Heart valves and Magnetic Braking

Artificial heart valves are usually safe for MRI, because any torque or attraction forces exerted by the external magnetic field are minimal compared to the force exerted by the beating heart. However, there is an interesting safety issue surrounding the induction of electric current in the artificial heart valve, which might theoretically compromise the function of some heart valves, in rare cases. The issue is magnetic breaking (aka the Lenz effect, or eddy current damping).

Forces which are described by the Lenz effect may restrict the movement of the moving parts of certain types of artificial heart valve, if those parts contain metal. This is particularly pertinent if the valve is in the mitral position, where the range of pressures experienced are lower (higher pressures are more likely to overcome any significant Lenz effect). It has been suggested that exclusion of borderline cardiac failure patients should be considered, in which the movement of the mitral valve is even more delicate because of especially low pressures. Certainly close clinical observation for signs of distress should be made. (Breathlessness and faintness would be caused first if the cardiac output was diminished because of an improperly functioning valve, a reaction which may incorrectly be attributed to anxiety.)

Where do these forces come from? When the magnetic flux (think of the number of magnetic field lines) through a circuit changes, an emf equal in magnitude to the rate of change of flux is induced in the circuit. (This is Faraday’s law.) This causes current to flow, and the induced emf and the induced current are oriented so as to oppose the change which produced them. (This is Lenz’s law. You can also use Fleming’s Right Hand (dynamo) Rule.) The direction of flow of the current that produces a magnetic field which reinforces* or opposes the external magnetic field can, therefore, be determined by Maxwell’s Right Hand Grip Rule. The opposition to motion comes from the effect of the external magnetic field on this new current (use Fleming’s Left Hand (motor) Rule). This force effects a torque on the circuit and therefore affects its movement.

This can be demonstrated (amusingly) with a slab of non-ferromagnetic conductor (e.g. aluminium or copper). Put the slab into the bore of the magnet on its end, and tip it over. It will fall over, but very slowly.

*Reinforces? Let’s consider the change which occurs when a conducting ring goes from a position normal to the direction of the external magnetic field (i.e. transverse plane) to parallel with the magnetic field (i.e. coronal plane), like the falling slab mentioned above. Note that the magnetic flux through the slab reduces as the slab falls over towards the coronal plane. This is the change which causes the current; the current is in such a direction that it generates a magnetic field which increases the flux, in this example. Whether the magnetic field reinforces or opposes B0 depends on the change in flux, which can be positive or negative and depends on the direction of movement.