Doppler Ultrasound for Heart Rate Assessment in a Porcine Model of Neonatal Asphyxia

Abstract

Objectives: Approximately 10% of newborn infants require resuscitation at birth. Accurate heart rate (HR) assessment guides resuscitation interventions, thereby reducing morbidities and mortality. While existing HR assessment methods have several limitations, the Doppler ultrasound (Doppler-US) might be a promising alternative. We aimed to evaluate accuracy and optimal use of Doppler-US for HR assessments during neonatal asphyxia in a pre-clinical model. Design: HR assessments were performed in 16 term newborn piglets that were anesthetized, intubated, and instrumented. Study I evaluated optimal transducer position, Study II compared aortic (AV) and pulmonary (PV) examination modes, and Study III examined accuracy during asphyxia, for HR assessment. Setting: Experimental setting. Subjects: Asphyxia-induced piglets. Interventions: Study I: Doppler-US (USCOM® 1A) HR was assessed on upper (A), middle (B), and lower (C) third of the sternum; study II: Doppler-US HR was assessed using AV and PV examination modes; study III: HR was assessed during asphyxia. Comparisons were made between Doppler-US and the clinical gold standard for HR assessments, electrocardiography (ECG). Measurements and Main Results: Study I: Mean (SD) Doppler-US HR at position A, B, and C showed no difference when compared to ECG HR. Study II: The mean (SD) Doppler-US HR using AV and PV modes also showed no difference when compared to ECG HR. Study III: Bland-Altman analysis revealed a mean difference (95% limits of agreement) between Doppler-US and ECG HR of 1.5 (-16 to 19) bpm. Additionally, motion artifacts produced false peaks and peak size was seen to decrease as bradycardia progressed. Conclusions: HR assessment using Doppler-US during asphyxia is accurate but has limitations and must be further evaluated prior to clinical use. Doppler-US can be positioned along the sternum and use either AV or PV mode for accurate assessments in a piglet model of neonatal asphyxia.

Keywords: Doppler ultrasound; heart rate; infants; neonatal resuscitation; newborn.

Figure 1

(a) Experimental scheme for the assessment of the optimal transducer position to acquire reliable heart rate (HR) data from piglets. The suprasternal notch is not available because of endotracheal intubation. Positions A, B, and C are located at increasing distances from the snout of the animal along the sternal area at 5–6, 6–7, and 7–8 inches (~13–15, ~15–18, and ~18–20 cm, respectively), respectively. For each piglet, a minimum of two HR recordings from each position were carried out by observers blinded from the USCOM monitor during signal acquisition. (b) Representative HR outcomes from position A, B, and C, showing no difference in HR detection, but more distinguished and clearer detection of the HR peak signal in position B. The images selected were judged to be best representative of signals obtained during stabilization.

Figure 2

Bland-Altman plots for Studies I and III. (A–C) Study I: Bland-Altman plot with correction for multiple observations per subject for Doppler ultrasound HR from respective positions A, B, or C vs. ECG HR. (D) Study III: Bland-Altman plot with correction for multiple observations per subject for Doppler ultrasound HR vs. ECG HR. ECG, electrocardiography; CBF, carotid blood flow; Doppler, Doppler ultrasound.

Figure 3

Study II: Representative HR outcomes from aortic valve (AV) and pulmonary valve (PV) examination mode, showing no difference in HR detection, but more distinguished and clearer detection of the HR peak signal in PV mode. The images selected were judged to be best representative of signals obtained during stabilization. ECG, electrocardiography; DUS, Doppler ultrasound; AV, aortic valve examination mode; PV, pulmonary valve examination mode.

Figure 4

(a) Influence of mechanical ventilation on heart rate (HR) signal detection by USCOM. Two distinct higher peaks caused by piglet's emphasized chest movement due to ventilation are observable in the monitor image. The automatic flowtrace function of USCOM calculates the average HR in real-time, but ventilation “peaks” can impair this measurement. In the displayed image, the calculated mean HR is 174 bpm, but after manual removal of the two ventilation-produced signals, this mean HR increases to 182 bpm. (b) False positive signals caused by mechanical ventilation and subsequent chest movement in a euthanized piglet. (c–e) Heart rate (HR) over the progression of bradycardia during asphyxia with HR at (c) 63 bpm, (d) 42 bpm, and (e) 23 bpm. As shown, the signal appears to lose clarity and decrease in peak size.