I am looking for some advice on pacing my long run as it relates to cardiac drift. I am training for my first 50k trail run and, this morning, did a comfortably paced 2.5 hour run that consisted of 5 loops. All 5 loops were within 1 minute of each other but my HR started in the 130s for number one and, by number five, was climbing into the high 160s. I believe I was adequately refueling and hydrating. I know the concept of cardiac drift is well described and I have seen it in myself for years. For whatever reason this has never been an issue racing IMs and I’m guessing that my HR gets a reset after the bike from using different muscles. I am concerned now because the 50k will probably take me close to 5 hours and I worry that it will drift right up to my max HR before I finish. Thoughts on pacing? Take it out slower? Let my pace slow as I run so that I stay in the 150s? Not worry about it and leave the HR monitor at home? All ideas appreciated.
Why do you think cardiac drift is an issue?
I would argue that muscular fatigue is your problem, not cardiovascular drift. You basically explained it to yourself but stating that it does not happen to you in IM because you use slightly different muscle firing patterns for biking as opposed to running. Not saying cardiovascular drift isn’t occuring to some extent, but I would guess just regular old fatigue is forcing your body to work harder the second half of your workout.
I would argue that muscular fatigue is your problem
How is that possible since the OP described he could maintain pace for the duration of his run?
I would recommend to use HR for a period of time and then due to CD ignore HR and go by pace. Even though your HR may be at or over your AT, in reality you’re not even close in reality.
During ultras what I do is use a HR cap. For the Canadian Death Race in August I will likely have my Suunto alert me if my HR hits 145. When that happens I walk. After a while when I start to notice the HR creeping when my effort level is low, I’ll turn the alerts off 
Don’t most people have heart rates slowing as the event goes longer? I doubt you’d reach your max heart rate in a 50 k run.
Wouldn’t you run by perceived exertion/pace for trail runs rather than heart rate (your Hr will be much lower on the dowhill vs uphill portions).
In hot conditions, or rather conditions that are hot for the person, heart rates will trend up. Depending on how hot and how far you are going they can trend up until a breaking point and then they start walking or getting off the bike or otherwise stopping. But that isn’t always the case, things can balance out. Heart rate trends up for a while and then settles.
On the other hand if you are low on fuel heart rate will tend to go down over the course of of an event. But that is almost always accompanied by slowing power or pace.
It’s dehydration. As you get hotter and less hydrated, your blood gets thicker. Your heart has to pump harder to get the thicker blood to where it needs to go. Add a little salt to your water and drink more and you won’t have this problem anymore.
This is a good reason for wearing HR monitors during long workouts. They don’t measure work done, but measure your body’s reaction to conditions. If you’re going at the same effort level as before and your HR starts creeping up, you’re either out of shape or getting dehydrated. If you can do the same workout sometimes without your HR going up, then you’re not out of shape - it’s hydration.
It’s dehydration. As you get hotter and less hydrated, your blood gets thicker. Your heart has to pump harder to get the thicker blood to where it needs to go. Add a little salt to your water and drink more and you won’t have this problem anymore.
This is a good reason for wearing HR monitors during long workouts. They don’t measure work done, but measure your body’s reaction to conditions. If you’re going at the same effort level as before and your HR starts creeping up, you’re either out of shape or getting dehydrated. If you can do the same workout sometimes without your HR going up, then you’re not out of shape - it’s hydration.
I’m by no means an expert, but what he said is what was explained to me. I had little cardiac drift beyond 1 hour of training. Then Florida summers hit and I sweat a lot. So within 45 minutes, my heart rate could jump 15 - 20 bpm on a 95 degree day. I then notice if I run in the morning or night, I don’t sweat as much and my heart rate for the same distances, pace and the same courses stays more consistent. Being 205 (lean) and a 90 degree plus day makes it very difficult to keep adequate hydration without upsetting the stomach.
Sport specific fitness plays a part as well as hydration. I was recently able to hit a new level of running fitness (2 min 10k Oly PR from same course/conditions), from a large amount of swimming and biking this winter, during which I was unable to run for a couple of months. From that lack of running I was seeing cardiac drift very early on for several weeks despite being able to race and train at a faster pace. After a few weeks, it gradually would happen later and later in the run as my run mileage went back up. My last long run before the injury I saw no cardiac drift (2 hours, mild temperature, mid 40 mpw).
Why do you think cardiac drift is an issue?
I’m interpreting your question to be “why do I care if my heart rate is drifting up”. I am mostly concerned about my inability to burn sugar/glycogen for upwards of 5 hours. I may be wrong but assume if I can keep my HR in the 140s I am utilizing a greater percentage of fat. Why I don’t know is if it matters if my HR drifts so long as I maintain the pace that initially put me in the 140s.
As for the mechanism, it may be dehydration but I was consuming 18oz per hour in foggy, 58deg F at 0630 in my local coastal hills with an average sweat rate. I know it is not blood viscosity but could be talked into decreased stroke volume.
As for fitness, I think this is a relative question. I am able to maintain 8:45 pace for 17 miles with 2000 ft climbing on a technical single track so I am not detrained but, yes, I am early in my build. I expect that as I gain fitness, the drift will persist but I will be executing the entire run at a faster pace.
Why do you think cardiac drift is an issue?
I’m interpreting your question to be “why do I care if my heart rate is drifting up”. I am mostly concerned about my inability to burn sugar/glycogen for upwards of 5 hours. I may be wrong but assume if I can keep my HR in the 140s I am utilizing a greater percentage of fat. Why I don’t know is if it matters if my HR drifts so long as I maintain the pace that initially put me in the 140s.
The percentage of fat you burn depends on many factors, the biggest ones being diet and exercise intensity. Heart rate is not a good measure of exercise intensity (as you found out). So an increasing HR is not really an issue.
As for the mechanism, it may be dehydration but I was consuming 18oz per hour in foggy, 58deg F at 0630 in my local coastal hills with an average sweat rate. I know it is not blood viscosity but could be talked into decreased stroke volume.
That is not enough fluid. You should be taking in twice as much as that. Probably the biggest factor in that cardiac drift.
I expect that as I gain fitness, the drift will persist but I will be executing the entire run at a faster pace.
Yes.