Myocardial function during bradycardia events in preterm infants

Highlights

• Myocardial function during sudden bradycardia events in preterm infants has not been described

• Speckle tracking analysis is a novel technique that can be applied retrospectively to examine myocardial function in detail

• Systolic function and stroke volume were maintained during moderate bradycardia events in preterm infants and atrial contractility was reduced

Abstract

Background

Transient bradycardia episodes are common in preterm infants and often secondary to apnea. Decreased ventilation with resultant hypoxemia is believed to be the predominant mechanism. Sudden bradycardias without apnea are also reported, possibly due to vagal stimulation. Point of care ultrasound is used to diagnose and follow cardiovascular complications in preterm infants. Inadvertently, the operator would sometimes capture bradycardia events. This study reports on left ventricular function during such events.

Methods

We retrospectively reviewed our cardiac ultrasound database for bradycardia events. Apical four or three chamber images before, during and after a bradycardia event were analysed with speckle tracking software which provides systolic and diastolic parameters of myocardial motion, deformation and volume.

Results

Over a 2 year period, 15 bradycardia events were noted in 14 patients with a median gestational age of 26 weeks (range 23 to 29). Heart rate decreased by an average of 43% (171/min to 98/min). Myocardial velocity and longitudinal strain rate during the atrial component of diastole were reduced during bradycardia. Longitudinal strain during systole was increased and radial deformation was unchanged. Ventricular volumes and ejection fraction did not change. Most parameters returned to baseline values after the event. Longitudinal systolic strain rate remained lower and stroke volume was 12% higher compared to baseline.

Conclusion

Parameters of systolic contractility and stroke volume were maintained and parameters of atrial contractility were reduced during mild to moderate bradycardia in preterm infants. Bradycardia reduces total cardiac output with a compensatory increase detected following the event.

Introduction

Transient bradycardia episodes are common in preterm infants and often secondary to apnea. Decreased ventilation with resultant hypoxemia is believed to be the predominant mechanism, although occasionally bradycardia may be found without preceding apnea [1], [2], [3]. The exact mechanism responsible for the bradycardia in this setting is not known, but likely involves inhibition of normal sinus node function by the autonomous nervous system with or without activation of primitive reflexes [4], [5], [6].

Several investigators have studied the cardiovascular effects of bradycardia and found a linear relation between the decrease in heart rate and organ blood flow. Changes in blood pressure were not consistent, and varied with the depth and duration of the bradycardia [7], [8], [9]. All investigators stress that periods of reduced blood flow could negatively affect brain development in preterm infants, and further studies into bradycardia mechanisms and cardiovascular function are warranted.

Point of care ultrasound is used to diagnose and follow cardiovascular status and hemodynamics in preterm infants. Common clinical indications for point of care cardiac ultrasound include hypotension, diagnosis and assessment of a patent ductus arteriosus, sepsis and pulmonary hypertension with associated difficulties in oxygenation [10]. Inadvertently, the operator may capture an abrupt bradycardic event before discarding the images as unsuitable for analysis. New non-Doppler echocardiography techniques such as speckle tracking echocardiography (STE) now make it possible to retrospectively examine myocardial function in further detail [11]. This technique uses computer software to analyse speckles generated by interaction of ultrasonic beam and the myocardium. It is able to track and follow the speckles within the myocardial tissue and borders, and thus produce parameters of myocardial motion, deformation and volume.

The aim of this study is to report on function of the left ventricle (LV) during bradycardia events in preterm infants using speckle tracking analysis.