Cerebral haemodynamic response to somatosensory stimulation in neonatal lambs

Cerebral haemodynamic response to somatosensory stimulation in preterm lambs and 7-10-day old lambs born at term: Direct synchrotron microangiography assessment

Neurovascular coupling has been well-defined in the adult brain, but variable and inconsistent responses have been observed in the neonatal brain. The mechanisms that underlie functional haemodynamic responses in the developing brain are unknown. Synchrotron radiation (SR) microangiography enables in vivo high-resolution imaging of the cerebral vasculature. We exploited SR microangiography to investigate the microvascular changes underlying the cerebral haemodynamic response in preterm (n = 7) and 7-10-day old term lambs (n = 4), following median nerve stimulation of 1.8, 4.8 and 7.8 sec durations.Increasing durations of somatosensory stimulation significantly increased the number of cortical microvessels of ≤200 µm diameter in 7-10-day old term lambs (p < 0.05) but not preterm lambs where, in contrast, stimulation increased the diameter of cerebral microvessels with a baseline diameter of ≤200 µm. Preterm lambs demonstrated positive functional responses with increased oxyhaemoglobin measured by near infrared spectroscopy, while 7-10-day old term lambs demonstrated both positive and negative responses. Our findings suggest the vascular mechanisms underlying the functional haemodynamic response differ between the preterm and 7-10-day old term brain. The preterm brain depends on vasodilatation of microvessels without recruitment of additional vessels, suggesting a limited capacity to mount higher cerebral haemodynamic responses when faced with prolonged or stronger neural stimulation.

The cerebral haemodynamic response to somatosensory stimulation in preterm newborn lambs is reduced with dopamine or dobutamine infusion

BACKGROUND

In the adult brain, increases in neural activity lead to increases in local blood flow. However, in the preterm neonate, studies of cerebral functional haemodynamics have yielded inconsistent results, including negative responses suggesting decreased perfusion and localised tissue hypoxia, probably due to immature neurovascular coupling. Furthermore, the impact of vasoactive medications, such as dopamine and dobutamine used as inotropic therapies in preterm neonates, on cerebrovascular responses to somatosensory input is unknown. We aimed to characterise the cerebral haemodynamic functional response after somatosensory stimulation in the preterm newborn brain, with and without dopamine or dobutamine treatment.

METHODS

We studied the cerebral haemodynamic functional response in 13 anaesthetised preterm lambs, using near infrared spectroscopy to measure changes in cerebral oxy- and deoxyhaemoglobin (ΔoxyHb, ΔdeoxyHb) following left median nerve stimulation using stimulus trains of 1.8, 4.8 and 7.8 s. The 4.8 and 7.8 s stimulations were repeated during dopamine or dobutamine infusion.

RESULTS

Stimulation always produced a somatosensory evoked response. Majority of preterm lambs demonstrated positive functional responses (i.e. increased ΔoxyHb) in the contralateral cortex following stimulus trains of all durations. Dopamine increased baseline oxyHb and total Hb, whereas dobutamine increased baseline deoxyHb. Both dopamine and dobutamine reduced the evoked ΔoxyHb responses to 4.8 and 7.8 s stimulations.

CONCLUSIONS

Somatosensory stimulation increases cerebral oxygenation in the preterm brain, consistent with increased cerebral blood flow due to neurovascular coupling. Notably, our results show that dopamine/dobutamine reduces oxygen delivery relative to consumption in the preterm brain during somatosensory stimulations, suggesting there may be a risk of intermittent localised tissue hypoxia which has clear implications for clinical practice and warrants further investigation.

Knowledge Gaps and Emerging Research Areas in Intrauterine Growth Restriction-Associated Brain Injury

Intrauterine growth restriction (IUGR) is a complex global healthcare issue. Concerted research and clinical efforts have improved our knowledge of the neurodevelopmental sequelae of IUGR which has raised the profile of this complex problem. Nevertheless, there is still a lack of therapies to prevent the substantial rates of fetal demise or the constellation of permanent neurological deficits that arise from IUGR. The purpose of this article is to highlight the clinical and translational gaps in our knowledge that hamper our collective efforts to improve the neurological sequelae of IUGR. Also, we draw attention to cutting-edge tools and techniques that can provide novel insights into this disorder, and technologies that offer the potential for better drug design and delivery. We cover topics including: how we can improve our use of crib-side monitoring options, what we still need to know about inflammation in IUGR, the necessity for more human post-mortem studies, lessons from improved integrated histology-imaging analyses regarding the cell-specific nature of magnetic resonance imaging (MRI) signals, options to improve risk stratification with genomic analysis, and treatments mediated by nanoparticle delivery which are designed to modify specific cell functions.