One of the basic bits of advice for people with ME/CFS is, “Don’t overdo it.” But, given the delayed reaction of PEM (post-exertional malaise), how can we know whether we’re exerting ourselves too much? Some medical professionals urge patients to monitor their pulse, or heart rate (HR), to stay within their energy envelope. Like many others, I follow this advice. But I believe that the focus on HR, and especially on a specific rate of beats per minute (BPM), is simplistic and actually can lead to ‘overdoing it.’
The first reason to not depend solely on heart rate is that psychological stress can cause PEM, too. This stress can be due to positive or negative situations, and the heart rate doesn’t have to rise as high as it does during overexertion to cause PEM. For example, while teaching a college class for 80 minutes twice a week (my only real activity), I feel low-level excitement, and my HR stays well below the rate that I can sustain for that length. Yet this excitement causes PEM. So exertion isn’t everything. (It would be great if researchers addressed this.)
The rest of this post discusses my approach to managing my exertion using heart rate. I list the equipment I use in another post.
Just a clue
First: a fast pulse is probably not the problem. Instead, it’s an indirect and imperfect clue to whatever actually causes PEM. Maybe the process starts with the buildup of lactic acid, or maybe something else; in any case, monitoring HR is simply an easy way to get a sketchy idea of what ‘overdoing it’ is.
Energy envelope: constantly changing
So I monitor more than my pulse. When feeling relatively well, I can walk for quite a while without my heart rate rising much, but I’ll get PEM if I go far at all. Thus, I need to watch the time, or my step count. Similarly, I can stand a short burst of higher-HR activity, such as climbing a flight of stairs, without provoking PEM. So my threshold for exertion is a sliding balance between time and intensity, as measured via HR.
Further, my capacity changes throughout the day. Some people plan their level of exertion for the day based on their resting heart-rate when they awaken. I’ve had days that started poorly or well and then changed for no discernible reason. It’s difficult emotionally to cancel an activity in the afternoon that I planned only that morning, especially when I’ve ‘saved up’ my energy for it, but it’s nice when I unexpectedly feel up to something. So my approach is to understand that all plans are provisional and to strike when the iron is hot, regardless of my resting heart-rate when I awaken.
When I’m in the midst of activity, watching my heart-rate can tell me when things start to go wrong. I’m not completely sure what it means, but, whenever I go for a long enough walk, I develop “post-exertional tachycardia”; that is, my HR goes up markedly (10-20 beats per minute) after I stop moving. The harder or longer the exertion, the stronger the effect; it’s also stronger during PEM. When I see this, I tell myself to back off the activity.
One Good Day rule
Finally, I have a “One Good Day” rule. Ideally, I have to experience 24 hours without PEM before I’ll get off my butt and do something taxing, such as shopping. My heart rate is higher during PEM—especially while doing things–even when other symptoms are less evident. So when my HR is high, regardless of other signs, I know that I should take it easy.