Venous system pathologies have increasingly been linked to clinically relevant disorders of CSF circulation whereas the exact coupling mechanisms still remain unknown. In this work, flow dynamics of both systems were studied using real-time phase-contrast flow MRI in 16 healthy subjects during normal and forced breathing. Flow evaluations in the aqueduct, at cervical level C3 and lumbar level L3 for both the CSF and venous fluid systems reveal temporal modulations by forced respiration. During normal breathing cardiac-related flow modulations prevailed, while forced breathing shifted the dominant frequency of both CSF and venous flow spectra towards the respiratory component and prompted a correlation between CSF and venous flow in the large vessels. The average of flow magnitude of CSF was increased during forced breathing at all spinal and intracranial positions. Venous flow in the large vessels of the upper body decreased and in the lower body increased during forced breathing. Deep respiration couples interdependent venous and brain fluid flow—most likely mediated by intrathoracic and intraabdominal pressure changes. Further insights into the driving forces of CSF and venous circulation and their correlation will facilitate our understanding how the venous system links to intracranial pressure regulation and of related forms of hydrocephalus.

Background: Deep inspiration acts as a driving force for eliciting an upward flow of CSF into the brain simultaneous to an increase of venous outflow to the heart. These findings suggest two interconnected fluid systems which together play a pivotal role in maintaining constant intracranial pressure. Moreover, venous system pathologies are increasingly connected with various disorders of CSF circulation, although exact coupling mechanisms remain unknown. The purpose of the present study was to explore the role of respiratory forces in linking both fluid systems in the upper and lower body.Methods: Twelve healthy subjects (2 females, age 23-38 years) were studied using real-time phase-contrast flow MRI at 3T. Subjects followed a breathing protocol with 40 s of normal and forced respiration. CSF flow was quantified at the aqueduct and spinal levels C3 and L3. Venous flow was studied in the internal jugular veins and cervical epidural veins and in the inferior vena cava and lumbar epidural veins. Flow values (ml s-1), ROI sizes (mm2) and flow frequency components (Hz) were determined and Wilcoxon signed-rank and paired t-tests employed to calculate p values.Results: Cardiac-related flow components prevailed during normal breathing. Forced respiration shifted the main frequency component for CSF and venous dynamics to 0.2 in line with the breathing protocol. Amplification of fluid flow during forced breathing reached significance at all positions except for lumbar and cervical epidural veins and the internal jugular vein which showed decreased flow rates. Veins of the superior and inferior parts of the body followed an opposite flow behavior.Conclusion: Our results support the notion that deep respiration acts as a coupling mechanism of the interdependent venous and CSF flow. Surpassing a certain threshold of intrathoracic and abdominal volume and pressure, deep breathing may perturb their cardiac-dominated fluid dynamics and prompt a synchronous increase of movements. Insights into the driving forces of CSF and venous circulation extend our understanding how the cerebral venous system may be conjunct to intracranial pressure regulations. It will further facilitate our understanding of the pathophysiology of related forms of hydrocephalus.