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A young woman in her 20s who was 8 months into her first pregnancy presented with periodic heart palpitations at rest.

The patient had been experiencing palpitations that lasted for a few minutes and then subsided on their own. They were not accompanied by any other symptoms such as sweating, fainting, or shortness of breath, reported Kapil Rajendran, MD, DM, of Government TD Medical College in Kerala, India, and colleagues in .

On further questioning, the patient described becoming winded when she lay down, but noted that sitting up relieved the breathlessness. Sometimes this shortness of breath would waken her a few hours after she had fallen asleep, forcing her to sit up or stand to regain her breath.

The patient's mother reported concern about a visible pulse that she had noticed in the patient's neck. The patient had no chronic medical conditions, nor was she taking any medications.

On initial physical examination, her blood pressure was 110/70 mm Hg and heart rate was 120 beats/min. Other findings were unremarkable. Clinicians noted that the jugular venous pulsations revealed regular cannon waves, and auscultation was significant for a variable first heart sound.

Lab tests revealed normal results. The patient had good biventricular function, and two-dimensional transthoracic echocardiography showed normally functioning valves.

The authors noted that a 12-lead ECG taken at admission demonstrated regular narrow QRS tachycardia (QRS duration 60 milliseconds) at a ventricular rate of 103 to 125 beats/min with a mean cycle length of 509.16 milliseconds, a QRS axis of +60°, and a QTc of 404 milliseconds.

"Regular P waves were evident marching onto the QRS complexes at a rate of 100 to 125 beats/min with mean cycle length of 517.16 milliseconds with varying PR intervals," they authors explained. "The atrial and ventricular rates were within the range of 103 to 125 beats/min and almost equal to each regular (with a marginal variation of 7.52 milliseconds), with varying PR intervals suggestive of an isorhythmic atrioventricular dissociation."

Discussion

Clinicians considered various differential diagnoses, including accelerated atrioventricular (AV) junctional tachycardia, paroxysmal junctional tachycardia, and focal ectopic tachycardia.

"Accelerated AV junctional tachycardia is characterized by a gradual mode of onset and offset, regular narrow QRS tachycardia at a ventricular rate of 70 to 130 beats/min, AV dissociation, and occasional retrograde atrial capture presenting with inverted P waves after the QRS complexes," the authors noted.

If AV dissociation is not complete or an antegrade exit block presents with Wenckebach periodicity, this may result in ventricular capture that interrupts the regular ventricular rate, they added. This often presents in patients who have just had heart surgery, or in those with digitalis toxicity, hypokalemia, myocarditis, or chronic obstructive lung disease with hypoxia.

Meanwhile, tends to present with "an abrupt onset and termination, regular narrow QRS tachycardia at a ventricular rate of 120 to 220 beats/min, AV dissociation with or without intermittent sinus capture (synchronization), or retrograde atrial capture," Rajendran and team explained. This is often a sequelae of cardiac surgeries, or the result of digitalis toxicity; adenosine treatment tends to be less than effective.

Focal ectopic tachycardia shows "a regular narrow QRS tachycardia with an atrial and ventricular rate of 100 to 200 beats/min, gradual warmth, and abrupt termination with positive P waves in V₁ if the origin is from a single focus," the clinicians noted. The rhythm may be irregular in patients with heart rates faster than 150 beats/min, and characterized by Wenckebach periodicity as a result of AV block. This often affects patients with significant structural heart disease, such as those with myocardial infarction, primum atrial septal defects, or cor pulmonale. Beta-blockers are effective in eliciting a prompt response, the authors said.

In the present case, the clinicians noted that while the P wave morphology seemed to be varying with varying PR intervals that mimicked multifocal atrial tachycardia, it was reasonable to exclude focal ectopic tachycardia in light of the fairly regular RR intervals and AV dissociation.

Evidence of regular narrow QRS tachycardia at a ventricular rate of 125 beats/min in this patient, along with the observed gradual onset and offset with isorhythmic AV dissociation, suggested possible accelerated AV junctional tachycardia, they noted.

Clinicians started the patient on oral metoprolol tartrate 12.5 mg twice daily and increased the dose to 25 mg twice daily on the following day. The ECG follow-up revealed "normal sinus rhythm at a rate of 75 to 80 beats/min, regular PR interval of 200 milliseconds, and QTc of 410 milliseconds with disappearance of jugular cannon waves and complete resolution of symptoms," they wrote.

"Marching of P waves onto QRS complexes simulates partial pre-excitation due to apparent shortening of PR interval," they explained. However this could be ruled out, because the beta-blocker treatment normalized the PR interval and there was no evidence of delta waves.

"Beta-blockers slow the rate of ectopic junctional pacemaker, allowing the sinoatrial nodal impulses to capture the ventricle," Rajendran and colleagues noted. They suggested that following the pregnancy, arrhythmia can be managed long-term with flecainide, beta-blockers, or nondihydropyridine calcium channel blockers.

Heart rate changes during pregnancy increase by 10% to 15% during the first trimester, and by 20% to 25% in the third trimester. This occurs as a result of a decrease in systemic vascular resistance, which is facilitated by surges in prolactin, prostaglandins, relaxin, and estrogen, Rajendran's group explained.

Patients at increased risk for developing arrhythmias in pregnancy include those who are 41 to 50 years of age; those with cardiovascular comorbidities such as diabetes, high blood pressure, or obesity; those younger than 30 years of age; those with structural heart diseases and conditions such as hyperthyroidism; and those who have previously been affected by arrhythmias.

Common observed in pregnancy include:

• Shortening of PR interval
• Leftward shift in QRS axis
• Prominent Q waves with T inversion in lead III
• QTc prolongation

"The present patient is unique in the sense that, to our knowledge, nonparoxysmal AV nodal tachycardia with isorhythmic AV dissociation has not been reported in pregnancy with a structurally normal heart," the authors concluded. In this case, beta-blocker treatment completely resolved her symptoms.

As this case illustrated, cardiovascular, autonomic, and hormonal changes may cause patients with a structurally normal heart to develop tachyarrhythmias in the third trimester.

"A ladder diagram could provide essential insights into the mechanisms underlying tachyarrhythmias and aids in planning an appropriate therapeutic strategy," they wrote.

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