Domande teoriche + Risposte, Biomacchine

Biomachines - possible theoretical questions

Outlet pressure during extracorporeal circulation procedures

Describe what determines the outlet pressure of either a volumetric or a centrifugal pump during extracorporeal circulation procedures. A volumetric pump, such as a peristaltic pump, is not afterload sensitive. This means that it’s characterized by a flow rate that remains almost constant in time (except for some oscillations) if the velocity doesn’t change. For what concerns the outlet pressure, it becomes higher for resistances that increase. So, since the flow rate never changes, if the resistances increase too much, we will have a disconnection of the pump. Roller pumps, when a meatus is allowed, are not volumetric anymore so by increasing the resistances the flow rate becomes smaller.

A centrifugal pump is instead afterload sensitive, which means that the flow rate does depend on the afterload of the circuit. For what concerns the head of the pump, it has its maximum when the flow rate is zero and then becomes smaller as we increase the flow rate. Moreover, for bigger resistances, H increases.

Positioning of biological and mechanical valves

One biological valve and one mechanical valve have to substitute mitral and aortic valves. Same diameters, discuss if any element exists to prefer a specific positioning of each prosthesis. In prosthetic heart valves, we have to be careful to back pressure. Pressure drop is almost the same for all valves if there is no pathology like stenosis.

In the left part of the heart, the back pressure is huge for the mitral valve; in fact, chordae tendineae help the valve not to have prolapse of the leaflets. In the mitral position, the mechanical valve is suggested since it can overcome big pressure differences and has big reliability. Instead, in the aortic position, the biological one would be a better choice since the fluid dynamics is almost perfect, with very low chances of blood clots since in this position they could migrate in all of the body or even obstruct the coronary circuit that starts just downstream the aortic valve.

Dimensions of venous and arterial cannulas in ECC

Explain how the dimensions of the venous and the arterial cannulas are chosen for a patient undergoing ECC. The dimensions of the cannulas used in ECC are usually different for the arterial and the venous side. For what concerns the venous cannulas, they usually have a diameter of 33 Fr (2/3 of the diameter of the vena cava), while arterial cannulas normally have a diameter that ranges from 28 to 30 Fr (1/3 of the aorta diameter).

The difference in diameter is due to the different functions carried out by the two cannulas. On the venous side, diameters are usually higher since a good venous return is needed and, given that the distance between the right atrium and reservoir is usually fixed, if the diameter was reduced there would be a bigger resistance to the flow.