PHYS-12 · Principle

My heart rate hit 183 at 44, above 220 minus age. Is that a problem?

Not on its own. Age formulas give a group average and missed endurance athletes by about 10 bpm either way in a large study, so a measured maximum above the formula is ordinary variation.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to everyoneEstablished — the maximum falls with age; well supported — formulas miss a person by about 10 bpm (one large study of endurance athletes, REF-3494); heat and fluid loss raise heart rate at the same work; well…

Practical Application

My heart rate hit 183 at 44, above 220 minus age. Is that a problem? Not on its own. Age formulas give a group average and missed endurance athletes by about 10 bpm either way in a large study, so a measured maximum above the formula is ordinary variation.
What it means for training: zones from a measured threshold (SYNTH-38); check heat, fluid, sleep, illness and caffeine before reading fitness into a high heart rate (SYNTH-41); race heart rate can run above interval values (arousal is the usual reading; no source in the base).
What it does not mean: that a high maximum marks a better heart: it did not differ with habitual activity between people (REF-1082).

Mechanism

Heart rate rises when stroke volume falls or drive rises: with heat and fluid loss (REF-228, REF-1341, PHYS-11), and after weeks off as blood volume falls (REF-1133). Day to day it varies "small" (a review, unsized, REF-1341) or 6-12 beats (a guideline: altitude, climate, hydration, glycogen, stress; REF-1077, C).
The maximum is about 0.7 bpm lower per year of age across populations (cross-sectional, REF-1082); the best formulas missed endurance athletes by 9-10 bpm RMSE (REF-3494); it differs between running and cycling (REF-537). Within a person it can move 3-7% (one cautious review), down with training, up with taper or detraining (REF-1342, C+; REF-1133); lower after overreaching (REF-1341).
A wrist sensor adds its own error on the bike (SYNTH-91).

Common misreading

"220 − age is my maximum"; "higher today, so I am unfit".
Safety (base practice): chest pain, palpitations or dizziness go to a doctor.

Backs

SYNTH-38 "Any day the athlete is not themselves" (practice, no entry) · SYNTH-41 "Check the ordinary causes first"

Sources

Tanaka, Monahan & Seals 2001 REF-1082 · Paper ↗
Achten & Jeukendrup 2003 REF-1341 (B+) · Paper ↗
REF-1342 (C+)
Gap, not yet in the base: reviews of sleep, caffeine, illness and race arousal