Session 2: Viability Index Scoring For Critical Cardiopulmonary and Extracorporeal Support Patients

Shock and ECMO Survival

The level of shock that a patient is enduring can be determined using the Viability Index (VI) with high scores correlating to a high level of shock. These charts contain the first VI score just before or just after patients were placed on ECMO. The patient population was pediatric with most patients weighing less than 10 kg. The diagnoses varied in two broad categories; respiratory and cardiac. The respiratory diagnoses included all the traditional neonatal ECMO conditions (with the exception of congenital diaphragmatic hernia); meconium aspiration, primary pulmonary hypertension, respiratory distress syndrome and sepsis. In older patients, diagnoses included primarily pneumonia, respiratory syncytial virus, sepsis and trauma. The most common cardiac diagnoses included pre and post surgery for congenital heart disease and myocarditis or cardiomyopathy.
The VI scores of 95% of the survivors fall into a relatively small area between the corrected anion gaps of 7 and 22 mEq/L and the venoarterial CO₂ gradients of 1 and 13 mmHg. This small area is known as the “lane” in reference to the rectangular area beneath a basketball goal from which a player is most likely to make a goal. Only 61% of the expired patients are located in the lane. The mortality for patients in the lane is 21%.
By contrast, only 5% of the survivors are located outside the lane along with 39% of the expired patients. The patients outside the lane have a mortality rate of 73%, 3.5 times higher than for patients in the lane.
Why such a difference? Patients outside the lane are at a higher level of shock than patients in the lane. Because the Viability Index score relates to the inadequacy of perfusion, the most likely cause for the increased mortality outside of the lane are complications from reperfusion injury caused by the sudden reperfusion of hypoxic ischemic tissues by the ECMO pump. This can manifest itself in many ways; brain hemorrhage or infarct, pulmonary hemorrhage, cardiac stun, renal failure, multiple organ failure or simply failure to improve from a known reversible condition. Rarely does the patient die of the original diagnosis of cardiac failure or pulmonary failure.