What are the likely causes of sinus bradycardia in a 67‑year‑old man with advanced head‑and‑neck cancer, chronic smoker, gastrostomy feeding tube receiving enteral nutrition, cirrhosis and on xifaximin, who is not taking any atrioventricular‑nodal blocking medications?

Bradycardia Etiology in a 67-Year-Old with Throat Cancer

In this 67-year-old man with head and neck cancer, cirrhosis, and enteral feeding, the most likely causes of sinus bradycardia are carotid sinus hypersensitivity from the neck mass compressing or involving the vagus nerve/carotid sinus, metabolic/electrolyte disturbances related to cirrhosis and tube feeding, and direct tumor infiltration of cardiac conduction tissue.

Cancer-Related Mechanisms

Direct Tumor Effects on Cardiac Conduction

• Neck mass involvement of the vagus nerve is a well-established cause of bradycardia and heart block in patients with head and neck malignancies 1
• Carotid sinus hypersensitivity secondary to head and neck tumors can cause paroxysmal bradycardia and hypotension, often vasodilatory in nature 2
• In one case series, 7 patients with head and neck cancer developed carotid sinus hypersensitivity with paroxysmal bradycardia associated with tumor compression of the carotid bifurcation 2
• A documented case showed a cervical lymph node metastasis causing incarceration and compression of the carotid sinus, resulting in extreme bradycardia requiring pacemaker placement 3

Infiltrative Disease

• Direct infiltration of the AV nodes by tumor (particularly lymphoma or amyloidosis) can cause brady-arrhythmias or heart block 1
• Infiltration of the myocardium by tumor can provoke both supraventricular and ventricular arrhythmias 4

Metabolic and Cirrhosis-Related Causes

Electrolyte Abnormalities

• Hypokalemia and hypomagnesemia are common in cancer patients and must be corrected, as they affect cardiac conduction 1
• Cirrhosis predisposes to multiple electrolyte disturbances including hypokalemia, hypomagnesemia, and hypocalcemia
• Tube feeding can cause refeeding syndrome with severe electrolyte shifts, particularly hypophosphatemia and hypomagnesemia

Cirrhosis-Specific Mechanisms

• Hypothyroidism associated with cirrhosis can cause bradycardia 1
• Increased vagal tone during feeding or defecation can trigger bradycardia 1
• Obstructive jaundice from cirrhosis is a recognized cause of sinus bradycardia 1

Chemotherapy and Radiation Effects

Chemotherapy-Induced Bradycardia

• Although the patient is not currently on chemotherapy, prior treatment with cisplatin, irinotecan, paclitaxel, mitoxantrone, 5-fluorouracil, or arsenic trioxide could cause persistent bradycardia 1, 5
• These agents can cause bradycardia and heart block even after treatment completion 5

Radiation Therapy Effects

• Radiation therapy for head and neck cancer affects all cardiac structures including the conduction system 1
• Radiation can cause chronic pericarditis and conduction abnormalities that may present years after treatment 1

Critical Diagnostic Approach

Immediate Evaluation Required

• Obtain baseline ECG to assess for heart block versus sinus bradycardia and measure QTc interval 1
• Check comprehensive metabolic panel including potassium, magnesium, calcium, and phosphorus 1
• Thyroid function tests to exclude hypothyroidism 1
• Review any prior chemotherapy or radiation exposure 1, 5

Imaging Studies

• Neck CT or MRI to evaluate for tumor compression of the carotid sinus or vagus nerve 3
• Echocardiogram to assess for infiltrative cardiomyopathy or pericardial disease 1

Specialized Testing

• Carotid sinus massage (performed cautiously with monitoring) may reproduce symptoms if carotid sinus hypersensitivity is present 2
• Holter monitoring to correlate symptoms with bradycardia episodes 1

Management Considerations

Symptomatic Bradycardia

• If the patient has fatigue, dizziness, presyncope, or syncope, pacemaker placement should follow ACC/AHA guidelines 1
• Carotid sinus hypersensitivity from head and neck tumors is often vasodilatory and does not respond well to pacemaker alone 2
• Periarterial nerve stripping or intracranial nerve root section may be necessary for severe carotid sinus hypersensitivity 2
• Radiation therapy can provide favorable response for tumor-related compression 3

Asymptomatic Bradycardia

• Most patients with chemotherapy-induced bradycardia are asymptomatic and can be monitored closely 1
• Correct all electrolyte abnormalities before considering other interventions 1

Important Clinical Pitfalls

• Do not assume bradycardia is benign in cancer patients—it may indicate tumor progression or compression of vital structures 3, 2
• Standard pacemaker therapy may be ineffective if the mechanism is vasodilatory carotid sinus hypersensitivity 2
• Xifaximin itself is not associated with bradycardia, but the underlying cirrhosis creates multiple risk factors
• Tube feeding can mask electrolyte abnormalities that develop gradually—regular monitoring is essential