Abstract
The small size and high heart rate of the neonatal mouse heart makes structural and functional characterisation particularly challenging. Here, we describe application of electrocardiogram-gated kilohertz visualisation (EKV) ultrasound imaging with high spatio-temporal resolution to non-invasively characterise the post-natal mouse heart during normal growth and regeneration after injury. The 2-D images of the left ventricle (LV) acquired across the cardiac cycle from post-natal day 1 (P1) to P42 revealed significant changes in LV mass from P8 that coincided with a switch from hyperplastic to hypertrophic growth and correlated with ex vivo LV weight. Remodelling of the LV was indicated between P8 and P21 when LV mass and cardiomyocyte size increased with no accompanying change in LV wall thickness. Whereas Doppler imaging showed the expected switch from LV filling driven by atrial contraction to filling by LV relaxation during post-natal week 1, systolic function was retained at the same level from P1 to P42. EKV ultrasound imaging also revealed loss of systolic function after induction of myocardial infarction at P1 and regain of function associated with regeneration of the myocardium by P21. EKV ultrasound imaging thus offers a rapid and convenient method for routine non-invasive characterisation of the neonatal mouse heart.
Highlights
Pedal reflex is absent in neonatal mice under typical anaesthesia concentration (»2%), we found that insertion of a neonatal mouse rectal thermometer still elicited reflexes at this level of anaesthesia
left ventricle (LV) mass obtained through electrocardiogram-gated kilohertz visualisation (EKV) high-resolution ultrasound imaging correlates with gravimetric LV mass during early post-natal growth
To investigate whether information obtained from EKV ultrasound imaging on cardiac structures correlated with ex vivo measurements during early life, cardiac growth from P2 to P42 was studied in vivo by ultrasound and compared with ex vivo metrics
Summary
The heart undergoes significant changes during the early period after birth that are important for subsequent function and susceptibility to disease (Soonpaa et al 1996; Soonpaa and Field 1998; Leu et al 2001; Hirschy et al 2006; Corrigan et al 2010; Piquereau et al 2010; Haubner et al 2012; Porrello et al 2013; Tian et al 2014; Blom et al 2016). Ultrasound has been extensively used in the context of cardiovascular research both in the mouse embryo and the adult (Phoon and Turnbull 2003; Corrigan et al 2010; Moran et al 2013) The strengths of this technique are its high resolution (50 mm), its rapidity (10À15 min scans per animal) and its relatively low cost (Olive and Tuveson 2006). High-frame-rate electrocardiogram-gated kilohertz visualisation (EKV) imaging enables postacquisition generation of a single typical cardiac cycle This is generated from many sequentially acquired M-modes that are spatially and temporally interleaved into an ultrasound B-mode image data set (Moran et al 2013). EKV imaging is known to increase spatial resolution of cardiac borders, as well as temporal resolution of the systolic and diastolic ends of the cardiac cycle (Lindsey et al 2018), but its use has not previously been reported in neonatal mice
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