| abstract |
An implantable medical device for electrically stimulating the heart to beat generally includes a processor, a plurality of electrodes, a sense amplifier, a pulse generator, and a heart status monitor. The processor can determine when the patient has entered an environment of high electromagnetic interference. When this occurs, the processor forces the implantable device into a safe noise mode. While in the same noise mode (which preferably continues while the patient is experiencing the electromagnetic interference), the implantable device paces the heart on demand and inhibits pacing during the vulnerable period. The processor determines when the vulnerable period is occurring and when the heart needs to be paced by monitoring a status signal from the heart status monitor. The status signal generated by the heart status monitor preferably is not sensitive to the electromagnetic interference, and thus the processor can determine the bio-mechanical state of the heart during a cardiac cycle even in the face of high electromagnetic interference. The heart status monitor preferably includes an impedance measurement circuit, but may include any type of cardiac sensor that can generate a status signal from which the processor can determine the beginning and ending of the vulnerable period. Accordingly, even during a period of high electromagnetic interference, the implantable device can provide on demand pacing support to the patient. |