II. Anatomy
A. Priming the circuit
Prior to cannulation (placing a patient on ECLS), the circuit must be
primed with an isotonic solution with 4-5 mEq/L potassium purging all of the gas
within the internal circuit and membrane lung. The circuit is also warmed to 37
degrees across the heat exchanger. For most adults, this “clear primed circuit” is
adequate; however for most children, especially neonates, a “blood primed
circuit” is preferable, bringing the hematocrit to 35-40%.[ 4 ] The volume of the
neonatal circuit is approximately 400-500 mL which is 1-2 times the newborn
blood volume. [ 3 ]
The circuit, therefore, must be primed carefully in order to perfuse the
neonate at onset of bypass with blood containing appropriate pH, hematocrit,
calcium, clotting factors, electrolytes, and temperature; however, ECLS may be
instituted in those patients over 35 kg in weight without addition of blood to the
prime. To prevent blood clots, heparin (1 unit/1mL prime) is added to the circuit
prior to cannulation and is closely monitored during the course of ECLS. Calcium
is added to replace that which is bound by the citrate in the bank blood.[ 4 ]
Finally, the gas of the primed circuit should resemble the patient’s physiologic
status (pH, carbon dioxide level) to avoid abrupt changes.
B. Cannulation techniques
Cannulation can be performed using cutdown or percutaneous
techniques. In neonates, a transverse neck incision is commonly used to access