Developmental changes in cerebral autoregulatory capacity in the fetal sheep parietal cortex

Abstract

We validated laser Doppler flowmetry (LDF) for long-term monitoring and detection of acute changes of local cerebral blood flow (lCBF) in chronically instrumented fetal sheep. Using LDF, we estimated developmental changes of cerebral autoregulation. Single fibre laser probes (0.4 mm in diameter) were implanted in and surface probes were placed on the parietal cerebral cortex at 105 +/- 2 (n = 7) and 120 +/- 2 days gestational age (dGA, n = 7). Basal lCBF was monitored over 5 days followed by a hypercapnic challenge (fetal arterial partial pressure of CO(2), P(a,CO2): 83 +/- 3 mmHg) during which lCBF changes obtained by LDF were compared to those obtained with coloured microspheres (CMSs). Mean arterial blood pressure (MABP) was increased and decreased using phenylephrine and sodium nitroprusside at 110 +/- 2 and 128 +/- 2 dGA. Intracortical and cortical surface laser probes gave stable measurements over 5 days. The lCBF increase during hypercapnia obtained by LDF correlated well with flows obtained using CMS (r = 0.89, P < 0.01). The signals of intracortical and surface laser probes also correlated well (r = 0.91, P < 0.01). Gliosis of 0.35 +/- 0.06 mm around the tip of intracortical probes did not affect the measurements. The range of MABP over which cerebral autoregulation was observed increased from 20-48 mmHg at 110 dGA to 35 to > 95 mmHg at 128 dGA (P < 0.05). Since MABP increased from 33 to 54 mmHg over this period (P < 0.01), the range between the lower limit of cerebral autoregulation and the MABP increased from 13 mmHg at 110 dGA to 19 mmHg at 128 dGA (P < 0.01). LDF is a reliable tool to assess dynamic changes in cerebral perfusion continuously in fetal sheep.

Figure 1. Representative tracing of a MABP (A) decrease below the lower limit of cerebral autoregulation and of corresponding lCBF changes measured by a cortical surface (B) and an intracortical (C) laser probe before artefact rejection in a fetal sheep at 110 dGA

SNP: infusion of sodium nitroprusside. The beginning of the lCBF decrease corresponding to the lower limit of cerebral autoregulation is clearly visible in spite of the spike-like artefacts. 100 % corresponds to baseline lCBF.

Figure 2. Long-term monitoring of baseline lCBF in the right parietal cortex of fetal sheep using laser Doppler flowmetry

A, cortical surface laser probes in fetuses at 110 dGA (n = 6). B, intracortical laser probes in fetuses at 110 dGA (n = 5). C, intracortical laser probes in fetuses at 128 dGA (n = 7). Data are means ±s.e.m. Note the small s.e.m. indicating the small variation of the LDF measurement. 100 % corresponds to baseline lCBF.

Figure 3. Example of the brain injury induced by an intracortical single fibre laser probe of 400 μm diameter (scale bar = 0.5 mm)

A: canal of the intracortical laser probe, arrow: surrounding gliosis.

Figure 4. Time course of lCBF changes during maternal hypercapnia (black bar) measured by cortical surface (○, n = 6) and intracortical (•, n = 5) laser probes in fetal sheep at 110 dGA

Data are means ±s.e.m. Both probes correlate with r = 0.91 (P < 0.01). Arrows indicate CBF measurement using coloured microspheres before (CMS1) and during hypercapnia (CMS2). 100 % corresponds to baseline lCBF.

Figure 5. Correlation of lCBF obtained by laser Doppler flowmetry (LDF) to P_a,CO2 (A) and to lCBF obtained by coloured microspheres (CMS, B) during a hypercapnic challenge

Measurements of the intracortical laser probes at both gestational ages are shown. Each point refers to one animal. 100 % corresponds to baseline. A: n = 13, r = 0.92; B: n = 9, r = 0.89, P < 0.05.

Figure 6. Time course of fetal MABP (line) and lCBF changes (circles) during sodium nitroprusside (SNP) and phenylephrine (PE) infusions (black bars) measured by an intracortical single fibre probe in a fetal sheep at 110 dGA (A) and at 128 dGA (B). C shows in higher time resolution the stepwise decrease and increase of MABP in the same animal as in A

Note the lower and upper limit of cerebral autoregulation (arrows). The upper limit of cerebral autoregulation was not reached at 128 dGA (B). 100 % corresponds to baseline lCBF.

Figure 7. Baseline mean arterial blood pressures (MABP, black bar), lower and upper limits of cerebral autoregulation in fetal sheep at 110 and 128 dGA

Data are means ±s.e.m., n = 7 in each group, *P < 0.05, **P < 0.01 compared to 110 dGA.

Figure 8. Developmental changes of the limits of cerebral autoregulation in the cortex of fetal sheep between 110 (A) and 128 dGA (B)

The range of MABP over which cerebral autoregulation was observed is indicated by the white (110 dGA) and black (128 dGA) bars. 100 % corresponds to baseline lCBF. Data are means with s.e.m. indicated unless it is smaller than the symbols, n = 7 in each group.

Figure 9. MABP (1) and lCBF (2) obtained by intracortical laser probes during infusion of sodium nitroprusside (black bar) directly into the fetal carotid artery at 128 dGA

Data are means ±s.e.m., n = 3. Note that neither the MABP nor the lCBF were affected by the infusion. 100 % corresponds to baseline lCBF.