Cerebral perfusion during sleep-disordered breathing

Pathogenesis of cognitive dysfunction in patients with obstructive sleep apnea: a hypothesis with emphasis on the nucleus tractus solitarius

OSA is characterized by the quintessential triad of intermittent apnea, hypoxia, and hypoxemia due to pharyngeal collapse. This paper highlights the upstream mechanisms that may trigger cognitive decline in OSA. Three interrelated steps underpin cognitive dysfunction in OSA patients. First, several risk factors upregulate peripheral inflammation; these crucial factors promote neuroinflammation, cerebrovascular endothelial dysfunction, and oxidative stress in OSA. Secondly, the neuroinflammation exerts negative impact globally on the CNS, and thirdly, important foci in the neocortex and brainstem are rendered inflamed and dysfunctional. A strong link is known to exist between neuroinflammation and neurodegeneration. A unique perspective delineated here underscores the importance of dysfunctional brainstem nuclei in etiopathogenesis of cognitive decline in OSA patients. Nucleus tractus solitarius (NTS) is the central integration hub for afferents from upper airway (somatosensory/gustatory), respiratory, gastrointestinal, cardiovascular (baroreceptor and chemoreceptor) and other systems. The NTS has an essential role in sympathetic and parasympathetic systems also; it projects to most key brain regions and modulates numerous physiological functions. Inflamed and dysfunctional NTS and other key brainstem nuclei may play a pivotal role in triggering memory and cognitive dysfunction in OSA. Attenuation of upstream factors and amelioration of the NTS dysfunction remain important challenges.

Sympathetic withdrawal augments cerebral blood flow during acute hypercapnia in sleeping lambs

STUDY OBJECTIVES

Cerebral sympathetic activity constricts cerebral vessels and limits increases in cerebral blood flow (CBF), particularly in conditions such as hypercapnia which powerfully dilate cerebral vessels. As hypercapnia is common in sleep, especially in sleep disordered breathing, we tested the hypothesis that sympathetic innervation to the cerebral circulation attenuates the CBF increase that accompanies increases in PaCO2 in sleep, particularly in REM sleep when CBF is high.

DESIGN

Newborn lambs (n = 5) were instrumented to record CBF, arterial pressure (AP) intracranial pressure (ICP), and sleep-wake state (quiet wakefulness (QW), NREM, and REM sleep). Cerebral vascular resistance was calculated as CVR = [AP-ICP]/CBF. Lambs were subjected to 60-sec tests of hypercapnia (FICO2 = 0.08) during spontaneous sleep-wake states before (intact) and after sympathectomy (bilateral superior cervical ganglionectomy).

RESULTS

During hypercapnia in intact animals, CBF increased and CVR decreased in all sleep-wake states, with the greatest changes occurring in REM (CBF 39.3% +/- 6.1%, CVR -26.9% +/- 3.6%, P < 0.05). After sympathectomy, CBF increases (26.5% +/- 3.6%) and CVR decreases (-21.8% +/- 2.1%) during REM were less (P < 0.05). However the maximal CBF (27.8 +/- 4.2 mL/min) and minimum CVR (1.8 +/- 0.3 mm Hg/ min/mL) reached during hypercapnia were similar to intact values.

CONCLUSION

Hypercapnia increases CBF in sleep and wakefulness, with the increase being greatest in REM. Sympathectomy increases baseline CBF, but decreases the response to hypercapnia. These findings suggest that cerebral sympathetic nerve activity is normally withdrawn during hypercapnia in REM sleep, augmenting the CBF response.

Reduced cerebral blood flow during wakefulness in obstructive sleep apnea-hypopnea syndrome

STUDY OBJECTIVES

To investigate changes in regional cerebral blood flow (rCBF) in patients with obstructive sleep apnea-hypopnea syndrome (OSAHS).

DESIGN

We compared the 99mTc-ethylcysteinate dimer (ECD) single photon emission computed tomography (SPECT) images of patients with OSAHS with those of age- and sex-matched healthy volunteers.

SETTING

University hospital.

PATIENTS AND PARTICIPANTS

Twenty-seven patients with severe OSAHS and 27 healthy volunteers underwent 99mTc-ECD brain SPECT studies.

INTERVENTION

For statistical parametric mapping analysis, all SPECT images were spatially normalized to the standard SPECT template and then smoothed using a 14-mm full-width at half-maximum Gaussian kernel. The Student t test was used for the statistical analysis.

MEASUREMENTS AND RESULTS

The mean age of patients and subjects was 44.3 years (range 31-58). All patients underwent overnight polysomnography. The mean apnea-hypopnea index of patients was 60.4 +/- 17.6 per hour (range 33 -104), indicating severe OSAHS. All patients snored heavily and had daytime sleepiness (mean Epworth Sleepiness Scale score, 10.7 +/- 3.7, range 6-12). Statistical parametric mapping analysis showed that rCBF in patients with OSAHS was significantly reduced in bilateral parahippocampal gyri and in the right lingual gyrus, as compared with that of healthy volunteers (P < 0.05 with false discovery rate correction). Moreover, apnea-hypopnea indexes of patients were negatively correlated with rCBF in the right pericentral gyrus and right cuneus at uncorrected P < 0.001.

CONCLUSIONS

Our results show the altered rCBF pattern in bilateral parahippocampal gyri, right lingual gyrus, pericentral gyrus, and cuneus in patients with severe OSAHS. These findings may partly explain the deficit in memory, spatial learning, executive function, and attention, which are frequently found in patients with OSAHS.

Cerebral blood flow response to isocapnic hypoxia during slow-wave sleep and wakefulness

Nocturnal hypoxia is a major pathological factor associated with cardiorespiratory disease. During wakefulness, a decrease in arterial O2 tension results in a decrease in cerebral vascular tone and a consequent increase in cerebral blood flow; however, the cerebral vascular response to hypoxia during sleep is unknown. In the present study, we determined the cerebral vascular reactivity to isocapnic hypoxia during wakefulness and during stage 3/4 non-rapid eye movement (NREM) sleep. In 13 healthy individuals, left middle cerebral artery velocity (MCAV) was measured with the use of transcranial Doppler ultrasound as an index of cerebral blood flow. During wakefulness, in response to isocapnic hypoxia (arterial O2 saturation −10%), the mean (±SE) MCAV increased by 12.9 ± 2.2% ( P < 0.001); during NREM sleep, isocapnic hypoxia was associated with a −7.4 ± 1.6% reduction in MCAV ( P < 0.001). Mean arterial blood pressure was unaffected by isocapnic hypoxia ( P > 0.05); R-R interval decreased similarly in response to isocapnic hypoxia during wakefulness (−21.9 ± 10.4%; P < 0.001) and sleep (−20.5 ± 8.5%; P < 0.001). The failure of the cerebral vasculature to react to hypoxia during sleep suggests a major state-dependent vulnerability associated with the control of the cerebral circulation and may contribute to the pathophysiologies of stroke and sleep apnea.