Hypoxia during VV ECMO
07 March 2018 (online)
Venovenous extracorporeal membrane oxygenation (VV ECMO) is the preferred mode for any kind of acute respiratory failure. The desired PO2 is > 50 and SPO2 > 88% but the acceptable values can be PO2 > 45 and SPO2 > 80%. In case if we are accepting lower PO2, we have to keep a higher hematocrit and meticulously monitor neurological status, lactates, and urine output to maintain oxygen delivery. Saturation during ECMO run depends on ECMO circulation, native circulation, and ratio of ECMO flow to cardiac output ([Table 1]). Whenever the effective ECMO circulation decreases, saturation decreases. So, decrease in ECMO flow, ECMO FiO2, failing membrane oxygenator, and increase in recirculation will lead to hypoxia. Similarly, anything that decreases the contribution from native circulation will also lead to hypoxia. So, decrease in ventilator settings and worsening lung status lead to hypoxia. Anything that increases metabolism, such as fever and restlessness, will also cause hypoxia due to increased consumption.
Recirculation is defined as the flow of oxygenated blood from the returning cannula to the draining cannula without entering systemic circulation. It decreases the efficacy of VV support. Around 30% of recirculation is average. The factors on which the recirculation depends are pump flow ([Fig. 1]), catheter position, cardiac output, and RA size or intravascular volume. The recirculation can be calculated with the help of following equation:
Management of hypoxia will depend on treating the underlying cause ([Fig. 2]). The usual strategy is to increase oxygen transfer by increasing ECMO flow, ECMO FiO2, or by increasing hematocrit and thereby improving oxygen delivery. Many a times hypoxia is secondary to increased metabolic rate and just controlling that (controlling fever, giving sedation) will improve saturation. Recirculation can be managed by adjusting the flow; sometimes a higher flow is the cause of recirculation and just by decreasing flow we can get better saturation. Too close placement of tip of drainage and return cannula may lead to recirculation and just repositioning of cannula can improve saturation. If the recirculation persists, the last solution is to put additional cannula for drainage and switch to VVV ECMO.
In spite of all the above measures if the patient remains hypoxic and lactates are rising, then the last resource is to induce hypothermia and ultimately switch to VA or VAV ECMO ([Table 2]).
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