Dynamic onset response of the internal carotid artery to hypercapnia is blunted in children compared to adults

Intracranial blood velocity reactivity to a steady state hypercapnic stimulus has been shown to be similar in children and adults, but the onset response to hypercapnia is slower in the child. Given the vasodilatory effect of hypercapnia on the cerebrovasculature, assessment of vessel diameter and blood flow are vital to fully elucidate whether the temporal hypercapnic response differs in children versus adults. Assessment of internal carotid artery (ICA) vessel diameter (ICAd), blood velocity (ICAv), volumetric blood flow (QICA), and shear rate (ICASR) in response to a 4 min hypercapnic challenge was completed in children (n = 14, 8 girls; 9.8 ± 0.7 y) and adults (n = 17, 7 females; 24.7 ± 1.8 y). The dynamic onset responses of partial pressure of end-tidal CO2 (PETCO2), QICA, ICAv and ICASR to hypercapnia were modelled and mean response time (MRT) computed. Following 4 min of hypercapnia, ICA reactivity and ICAd were comparable between the groups. Despite a similar MRT in PETCO2 in children and adults, children had slower QICA (children 108 ± 60 s vs. adults 66 ± 37 s; P=.023), ICAv (children 120±52 s vs. adults 52±31 s; P=.001) and ICASR (children 90 ± 27 s vs. adults 47 ± 36 s; P=.001) MRTs compared to adults. This is the first study to show slower hypercapnic hyperemic kinetic responses of the ICA in children. The mechanisms determining these differences and the need to consider the duration of hypercapnic exposure when assessing CVR in children should be considered in future studies.ICA), shear rate (ICASR), and diameter (ICAd) to hypercapnia provides information on the responses prior to steady state and potentially allows interrogation of the temporality of ICA vasomotion. 

Therefore, the purpose of this study was to compare child hypercapnic hyperemic ICA responses and their temporal order with adult responses. We hypothesized that (i) in response to 4 min of steady state hypercapnia the magnitude of the increase in ICAv, QICA, and ICASR would not differ between children and adults; and (ii) that 4 min of sustained hypercapnia would result in similar increases in ICAd in adults and children; however, (iii) modelling of the dynamic onset response would result in a slower MRT for ICAd, QICA and ICASR  in children compared to adults and (iv) the MRT for ICASR would precede ICAd in both children and adults.