Expert: John Locke - 1/8/2011

Question
Hi,
I have been studying the heart and have come to certain parts which I just cannot get my head around. I understand when you have a valsalva's manoeuvre and cold pressure test your heart rate should increase leading to an increase in blood pressure, as they are both putting pressure in the heart in different ways.
What I don't understand is that how is it possible for a persons heart rate to go unchanged whilst having these tests done, and would exercising give the same response then?

Many thanks
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Answer
Thanks for using AllExperts. First, a correction to your question: in the normal physiologic response to a Valsalva maneuver, the blood pressure falls and heart rate rises in compensation to maintain a constant cardiac output. This rise in heart rate is mediated by epinephrine, a hormone that is released by the adrenal medulla and (particularly important for the heart) sympathetic neurons arising from the spinal nerves T2-T5, which release epinephrine also directly onto the heart muscle and pacemaker centers. The release of epinephrine--which occurs during times of excitement, stress, fear, and agitation, in addition to the Valsalva maneuver--has the effect of increasing the rate and strength of heart muscle contraction. This effect can be blocked if the effect of the epinephrine can be blocked. This can be done with drugs known as beta blockers: these drugs block the effect of epinephrine and related compounds on heart muscle and other tissues in they body. Any response that would normally cause a rise in heart rate does not in the presence of beta blockers--this includes the Valsalva maneuver and exercise. That is the major situation I am aware of in which the Valsalva maneuver would not yield an increase in heart rate. The other situation in which the heart would react abnormally is a transplanted heart. A transplanted heart has been removed from the nerves that control heart rate; it would be responsive to physical exercise (it can still respond to epinephrine released from the adrenal glands) but it would be less responsive to the Valsalva maneuver. The Valsalva maneuver requires what is essentially a reflex nerve loop involving blood pressure control and heart rate, and those nerve connections no longer exist after a heart transplant.