Question explored with the scientific record
what does a normal pulse of 43 indicate
A pulse of 43 is low enough to need an explanation, but the cause determines whether it is a sign of fitness or a medical problem.
A resting heart rate of 43 beats per minute (bpm) is called bradycardia. In a healthy, well-trained endurance athlete, this can be normal. Studies of elite runners show resting heart rates in the 30s and 40s, driven by a larger, more efficient heart and increased vagal (parasympathetic) tone [6, 7]. One study of veteran endurance athletes found a mean 24-hour heart rate of 59 bpm and a minimum of 37 bpm, with some showing pauses or even Mobitz I (Wenckebach) heart block during sleep [8]. In these cases, the low pulse is a sign of adaptation, not disease.
Outside of athletic training, a sustained pulse of 43 is more concerning. It can indicate sick sinus syndrome, where the heart's natural pacemaker fails, or heart block, where the electrical signal is delayed or blocked between the upper and lower chambers. One study found that former professional endurance athletes who developed sinus node or AV node dysfunction needed pacemakers at a younger age (average 65 years) than non-athletes [1]. Other causes include medication side effects (beta-blockers, calcium channel blockers, some antiarrhythmics), electrolyte disturbances like severe hypokalemia (potassium of 1.56 mmol/L in one case caused bradycardia and AV block) [2], Lyme carditis (a 17-year-old presented with a heart rate of 30 bpm) [5], and even a rare reflex from a throat mass [3].
The key question is whether the person has symptoms. A pulse of 43 in an athlete who feels fine, exercises without trouble, and has no dizziness or fainting is likely benign. The same number in a non-athlete, or anyone with fatigue, lightheadedness, near-syncope, or shortness of breath, warrants a full workup: an ECG, a Holter monitor to catch the rhythm over 24 hours, blood tests for electrolytes and thyroid function, and possibly an echocardiogram. The evidence here does not provide a single rule for when a pacemaker is needed, but a resting rate of 50 bpm or less in children with congenital heart block was significantly linked to syncope [4].
My call: a pulse of 43 is a signal that must be interpreted in context. In a fit athlete without symptoms, it is likely normal. In anyone else, or with any symptom, it requires a medical evaluation to rule out sick sinus syndrome, heart block, medication effects, or an underlying infection. Confidence: high.
Sources used 8
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Intrinsic Sinus Node/Atrioventricular Node Dysfunction Requiring Pacemaker Implantation: Role of Former Professional Sport Activity
Former professional endurance athletes with intrinsic sinus node/AV node dysfunction required pacemaker implantation at younger ages and more often for AV block than former power/mixed athletes and non-athletes.
DOI: 10.3390/jcm13010203 -
Severe Limb Weakness, Bradycardia, and AV Block in Periodic Paralysis Hypokalemia
A 32-year-old Javanese man with hypokalemic periodic paralysis presented with acute limb weakness, significant bradycardia and AV block, and severe hypokalemia; ECG later normalized and he was discharged without neurologic deficits.
DOI: 10.69868/ani.v2i02.27 -
Laryngocardiac Reflex: A Case Report and Review of the Literature
This case report describes a 45-year-old man with exercise-induced bradycardia caused by the laryngocardiac reflex from a supraglottic lymphovascular malformation, whose symptoms were reversed by a unilateral superior laryngeal nerve block and resolved after CO2 laser excision, …
DOI: 10.1016/j.jvoice.2017.07.022 -
Congenital complete atrioventricular block: Clinical and electrophysiologic predictors of need for pacemaker insertion
This study investigates the clinical and electrophysiologic predictors of the need for pacemaker insertion in children with congenital complete atrioventricular block, finding that a resting heart rate of 50 beats/min or less is significantly correlated with the incidence of syn…
DOI: 10.1016/0002-9149(81)90326-x -
Lyme carditis
This case report discusses a 17-year-old girl who presented with syncope and was diagnosed with Lyme carditis, highlighting the importance of recognizing cardiac manifestations of Lyme disease in emergency settings.
DOI: 10.1007/s11739-009-0285-9 -
Pronounced resting bradycardia in male elite runners is associated with high heart rate variability
This study investigates the pronounced resting bradycardia and high heart rate variability in elite male middle- and long-distance runners, suggesting an increased parasympathetic tone as a contributing factor.
DOI: 10.1111/j.1600-0838.1997.tb00152.x -
Heart rate variability in athletes and nonathletes at rest and during head-up tilt
In a cross-sectional study of 11 sedentary men and 10 endurance-trained cyclists, resting heart rate variability (HRV) was higher in time-domain for athletes with no group difference in frequency-domain HRV at rest or during head-up tilt (HUT); during HUT both groups showed simi…
DOI: 10.1590/S0100-879X2005000400019 -
Electrocardiographic findings in male veteran endurance athletes.
This study compared 20 male veteran endurance runners with 20 age-matched sedentary controls using resting, exercise, and ambulatory electrocardiography to characterize bradyarrhythmias, AV block, and other conduction/arrhythmia manifestations associated with long-term endurance…
DOI: 10.1136/hrt.61.2.155