What causes a 22week fetus HR to drop from 150bpm to under 40bpm?

My obstetrician sent me to a perinatologist because at 22 1/2 weeks my baby's heart rate is dropping from about 150 to less then 40 bpm about every 3-5 minutes, it lasts about 20 seconds and then starts to climb back to normal.  The specialist determined that the episodes are both induced by pressure and spontanious. The babies heart structure is looks good and is growing normally and my placenta is in front of the baby. There were no signs of umbilical cord compression. The episodes happen when the baby makes a big movement or the Doctor pushes on my belly. The Doctor's first thought is that the baby has vagus nerve over-sensitivity but says she has not had a fetus have this frequent of episodes nor any spontanious episodes.  She has tested me for Sjogrens, and it came back negative. Both the specialist and my obstetrician say that at this point in gestation there is nothing they can do. I can't find any literature or personal stories that resemble mine. Can you provide any additional thoughts to this??  Could the problem be with the placenta?

[Jeffrey R Boris, M.D.]

Dear Triantiums

My first question here is whether your baby’s heart rate is a normal sinus rhythm.  This means does the normal heartbeat initiate in the sinus node, the natural pacemaker of the heart, travel through the atria (the upper chambers of the heart), go through the A-V node (the single wire that connects the upper and lower chambers of the heart electrically), and then go to the ventricles (the lower chambers)?  Or, does it mean that the A-V node is shut down and the electrical signal is not reaching the lower chambers at all?  This would mean that the atria and the ventricles are beating at two different rates, with the ventricles going at <40 beats/minute and the atria going along at a regular rate in the 120-180’s; this is called complete heart block.  One possible reason for this can include Sjogren’s syndrome or systemic lupus erythematosus, which would create antibodies that damage and destroy the A-V node; however, it sounds like you had the test to rule out those antibodies.  Can the vagus nerve slow the heart beat?  It can certainly slow both the sinus and A-V nodes, although there can also be some intrinsic damage to the A-V node that allows it to shut off all the way with increased vagal nerve tone, causing intermittent complete heart block.  Certain anatomic abnormalities can lead to a structurally abnormal A-V node, although sometimes the A-V node just doesn’t form completely correctly, despite a structurally normal heart.  I do not believe that this is specifically a placenta problem, as those tend to me more chronic and less spontaneous.

In this case, I would recommend that your baby undergo fetal echocardiography, which is a specialized fetal ultrasound of the heart, to better assess the rhythm and the anatomy of the heart.  That would most likely help out with the diagnosis of what is going on and help to narrow things down.