What could be causing a patient's symptoms of intermittent tachycardia (rapid heart rate) and perceived cardiac pauses, despite normal diagnostic test results and a chronically elevated white blood cell count (leukocytosis)?

Evaluation and Management of Intermittent Tachycardia with Normal Test Results

Initial Assessment

Inappropriate Sinus Tachycardia (IST) is the most likely diagnosis for this patient with persistent symptoms of tachycardia despite normal diagnostic test results. 1 This condition is characterized by unexplained sinus tachycardia that causes debilitating symptoms including palpitations, weakness, fatigue, lightheadedness, and sensations of "heart racing" - matching this patient's description of her heart "speeding up and stopping."

IST is a diagnosis of exclusion that requires ruling out secondary causes of tachycardia, which appears to have been done through the patient's multiple ER visits and cardiac monitoring.

Diagnostic Considerations

Key Features Supporting IST Diagnosis:

• Patient reports episodes of rapid heart rate
• Normal cardiac monitor results (suggesting sinus mechanism)
• Significant psychological distress and anxiety about symptoms
• Persistent symptoms despite reassurance
• Symptoms affecting quality of life (fear of sleeping, impact on childcare)

Differential Diagnosis to Consider:

1. Inappropriate Sinus Tachycardia (IST)

   • Defined as sinus tachycardia unexplained by physiological demands
   • Characterized by resting heart rates >100 bpm and average rates >90 bpm over 24 hours
   • Associated with debilitating symptoms despite benign prognosis 1

2. Physiological Sinus Tachycardia

   • Secondary to underlying conditions like:
     • Infection/inflammation (suggested by elevated WBC count)
     • Anemia
     • Dehydration
     • Hyperthyroidism
     • Medications or stimulants 1

3. Paroxysmal Supraventricular Tachycardia (PSVT)

   • Brief episodes may be missed on standard monitoring
   • May require longer monitoring periods 1

4. Anxiety-Related Tachycardia

   • Important to note that anxiety can be both a cause and result of tachycardia
   • IST patients often have associated anxiety disorders 1

Evaluation Plan

Immediate Steps:

1. Extended Cardiac Monitoring

   • Consider 14-30 day external loop recorder or implantable loop recorder based on symptom frequency 1
   • Aim to capture correlation between symptoms and actual rhythm

2. Laboratory Evaluation

   • Complete blood count (investigate elevated WBC count)
   • Thyroid function tests
   • Basic metabolic panel
   • Inflammatory markers

3. Cardiac Evaluation

   • Complete the scheduled echocardiogram to rule out structural heart disease
   • Consider exercise stress test to evaluate heart rate response to exertion

Addressing the Elevated WBC Count:

• The chronically elevated white count requires investigation as it could indicate:
  • Underlying infection/inflammation contributing to tachycardia
  • Stress response
  • Other systemic conditions 2, 3

Management Recommendations

Ivabradine is reasonable as first-line therapy for ongoing management of symptomatic IST if diagnosis is confirmed. 1

Pharmacological Options:

1. Ivabradine (First-line)

   • Inhibits the "If" channel responsible for sinus node automaticity
   • Class IIa recommendation with Level B-R evidence 1
   • Effectively reduces heart rate without affecting blood pressure

2. Beta Blockers (Second-line)

   • May be considered for symptomatic IST (Class IIb, Level C-LD)
   • Often limited by side effects like hypotension 1

3. Combination Therapy

   • Beta blockers plus ivabradine may be considered in resistant cases (Class IIb, Level C-LD) 1

Non-Pharmacological Approaches:

1. Identify and treat reversible causes (Class I recommendation) 1

   • Address the elevated WBC count
   • Evaluate for occult infection or inflammation

2. Psychological Support

   • Acknowledge the reality of symptoms despite normal test results
   • Consider referral for cognitive behavioral therapy to address anxiety
   • Validate patient's concerns rather than dismissing them as "just anxiety"

Important Considerations and Pitfalls

1. Avoid dismissing symptoms as "just anxiety"

   • The patient's distress is real and affecting quality of life
   • Anxiety can be both a cause and consequence of tachycardia

2. Symptom-rhythm correlation is crucial

   • Extended monitoring may be needed to capture episodes
   • Consider monitoring options based on symptom frequency 1

3. Investigate the elevated WBC count

   • Could indicate underlying condition contributing to symptoms
   • Persistent leukocytosis warrants evaluation

4. Recognize diagnostic challenges

   • Heart failure and other cardiac conditions can be misdiagnosed initially 4
   • Consider referral to electrophysiology if symptoms persist despite treatment

5. Acknowledge limitations of standard testing

   • Brief arrhythmia episodes may be missed on standard monitoring
   • Cardiologist evaluation improves diagnostic accuracy in challenging cases 5

Follow-up Plan

1. Short-term follow-up (1-2 weeks)

   • Review results of extended monitoring and laboratory tests
   • Assess response to any initiated treatments
   • Provide reassurance and psychological support

2. Medium-term follow-up (1-3 months)

   • Complete cardiac evaluation including echocardiogram
   • Adjust medication if needed
   • Consider referral to electrophysiology if diagnosis remains unclear

3. Long-term management

   • Focus on symptom control and improving quality of life
   • Educate on prognosis (generally benign but symptoms can be persistent)
   • Consider exercise training as adjunctive therapy

By taking this patient's symptoms seriously and pursuing appropriate evaluation and management, we can address both the physiological and psychological aspects of her condition to improve her quality of life.