve PAO2
°     Increase PEEP
°     surface area for gas exchange
°     Improvement of atelectasis
°     Redistribution of lung water

General Measures
°     Physio / suctioning
°     Sedation / Consider Paralysis: Decrease O2 requirements and CO2 production
°     Treat factors that increase metabolic demand, sepsis etc: Decrease O2 requirements and CO2 production
°     Optimise fluid balance: balance of interstitial overload and maximising cardiac output and DO2
°     Optimise Haemoglobin, optimal oxygen carriage / viscosity combination and minimise immune and volume effects of transfusion

Optimise Recruitment and FRC
°     Lung recruitment manoeuvre: Opening collapsed alveoli, increasing FRC and area available for gas exchange
°     Increase I:E Ratio towards 1:1: Increased FRC as above, recruitment, increase PAO2
°     Inverse ratio ventilation: Longer in inspiration with potential to gas trap and provide autoPEEP above set PEEP with subsequent increase FRC and area for gas exchange.
Positioning
°    Prone position: better VQ matching, improved mechanical advantage, less lung compression from abdominal and mediastinal contents

Optimise Flow to Ventilated Alveoli
° Inhaled Nitric oxide: inhaled dilator delivered only to ventilated alveoli
° Prostacyclin: improved perfusion to ventilated alveoli

Last Resorts
°     Tracheal Gas insufflation and other measures to decrease circuit dead space: reduced dead space means lower CO2 with relative increase in partial pressure of O2
°     HFOV
°     ECMO, external membrane oxygenation and CO2 removal: lung rest and minimisation of VALI.

Discussion

This question is about the ventilation strategies in ARDS, which is a topic discussed elsewhere:

In summary:

Initial ventilator strategy:

Use a Pressure Control mode (it may be safer, though the evidence is not strong)
Lung-protective ventilation: use low tidal volumes (6ml/kg)
Open-lung ventilation: avoid derecruitment by using a high PEEP
- The ideal PEEP can be found either by finding the lower inflection point or the pressure-volume curve or by observing a stepwise decrease in PEEP after a recruitment manoeuvre.
- As the ARDS severity increases, consider using a higher PEEP.
Use a lower driving pressure (ΔP) -Amato et al, 2015. That means, using a higher PEEP and aiming for a lower plateau pressure
Accept a level of "permissive hypercapnea"

Additional ventilator manoeuvres to improve oxygenation:

Use an I:E ratio of 1:1, even though manipulating the I:E ratio does not seem to improve survival, even though it may improve oxygenation.
One might attempt some recruitment manoeuvres if hemodynamics permit. Again, these offer a transient improvement in oxygenation, but do not influence survival.

Non-ventilator adjunctive therapies for ARDS:

Minimization of dead space ventilation - Remove as much tubing as you can.
Low-carbohydrate high-fat nutrition - Keep them off the carbs, and don't overfeed.
Neuromuscular blockade improves survival, not just gas exchange.
Sedation decreases energy expenditure and improves ventilator synchrony
Fluid management should have a goal of neutral balance (keep em dry)

Ventilator strategies to manage refractory hypoxia

Prone ventilation, for at least 16 hours a day (PROSEVA, 2013)
High frequency oscillatory ventilation may not improve mortality among all-comers (OSCAR, 2013) or it may actually increase mortality (OSCILLATE, 2013) but some authors feel that there were problems with methodology.

Non-ventilator adjuncts to manage refractory hypoxia

Nitric oxide was a cause for some excitement, but is no longer recommended.
Prostacyclin is still a cause for excitement, and is still vaguely recommended.
- Neither agent improves mortality, but prostacyclin can improve oxygenation.
ECMO may improve survival (CESAR, 2009) but again there were problems with methodology.

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