Beta-Blocker, HIIT, and a Heart Rate That Lies
Marcus, 54, has trained with you three times a week for eight weeks. He takes metoprolol succinate 50 mg daily for hypertension and has written clearance from his physician for "moderate-to-vigorous exercise." At intake his resting heart rate was 58 bpm and blood pressure 128/78. He now wants to add 20 minutes of HIIT twice a week to prepare for a 10 km race in 10 weeks. Using 220 minus age, he calculated a maximum of 166 bpm and a "hard" interval target of 141 bpm, but he reaches 141 during a brisk walk while still able to speak in full sentences. He also mentions feeling briefly dizzy and "greyed out" twice when he stopped intervals abruptly and stood still. He has no chest pain, no shortness of breath at rest, and no known arrhythmia. He is motivated but fixated on the heart-rate numbers on his watch.
1. Marcus wants to hold 141 bpm during every work interval. Explain why that number is unreliable for him and what you will use instead.
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Why the number fails. The 220-minus-age estimate already carries roughly a 10-12 bpm standard deviation in healthy, unmedicated adults. Metoprolol blunts the chronotropic response, so his true maximum is likely well below 166. That means 141 could sit at or above his actual ceiling rather than in a hard-but-sustainable zone. The formula does not over-protect him here; it can push him past maximal effort while he believes he is being conservative.
The confirming clue. He talks in full sentences at 141. Ventilation and perceived effort are not blunted by the drug, so the talk test and RPE remain valid even when heart rate is not.
What I use instead. Session RPE on the 6-20 Borg scale, targeting 13-15 during work intervals; the talk test (short phrases, not full conversation); and, if his physician agrees, a supervised submaximal test so I can build a heart-rate reserve using data from him rather than from a formula.
Documentation. I record the medication, the discrepancy, and the method chosen, and I tell him to stop if he feels light-headed rather than chasing a watch number.
2. What is your working explanation for the dizziness, and what do you change immediately?
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Most likely cause. Beta-blockers reduce heart rate and can blunt the vasoconstrictive compensation to standing, producing orthostatic symptoms. His description fits: symptoms came on when he stopped abruptly and stood still, which pools blood in the legs right after vasodilation from hard effort.
Immediate changes.
- Replace abrupt stops with a 3-5 minute active cooldown (walking), never standing still.
- Coach breathing so he does not Valsalva during intervals.
- Emphasize hydration and rising slowly from the floor or bike.
- Schedule intervals mid-session, not as the final minutes of a session he then walks straight to his car.
What triggers referral. Any dizziness at rest, near-syncope, visual change that does not resolve within a minute of walking, or a repeat episode despite these changes goes back to his physician. I do not adjust or speculate about his medication, and I do not frame this as "just dehydration" without a plan and a referral threshold.
3. Outline the first three weeks of interval progression and justify the rate of change.
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Week 1. 4 x 1 minute at RPE 13-14 with 2 minutes of easy recovery, inside a 20-minute total block. Work:rest of 1:2 keeps the cardiovascular load modest while he learns pacing without heart-rate feedback.
Week 2. 5-6 x 1 minute at RPE 14 with 90 seconds recovery (roughly 1:1.5). Same total work duration, slightly denser.
Week 3. 6-8 x 1 minute at RPE 14-15 with 60-90 seconds recovery, but cap total work at 8-10 minutes. Keep one continuous steady session each week so his 10 km base still develops.
Justification. He is eight weeks into training, on a rate-limiting medication, and has had two orthostatic episodes. Total interval work is the variable I progress; session count, terrain, and watch-based targets stay fixed. I hold increases to roughly one additional work interval per week and check his 24-hour response (sleep, soreness, resting heart rate, any dizziness) before each progression. If the dizziness recurs, I hold the current week rather than advancing.