An Anaesthetist from a provincial hospital has a 20 year old man with suspected fat embolism syndrome following an isolated femoral fracture that was repaired earlier that day. The patient has become increasingly hypoxic and difficult to ventilate, but transfer to a metropolitan centre has been delayed for 12 hours due to bad weather.
His arterial blood gases on SIMV mode of ventilation are as follows: FiO2 1.0, pH 7.21, PaO2 65 mmHg (8.6 kPa), PaCO2 72 mmHg (9.3 kPa), HCO3 28 mmol/L. He has a four quadrant infiltrate on his chest X-Ray.
Outline the advice that you would give to help your colleague manage this patient’s ventilation.
• Confirm Diagnosis
- ARDS criteria: CXR, PF ratio, Etiology, no overload
• exclude other etiologies - where is the ETT (not RMB), no pneumothorax, aspiration etc.
• What ventilator is he using, are you familiar with it’s modes (such as pressure control, volume control)
• Ventilatory strategy –pressure and volume limitation to minimise barotrauma)
• PEEP increments to effect, ensuring Plateau Pressure < 30 cm H20
• Heavy sedation and paralysis to minimize O2 consumption and CO2 generation to GCS 3 and no spontaneous ventilation
• Targets for ventilation SpO2 > 90-95 and PO2 > 60
• permissive hypercapnia as long as pH > 7.1
• prone position probably not appropriate (if staff not experienced)
• recruitment manoeuvres
-CVP only to ~PEEP+2 as maximum
- consider frusemide if CVP PEEP +5
-use inotrope to maintain MAP > 60 - suggest noradrenaline
-Transfuse only for Hb approaching 7
• Reassure him and make yourself available for advice 24/7 (Mention of NO, liquid ventilation, surfactant, TGI – no role in this setting)
This is a pretty realistic scenario; as if you were on call and managing a patient over the phone, giving advice to your registrar.
The patient is acidotic, hypoxic and hypercapneic.
What do you do?..
- Confirm that the ETT is in the right position and is not blocked up with secretions
- Confirm diagnosis of ARDS by performing a TTE, looking for significant cardiac dysfunction
- Exclude immediately reversible problems, eg. pneumothorax
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.
Note how useless it would have been to digress into specifics of management for fat embolism syndrome.