6.3 Example of Biocoating That Is Used During ECMO
6.3.4 Cannulas for Adult Patients
The blood fl ow in the ECMO circuit is determined by the size of the cannula (inter- nal diameter and length), the design, the pressure drop, and the positioning. The choice of the cannulas is made according to the mechanical and fl uid characteristics (pressure, fl ow, etc.) (Table 6.2 ). It takes into account surgical needs, cannula place- ment (central or peripheral), the quality of the patient’s blood vessels, and the type of ECMO to be used (e.g., VA or VV). Certain ECMO confi gurations are especially designed to suit the type of cannulation, for example, to suit the dual-lumen single cannulation of the jugular vein or the double femoro-jugular cannulation (Fig. 6.6 ).
The cannulas designed for circulatory assistance are designed for percutaneous access into the arteries or veins of the patient. They usually have a wire enforced body on the longest part of the cannula in order to prevent changes in their hemody- namic characteristics during the mobilization of the patient (Fig. 6.7 ).
Additionally, the temperature of the circulating blood in the ECMO circuit can change the resistance of the cannula. The hemodynamics of ECMO can also be affected by compression of one or more of the cannulas by intrathoracic and/or intra- abdominal pressure (cough, pleural effusion, and/or intra-abdominal hypertension).
Venous
Pressure drop vs. flow
Arterial
Pressure drop vs. flow 200
180 160 140 120 100 80 60 40 20
200 180 160 140 120 100 80 60 40 20
0 1
Pressure drop (mmHg) Pressure drop (mmHg)
2 3
Flow rate (l/min. of water) at room temperature Flow rate (l/min. of water) at room temperature
Outflow cannula Inflow cannula
4 5 6 7
29 fr.
27 fr.
22 fr.
23 fr.
21 fr.
19 fr.
23 fr.
21 fr.
19 fr.
17 fr.
15 fr.
17 fr.
15 fr.
0 1 2 3 4 5 6 7
Table 6.2 Example of pressure drop for arterial and venous cannulas
a b
Fig. 6.6 ( a ) VV ECMO with a double-lumen cannula. ( b ) VV ECMO with two cannulas placed in the jugular and femoral veins
Fig. 6.7 Example of percutaneous cannulas
Logically, the use of infl ow and outfl ow cannulas with maximal diameter would reduce the pressure drop in the circuit. However, like often in ECMO, a balance has to be found between treating the pathology and minimizing trauma and discomfort to the patient caused by the presence of the cannulas in the vascular system.
The correct selection of in- and outfl ow cannulas, as well as their positioning, is very important. They help to optimize the hemodynamics of ECMO in relation to the patient and ensure an adequate fl ow able to satisfy the physiological demands to be met. This results in a reduction in the trauma to the blood cells, thereby reducing hemolysis and gaseous microemboli [ 13 , 14 , 17 ], by preventing an excessive increase in the P 1 or P 3 (Fig. 6.2 .).
6.3.4.1 Venous Cannulas
Venous cannulas (the infl ow cannula) are longer than the outfl ow cannulas (±55 cm) as they need to reach from their insertion point in the common femoral vein to adjacent to the right atrium (RA). They have a larger diameter than outfl ow cannulas (15–29 Fr) and a larger proportion of their length is multiperforated. These features reduce the pressure drop and limit the phenomena of chattering avoiding a signifi cant collapse of the wall of the right atrium (RA) or the inferior vena cava (IVC). The result of these characteristics will allow the observation of a decreased pressure drop at in the pres- sure drop at P 1 (Fig. 6.2 .) by increasing the drainage fl ow while maintaining a constant preload and the RPM of the centrifugal pump. In certain circumstances, the cannula design allows the use of smaller infl ow cannulas without causing deleterious hemody- namic consequences while maintaining the same fl ow (Table 6.2 ). During pulmonary assistance (VV) using a femoro-jugular cannulation, the infl ow cannula is placed in the inferior vena cava with its distal tip at the level of the subhepatic veins, while the outfl ow cannula is placed via the jugular vein into the superior vena cava (SVC) with its distal tip at the level of the right atrium.
To eliminate the discomfort that is caused by the presence of the cannula in the jugular vein, or for certain therapeutic reasons, some centers cannulate the right and left femoral veins. This requires the use of two different cannulas with different design and diameters. The distal tip of the infl ow cannula is positioned through the left femo- ral vein at the level of the subhepatic veins, while the distal tip of the outfl ow cannula is positioned through the right femoral vein at the level of the right atrium. This com- bination can reduce the effect of shunt (recirculation) between the two cannulas.
6.3.4.2 Arterial Cannulas
Arterial cannulas (outfl ow cannula) are smaller than venous cannulas both in terms of diameter (15–23 Fr) and overall length. They have some perforations at their distal tip, though these are not as extensive as in the venous cannulas. According to their charac- teristics, a pressure drop at the outfl ow cannula can be observed when P 3 increases (Fig. 6.2 ) causing a decrease in fl ow despite a constant afterload and RPM of the cen- trifugal pump. During femoro-femoral VA ECMO, the outfl ow cannula is placed into the common femoral artery. The particular position of the cannula can cause ischemia of the ipsilateral lower limb. This can be prevented by the introduction of a 6 fr catheter for a distal perfusion [ 18 ] connected to the Luer connector of the outfl ow cannula and introduced few centimeters downstream the superfi cial femoral artery.
The placement of the cannulas is guided by echocardiography (TEE) and/or fl uoroscopy.
In general, the cannulas have a radiopaque marker, though the distal few centi- meters are often radiotransparent.