To the Editor,
Arrhythmias and other cardiovascular symptoms in patients with COVID-19 are frequently
reported and are likely associated with infection-related myocarditis, ischemia, and/or
systemic proinflammatory stimulation (“cytokine storm”). In a case series including
138 hospitalized patients, 17% (and 44% of the patients admitted to the intensive
care unit) had an (unspecified) arrhythmia . Moreover, medications used in COVID-19
patients may increase arrhythmic risk. We have observed fever plus relative bradycardia,
i.e. an inappropriately low heart rate response to increased body temperature ,
in several hospitalized COVID-19 patients. Our primary objective was to assess the
prevalence of relative bradycardia in patients with COVID-19.
We retrospectively reviewed the electronic medical records of the first 174 patients
with confirmed COVID-19 (detection of severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2) by RT-PCR from nasopharyngeal swabs) admitted to the University Hospital
Basel, Switzerland, from February 27, 2020 to April 15, 2020. During this period,
symptomatic inpatients without contraindications were routinely treated with lopinavir/ritonavir
for 5-7 days and hydroxychloroquine for two days. In addition, patients with severe
disease received tocilizumab. An ECG was routinely performed on admission in all patients
and again on the third day of hospitalization in all patients on treatment. We included
in our analysis only patients primarily admitted to the ward (162 patients), and excluded
further 52 patients for the following reasons: treatment with heart rate lowering
agents (e.g. beta blockers, non-dihydropyridine calcium channel blockers, amiodarone,
digoxin) and conditions associated with bradycardia (e.g. hypokalemia <3.0 mmol/L)
(33); non-sinus rhythm on ECG (7); missing ECG (1); refusal of consent (11).
Relative bradycardia was defined as a heart rate < 90/min and concomitant fever (tympanic
temperature ≥ 38.3°C), measured at least twice within 24 hours. If more measurements
met these criteria, we included the measurements with the highest body temperature.
The local ethical board approved the study (EKNZ 2020-00769).
110 patients with COVID-19 (median age: 59 years, males: 60%) were evaluated for bradycardia.
71 of the 110 patients (64%) had fever during hospitalization. 40 patients had relative
bradycardia (36% of all COVID-19 patients and 56% of COVID-19 patients with fever).
Relative bradycardia occurred a median of 9 days (IQR: 6-11) after symptoms onset.
Moreover, 38 of the 110 patients (34%) had at least once a heart rate of < 60/min
during the hospital stay irrespective of the body temperature (18 of the relative
bradycardia group, 20 without relative bradycardia). None of the 110 patients had
a QT-prolongation. The temperature-heart rate relationship in patients with relative
bradycardia is reported in Figure 1
Temperature-heart rate relationship in patients with COVID-19 and a relative bradycardia
(for each patient the two values with the highest temperature ≥ 38.3°C and a heart
rate < 90 bpm measured in the consecutive 24 hours are reported) (red). Physiologically
appropriate temperature-heart rate relationship (blue), adapted from . Temperature-heart
rate relationship in patients with COVID-19 without relative bradycardia (for each
patient the two values with the highest temperature are reported) (green). Dotted
lines represent linear regression between temperature and heart rate. RB: relative
Patients with relative bradycardia were significantly older (median age: 62 years)
and presented with significantly higher maximal temperatures (median: 39.3 °C) compared
to patients with fever and an appropriate heart rate response (49 years; 38.7 °C).
Otherwise the two groups did not differ significantly regarding off-label drug treatment,
oxygen therapy or laboratory findings. The clinical outcome (intensive care unit admission,
intubation, death) was similar in patients with fever and relative bradycardia (20%,18%
and 3% of patients respectively), and in patients with fever and appropriate heart
rate response (19%, 13% and 6%).
We found that applying a conservative definition, 56% of hospitalized COVID-19 patients
with fever had relative bradycardia. A recently released study of 54 Japanese patients
with mild to moderate COVID-19 used a broader definition (not requiring the presence
of fever or a minimal temperature) and also showed that relative bradycardia was a
common characteristic .
Typically, the heart rate increases by about 10/min for each Fahrenheit degree increase
in body temperature above 101 °F (38.3 °C). The appropriate heart rate with a body
temperature of 38.3 °C is about 110/min . The term relative bradycardia describes
the failure of the heart rate to rise when body temperature is elevated.
Many infectious and non-infectious causes of relative bradycardia in febrile patients
have been described (e.g. typhoid fever), but the pathogenesis of this phenomenon
is still unknown. Direct pathogen effects on the sinoatrial node and effects of inflammatory
cytokines are among proposed mechanisms . Interestingly, interleukin-6 (IL-6) is
the cytokine reported to exhibit the strongest correlation with depressed heart rate
variability, which in turn may predict relative bradycardia . On the other hand,
IL-6 appears to play also an important role in the “cytokine storm” caused by SARS-CoV-2.
A recent report described the development of sinus bradycardia in 8 of 26 patients
with severe COVID-19 pneumonia (without mentioning the presence of fever). The authors
postulated an inhibitory effect of SARS-CoV-2 on sinus node activity .
In conclusion, relative bradycardia is a frequent clinical feature of COVID-19, occurring
in 56% of febrile patients hospitalized on our hospital’s wards. This fact should
be taken into account while evaluating febrile patients in the context of a possible
SARS-CoV-2 infection. Relative bradycardia in non-critically ill patients does not
appear to be associated with a worse clinical outcome.
GC: Conceptualization, Methodology, Investigation, Formal analysis, Writing - Original
Draft; MO: Formal analysis, Writing – Reviewing and Editing; AE, MS: Writing – Reviewing
and Editing; SB: Conceptualization, Methodology, Supervision, Writing - Original Draft
and Reviewing and Editing.
The authors declare no conflicts of interest. No external funding was received for