Relationship of cerebral blood volume with arterial and venous flow velocities in extremely low-birth-weight infants

Unstable cerebral blood flow is theorised to contribute to the occurrence of intraventricular haemorrhage (IVH) in extremely low-birth-weight infants (ELBWIs), which can be caused by increased arterial flow, increased venous pressure, and impaired autoregulation of brain vasculature. As a preliminary step to investigate such instability, we aimed to check for correlations of cerebral blood volume (CBV), as measured using near-infrared spectroscopy, with the flow velocities of the anterior cerebral artery (ACA) and internal cerebral vein (ICV), as measured using Doppler ultrasonography. Data were retrospectively analysed from 30 ELBWIs uncomplicated by symptomatic patent ductus arteriosus, which can influence ACA velocity, and severe IVH (grade ≥ 3), which can influence ICV velocity and CBV. The correlation between tissue oxygen saturation (StO2) and mean blood pressure was also analysed as an index of autoregulation. CBV was not associated with ACA velocity; however, it was significantly correlated with ICV velocity (Pearson R = 0.59 [95% confidence interval: 0.29–0.78], P = 0.00061). No correlation between StO2 and mean blood pressure was observed, implying that autoregulation was not impaired. Conclusion: Although our findings are based on the premise that cerebral autoregulation was unimpaired in the ELBWIs without complications, the same result cannot be directly applied to severe IVH cases. However, our results may aid future research on IVH prediction by investigating the changes in CBV when severe IVH occurs during ICV velocity fluctuation. What is Known: • The pathogenesis of IVH includes unstable cerebral blood flow affected by increased arterial flow, increased venous pressure, and impaired cerebral autoregulation. • The approaches that can predict IVH are under discussion. What is New: • ACA velocity is not associated with CBV, but ICV velocity is significantly correlated with CBV. • CBV measured using NIRS may be useful in future research on IVH prediction. What is Known: • The pathogenesis of IVH includes unstable cerebral blood flow affected by increased arterial flow, increased venous pressure, and impaired cerebral autoregulation. • The approaches that can predict IVH are under discussion. What is New: • ACA velocity is not associated with CBV, but ICV velocity is significantly correlated with CBV. • CBV measured using NIRS may be useful in future research on IVH prediction.


tHb
Total haemoglobin TRS Time-resolved spectroscopy

Introduction
Survival rates among extremely low-birth-weight infants (ELBWIs) have improved with advances in neonatal medicine. Nonetheless, such progress is yet to overcome one serious complication associated with poor neurological outcomes, which remains a major problem: intraventricular haemorrhage (IVH) [1][2][3]. Generally, IVH occurs in the capillary network of the germinal matrix, a vascular bounding zone between cerebral arterial and cerebral venous beds. Blood entering this region via arteries ultimately empties into the internal cerebral veins. When its flow becomes congested or impeded, the resulting increase in intravenous pressure can lead to a matrix rupture. The fundamental aetiology of IVH lies in the weakness of the immature germinal matrix: it is theorised to result from cerebral blood flow (CBF) instability caused by increased arterial flow, increased venous pressure, and/or impaired autoregulation in the cerebral vasculature [4]. Preterm infants must be monitored to prevent IVH, but in the case of ELBWIs, such approaches should be non-invasive because of their immature skin and organs. Doppler ultrasonography has been explored as a useful modality for measuring CBF through arteries and veins. Research on arterial flow has focused on the anterior cerebral artery (ACA) [5,6], while research on venous flow has linked perfusion waveform instability in the internal cerebral vein (ICV) to IVH risk [7]. Despite the utility of ultrasound as a visualisation and screening tool, its inability to monitor continuously poses a major disadvantage. Near-infrared spectroscopy (NIRS) is capable of continuous monitoring, but useful results have not yet been achieved by investigations of this alternative approach [8,9]. This could be because the measurement principle used by conventional NIRS-spatially resolved spectroscopy [10]-is affected by limited quantifiability and reproducibility compared with those of methods based on time-resolved spectroscopy (TRS). The NIRS applied by our clinical team is based on TRS, which is both quantitative and reproducible [11]: in addition to brain tissue oxygen saturation, our NIRS approach could serve as a valuable technique for monitoring cerebral haemodynamics in neonates to prevent IVH, if the extent of correlation between flow velocities of the ACA and ICV and local cerebral blood volume (CBV), i.e., the volume of the blood supplied by the ACA, before draining via the ICV, are confirmed. Some comorbidities could complicate these relationships, i.e., symptomatic patent ductus arteriosus (PDA), which can influence ACA velocity [12], and severe IVH (grade ≥ 3), which can influence ICV velocity and CBV [13]. Thus, as a preliminary step in this direction, we aimed to explore such associations in ELBWIs lacking these complications, with a high likelihood of healthy development.

Patients and methods
This retrospective observational study examined ELBWIs born at 30 weeks or earlier and weighing under 1000 g admitted to the neonatal intensive care unit of Saitama Children's Medical Center between April 2020 and August 2021. This study was approved by the Institutional Research Ethics Board of Saitama Children's Medical Center. Informed consent was obtained from the families of all patients included in the study. Regardless of mechanical ventilation, only neonates in a stable condition not requiring anti-hypotensive drugs were included in the analysis. Cases of congenital heart disease or chromosomal abnormality were excluded, as were cases of symptomatic PDA requiring treatment at the time of testing, severe IVH cases rated grade 3 or higher [13], or unstable cases due to conditions such as pneumothorax and intestinal perforation. Cases were also excluded if their monitoring data were not saved or no consent was obtained from their parents.
The gestational age (weeks), birth weight (g), appropriate for gestational age (AGA) or small for gestational age (SGA) status, sex (M/F), birth multiplicity (singleton/twins/ triplets), age (days), presence of ventilation and severe IVH (grade ≥ 3), corrected gestational age (weeks), and weight (g) at the time of testing of ELBWIs were extracted from their medical records on the day of testing. Haemoglobin levels in venous blood (g/dL) at the time of testing and mean blood pressure (mmHg), as measured by non-invasive blood pressure monitoring, were analysed in tandem.
Ultrasonography was performed using the Philips Affiniti 50 system (Philips Healthcare, Andover, MA, USA). Flows through the left ACA and left ICV were visualised in the sagittal plane using pulsed Doppler imaging via a 12-MHz probe placed over the fontanelle (Fig. 1). Regarding the ACA, peak systolic velocity (PSV; cm/s) and end-diastolic velocity (EDV; cm/s) were measured (Fig. 2a). The resistance index (RI) was automatically calculated from these values using the following equation: RI = (PSV − EDV)/PSV. Regarding the ICV, velocity (cm/s) was measured (Fig. 2b).
Each infant underwent testing at an age in which their condition was judged to be clinically stable based on vital signs and other considerations. On this day, ACA and ICV velocities were measured by cranial ultrasound at the same time as NIRS-related variables by tNIRS-1, three times on the same day by the same operator. For each variable, the mean of these three observations was recorded for statistical analysis.
The correlation coefficient between CBV and ACA or ICV velocity was investigated. The correlation among several factors was also investigated. Particularly, this study focused on StO 2 as an index for changes in regional cerebral oxygenation, as reported previously [16,17]. In addition, we focused on the correlation coefficient between mean blood pressure and StO 2 because r > 0.50 was reported in previous studies as an index of dysfunctional cerebral autoregulation [17][18][19].
All statistical analyses were performed with EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria). More precisely, it is a modified version of R commander designed to add statistical functions frequently used in biostatistics [20]. Data are described as frequency (percentage), mean ± standard deviation for parametric data, or median (range) for nonparametric continuous data. Normally distributed continuous variables were tested using the Shapiro-Wilk normality test. The outliers were detected using the Smirnov-Grubbs test. The correlation coefficient was explored using Pearson product-moment correlation coefficient for normally distributed continuous variables and Spearman rank correlation coefficient for non-normally distributed continuous variables. The relationship between the absolute values of the correlation coefficient and the strength of the correlation was defined as poor correlation for values below 0.4, good correlation for values 0.4-0.7, and strong correlation for values over 0.7.
ICV velocity was measured as 5.01 ± 1.07 cm/s; all such measurements were performed under continuous flow. The following values were obtained using simultaneous NIRS: O 2 Hb, 27.8 ± 4.9 μmol/L; HHb, 17.2± 3.4 μmol/L; tHb,    (Table 3). ICV was found to significantly correlate with CBV: 0.59 [0.29−0.78], P = 0.00061 (Table 3; Fig. 4). Both CBV and ICV velocities were tested for correlations with all patient and ACA-related variables. Regarding ACA-related variables, gestational age, birthweight, corrected gestational age at measurement, and weight at measurement were well correlated with CBV. Venous Hb and tHb concentrations, including the calculating equation of CBV, were well correlated with each factor. Corrected gestational age at measurement was well correlated with PSV. PSV and EDV, including the calculating equation of ACA RI, were well correlated with each factor. In ICV-related variables, birthweight, corrected gestational age at measurement, and weight at measurement were well correlated with CBV. StO 2 was not correlated with CBV or mean blood pressure (Table 4).

Discussion
IVH can be caused by several factors, but its fundamental aetiology lies in the weakness of the immature germinal matrix. It is theorised to result from CBF instability caused by increased arterial flow, increased venous pressure, and/or impaired cerebral autoregulation [4]. As a preliminary step to investigate such instability, this study suggested that CBV was significantly associated with ICV velocity, but not ACA velocity, as cerebral autoregulation was not estimated to be impaired in the ELBWIs uncomplicated by symptomatic PDA and severe IVH (grade ≥ 3).
CBF can be destabilised by dysfunctional cerebral autoregulation. Previous NIRS studies of preterm infants found that impaired autoregulation, which is likely to precede severe IVH, can be indicated by a correlation between CBF and mean arterial pressure [21,22]. However, the differences in NIRS devices and the use of a non-invasive blood pressure-monitoring technique preclude a direct comparison of our results and those of previous studies; nevertheless, we did not observe a correlation of CBV with mean blood pressure in a manner suggestive of impaired autoregulation (Table 3). Our CBV observations were obtained simultaneously with ACA velocity (mean: 2.14 ± 0.33 mL/100 g) and ICV velocity (2.15 ± 0.34 mL/100 g); they do not deviate greatly from the values reported in previous studies of preterm infants (1.7 ± 0.8 mL/100 g [23]) and neonates (2.3 ± 0.6 mL/100 g [15]), respectively, using TRS. The CBV values in this study were estimated to be within normal limits under proper cerebral autoregulation because StO 2 was not correlated with mean blood pressure (Table 3), as reported previously [17][18][19], showing that r > 0.50 is judged as the lack of cerebral autoregulation. Of note, CBV was not correlated with StO 2 (Table 3), estimated as an index for changes in regional  cerebral oxygenation [16,17]. These findings indicate that hyperperfusion does not exist in this condition. Regarding arterial flow, patients with symptomatic arterial complications that could affect ACA velocity were excluded from our investigation of potential associations between ACA velocity and CBV, as measured using NIRS. Fluctuating ACA velocity in the presence of symptomatic PDA is hypothesised to provoke IVH in which pressure spikes cause germinal matrix capillaries to rupture [24]. ACA velocity reduces as the systolic flow pumps out via the opening, decreasing EDV and consequently increasing RI. In fact, RI ≥ 0.8 has been reported as an especially strong predictor of severe IVH [12]. However, we did not find CBV to significantly correlate with any ACA flow-related variable examined (i.e., PSV, EDV, or RI; Table 4a). Before our study, ACA flow velocity had not been tested for associations with CBV, but one group comparing it with cerebral saturation as measured by NIRS did report a significant correlation between cerebral saturation and RI (i.e., of the ACA) [6]. In the tNIRS-1 system that we used, cerebral saturation corresponds to "StO 2, " estimated as an index for changes in regional cerebral oxygenation [16,17]; however, StO 2 did not correlate with either PSV, EDV, or RI in our analysis (Table 4a). These findings suggest that regardless of IVH and/or symptomatic PDA, the correlation between ACA velocity and CBV is less likely to be useful in future research, which is consistent with Perlman and Volpe's [25] report that suggested ACA velocity is not reliable for predicting IVH.
Regarding venous pressure, ICV flow in new-born infants is typically continuous, i.e., non-pulsatile [26]. However, ICV fluctuations have been documented in ELBWIs who later develop IVH [7]. While the mechanism underlying these fluctuations is unclear, the junction of the ICV with the upstream subependymal vein (SEV), which receives venous blood from the germinal matrix before it, may be involved. This junction is highly vulnerable to hemodynamic changes; thus, IVH may be easily provoked by venous congestion in this region [4,27]. Our examination of potential associations between ICV haemodynamics and CBV as measured by NIRS-a topic never investigated in prior research-found CBV to significantly correlate with ICV velocity in ELBWIs in whom ICV flow was confirmed to be continuous (Table 4b; Fig. 4). This result is based on the premise that cerebral autoregulation was not impaired. The correlation observed does not constitute a causal relationship. However, this correlation leads CBV cerebral blood volume, ACA anterior cerebral artery, PSV peak systolic velocity, EDV end-diastolic velocity, RI resistance index, ICV internal cerebral vein, CI confidence interval * Denotes statistical significance in Pearson correlation coefficient ** Denotes statistical significance (P < 0.05) to the possibility that ICV velocity may be continuously estimated by CBV monitored continuously using NIRS, although ICV flow velocity is essentially difficult to monitor continuously via ultrasonography. As the blood supply to the germinal matrix by the ACA exits via the ICV, NIRS-based CBV monitoring has the potential of identifying warning signs of increased venous pressure by providing a proxy for IVH prediction, with higher ICV velocity corresponding to increased CBV perfusion upstream. This study has some limitations. First, we could not eliminate possible interaction effects because birth weight, corrected gestational age at measurement, and weight at measurement were well correlated with CBV but not with ICV. Other limitations are a small sample size, the possibility that the brain tissue measured using tNIRS-1 extended beyond the germinal matrix, and the fact that the presence of a correlation does not prove causation. In addition, the measurement principle of the NIRS device in this study is different from that of NIRS devices in previous studies. Finally, our findings do not necessarily guarantee the premise that the results of the study population, with excluded important variables related to severe IVH such as symptomatic PDA and hypotension requiring the use of inotropic drugs, can be applied to severe IVH cases.   In conclusion, a significant correlation between ICV velocity and CBV was confirmed in this investigation. Given that this finding is based on the condition that cerebral autoregulation was not estimated to be impaired in the ELBWIs uncomplicated by symptomatic PDA and severe IVH (grade ≥ 3), the same result cannot be directly applied to severe IVH cases. However, given that it is difficult to continuously monitor ICV velocity using ultrasonography, our results may aid future research on IVH prediction using NIRS-measured CBV to estimate ICV velocity by investigating CBV changes when severe IVH occurs as ICV velocity fluctuates.