Introduction
Extracorporeal membrane oxygenation (ECMO) is a form of temporary
cardiopulmonary support used in severe cardiovascular or respiratory
failure. Using large cannulae placed in major blood vessels,
deoxygenated venous blood is circulated through an extracorporeal
circuit, which includes an oxygenator that conducts efficient gas
exchange. This circulation is supported by pressure generated by a pump.
The oxygen-rich blood is then reinfused back into the body via another
large blood vessel.
Veno-venous (VV) ECMO is used primarily for respiratory support whereby
oxygen-rich blood is reinfused into a large vein. Veno-arterial (VA)
ECMO provides hemodynamic support by reinfusing this oxygen rich blood
into the arterial circulation through a large cannula usually placed in
a major distributary of the aorta. It is not uncommon for patients who
primarily have respiratory failure to also have significant
cardiovascular compromise. The latter usually improves once adequate gas
exchange is restored on VV ECMO. However, not infrequently there is
concomitant and significant cardiovascular and respiratory failure. For
example, patients with severe respiratory failure on VV ECMO may develop
cardiomyopathy requiring significant cardiovascular support. Conversely,
patients with primarily cardiovascular failure on VA ECMO may have or
develop concomitant severe respiratory failure due to pulmonary edema
from elevated left atrial pressure or a primary injury to the lungs. In
the absence of significant cardiovascular recovery, oxygenation of the
proximal aorta and its distributaries is largely from the oxygen rich
blood reinfused by the ECMO circuit. In patients on VA ECMO, recovery of
heart function with persistent severe lung injury leads to
“differential hypoxemia” where ejection of poorly oxygenated blood
from the left ventricle competes with retrograde oxygenated blood flow
from the ECMO circuit, leading to myocardial and cerebral ischemia.(1-8)
This is detected by comparing oxygenation from a right radial arterial
sample with one obtained from the left. As LV function recovers, the
right sided arterial blood sample will have relatively lower oxygen
content reflecting native lung dysfunction. With further recovery, the
“mixing point” of blood may move further down the aortic arch with
lower oxygen content of both compared to the femoral arteries.
To address these perfusion requirements, clinicians have developed
“hybrid” configurations by placing additional cannulae.(9) The most
common of these is venoarterio-venous (VA-V) ECMO, which consists of a
single venous drainage cannula and both arterial and venous reinfusion
limbs. In patients with secondary heart failure (on VV ECMO), a femoral,
subclavian, or axillary artery cannula is added to provide adequate
circulatory support. In differential hypoxemia (on VA ECMO) an internal
jugular or subclavian vein catheter is added to deliver oxygenated blood
to the pulmonary circulation.(4) Using an adjustable clamp on the venous
reinfusion limb, arterial and venous reinfusion flows are titrated until
adequate hemodynamics and oxygenation are obtained.(10, 11)
The number of patients being treated with ECMO has dramatically
increased over the last decade, and its use in our ICU has followed a
similar trend.(4, 12) While a few case series have provided initial
descriptions of patients requiring VA-V ECMO, there remains a paucity of
literature on the subject.(10, 11, 13, 14) We report our hospital’s
experience with VA-V ECMO to further characterize the patient
population, indications, and outcomes associated with this hybrid
configuration.