Apnea Test on VA ECMO support
In recent years, the application of Extra Corporeal Life Support (ECLS) experienced a significant increase in reported cases/indications. Despite advancements, ECLS comes with potentially severe neuro-complications, mainly hemorrhagic or ischemic. Furthermore, in settings such as Extra Corporeal Cardio Pulmonary Resuscitation (ECPR), the underlying pathological process could per sé evolve to severe, diffuse, and irreversible brain injury. Determination of death, according to neurological criteria may, therefore, be required in this population.
Evaluating complete termination of brain function at the bedside usually includes Apnea Test (AT) to assess the absence of spontaneous ventilatory efforts, in the presence of PaCO2 elevation with no pH increase. Ihie et al (2019) argued that, on peripheral Veno-Arterial (V-A) ECMO, the PaCO2 sampled at one single point of patient/circuit system may not reflect cerebral PaCO2. In this ECLS configuration, if contractility is partially preserved or recovering, the returning extracorporeal blood flow re-perfuses aorta mixing with blood ejected from left ventricle. The location of the melting point (or mixing cloud) depends from Left Ventricular (LV) contractility and pump speed. According to this peculiar physiology, cerebral blood supply could arise from LV, with PaCo2/PaO2, depending on native lung function and on mechanical ventilation settings/spontaneous breathing efforts, &/or from ECMO, with PaCo2/PaO2, depending on ECLS settings/Membrane Lung (ML) function.
Unless there is evidence of cardiac standstill with closed aortic valve, the exact location of the cloud (the exact source of cerebral blood flow) could not be precisely predicted. This study reports on the development and implementation of AT protocol, including measurement of both systemic distal arterial and post-ML blood, to ensure cerebral CO2 tension to be reliably determined, while ECMO settings are progressively adjusted to decrease CO2 removal.
STUDY STRENGTHS & LIMITATIONS
A strong physiological rationale supports the proposed protocol for AT on V-A ECMO. It could be implemented at the bedside, without the need for moving the patient to perform ancillary tests, or manipulating the circuit.
Gradual de-escalation of fresh gas flow to ML and adjustment of blood flow can prevent systemic hypoxemia and hemodynamic instability eventually. The latter impact negatively on procurement procedures and/or graft function. However, this single centre experience only reports three cases (just two underwent radiological confirmation), with actual inconsistency between the two sampling sites observed once.
TAKE HOME MESSAGE
On peripheral V-A ECMO, blood perfusing the brain may derive from extracorporeal system, LV or both. Nevertheless, Apnea Test could be performed with ease and safety, but simultaneous arterial blood sampling, both on the patient and on the circuit (post-ML), may be required for the test to be reliable.
This article review was prepared and submitted by Marta Velia Antonini @FOAMecmo, CCN/CCP on behalf of N&AHP ESICM.
1) Ihle JF, Burrell AJC, Philpot, Pilcher, Murphy D, Pellegrino VA. A Protocol that mandates post-oxygenator and arterial blood gases to confirm brain death on veno-arterial extracorporeal membrane oxygenation. ASAIO J. 2019 Oct 11.
2) Appendix to the previous article: Apnea Testing on veno-arterial extracorporeal membrane oxygenation (ECMO) Protocol.
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