Author: Chris Nickson
Reviewer: Aidan Burrell
Everything ECMO 002
The 26 year-old man from Everything ECMO 001 received VV ECMO for severe ARDS secondary to influenza. Fortunately he has improved since you corrected his persistent hypoxaemia.
Q1. What is the objective of mechanical ventilation in an ARDS patient on VV ECMO?
The main objective is to prevent ventilator-induced lung injury (VILI) and to allow the lungs to recover.
VV-ECMO is used to provide adequate circuit blood flow for oxygenation and fresh gas flow* (FGF) for carbon dioxide (CO2) removal.
*Fresh gas flow is also called the sweep gas flow or simply “the sweep”
Q2. What are the typical ventilator settings for a ARDS patient on VV ECMO?
Typical ventilator settings for severe lung disease in the context of VV ECMO support are:
- low-level pressure control (peak airway pressure <25 cmH2O)
- reduces VILI due to alveolar strain
- high PEEP (10-15 cmH20)
- reduces VILI due to lung derecruitment/ recruitment (atelectrauma) and shearing injury at the interface between collapsed and ventilated lung
- low tidal volumes
- so-called lung rest or ultra-protective ventilation generally refers to tidal volumes <4 mL/kg predicted body weight (PBW)
- reduces VILI caused by alveolar strain
- low respiratory rates may also be considered to help decrease mechanical lung stress
- FiO2 < 0.6 if sufficient circuit flow, keeping SaO2 >85% in most circumstances
- prevents resorption atelectasis and oxygen toxicity from high lung FiO2 (which contributes little to oxygenation in the context of severe hypoventilation due to ‘lung rest’)
In the severe phase of respiratory failure, tidal volumes may be <2mL/kg at the above pressure settings. In addition to lowering the risk of VILI, the use of pressure controlled ventilation allows daily monitoring of tidal volumes, which increase as the patient improves. As lung compliance improves (e.g. tidal volumes >2 mL/kg), trials of de-sedation and spontaneous breathing can be started.
Significant regional variation exists on how to ventilate patients lungs on VV ECMO. A strategy of lower ECMO flows only allows a small reduction in lung ventilation parameters, while higher support with ECMO (eg with high flow configurations) allow ultraprotective lung ventilation and lung rest. Further studies are needed to determine which strategy is better.
Q3. When should weaning from VV ECMO be commenced?
Weaning can be commenced when lung recovery has begun and lung rest is no longer necessary. This is determined by the patient’s underlying condition. Different causes and different severity of lung disease will require different lengths of time with VV ECMO support. Weaning is typically started when the patient’s lung compliance improves on the ventilator along with an improving chest x-ray. This requires clinical judgement and careful monitoring of the patient’s progress.
Q4. How is weaning from VV ECMO performed?
Weaning from VV ECMO is performed by progressively decreasing the FGF to the oxygenator. At the same time lung ventilation must be increased to maintain adequate CO2 clearance.
Succesful weaning is confirmed if the patient remains stable at a FGF of 0 L/min for a period of 4 to 24 hours (at the ICU Consultant’s discretion). At an FGF of 0 L/min the patient is effectively ‘off ECMO’ .
Note that for VV ECMO:
- Oxygen blender FiO2 does not need to be weaned – when the sweep gas or FGF is off, the patient is off ECMO.
- Circuit blood flow does not need to be changed. However, when pulmonary oxygenation has improved there may be a rationale to reduce circuit flow rates with the aim of reducing access insufficiency (thus facilitating diuresis and spontaneous breathing trials)
- Echocardiography is not required for weaning (unlike VA ECMO)
Q5. How are peripherally inserted VV ECMO cannulae removed?
Peripherally inserted VV ECMO cannulae removal at The Alfred ICU is performed as follows:
- Explain procedure to patient and/or family
- Remove dressings
- Clamp all access and return lines
- Power off the ECMO console
- Remove the cannulae simultaneously and immediately apply pressure to the puncture sites with sterile gauze
- Pressure is maintained for 20 minutes
- The patient remains supine and still for 4 hours and the cannula sites are monitored for bleeding
- A lower limb ultrasound is arranged within 24-48 hours of decannulation to exclude venous thrombosis.
Note that this only applies to peripherally inserted VV cannulae. VA cannulae, central cannulae or VV cannulae inserted using a cut-down technique should be removed in the operating theatre by a vascular surgeon.
References and Links
- Kredel M, Bierbaum D, Lotz C, Kuestermann J, Roewer N, Muellenbach RM. Ventilation during extracorporeal membrane oxygenation for adult respiratory distress syndrome. Crit Care. 2014 Jun 30;18(3):442. doi: 10.1186/cc13954. PubMed PMID: 25041935; PubMed Central PMCID: PMC4223403.
- Schmidt M, Pellegrino V, Combes A, Scheinkestel C, Cooper DJ, Hodgson C. Mechanical ventilation during extracorporeal membrane oxygenation. Crit Care. 2014 Jan 21;18(1):203. doi: 10.1186/cc13702. PubMed PMID: 24447458; PubMed Central PMCID: PMC4057516.