Hi there -
Given the sudden increase in intensity of your workout from 3.8 to 6.0, you may have simply surprised your heart and changed your threshold for heart rate. While we generally observe the theoretical maximum rate as 220 minus one's age (which would give you a "maximum" heart rate of 178), and try to reach a person's maximum rate during routine stress testing (barring any untoward symptoms which would cause us to slow or stop the test), you actually "jumped" into a peak stress test situation on that first big step up in speed. It's been my observation that many people react to this by a sort of reflex tachycardia that doesn't slow down as quickly as one would expect ideally, but without significant changes in EKG waveform nor any untoward symptoms.
The fact that your rate hits 145 - 150 while walking on the treadmill is a little bit odd, and for this reason, primarily, you might want to have a physical exam to determine if you might have something else going on, such as a viral infection, an unrecognized anemia, or some other indirect cause of this change. The other numbers aren't too surprising, but the last number you mentioned is rather unusual and may represent some undue fatigueability.
After 14 years you're also entitled to a follow-up echocardiogram, although trivial mitral regurge is extremely common and usually "normal" among the general, healthy adult population. It could ease your mind to know there's nothing going on with the mitral valve, or, if by the extremely off-chance there is, that would also be useful information. However, I'd almost be willing to bet that hasn't changed enough to account for your numbers (and I say "numbers" because other than the sign of increased peak heart rate and slower recovery time, you appear to have zero symptoms). It's just prudent to have an occasional complete physical and it wouldn't be wrong to have that echo as well, again, if only to ease your mind.
Sounds as though you're in good shape. Only one way to be certain, of course. Make an appointment and get a complete checkup.
I hope this is helpful to you. Best of luck, and please do follow up with us as needed.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist