Heart and Brain Blood Flow in Obstructive Sleep Apnea

Background:

Obstructive sleep apnea (OSA) is characterized by recurrent upper airway obstruction during sleep, accompanied by continued inspiratory effort, intermittent hypoxemia, and sleep fragmentation. OSA is associated with cardiovascular and cerebrovascular disease, but the physiological mechanisms linking obstructive respiratory events to altered cerebral circulation remain incompletely understood.

During obstructive apnea, inspiratory efforts against an occluded airway generate substantial negative intrathoracic pressure. These pressure changes may affect cardiac filling, arterial blood flow, and cerebral venous drainage. In combination with changes in oxygenation, carbon dioxide, and autonomic activity, obstructive respiratory efforts may disturb the balance between cerebral arterial inflow and venous outflow, with potential consequences for intracranial pressure and cerebrospinal fluid dynamics.

Conventional assessments of OSA primarily describe respiratory event frequency and oxygen desaturation. These measurements do not directly characterize the accompanying changes in cerebral and central hemodynamics. Phase-contrast magnetic resonance imaging (MRI), including four-dimensional flow MRI and targeted two-dimensional acquisitions, enables noninvasive assessment of blood flow in the thoracic, cervical, and intracranial circulations.

Continuous positive airway pressure (CPAP) stabilizes the upper airway and reduces obstructive respiratory events. Comparing hemodynamics without and with CPAP may therefore help clarify the acute contribution of obstructive breathing to altered cerebral arterial inflow and venous drainage.

Study hypothesis:

Obstructive respiratory efforts produce transient changes in central and cerebral hemodynamics, including impaired cerebral venous drainage and altered arterial inflow. These responses differ between participants with severe OSA and controls without OSA and are attenuated during CPAP.

Furthermore, OSA severity and hemodynamic disturbances may be associated with structural and diffusion-based MRI markers related to perivascular fluid dynamics.

Objectives:

1. To characterize changes in arterial and venous blood flow during obstructive respiratory events in participants with severe OSA.
2. To compare resting cerebral and central hemodynamics between participants with severe OSA and controls without OSA.
3. To investigate arterial and venous flow responses to standardized Müller and Valsalva maneuvers and compare these responses between groups.
4. To assess the acute effects of CPAP on cerebral arterial inflow and venous outflow in participants with OSA.
5. To explore associations between OSA severity, hemodynamic measurements, and complementary structural, diffusion-based, and spectroscopic MRI measures.

Study design and examinations:

This is a prospective, single-center physiological study comprising 50 adults aged 18-65 years:

25 participants with severe OSA, defined by an apnea-hypopnea index (AHI) of at least 30 events per hour 25 controls without OSA, defined by an AHI below 5 events per hour.

Following screening, participants attend one MRI visit at Rigshospitalet, Copenhagen, Denmark. Imaging is performed using a 3-tesla MRI scanner. The examination combines structural brain imaging, diffusion imaging, magnetic resonance spectroscopy, four-dimensional flow MRI, and targeted two-dimensional phase-contrast MRI.

Flow measurements target the ascending aorta and superior vena cava in the thorax; the bilateral internal carotid arteries, vertebral arteries, and internal jugular veins in the neck; and the superior sagittal sinus intracranially. Total cerebral blood flow is calculated as the sum of flow through the bilateral internal carotid and vertebral arteries.

In the OSA group, additional measurements are obtained during sleep to capture spontaneous obstructive respiratory events. Selected acquisitions are repeated during CPAP to allow within-participant comparisons. Physiological monitoring is used to relate MRI measurements to respiratory effort, oxygen desaturation, and recovery following respiratory events.

Endpoints:

Primary endpoint:

Changes in volumetric blood flow rate, Q(t), and peak velocity across the phases of obstructive respiratory events in the ascending aorta, superior vena cava, internal carotid arteries, vertebral arteries, internal jugular veins, and superior sagittal sinus. Event-related analyses will distinguish obstructive effort, desaturation, and arousal or recovery phases where supported by the physiological recordings.

Secondary endpoints:

1. Hemodynamic responses to respiratory maneuvers: Changes in volumetric blood flow rate and peak velocity during Müller and Valsalva maneuvers, including comparisons between the OSA and control groups.
2. Resting total cerebral blood flow: Total cerebral blood flow measured using four-dimensional flow MRI, including comparisons between groups and within-participant changes during CPAP in the OSA group.
3. Resting arterial and venous flow:

Mean flow in the intracranial, cervical, and thoracic vessels examined, including comparisons between groups and within-participant changes during CPAP in the OSA group.

Additional exploratory assessments:

1. Enlarged perivascular spaces: Structural MRI will be used to characterize enlarged perivascular spaces, including overall burden and regional measures in the basal ganglia and centrum semiovale.
2. Perivascular water diffusion: Diffusion imaging will be used to calculate the diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) index, an indirect research marker of directional water diffusion in periventricular regions.
3. Cerebral lactate:

Magnetic resonance spectroscopy will be used to explore cerebral lactate signals at baseline and during apnea-related acquisitions.

4 BrainAGE T1-weighted structural MRI will be used to estimate brain age and calculate the difference between predicted brain age and chronological age (BrainAGE), an exploratory imaging marker of structural brain ageing.

1. Associations with OSA severity: Hemodynamic and complementary MRI measures will be explored in relation to respiratory event frequency and measures of nocturnal oxygen desaturation.