Bradycardia in acute haemorrhage
Executive Summary
Traditional medical teaching suggests that acute blood loss is invariably accompanied by tachycardia, hypotension, and poor peripheral perfusion. However, clinical evidence indicates that a significant subset of patients—ranging from 28.9% to 35% in trauma cases—exhibit a "relative" or "paradoxical" bradycardia during acute haemorrhage. This absence of a tachycardic response can lead to dangerous delays in diagnosis and treatment.
This document synthesizes clinical case reports and physiological research to outline the biphasic cardiovascular response to hypovolaemia. The core takeaway for clinicians is that hypotension accompanied by a normal or low heart rate after surgery must be treated as a potential sign of occult bleeding, requiring prompt circulatory volume replacement.
The Traditional Paradigm vs. Clinical Reality
In a healthy adult, the standard response to decreased stroke volume and hypovolaemia involves neural and hormonal mechanisms that trigger tachycardia (a heart rate exceeding 100 beats/min) to maintain cardiac output. This is mediated by arterial baroreceptors.
Despite this "accepted" sign, clinicians frequently encounter patients who do not manifest this initial tachycardic response. This phenomenon is variously termed:
Relative bradycardia
Absence of tachycardic response
Paradoxical bradycardia
Failure to recognize these labels as indicators of acute haemorrhage is potentially life-threatening, as it may result in the misinterpretation of vital signs during the critical window for intervention.
Clinical Case Analyses
The following cases, observed within a single year at the Royal United Hospital (Bath) and Weston General Hospital, illustrate the diagnostic challenges of relative bradycardia.
Physiological Mechanisms of the Haemorrhagic Response
Research suggests that the heart rate response to a reduction in circulating blood volume is typically biphasic when resuscitation is not immediately provided.
The Biphasic Response
Phase One: Characterized by baroreceptor-mediated reflex vasoconstriction and cardio-acceleration. The patient exhibits tachycardia while attempting to maintain normotension.
Phase Two: Occurs when approximately one-third of the circulating blood volume is lost. Sympathetic drive falls abruptly, and cardiac vagal drive increases. This results in a simultaneous drop in both mean arterial pressure and heart rate.
Distinguishing Relative Bradycardia
While Phase Two naturally involves bradycardia, "relative bradycardia" is distinct because the patient entirely skips the initial tachycardic response of Phase One. The specific reason why some patients do not show initial tachycardia remains unknown, though several theories exist:
Ventricular Mechanoreceptors: Activation of a vagally mediated reflex arc originating from mechanoreceptors in the left ventricle may increase diastolic filling time to boost stroke volume.
Peritoneal Irritation: A parasympathetic reflex mediated via the vagus nerve may be triggered by the presence of blood in the peritoneum.
Vagal Nerve Involvement: Experimental evidence in cats shows that if the vagus nerves are cooled or cut, the bradycardic response to haemorrhage disappears, though the fall in blood pressure remains.
Statistical Prevalence and Prognosis
Retrospective studies of trauma patients highlight that bradycardia in the face of hypotension is a common clinical finding:
Isolated Trauma Study: In a study of 71 patients with isolated extremity or penetrating abdominal trauma and systolic blood pressure <100 mm Hg, 35% (25 patients) had a pulse rate of fewer than 100 beats/min.
Trauma Centre Study: A larger review of 750 patients with systolic blood pressure <90 mm Hg found that 28.9% (217 patients) had a pulse rate of fewer than 90 beats/min.
Interestingly, some data suggests that bradycardia may be associated with a better prognosis in certain patient subgroups, likely due to the increased diastolic filling time allowing for higher stroke volume.
Conclusion and Clinical Recommendations
The assumption that tachycardia is the only haemodynamic response to acute central hypovolaemia is a clinical fallacy. To ensure patient safety, the following must be integrated into post-operative and emergency care:
Broaden Diagnostic Criteria: Consider acute haemorrhage in any patient exhibiting hypotension, even if the heart rate is normal or low.
Prioritize Volume Replacement: Do not delay the increase of circulatory volume in critically ill patients while waiting for "traditional" signs like tachycardia to appear.
Monitor Haemoglobin: In the presence of unexplained hypotension, rapid haemoglobin assessment is vital, as seen in the cases where levels dropped significantly (as low as 55 g/l).
