A 6-month-old boy is brought to the emergency department due to lethargy for several hours. His mother says he is usually a "very active baby," and she is afraid that he is "coming down with something." The patient's birth history is unremarkable, and he was born at 38 weeks gestation. He is afebrile. Blood pressure is 70/44 mm Hg, pulse is 240 beats/min, and respirations are 52/min. The patient appears pale and is perspiring. Capillary refill time is less than 2 seconds. The lungs are clear to auscultation. No murmurs are heard. An ECG shows a regular heart rate of 240/min with narrow QRS complexes and no identifiable P waves. Which of the following is the most likely cause of this patient's hypotension?

Question

A 6-month-old boy is brought to the emergency department due to lethargy for several hours.  His mother says he is usually a "very active baby," and she is afraid that he is "coming down with something."  The patient's birth history is unremarkable, and he was born at 38 weeks gestation.  He is afebrile.  Blood pressure is 70/44 mm Hg, pulse is 240 beats/min, and respirations are 52/min.  The patient appears pale and is perspiring.  Capillary refill time is less than 2 seconds.  The lungs are clear to auscultation.  No murmurs are heard.  An ECG shows a regular heart rate of 240/min with narrow QRS complexes and no identifiable P waves.  Which of the following is the most likely cause of this patient's hypotension?

Quizplus · Accepted Answer

11ec160b_3a9f_6ace_9249_cde6db0b09d2_MD0001_00 This patient is pale, lethargic, and diaphoretic with ECG findings consistent with supraventricular tachycardia (SVT), the most common pediatric arrhythmia. Normally, depolarization impulses originate at the sinoatrial node and travel through the atrial myocardium to the atrioventricular (AV) node, where the signal pauses to allow ventricular filling following atrial contraction. The impulse then continues through the His-Purkinje system to the ventricular myocardium. SVT occurs when an impulse originates at an abnormal focus above the ventricles, most commonly a reentrant circuit within the AV node (ie, AV nodal reentry tachycardia). Heart rate is elevated (usually >150/min in adults or >220/min in infants), but rhythm is usually regular. QRS complexes are narrow, and P waves are generally buried within the QRS complex due to retrograde atrial depolarization occurring in conjunction with anterograde ventricular depolarization. With persistent tachycardia, ventricular diastole shortens, allowing less time for ventricular relaxation and filling. Stroke volume and cardiac output decrease, leading to hypotension and poor peripheral perfusion. Infants may become altered (eg, lethargic, poor feeding) and develop signs of heart failure (eg, tachypnea, crackles, hepatomegaly). (Choice A) Decreased effective intravascular volume is the mechanism for hypotension associated with hypovolemic shock. This patient has normal capillary refill without a history of volume depletion (eg, diarrhea, vomiting, blood loss). (Choice B) Hypotension due to decreased myocardial contractility is seen with a myocardial infarction, which is extremely rare in infancy, or a viral-induced cardiomyopathy, which is unlikely in the absence of infectious symptoms (eg, fever, rhinorrhea). (Choice C) Low cardiac output associated with SVT would lead to a compensatory increase, not a decrease, in peripheral vascular resistance. (Choice D) A large left-to-right intracardiac shunt (eg, ventricular septal defect [VSD]) can cause hypotension and tachycardia, but signs of heart failure (eg, crackles, hepatomegaly) would be expected. In addition, depending on the size of VSD, ECG may show left or biventricular hypertrophy, not narrow QRS complexes with absent P waves. Educational objective: Supraventricular tachycardia is characterized by an elevated heart rate with a regular rhythm and narrow QRS complexes due to an abnormal electrical impulse originating above the ventricles. Sustained tachycardia shortens ventricular diastole, leading to decreased stroke volume and cardiac output and subsequent hypotension. __________ References: Supraventricular tachycardia (SVT). (http://www.ncbi.nlm.nih.gov/pubmed/28723001)

A 6-Month-Old Boy Is Brought to the Emergency Department Due