Discussion
To our knowledge, we report the largest study focusing on outcomes of
VA-ECMO treatment considering blood pH at VA-ECMO initiation. Our
findings suggest: 1) the survival at one year of patients with a blood
pH at cannulation <7 is low but does exist. However, patients
were younger and treated mainly for refractory cardiac arrest, but after
adjustment blood pH remains associated with mortality. 2) The lactate
level is highly correlated with the pH level.
The use of ECMO has been used exponentially since the early 2000s in
western countries, allowing to treat even patients in refractory
out-of-hospital cardiopulmonary arrest with better results than standard
resuscitation (43% vs 7% at discharge and 43% vs 0% at 3 months)(7).
VA-ECMO might become in a near future the standard treatment for
refractory cardiorespiratory arrest, however clinical results remain
questionable in many centers. Mørk and al. recently reported a thirty
day survival rate of 26% after ECMO initiation for out of hospital
refractory cardiac arrest(8). Predicted scores have been proposed but
the optimal tool remains an unmet need (9). We believe pH and lactates
may represent interesting parameters to consider to
help in this goal research.
The survival rate in group 1 (lowest pH) was significantly lower than in
the other 2 groups (p<0.001). In our experience, having a pH
lower than 7 at implantation multiplies the risk of death by four (Table
3). In the literature, survival in the adult population treated with
VA-ECMO for cardiogenic shock is similar than in our study(10). However,
we do report here our real-life daily practice without excluding
patients with high acidity, even those who were supported for a few
hours. Interestingly, Mørk et al. reported good outcomes on a population
similar to our group 1(8). In our study, the survival rate, in group 1
although very low, is not negligible. Indeed, in this group one patient
out of 6 has been discharged alive from the hospital despite the extreme
acidity at cannulation.
It is important to note that the high mortality rate occurs within the
first 30 days post-cannulation and that the median survival is
respectively 1, 14 and 33 days for groups 1, 2 and 3. These results
indicate that financial and human resources require to manage such
patients, although important, might not last for a long time of
mechanical support, especially for group 1 patients. The most
critically-ill ones die promptly, mainly within the first days. An
ongoing study (ECMONOMY) at the Rennes’s University Hospital, aims to
accurately study the cost of a hospital stay for a patient treated with
VA ECMO (NCT03686540 ). This study will eventually help to
explore the financial ratio in order to avoid unnecessary expenses.
Patients in group 1 had fewer complications, whether thrombotic or
infectious, than the others groups. One may consider it is because these
patients die more quickly and did not have time to develop such
complications.
As in daily life practice we demonstrated a strong correlation between
the blood pH level and the lactate level (-0.66). This strong
correlation is interesting as one or both parameters might be part of a
new decision-making algorithm at cannulation. There are already scores
such as the SAVE score which predicts the survival rate in patients with
refractory cardiogenic shock requiring ECMO(3). This score requires a
blood sample to get serum creatinine or bicarbonate levels, excluding
its easy use out of hospital and requiring a minimal delay. We believe
pH and lactates dosage can be obtained almost instantaneously, even
outside hospital with small ambulatory biological samples devices. Such
strategy may allow to help in the difficult decision-making process,
especially in high emergent cases, out of hospital or in the emergency
room. Indeed, our study showed that when blood pH level is low
(<7) and blood lactate level is high (>7) the
survival prognosis is low (<7%). This simple algorithm with
“the three seven rule” may be very helpful for emergent real-life
clinical.
Moreover Mungan et al. indicate that in addition to the lactate level at
ECMO initiation, the evolution of lactate clearance is also important as
these 2 factors allow to estimate a reliable prognosis of the patient’s
outcome(11). To be more precise, Li et al. indicate from their
univariate and multivariate analyses that the important factors are the
mean lactate concentration and lactate clearance at 6 and 12 hours after
the initiation of ECMO(12). This approach integrating pH and lactates
levels at ECMO initiation seems logical as the elevation of lactate
levels is related to tissue damage and that lactate clearance therefore
indicates a restoration of tissue oxygenation and hemodynamic
circulation. In our study low pH and high lactate rates are mortality’s
predicting factors as demonstrated by Seeger and al(6).
We believe our study pleads strongly for further investigations to
precise utility or futility of ECMO treatment using pH and lactates
level. These additional investigations seem to be essential because new
indications of short-term assistance (ECMO or Impella®) may develop such
as High Risk Percutaneous Coronary Indication (HR PCI), Transcatheter
Aortic Valve Implantation (TAVI) or Mitraclip®. Moral and ethical
obligations will require careful evaluation of these possible new
indications to avoid futile results. Heart team approach will be even
more essential to define possible new indications in a very severe
population.
Our study has some limitations, including its retrospective nature and
the heterogeneity of its cohort. The retrospective character is somewhat
mitigated by the fact that the data in the database are entered
prospectively by nurses dedicated to the database. Moreover, our study
compares survival outcomes between different subgroups of patients in a
tertiary high-volume ECMO center. Our database does not contain the data
of all the requests for ECMO procedure but only the patients implanted;
it is therefore possible that a selection was made at the discretion of
the surgical team when managing the call for ECMO patient, especially
given the patient’s age and comorbidities.