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Obstructive sleep apnea (OSA) is associated with extensive neurologic comorbidities. It is hypothesized that the repeated nocturnal apneas experienced in patients with OSA may inhibit the normal apneic response, resulting in hypoxic brain injury and subsequent neurologic dysfunction. In this study, we applied the recently developed OxFlow MRI method for rapid quantification of cerebral metabolic rate of oxygen (CMRO2) during a volitional apnea paradigm. MRI data were analyzed in 11 OSA subjects and 10 controls (mean±SD apnea-hypopnea index (AHI): 43.9±18.1 vs. 2.9±1.6 events/hour, P<0.0001; age: 53.8±8.2 vs. 45.3±8.5 years, P=0.027; BMI: 36.6±4.4 vs. 31.9±2.2kg/m2, P=0.0064). Although total cerebral blood flow and arteriovenous oxygen difference were not significantly different between apneics and controls (P>0.05), apneics displayed reduced baseline CMRO2 (117.4±37.5 vs. 151.6±29.4µmol/100g/min, P=0.013). In response to apnea, CMRO2 decreased more in apneics than controls (−10.9±8.8 % vs. −4.0±6.7 %, P=0.036). In contrast, group differences in flow-based cerebrovascular reactivity were not significant. Results should be interpreted with caution given the small sample size, and future studies with larger independent samples should examine the observed associations, including potential independent effects of age or BMI. Overall, these data suggest that dysregulation of the apneic response may be a mechanism for OSA-associated neuropathology.
Obstructive sleep apnea (OSA) is defined by structural and functional failure of the upper airway to maintain patency during sleep, resulting in periodic cessations or reductions in breathing and subsequent arterial desaturations. One in five adults in the Western world is believed to have at least mild OSA,1 a figure that is rapidly increasing as obesity, the main risk factor for OSA, becomes more prevalent. In addition to the typical symptoms of daytime sleepiness, snoring, and disturbed sleep, OSA is also associated with significant systemic comorbidities, including hypertension, myocardial infarction and congestive heart failure, stroke, and type 2 diabetes.2 Of particular relevance to this study, patients with OSA have a high prevalence of central nervous system dysfunction, including depression, dementia, and diminished cognitive performance.3 Magnetic resonance imaging (MRI) studies of OSA patients have detected neurologic lesions suggestive of hypoxic damage, including focal loss of grey matter4 and white matter.5
The etiology of OSA-associated neurologic comorbidities is not well understood. Brain tissue is particularly sensitive to hypoxic damage and rapid reperfusion,6 and brain regions known to be more acutely affected by hypoxia, such as the hippocampus, are among those identified as having grey matter loss in OSA.4 In normal physiology, apnea-induced hypercapnia and hypoxia cause chemoreceptor-mediated central vasodilation and concurrent peripheral vasoconstriction, preferentially conserving oxygen delivery to the brain to prevent hypoxic brain injury.7 In fact, recent work by our group has demonstrated that the cerebral metabolic rate of oxygen (CMRO2) is not just maintained, but slightly increased in young healthy subjects in response to 30-s volitional apnea.8 This may represent a mechanism for increasing energy stores in anticipation of prolonged apnea, and is consistent with gas-mixture breathing studies demonstrating increased CMRO2 in response to steady-state hypoxia.9 However, it is possible that patients with OSA do not possess a normal apneic response, allowing hypoxic damage to occur during OSA-associated nocturnal apneas.
In support of this hypothesis are studies associating OSA with blunted cerebrovascular reactivity (CVR),10–12 typically defined as the cerebral blood flow (CBF) change in response to a vasoactive stimulus, such as hypercapnia11 or apnea,10,12 the latter of which is particularly pertinent to OSA pathophysiology. In one study, CVR assessed in response to breath-hold by Doppler ultrasound was found to be significantly lower in OSA subjects, and more so in the morning, indicating that their diminished vasodilatory response is worsened by more recent exposure to nocturnal apneas.10 Recently, blood-oxygen-level dependent (BOLD) functional MRI (fMRI) detected reduced CVR in select brain regions of apneics, including the hippocampus.12 Blunted cerebrovascular responses to autonomic challenges13,14 (e.g., orthostatic hypotension, cold pressor challenge, etc.) suggest a mechanism of brain injury even during wakefulness, whereby day-to-day activities (e.g., standing) may not be met with an adequate cerebrovascular response to preserve central oxygen delivery. Finally, studies in both animals15 and healthy humans16 exposed to chronic intermittent hypoxia (CIH) paradigms indicate a causal link between exposure to cyclic hypoxia and impaired vascular reactivity. If initial hypoxic injury itself leads to further blunting of the apneic response, a negative feedback cycle of worsening hypoxic damage could ensue.
Though supporting a mechanism for hypoxic brain injury in OSA, these previous studies all measure surrogate markers of brain oxygen metabolism (i.e., blood flow, perfusion, or BOLD fMRI signal), reductions of which do not necessarily correlate with decreased oxygen delivery and consumption. Of more central interest is whether oxygen consumption itself is maintained. Direct quantification of CMRO2 requires quantification of both cerebral blood flow and oxygen extraction, the latter posing the greater technical challenge. CMRO2 is less variable than blood flow or oxygen extraction in healthy subjects at baseline,17,18 and relatively conserved in response to physiologic challenges such as hypercapnia19,20 and hypoxia,9 suggesting that CMRO2 may be a more significant index for assessing neurovascular dysfunction than either blood flow or oxygenation alone. CMRO2 reduction has been associated with many of the most common neurologic disorders, including Alzheimer’s disease,21 Parkinson’s disease,22 and multiple sclerosis.23
Quantifying CMRO2 in response to apnea requires temporal resolution on the order of several seconds. Although this temporal resolution can be achieved with BOLD fMRI, attempts to ‘calibrate’ the BOLD signal (i.e., convert fractional BOLD signal change to fractional change in CMRO2) rely on models with many physiologic assumptions and complex experimental setups involving gas-mixture breathing.24 Moreover, such calibration techniques still provide only relative changes in CMRO2, with additional calibration needed to quantify baseline values in absolute physiologic units.
Recently, we have introduced a MRI technique for rapid whole brain CMRO2 quantification based on simultaneous susceptometry-based oximetry (SBO) and phase-contrast (PC)-MRI blood flow quantification – termed OxFlow.18 Subsequent iterations of the technique have employed view-sharing to achieve temporal resolution as low as 3s for whole brain CMRO2 quantification.8,25 Unlike all previous CMRO2 measurement techniques, this method has sufficient temporal resolution to detect CMRO2 changes in response to apnea, allowing direct evaluation of the relationship between apnea, cerebrovascular reactivity, and brain oxygen delivery and consumption.
In this study, OxFlow was applied to compare the CMRO2 response to apnea in OSA subjects and healthy controls. We hypothesized that OSA would be associated with reduced baseline CMRO2, as well as a blunted CMRO2 response to volitional apnea, and that this blunting would correlate with disease severity as measured by the apnea hypopnea index (AHI).
Susceptometry-based oximetry (SBO) exploits the relative paramagnetism of hemoglobin in the deoxygenated state, which results in a linear relationship between venous blood oxygen saturation (Yv) and venous blood magnetic susceptibility relative to surrounding tissue (Δχ):
where Hct is the hematocrit and Δχdo and Δχoxy are the volume susceptibility differences between fully oxygenated and deoxygenated packed red blood cells and between fully oxygenated packed red blood cells and water, respectively. Values of 4π×0.273 and 4π×0.008p.p.m. (SI units) are used for Δχdo and Δχoxy, based on ex vivo calibration experiments.26 The susceptibility offset (Δχ) induces a field shift (ΔB), which causes an increase in MR signal phase (Δ) between blood and surrounding ‘reference’ tissue as a function of echo spacing (ΔTE) in a multi-echo gradient echo imaging sequence
where γ is the proton gyromagnetic ratio.
Solving for Yv thus hinges on determining Δχ from the measured ΔB, an inversion problem that is mathematically ill-posed in the general case. However, by modeling the blood vessel of interest as a long straight cylinder27 with defined tilt angle (θ) relative to the main magnetic field (B0), an expression relating the susceptibility and field offsets can be derived analytically
To quantify global Yv, this long paramagnetic cylinder model is applied to the superior sagittal sinus (SSS), the largest cerebral vein, which drains about 50% of total cerebral outflow. Yv in the SSS is nearly identical to global cerebral venous oxygenation measured in the internal jugular veins as shown by T2-based oximetry methods.17 However, while trachea-induced susceptibility artifacts complicate SBO in the jugular veins, the field adjacent to the SSS is relatively homogeneous, making it the ideal candidate for global Yv quantification via SBO.
SBO and PC-MRI can be readily combined as a single gradient echo sequence by applying flow encoding before a multi-echo gradient-recalled echo readout, achieving simultaneous quantification of blood oxygenation and flow (Figure 1(a) and ((cc)).8,18 In this study, OxFlow was implemented with BRISK Cartesian view-sharing,28 with one-quarter k-space acquired at each time point and a resulting temporal resolution of 2s for each simultaneously acquired pair of field and velocity maps. BRISK is more robust against subject motion compared to previous implementations of OxFlow using Keyhole view-sharing.29,30 Other OxFlow pulse sequence parameters used in this study were: reconstructed matrix=208×208, resolution=0.85×0.85×5.00mm, TR/TE1/ΔTE=19.23/5.73/7.04ms, bandwidth=321Hz/pixel, and VENC=50cm/s.
SSS blood flow (SSSBF) can be converted to total CBF (tCBF) via multiplication by a calibration factor determined through a separate two-slice interleaved PC-MRI acquisition toggled between the internal carotid and vertebral arteries in the neck (which comprise tCBF) and the SSS in the head (Figure 1(b) and ((cc)).8 This calibration step can be run before subsequent OxFlow experiments, allowing tCBF to be quantified as
CMRO2 can then be quantified via the Fick principle
where Ca is the hematocrit-dependent arterial oxygen content of blood in µmol O2/100mL and Ya is the arterial oxygen saturation in percent hemoglobin oxygen saturation (%HbO2), which can be measured by digital pulse oximetry. The tCBF:SSSBF calibration pulse sequence parameters used in this study were: reconstructed matrix=208×208, resolution=0.85×0.85×5.00mm, TR/TE=12.02/5.73ms, bandwidth=321Hz/pixel, VENC=50cm/s (head slice)/80cm/s (neck slice), temporal resolution=10s, and averages=4.
Subjects were recruited based on results of a clinically indicated sleep study (in-lab attended polysomnography) performed at the University of Pennsylvania Sleep Center. The AHI was calculated as the mean number of apnea and hypopnea events per hour of sleep. Obstructive apneas were defined as at least a 90% drop in the thermal sensor excursion of baseline lasting at least 10s; hypopneas were defined as a 50% reduction in airflow for greater than 10s and associated with greater than 3% decrement in oxyhemoglobin saturation and/or an arousal. Nasal pressure monitors were used in all subjects to measure airflow.
Thirteen newly diagnosed apneics (AHI>15 events/h) and 10 non-apneic controls (AHI<10 events/hour) were selected after screening for standard MRI exclusion criteria (claustrophobia, metal implants, pregnancy, etc.) and excluding diseases expected to affect cerebral metabolism and/or cerebrovascular reactivity, including congestive heart failure, chronic obstructive pulmonary disease, stroke, head trauma, and other significant neurological diseases. Cigarette smokers or users of other nicotine products were excluded as smoking can affect vasodilation. Subjects had no prior history of OSA diagnosis or continuous positive airway pressure (CPAP) use. Subject demographics are displayed in Table 1.
All imaging protocols were approved by the Institutional Review Board of the University of Pennsylvania according to the ethical standards of the Belmont Report, and subjects provided written informed consent. Prior to scanning, a capillary blood sample was obtained and analyzed using an Hb 201+ (HemoCue, Brea, CA, USA) portable hemoglobin measurement device for determination of Hct in equation (1) and Ca in equation (5).
The apnea paradigm consisted of 30-s breath-holds at end-expiratory volume to mimic nocturnal apneas experienced in OSA. Coaching was used to maximize intra- and inter-subject repeatability and consistency of the apneas. Prior to scanning, breathing at normal end-expiratory volume was explained and demonstrated. During all breath holds, subjects were verbally instructed to “breathe in”, “breathe out”, and “stop breathing” at 6, 3, and 0s, respectively, before the designated start of each apnea period, and instructed to “breathe normally” at the end of the apnea period. Each subject performed two practice apneas in the MRI scanner prior to OxFlow scanning and three during OxFlow scanning. Verbal instructions were given via MRI-compatible headphones. Breath-hold compliance was monitored by respiratory bellows.
To minimize biological confounds and normal variations that might occur during the diurnal cycle, all subjects were scanned in the afternoon and instructed to abstain from caffeine (which promotes vasoconstriction) on the day of the study. All MR-imaging studies were performed on a 1.5T wide-bore (70cm) Siemens Espree system (Siemens Medical Solutions, Erlangen, Germany) using vendor-supplied 12-channel head and 2-channel neck receive coils. Subjects were fitted with pulse oximetry (Expression, Invivo Research Inc., Orlando, FL, USA) and respiratory bellows before performing the first practice breath-hold. A vendor-provided axial localizer scan was run for subsequent slice planning, followed by a second practice breath-hold. To allow tCBF normalization to brain mass, a 1-mm isotropic, 3D T1-weighted MPRAGE data set was acquired. Next, the tCBF:SSSBF calibration scan was run, followed by second-order shimming over the brain volume. Finally, the OxFlow sequence was run continuously for 9 min, during which the subjects completed three coached 30-s apneas, each followed by 2min of normal breathing recovery. The entire MRI protocol lasted approximately 20min (Figure 2).
All subjects were able to successfully complete each breath-hold. However, two subjects (both apneics) failed to remain awake and experienced obstructive apneas during the recovery portions of the OxFlow acquisition, resulting in periodic desaturations throughout the paradigm. Their data were excluded from further analysis.
All image reconstruction was performed with in-house-written MATLAB (Mathworks, Natick, MA) scripts. BRISK-sampled raw OxFlow data were first reordered to create full k-space data sets corresponding to each echo at 2-s temporal resolution. Velocity maps were obtained from the phase difference between flow-encoded and flow-compensated images reconstructed from data acquired at TE1. Field maps were generated from the phase difference between flow-compensated images reconstructed from data acquired at TE1 and TE2. Magnitude images at each time point were used to motion-correct the time series velocity and field maps using the StackReg plugin for ImageJ.31
OxFlow-derived SSSBF was determined by integration of the velocity map over an ROI fully containing the SSS. Data from the two-slice interleaved calibration sequence was processed analogously – with tCBFcal quantified by integration over the internal carotid and vertebral arteries – to calculate the tCBFcal/SSSBFcal calibration factor to upscale OxFlow-derived SSSBF and determine tCBF in equation (4). Total brain volume was determined from the T1-MPRAGE data using the BET tool in FSL,32 and converted to mass based on an average brain density of 1.05g/mL.33
For Yv quantification, bulk susceptibility effects were removed from the field maps via second-order polynomial fitting of the field in brain tissue surrounding the SSS. Average phase was measured in two ROIs, one entirely within the SSS and another in a small reference region of brain tissue immediately surrounding the SSS approximately one vessel radius in width and located one vessel radius anterior to the SSS border. The difference in phase between these regions provides Δ in equation (2).
Ya values obtained via pulse oximetry were recorded at 2-s intervals matching each OxFlow time point. To correct for the temporal delay between central and peripheral blood arrival from the lungs, the pulse oximetry data was time-shifted for each subject such that the initial resaturation following apnea occurred 7s after apnea cessation. This timing corresponds to the known circulatory transport delay between the lungs and brain34 to within the temporal resolution of the pulse sequence. The arteriovenous oxygen difference (AVO2D) was quantified as Ya−Yv. Combination of equations (1) to (5) was used to determine temporally resolved CMRO2.
For each subject, time-course data were averaged over the three repeated blocks of the paradigm to improve signal-to-noise (SNR) and remove physiologic variation unrelated to apnea. For all parameters, average baseline values were quantified over the 24s (12 data points) immediately preceding the “breathe in” command. For parameters that change monotonically in response to apnea, maximum (tCBF, Yv) or minimum (Ya, AVO2D) percent changes relative to the average baseline values were quantified. To characterize the CMRO2 apneic response, data were averaged over the second half (final 14s, seven data points) of the apnea period to generate average end-apnea parameter values. The second half of the apnea period was used to eliminate residual breathing effects and because physiologic changes from apnea are not expected to occur instantaneously. The CMRO2 apneic response was quantified as the percent change from the average baseline to the average end-apnea period.
Continuous outcomes were summarized using means and standard deviations (SDs) and categorical outcomes using frequencies and percentages. Given the relatively small number of apneics (N=11) and controls (N=10) in this study, summary measures were compared between groups using Wilcoxon two-sample exact tests (for continuous variables) and Fisher’s exact tests (for categorical variables). Baseline CMRO2 and the CMRO2 apneic response values were correlated with AHI using Spearman’s rank correlations. Statistical significance was defined as P<0.05. Throughout the manuscript, parameter values are reported as mean [SD] and all P-values are two sided.
Given the limited overlap in age and BMI between OSA subjects and controls, it was not possible to model the independent effect of these variables on CMRO2 or the CMRO2 apneic response within the entire sample. Instead, the potential effect of between-group differences in these variables was assessed in a secondary analysis within a small sample of apneics and controls (N=four pairs), matched for age (within 2.5 years) and BMI (within 2.5kg/m2). Differences in traits of interest were calculated within each pair, as the value in the apneic subject minus that in the control. Observed differences were tested for significance using an exact P-value from the non-parametric signed rank test on the difference. The calculated differences in the matched sample were compared to that in the overall population. If a similar magnitude was observed, it was concluded that results in the overall population were unlikely to be primarily driven by differences in age and BMI. As a further step toward understanding potential confounding effects, age and BMI were associated with CMRO2 and the CMRO2 apneic response using Spearman’s rank correlations. These tests were performed across all subjects (N=21), apneics only (N=11), and controls only (N=10).
AHI was significantly higher (43.9 [18.1] vs. 2.9 [1.6] events/hour, P<0.0001) and Ya nadir lower (77.5 [8.5] vs. 89.0 [3.7] %HbO2, P=0.0001) in apneics relative to controls. Subjects in the OSA group (AHI>15 events/hour) were of greater age (53.8 [8.2] vs. 45.3 [8.5] years, P=0.027) and BMI (36.6 [4.4] vs. 31.9 [2.2] kg/m2, P=0.0064), and had slightly though non-significantly larger brain mass (1437  vs. 1376  g, P=0.39).
Time-course plots of the MR- and pulse oximetry-measured parameters in a single OSA subject (male, 63 years old) (Figure 3a) demonstrate the expected increase in Yv and tCBF and decrease in Ya in response to apnea (red shading), as previously observed in young healthy subjects.30 The resulting CMRO2 time-course in the same OSA subject (Figure 3b) shows a CMRO2 reduction from baseline (denoted ‘Base’) to end-apnea (denoted ‘EA’) of 12.1%.
Group time-course plots of measured parameters (Figure 4a) demonstrate a lower tCBF and Ya and higher Yv in apneics versus controls, as well as a considerably lower CMRO2 in apneics (Figure 4b) throughout the paradigm. Although tCBF, Ya, and Yv baseline-averaged parameters values were not statistically different between groups (Table 2), they synergistically resulted in a significantly lower CMRO2 in apneics versus controls (117.4 [37.5] vs. 151.6 [29.4] µmol/100g/min, P=0.013). The initial rise and fall in tCBF and Yv observed at the beginning of the apneic period is attributable to breath-hold-induced intrathoracic pressure changes causing modulations in cerebral venous return, as previously observed.8
To better illustrate the apneic response, measured parameters (Figure 4c) and CMRO2 (Figure 4d) are displayed in terms of percent changes relative to average baseline values. CVR – the change in tCBF in response to apnea – was not different between groups. However, there was a trend toward a larger decrease in oxygen extraction (AVO2D) in apneics (−40.9 [14.0] vs. 32.0 [6.4]%, P=0.099). CMRO2 decreased significantly in apneics (−10.9 [8.8]%, P=0.0049) but not in controls (−4.0 [6.7] %, P=0.13), with a significant group difference (P=0.036). Ya reduction in response to apnea was greater in apneics (−6.8 [0.5] vs. −6.1 [1.4] %, P=0.036), although it should be noted that the magnitude of this difference was quite small, and thus had little impact on the observed differences in the CMRO2 apneic response.
To examine the sensitivity of CMRO2 to OSA disease severity, AHI was correlated with both baseline CMRO2 (Figure 5a) and the CMRO2 apneic response (Figure 5b). When including all subjects (N=21), AHI correlated significantly with baseline CMRO2 (Spearman’s ρ=−0.65, P=0.0014) and the CMRO2 apneic response (Spearman’s ρ=−0.53, P=0.013). When restricted to apneics only, AHI correlation with baseline CMRO2 was only marginally significant (Spearman’s ρ=−0.61, P=0.047), and AHI correlation with the CMRO2 apneic response (Pearson’s ρ=−0.47, P=0.14) was only a trend.
Examination of a sub-sample of age- and BMI-matched subjects provided insight into possible confounding effects of the slight mismatch in these group characteristics. As expected given matching, pairs were similar with respect to age (mean [SD] difference: −0.25 [1.26] years, P>0.99) and BMI (1.2 [1.5] kg/m2, P=0.375). Within the matched sample, apneics had a baseline CMRO2 45.9µmol/100g/min lower on average compared to matched controls (P=0.125) and a CMRO2 apneic response 10.4% lower (P=0.375). These differences are greater than those observed between apneics and controls in the overall sample, supporting the effect size seen in that population. This suggests that although statistical significance was not achieved in this small matched sample, associations in the overall sample were not completely driven by imbalances in age and BMI.
Though not reaching significance, the data were suggestive of a negative correlation between CMRO2 and BMI (ρ=−0.40, P=0.070) across the entire sample. In contrast, correlation between CMRO2 and BMI in apneics only (ρ=−0.38, P=0.25) or controls only (ρ=0.25, P=0.49) was not significant, and correlations between BMI and the CMRO2 apneic response were not significant in any group. Across all subjects, correlations between age and both baseline CMRO2 and CMRO2 apneic response were small and non-significant. Age correlated significantly only with the CMRO2 apneic response in control subjects (ρ=0.73, P=0.016), and approached significance when correlated with baseline CMRO2 in control subjects (ρ=0.54, P=0.105).
While a range of technologies have been used to study the pathophysiology of OSA, to the best of our knowledge, this is the first study to directly measure CMRO2 and its change in response to apnea in OSA subjects. We highlight two main findings: (1) baseline CMRO2 is lower in OSA subjects relative to controls and (2) there is a larger CMRO2 decrease in response to apnea in OSA subjects. Given the small sample size of the study, and that confounding by age and/or BMI cannot entirely be excluded given that groups were not fully matched, these preliminary findings should be interpreted with caution and replicated in larger samples. However, the results are consistent with growing evidence that blunted autoregulatory mechanisms in OSA may contribute to OSA-associated neuropathology,10–14,16 and suggest a potential role for CMRO2 in studying these mechanisms.
The observed reduction in baseline CMRO2 in OSA subjects is a consequence of both oxygen delivery (tCBF) and oxygen extraction (AVO2D) reduction. Although OSA subjects had lower values in both tCBF and AVO2D on average, results did not reach statistical significance. As mentioned, these negative results must be interpreted with caution given the limited sample size. Nevertheless, they suggest that CMRO2 may provide a more sensitive marker of baseline metabolic dysfunction than blood flow or oxygen extraction alone. Recently, a similar study of OSA subjects and controls also found no group differences in baseline blood flow and no change in baseline blood flow in apneics treated with CPAP.12 We emphasize that our observed CMRO2 reduction in OSA subjects cannot be explained by brain atrophy, as the CMRO2 is normalized to brain volume, and furthermore, brain mass was not significantly different between groups.
In additional to lower baseline CMRO2, OSA subjects had a significantly larger decrease in CMRO2 in response to apnea compared to controls. In contrast, the increase in flow in response to apnea (CVR) was nearly identical between groups, with the reduced CMRO2 apneic response largely driven by a greater reduction in AVO2D in apneics. Just as CMRO2 has been proposed as a better measure of baseline neuronal function than blood flow, our results suggest that the CMRO2 response to vasoactive challenges may provide a more sensitive marker of regulatory dysfunction than flow-based CVR. While some studies have associated OSA with reduced CVR,10–12 others, including the present study, have not.13,35 More so than CVR reduction alone, inability to maintain CMRO2 during apnea provides a potential mechanism to explain the development of hypoxic brain damage in OSA.
Although not reaching statistical significance, the control group in this study also displayed a negative CMRO2 response to apnea. This contrasts with previous data in 10 young, non-obese healthy subjects with no major underlying medical conditions, where a small but significant (6.0 [3.5] %, P=0.0004) increase in CMRO2 was observed during apnea using the same imaging protocol.30 One likely explanation for the different CMRO2 apneic response in these two control groups is the differences in their clinical characteristics. Controls in the present study were recruited among older, relatively obese subjects from a Sleep Clinic. These subjects likely have a higher prevalence of obesity, high blood pressure, and metabolic syndrome compared to the previously studied young healthy cohort.
AHI correlated significantly with baseline CMRO2 across all subjects as well as in apneics only, demonstrating that baseline CMRO2 may be sensitive not just to apnea status but also apnea disease severity. In contrast, AHI correlation with the CMRO2 apneic response was significant among all subjects, but only a trend when restricted to apneics. Thus, studies with more subjects are needed to determine the relative sensitivity of baseline vs. apneic response CMRO2 to OSA disease state.
Our study has several limitations. First, it was performed in a relatively small sample of apneics and controls. A larger sample size would increase power to detect group differences, some of which were marginally significant. Thus, negative results (P>0.05) should be interpreted with caution. However, our study did observe significant differences in our primary outcomes of interest, and, in general, power was approximately 75% to observe a mean difference between apneics and controls of 1.25 standard deviations.
In this study, OSA subjects were significantly older and more obese than controls. While these differences reflect differences between OSA and non-OSA patients in the general population, the small sample and limited covariate overlap restricted our ability to control for potential confounders which could affect CMRO2 – such as age and BMI – in statistical models.
While some recent work suggests that baseline CMRO2 may increase slightly with age,36 such a trend would have biased our results towards the null. In our data, there was a lack of association between CMRO2 measures and age in the overall sample, further suggesting that age did not confound the relationship between OSA and CMRO2 measures in our study. While a significant positive correlation was observed between age and the CMRO2 apneic response in controls only (ρ=0.73, P=0.016), it is unclear why this effect would exist only in controls, and, therefore, this result should be interpreted with caution.
The relationship between BMI and CMRO2 has not been specifically investigated previously, and presents an added complication in our study, as obesity may affect the subjects’ ability to hold their breath. For instance, obesity lowers functional residual capacity, which could potentially cause more significant hypoxia or hypercapnia to develop during volitional apnea, impacting the apneic response. While we observed a negative correlation between CMRO2 and BMI across the entire sample (ρ=−0.40, P=0.070), this does not necessarily demonstrate confounding by BMI, as such a trend would in fact be expected if CMRO2 were independently associated with OSA, given the BMI mismatch between apneics and controls. Furthermore, the fact that the same trend was not observed among control subjects only (ρ=0.25, P=0.49) suggests that in the absence of OSA pathology, CMRO2 is not independently associated with lower BMI, thus arguing against BMI effects driving our observed group differences. Finally, the observed correlation between BMI and CMRO2 was only 62% as strong as the correlation between AHI and CMRO2 (ρ=−0.65, P=0.0014). Thus, it is possible that the observed CMRO2 correlation with BMI is due to an independent CMRO2 association with AHI, rather than BMI. Future, larger studies including subjects and controls with a similar range of BMIs could better assess whether BMI independently affects CMRO2, and whether any such effects exist across the general population or are restricted to apneics alone.
CMRO2 group differences observed in the overall study sample were similar to those in the subset of age and BMI matched subjects, again arguing in favor of OSA independently lowering baseline CMRO2 and the CMRO2 apneic response. However, it will be essential for larger independent studies to confirm the observed associations. Furthermore, future studies should examine the possible effects of additional OSA-associated co-morbidities, including type 2 diabetes.
Two subjects (both apneics) were excluded from data analysis due to inability to stay awake between breath-holds. Compliance could be improved in future studies via modification of the breath-hold protocol to include more frequent, shorter apneas, or by introducing visual cuing of the breath-holds, which, in addition to sustaining subject attention, has been shown to improve breath-hold reproducibility.37 However, even with perfect subject compliance, volitional apnea may not elicit the same neurovascular response as true sleep-associated nocturnal apneas. To address this, our methodology could be applied during sleep to capture true apneic events in OSA subjects. The feasibility of high temporal resolution MR imaging during sleep has been demonstrated in previous studies examining airway closure in OSA.38
A limitation of the OxFlow technique is that it is confined to quantification of global blood flow, oxygenation, and CMRO2. Though apnea, like hypercapnia and hypoxia, can be thought of as a global cerebrovascular challenge, OSA has been associated with focal brain lesions,4,5 and a recent study employing BOLD fMRI detected regional differences in CVR in OSA subjects.12 Unfortunately, CMRO2 mapping techniques are still at a developmental stage, with temporal resolutions on the order of many minutes and image noise levels often requiring whole-brain averaging to achieve physiologically plausible parameter values. While mapping of BOLD fMRI signal is possible at high temporal resolution, it does not provide a direct measure of either brain oxygen metabolism or blood flow, but rather reflects a complex interplay between blood flow, tissue properties, and CMRO2. A combination of both quantitative global measures, such as OxFlow, and qualitative but spatially resolved measures such as BOLD fMRI, could offer an ideal approach for future studies of vascular and metabolic dysfunction in OSA.
Interpreting changes in baseline and apneic tCBF and CMRO2 is challenging, as such alterations could be viewed as either a cause or effect of underlying neuropathology. In a study of baseline CMRO2 and hypercapnic CVR in subjects with mild cognitive impairment (MCI),39 reduced CMRO2 in the setting of maintained CVR was interpreted as suggesting less demand for oxygen due to primary neural dysfunction, as opposed to failure to meet demand due to vascular dysfunction. In our study, reduced baseline CMRO2 and maintained CVR were also observed, though the CMRO2 apneic response was reduced, suggesting that there may be a component of supply-side deficiency in OSA not accounted for by CVR. Longitudinal monitoring of CMRO2 in OSA could help to discriminate between these various interpretations, for instance, by determining whether changes in baseline CMRO2 and CMRO2 apneic response occur concurrently or serially in OSA disease progression, and the extent to which CPAP therapy and resulting neurocognitive improvements are reflected by CMRO2 metrics. The simplicity, speed, and robustness of the OxFlow technique make it well suited for such applications.
In summary, our results suggest that baseline CMRO2 is reduced in OSA subjects relative to controls, and, furthermore, that OSA subjects may fail to maintain normal CMRO2 during apnea. These findings add to the growing evidence that OSA-associated neuropathology is a consequence of autoregulatory dysfunction. MRI-based quantification of CMRO2 may offer a new method for better understanding the mechanisms of neurologic impairment in patients with sleep apnea.
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by NIH T32-EB000814, R21-HD069390, R01-HL109545, and R01-HL122754, and by the Institute of Translational Medicine and Therapeutics of the University of Pennsylvania (grant number UL1RR024134 from the National Center for Research Resources).
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
ZBR designed the study and study methods, performed the study experiments, analyzed the data, and wrote the manuscript; SEL designed the study and study methods, performed the study experiments, and analyzed the data; BTK analyzed the data and edited and reviewed the manuscript; LGK designed the study methods; RJS and FWW designed the study and study methods and edited and reviewed the manuscript.