Vascular Reactivity to Hypercapnia Is Impaired in the Cerebral and Retinal Vasculature in the Acute Phase After Experimental Subarachnoid Hemorrhage

Abstract

Objective: Impaired cerebral blood flow (CBF) regulation, such as reduced reactivity to hypercapnia, contributes to the pathophysiology after aneurysmal subarachnoid hemorrhage (SAH), but temporal dynamics in the acute phase are unknown. Featuring comparable molecular regulation mechanisms, the retinal vessels participate in chronic and subacute stroke- and SAH-associated vessel alterations in patients and can be studied non-invasively. This study is aimed to characterize the temporal course of the cerebral and retinal vascular reactivity to hypercapnia in the acute phase after experimental SAH and compare the potential degree of impairment.Methods: Subarachnoid hemorrhage was induced by injecting 0.5 ml of heparinized autologous blood into the cisterna magna of male Wistar rats using two anesthesia protocols [isoflurane/fentanyl n = 25 (Sham + SAH): Iso—Group, ketamine/xylazine n = 32 (Sham + SAH): K/X—Group]. CBF (laser speckle contrast analysis) and physiological parameters were measured continuously for 6 h. At six predefined time points, hypercapnia was induced by hypoventilation controlled via blood gas analysis, and retinal vessel diameter (RVD) was determined non-invasively.Results: Cerebral reactivity and retinal reactivity in Sham groups were stable with only a slight attenuation after 2 h in RVD of the K/X—Group. In the SAH Iso—Group, cerebral and retinal CO2 reactivity compared to baseline was immediately impaired starting at 30 min after SAH (CBF p = 0.0090, RVD p = 0.0135) and lasting up to 4 h (p = 0.0136, resp. p = 0.0263). Similarly, in the K/X—Group, cerebral CO2 reactivity was disturbed early after SAH (30 min, p = 0.003) albeit showing a recovery to baseline after 2 h while retinal CO2 reactivity was impaired over the whole observation period (360 min, p = 0.0001) in the K/X—Group. After normalization to baseline, both vascular beds showed a parallel behavior regarding the temporal course and extent of impairment.Conclusion: This study provides a detailed temporal analysis of impaired cerebral vascular CO2 reactivity starting immediately after SAH and lasting up to 6 h. Importantly, the retinal vessels participate in these acute changes underscoring the promising role of the retina as a potential non-invasive screening tool after SAH. Further studies will be required to determine the correlation with functional outcomes.

Opening a window to the acutely injured brain: Simultaneous retinal and cerebral vascular monitoring in rats

“…The measurements of the vessel diameter of the retinal vessels did not give as homogeneous results as those of the cerebral vessels. The detailed analysis provided in our recent publication (see Figure 2 of the Supplementary material in Warner et al, 2022) showed stable reactivity over time with only selecting vessels with good reactivity at the beginning of the measurement phase. Considering all vessel segments at baseline, a higher variability is evident.…”

“…In addition, two anesthetic protocols and the narcotics' effects on the cerebral and retinal vessels are described. The demonstrative examples used in this article are taken from the analysis performed in the context of our previous, already published study (Warner et al, 2022). Statistical analyses between and within anesthetic groups can be found in that study.…”

“…The moving average was set to 10, and an exposure time of 10,000 µs was used. -Place the animal under the STIS while remaining in the stereotactic frame to constantly measure and record CBF free of moving artifacts (Conzen et al, 2019;Warner et al, 2022). -Note: Avoid artificial movements, e.g., for controlling the ventilation tubing or accidental bumping on the stereotactic frame as these interfere with the measurement.…”

“…For illustration, two typical examples from both groups of anesthesia are given in Figure 5. A detailed analysis of the CBF responses to repetitive hypercapnic stimulation events is provided in our recent publication (see Figure 2 of the Supplementary material in Warner et al, 2022), demonstrating stable reactivity over time with a tendency toward smaller responses in K/X compared to Iso.…”

“…Recently, our group observed the first clinical evidence of retinal vasculature changes after aneurysmal subarachnoid hemorrhage (SAH) (Albanna et al, 2016(Albanna et al, , 2021Conzen et al, 2018). Finally, we have presented another link between cerebral and retinal vasculatures in an acute neurovascular experimental set-up: The vascular reactivity to hypercapnia was simultaneously impaired in both vascular beds, retinal and cerebral, in the acute phase after experimental SAH in rats (Warner et al, 2022). Our results showed that minutes after SAH, retinal vessels participated in changes of the cerebral vessels, suggesting that the retinal vessels were directly affected by the same impact and therefore may serve as easily accessible proxies for acute cerebral vascular changes.…”