|My treasured heart rate monitor tells me when I've gone past my limits|
I have no medical background (even though my family calls me Dr. Sue, Medicine Woman now!), so I will try to keep this simple, as I understand it, without too much medical jargon.
Why Do We Crash After Exertion?
We all know that one of the biggest and worst parts of ME/CFS is the exercise intolerance, also known as post-exertional malaise. When we do too much, we feel terrible the next day (and often the day after that, and so on). And "doing too much" for someone with ME/CFS could mean taking a shower or fixing lunch or taking a 10-minute walk. If you want to know more about post-exertional malaise, I recommend Jennie Spotila's excellent series of articles on PEM.
The reason WHY we feel so bad after exertion is because our cells don't handle oxygen the way they're supposed to, a condition known as oxidative stress. When normal, healthy people exercise, their heart rates gradually increase (which is good for them!), in a process called aerobic exercise. During this process, the cells are using oxygen in the energy-creating metabolic process within the mitochondria (kind of like tiny energy engines in our cells). If a healthy person exercises for a long time and/or at high intensity, their cells switch to anaerobic metabolism.
People with ME/CFS have an extremely low anaerobic threshold because our cells don't handle oxygen appropriately, so that our bodies switch to anaerobic metabolism with very little exertion. So, the effect of a short walk in someone with ME/CFS can be similar to the effects of a healthy person running a marathon.
You can estimate your anaerobic threshold (AT) using this formula:
(220 - your age) * 0.6 = anaerobic threshold or AT, in beats per minute
(that is, 60% of your maximum heart rate)
See my recent article for more detailed formulas, with more options for women and for those more severely ill.I'm 45, so my anaerobic threshold is 105 beats per minute (note, that was in 2011 when I first wrote this post. As I am updating it here in 2019 at age 54, my AT is now estimated at 100 bpm, though in reality, it is higher than that - see below).
Note that this formula only provides an estimate of AT. The most accurate way to find out what yours is is to take part in a 2-day exercise test specifically designed for ME/CFS, but most of us don't have that opportunity. You can use the formula as an estimate and then test it out for yourself.
How Does OI Affect Post-Exertional Crashes?
Another significant factor in post-exertional crashes is Orthostatic Intolerance (OI), a condition that affects more than 97% of patients with ME/CFS (and many of those with fibromyalgia and Lyme, too). OI is basically an inability to maintain a steady heart rate (HR) and/or blood pressure (BP) while upright, and it is behind many of the symptoms we experience as ME/CFS, even if you don't realize it (check out this past post on OI for more information).
Typically in ME/CFS (though not always), OI causes our heart rate to increase and our blood pressure to decrease, often substantially, when we are upright, either sitting or standing (though there are some rare cases in ME/CFS where the BP increases when upright or in some cases BP and HR jump around - those are still OI). So, while a healthy 45-year old would probably not hit 105 beats per minute until she'd been exercising for a while, I can hit 105 just standing up. The result is that even minor exertion can put someone with ME/CFS over their anaerobic threshold.
Tracking Heart Rate
So, monitoring or keeping track of your heart rate can give you a solid, quantifiable way to know when you've passed your limits and are in danger of experiencing post-exertional malaise (i.e. a crash). You can check your pulse rate simply by touching your wrist or neck and counting the beats, but I highly recommend using a heart rate monitor.
I asked for and got a heart rate monitor for Christmas (thanks, honey!), and wearing it during my normal daily life has been enlightening. I planned to wear it when I took a walk or went shopping or something else exerting, so I could tell when I had done too much. On my first day, I decided to take a walk in our neighborhood, so I strapped it on, after calculating my anaerobic threshold (105). I was shocked to see that my heart rate just standing in the kitchen was already over 100! I bent down to tie my shoes, and my heart rate jumped up to 115!! And I hadn't even left the house yet.
I did take a 15-minute walk that day, as slowly and carefully as I could, but my average heart rate was 103 and the maximum (after I had my shoes on) was 110.
Over the course of the next week, I wore the heart rate monitor almost all the time during the day and watched it constantly (some might say obsessively). I found that it provided hard data to back up what I already knew about OI (I love data!).
Even lying down (what's known as resting heart rate), my heart rate rarely went below 90. Sitting on the couch with my feet up, it was often in the 90's or near 100. Standing up to make a cup of tea could drive it over 100. Taking a shower or putting in a load of laundry could spike it up to 130 or higher! One day in the library, I noticed it was close to 100 just walking around slowly, but when I crouched down to see something on a low shelf, it dropped to 75-80. I knew from Dr. Rowe's article on OI that crouching helped, but it was illuminating to see that for myself. I also noticed that it varied from day to day, depending on how I felt. Research bears this out - your AT is even lower and your HR even higher when you feel worse.
On a grocery shopping trip, I discovered - much to my surprise - that the part of shopping that drove my heart rate highest was carrying the groceries into the house. This was much more exerting to me than pushing the cart or walking around the store.
It was fascinating for me to see that my quietest day could easily put me over my anaerobic threshold fairly often. No wonder I rarely felt good - I was in a constant cycle of post-exertional crashes without even "doing" anything! I really had no idea that was happening.
Try it for yourself - monitor your heart rate for a few days. Find out where your own anaerobic threshold is and how much exertion it takes to put you over it. Experiment! Try various normal daily activities and see which ones are within your limits and which push you beyond them. Figure out how some simple changes can keep you from going past your threshold - maybe dividing tasks up into parts or resting in the middle or delegating certain things.
Choosing a Heart Rate Monitor (updated)
Although there are now many different kinds of wearable devices that can measure your heart rate, there are a couple of things to look for that you'll need specifically for ME/CFS. You want a heart rate monitor that continuously monitors your heart rate, giving a constant read-out. The kinds of devices that require you to push a button to check your heart rate aren't adequate for us. A large read-out that you can easily see is best. You also want to make sure that you can set audible alarms for heart rate that are custom to you. Some devices may just calculate your limits based on your age, but with ME/CFS, we are using it differently than that. You will set your upper limit to whatever your anaerobic threshold (AT) is, according to the formula above, and you want an audible alarm to inform you immediately if you hit that limit. All models can link to a smart phone or computer, so you can see your data.
I have been very happy with my Alpha Mio heart rate monitor. There are, of course, now plenty of fitness trackers available, like Fitbit, that measure heart rate, though I find their HR read-outs quite small and hard to see while you are moving around, and they are designed for athletes and active people, with different settings for different sports and activities. They do, however, have additional features like sleep tracking, so you can compare models and brands and decide for yourself.
Treating OI to Improve Stamina
For me, the monitoring only did so much - I was still over my limit during most daily tasks and could not manage even a short walk without crashing afterward. Then, I tried a new treatment that turned out to be life-changing for me! I now take beta blockers for my OI. Beta blockers block certain hormones in the body to bring heart rate down to more normal levels. As with everything else in ME/CFS, a lower dose is best for us, and it only takes a little bit to be effective (and too much beta blocker actually increases fatigue). Once I found the right beta blocker (there are several dozen to try) and the right dose for me (see tips in that link), my life was transformed. I could go grocery shopping, take walks, and get through normal daily activities, all without going over my limits and crashing. I still use my heart rate monitor, but now I only need it when I am doing something especially exerting, like walking or hiking. I am able to stay below my AT in my normal daily life and even cook again!
I have also found, as noted above, that my AT has actually increased, even though I am getting older, because I am now able to be more active and improve my fitness (all while staying below my limits). This has been born out in research, as explained in my recent ProHealth article: staying below your limits and preventing PEM (crashes) allows you to begin to improve your fitness level, which will gradually increase your AT...allowing you to do even more! It's a positive domino effect that I have seen work in real life.
Once you've determined where your limits are, you can try to stay within them as much as possible. This should result in feeling better, and then you can try to very slowly and gradually do a bit more. This is NOT the dreaded Graded Exercise Therapy (GET), which can worsen your condition, but using science and quantifiable measurements to stay below your limits and prevent crashing. Treating Orthostatic Intolerance can help you to become even more active and further reduce post-exertional crashes. Good luck! Let me know about your own experiences in monitoring heart rate. And if you want to dig more in-depth into this topic or chat with others with ME/CFS using heart rate monitors, join the Facebook Group ME/CFS - Pacing with a Heart Monitor.
UPDATE AS OF 8/21/19: Seven years after starting beta blockers - and with the help of my heart rate monitor - I have been able to greatly increase my physical stamina. I can now manage up to two hours of walking/hiking at a time (wearing my heart rate monitor and staying below my AT, of course!), without crashing afterward. I love the outdoors, and I have been gradually increasing the lengths of hikes I can manage - I did 1.5 miles on hilly terrain last summer! I have also started slowly doing some weight training - mostly on my back to keep my heart rate low and in the morning when my beta blockers are working best. I can now work out with weights for about 20-30 minutes most mornings and have increased my muscle tone substantially (which further improves OI). I rarely crash from over-exertion anymore, which has been life-changing for me. I am planning to walk a 5k this fall, which will be a big milestone for me! None of this would have been possible without first treating OI and using my heart rate monitor to stay within my limits.
This is the latest version of the heart rate monitor I now use - I got mine 5 years ago, and it still works great (and no chest strap like the old ones!). I've found it to be very reliable and has a long battery life (and rechargeable battery):