Fig. 27.1
Example of daily ventilatory management in an ARDS patient. Main parameters of ventilator and ECMO setting, gas exchange, and hemodynamics are shown day by day. As soon as the native lung improves, the patient is switched to assisted mechanical ventilation (dotted line), and mechanical and ECMO GF support is progressively reduced
On the other hand, in purely hypercapnic patients, the extracorporeal blood flow is low from the beginning and the magnitude of the assist will depend on the sweep gas flow.
In the first phases, the FiO2 of the sweep gas is usually kept at 100 %, especially in hypoxemic patients. As gas exchanges improve, it is suggested to reduce the FiO2 of the ventilator before that of the sweep gas, to avoid oxygen-related toxicity on the native lung.
During the weaning phase, different strategies of mechanical ventilation can be adopted.
In some patients, for example those with serious bleeding complications, the priority is to discontinue ECMO as soon as possible; in these cases, patients will be disconnected from ECMO while still on controlled mechanical ventilation.
More commonly, ECMO is used to facilitate the switch from controlled mechanical ventilation to an assisted spontaneous breathing mode, such as Pressure-Support Ventilation (PSV) or Neurally Adjusted Ventilatory Assist (NAVA) [3, 4]. In this case, there is a complex interplay between sedation, respiratory drive, and ventilation of the artificial lung; for example, increasing the sweep gas is a very efficient mode to control the respiratory drive of the patient and may allow a reduction of sedative drug dosage. In other words, modulating the extracorporeal assist becomes a strategy to facilitate the patient’s weaning from the ventilator.
Finally, there are situations where it is desirable to reduce as much as possible the duration of invasive mechanical ventilation, such as in presence of severe immunocompromise: in these cases, patients can be extubated while still on ECMO, and the extracorporeal support is discontinued only after separation from the ventilator.
27.4 Trial of ECMO Discontinuation
Once the patient is judged as ready for weaning according to the criteria listed above, it is recommended to perform a trial of temporary discontinuation of the extracorporeal support. By definition, venovenous ECMO does not provide hemodynamic support: for this reason, unlike with venoarterial ECMO, there is no need for stopping or reducing extracorporeal blood flow at the time of the trial off.
The trial of venovenous ECMO discontinuation should be performed as follows:
If the patient is on controlled mechanical ventilation, the ventilator settings (respiratory rate, plateau pressure, FiO2, and PEEP) should be adjusted to values that are considered acceptable off ECMO. If the patient is on assisted spontaneous ventilation (e.g., PSV, ACV, NAVA), an adjustment of the level of inspiratory assist and a careful modulation of the level of sedation may be required.
Once the ventilator settings have been adjusted as described before, the sweep gas to the oxygenator is turned off. It should be remembered that it is not enough to turn the flowmeter to zero, but it is necessary to clamp the gas tubes, since oxygen can leak around the flowmeter even when it appears to be off. Once the sweep gas flow is stopped, the oxygen will be fully consumed after about 20 min: monitoring of venous oxygen saturation on the extracorporeal circuit will indicate when the excess oxygen in the circuit has been used up.
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