The ECMO blood pump controls the blood flow for the patient. Many pumps exist and differ in a number of ways, e.g. design, haemo-compatibility, durability and availability. There are two basic subgroups, namely occlusive and non occlusive.
- Occlusive pumps are positive displacement pumps and include roller pumps. These pumps will displace a volume of fluid depending on the speed of the pump, and are insensitive to afterload.
- Non occlusive pumps / rotary pumps can be centrifugal, diagonal or axial. These pumps are all preload dependent and afterload sensitive. Importantly, these pumps are limited in the amount of pressure they can generate.
Occlusive roller blood pumps are peristaltic blood pumps that produce a hydrodynamic pressure gradient through compression of a circular segment of tubing with a roller (wiper) that rotates and positively displaces the fluid in the tube.
Non occlusive centrifugal pumps are the pumps most commonly used in adults. Centrifugal pumps tend to generate a higher pressure, but with a lower revolutions per minute (rpm) than axial pumps. They produce a hydrodynamic pressure gradient through rotational kinetic energy through the use of an impeller assembly. The impeller is sealed within an operating chamber and magnetically coupled to the drive motor. Centrifugal blood pumps generate flow of blood by a spinning rotor which generates suction in the centre inlet and then propels the blood outward from the pump housing thus creating a positive pressure at the periphery of the pump head. There is no tubing compression and this allows for longer continuous pump operation without the need for replacement. It allows safe clamping of the circuit. The major benefit of current centrifugal blood pumps is the lower rpm required to generate a higher pressure. Newer pumps are also designed so as to minimize local heat generation around the impeller which minimizes thrombosis. They also have a lower rate of haemolysis compared to earlier generation pumps because of a pivot bearing and bearing free magnetic levitation design.
Each ECMO circuit includes a gas exchange device, which is known as an oxygenator or a membrane lung. Many are available for clinical use worldwide and differ in technical characteristics.
The Maquet Quadrox iD is the current gas exchange device in use at the Alfred ICU for the PLS ECMO console. These are made from plasma resistant hydrophobic PMP (poly-methyl pentene) hollow fibres and contain an integrated heat exchanger. They are designed in order to reduce the formation of microbubbles and protect against bacterial contamination. They have a low pressure drop across the fibre bundle and a small priming volume of 252mls.
The Maquet Cardiohelp Advanced is the other gas exchange device in use at the Alfred ICU. These are exclusively used with the Cardiohelp console. These oxygenators differ from the PLS ones as they incorporate a pump, a gas exchanger and a heat exchanger into a single product. There are sensors to detect inlet saturation, haemoglobin, haematocrit and temperature. The system is coated with Bioline thromboresistant surface. The integrated system is easier to set up, prime and transport.
Circuit / tubing
Most of the circuit tubing in ECMO circuit is made from polyvinylchloride (PVC), mixed with a plasticizer. The amount of plasticizer added determines the flexibility of the tubing. Currently, diethylhexyl phthalate (DEHP) is used as a plasticizer to ensure ECMO circuits are flexible. There has been some concern that leaching of DEHP occurs from ECMO circuits, and adverse effects have been reported in laboratory animals exposed to DEHP. The risk is highest in prematurely born male neonates. There are efforts underway to find alternatives.
Tubing length and diameter contribute to blood flow. Achieving the desired flow is determined by the vascular access site, the diameter of drainage tubing, the diameter of the access (drainage) cannula, resistance and certain pump properties. The size of the tubing is chosen so as to minimise resistance to venous drainage. Typically the tubing in our adult ECMO circuits has an external diameter of 3/8th of an inch. Blood flow through one meter of this size tubing, with a pressure gradient of 100 mmHg is typically around 5 L/min.
Cannulae are plastic tubes that are inserted into the vascular system for drainage or re-infusion of blood. They are constructed with wound, flattened metal reinforcement and a thin flexible plastic outer layer that permits curving but prevents kinking. Cannulae are very susceptible to puncture. Contact with sharp instruments (needles and scalpels) must be avoided as even minor cannula damage on ECMO may cause death. Percutaneous cannulae are sold with a custom introducer which includes a guidewire channel. Two brands of percutaneous cannulae are used. Our currently used cannulae are described in the equipment section.
Access (drainage) Cannula drains blood from the venous system into the ECMO circuit in both VV or VA ECMO.
Return (re-infusion) Cannula delivers blood back to the patient from the ECMO circuit. Only single stage cannulae that expel blood from the cannula tip are used for this purpose (see below).
Distal Perfusion Cannula delivers blood antegrade into the superficial femoral artery distal to the ECMO return cannula to maintain perfusion to the leg in a femoral cannulation. In case of complications or by preference of the ECMO centre perfusion to the perfusion cannula can also be inserted into a distal artery (e.g. dorsalis pedis) and blood is supplied to the leg retrograde.
Specification of cannulae
Manufacturers use different terminology to describe their cannulae. However, in principle there are:
- Single-stage cannulae: these have a short perforated region near the tip to return or access blood. Depending on their length they are often referred to as venous (longer length e.g. 50-60 cm – see figure 1) and arterial (short e.g. 15-20 cm – see figure 2) with a side port for the connection of a distal perfusion cannula.
- Multi-stage cannulae: these are able to drain blood through side holes over a long (20-25cm) region of the distal cannula including to the tip. Theoretically, they allow greater flow for lower suction pressure. In practice, most blood enters the cannula via the most proximal side ports. This has major implications for siting the cannula (see VV ECMO Configurations). They are also referred to as venous cannulae.