Cerebral hemodynamics, blood gases, and electrolytes during breath-holding and the Valsalva maneuver

Portable, high speed blood flow measurements enabled by long wavelength, interferometric diffuse correlation spectroscopy (LW-iDCS)

Diffuse correlation spectroscopy (DCS) is an optical technique that can be used to characterize blood flow in tissue. The measurement of cerebral hemodynamics has arisen as a promising use case for DCS, though traditional implementations of DCS exhibit suboptimal signal-to-noise ratio (SNR) and cerebral sensitivity to make robust measurements of cerebral blood flow in adults. In this work, we present long wavelength, interferometric DCS (LW-iDCS), which combines the use of a longer illumination wavelength (1064 nm), multi-speckle, and interferometric detection, to improve both cerebral sensitivity and SNR. Through direct comparison with long wavelength DCS based on superconducting nanowire single photon detectors, we demonstrate an approximate 5× improvement in SNR over a single channel of LW-DCS in the measured blood flow signals in human subjects. We show equivalence of extracted blood flow between LW-DCS and LW-iDCS, and demonstrate the feasibility of LW-iDCS measured at 100 Hz at a source-detector separation of 3.5 cm. This improvement in performance has the potential to enable robust measurement of cerebral hemodynamics and unlock novel use cases for diffuse correlation spectroscopy.

Open-source FlexNIRS: A low-cost, wireless and wearable cerebral health tracker

Currently, there is great interest in making neuroimaging widely accessible and thus expanding the sampling population for better understanding and preventing diseases. The use of wearable health devices has skyrocketed in recent years, allowing continuous assessment of physiological parameters in patients and research cohorts. While most health wearables monitor the heart, lungs and skeletal muscles, devices targeting the brain are currently lacking. To promote brain health in the general population, we developed a novel, low-cost wireless cerebral oximeter called FlexNIRS. The device has 4 LEDs and 3 photodiode detectors arranged in a symmetric geometry, which allows for a self-calibrated multi-distance method to recover cerebral hemoglobin oxygenation (SO2) at a rate of 100 Hz. The device is powered by a rechargeable battery and uses Bluetooth Low Energy (BLE) for wireless communication. We developed an Android application for portable data collection and real-time analysis and display. Characterization tests in phantoms and human participants show very low noise (noise-equivalent power <70 fW/√Hz) and robustness of SO2 quantification in vivo. The estimated cost is on the order of $50/unit for 1000 units, and our goal is to share the device with the research community following an open-source model. The low cost, ease-of-use, smart-phone readiness, accurate SO2 quantification, real time data quality feedback, and long battery life make prolonged monitoring feasible in low resource settings, including typically medically underserved communities, and enable new community and telehealth applications.

Superconducting nanowire single-photon sensing of cerebral blood flow

Significance: The ability of diffuse correlation spectroscopy (DCS) to measure cerebral blood flow (CBF) in humans is hindered by the low signal-to-noise ratio (SNR) of the method. This limits the high acquisition rates needed to resolve dynamic flow changes and to optimally filter out large pulsatile oscillations and prevents the use of large source-detector separations ( ≥ 3    cm ), which are needed to achieve adequate brain sensitivity in most adult subjects. Aim: To substantially improve SNR, we have built a DCS device that operates at 1064 nm and uses superconducting nanowire single-photon detectors (SNSPD). Approach: We compared the performances of the SNSPD-DCS in humans with respect to a typical DCS system operating at 850 nm and using silicon single-photon avalanche diode detectors. Results: At a 25-mm separation, we detected 13 ± 6 times more photons and achieved an SNR gain of 16 ± 8 on the forehead of 11 subjects using the SNSPD-DCS as compared to typical DCS. At this separation, the SNSPD-DCS is able to detect a clean pulsatile flow signal at 20 Hz in all subjects. With the SNSPD-DCS, we also performed measurements at 35 mm, showing a lower scalp sensitivity of 31 ± 6 % with respect to the 48 ± 8 % scalp sensitivity at 25 mm for both the 850 and 1064 nm systems. Furthermore, we demonstrated blood flow responses to breath holding and hyperventilation tasks. Conclusions: While current commercial SNSPDs are expensive, bulky, and loud, they may allow for more robust measures of non-invasive cerebral perfusion in an intensive care setting.

Brain oxygenation responses to an autonomic challenge: a quantitative fMRI investigation of the Valsalva manoeuvre

In late age, the autonomic nervous system (ANS) has diminished ability to maintain physiological homeostasis in the brain in response to challenges such as to systemic blood pressure changes caused by standing. We devised an fMRI experiment aiming to map the cerebral effects of an ANS challenge (Valsalva manoeuvre (VM)). We used dual-echo fMRI to measure the effective transverse relaxation rate (R2*, which is inversely proportional to brain tissue oxygenation levels) in 45 elderly subjects (median age 80 years old, total range 75-89) during performance of the VM. In addition, we collected fluid-attenuated inversion recovery (FLAIR) data from which we quantified white matter hyperintensity (WMH) volumes. We conducted voxelwise analysis of the dynamic changes in R2* during the VM to determine the distribution of oxygenation changes due to the autonomic stressor. In white matter, we observed significant decreases in oxygenation levels. These effects were predominantly located in posterior white matter and to a lesser degree in the right anterior brain, both concentrated around the border zones (watersheds) between cerebral perfusion territories. These areas are known to be particularly vulnerable to hypoxia and are prone to formation of white matter hyperintensities. Although we observed overlap between localisation of WMH and triggered deoxygenation on the group level, we did not find significant association between these independent variables using subjectwise statistics. This could suggest other than recurrent transient hypoxia mechanisms causing/contributing to the formation of WMH.

Cerebral hemodynamics during graded Valsalva maneuvers

The Valsalva maneuver (VM) produces large and abrupt changes in mean arterial pressure (MAP) that challenge cerebral blood flow and oxygenation. We examined the effect of VM intensity on middle cerebral artery blood velocity (MCAv) and cortical oxygenation responses during (phases I–III) and following (phase IV) a VM. Healthy participants (n = 20 mean ± SD: 27 ± 7 years) completed 30 and 90% of their maximal VM mouth pressure for 10 s (order randomized) whilst standing. Beat-to-beat MCAv, cerebral oxygenation (NIRS) and MAP across the different phases of the VM are reported as the difference from standing baseline. There were significant interaction (phase ^* intensity) effects for MCAv, total oxygenation index (TOI) and MAP (all P < 0.01). MCAv decreased during phases II and III (P < 0.01), with the greatest decrease during phase III (−5 ± 8 and −19 ± 15 cm·s⁻¹ for 30 and 90% VM, respectively). This pattern was also evident in TOI (phase III: −1 ± 1 and −5 ± 4%, both P < 0.05). Phase IV increased MCAv (22 ± 15 and 34 ± 23 cm·s⁻¹), MAP (15 ± 14 and 24 ± 17 mm Hg) and TOI (5 ± 6 and 7 ± 5%) relative to baseline (all P < 0.05). Cerebral autoregulation, indexed, as the %MCAv/%MAP ratio, showed a phase effect only (P < 0.001), with the least regulation during phase IV (2.4 ± 3.0 and 3.2 ± 2.9). These data illustrate that an intense VM profoundly affects cerebral hemodynamics, with a reactive hyperemia occurring during phase IV following modest ischemia during phases II and III.

Blood flow MRI of the human retina/choroid during rest and isometric exercise

PURPOSE

To investigate blood flow (BF) in the human retina/choroid during rest and handgrip isometric exercise using magnetic resonance imaging (MRI).

METHODS

Four healthy volunteers (25-36 years old) in multiple sessions (1-3) on different days. MRI studies were performed on a 3-Tesla scanner using a custom-made surface coil (7×5cm in diameter) at the spatial resolution of 0.5×0.8×6.0 mm. BF was measured using the pseudo-continuous arterial-spin-labeling technique with background suppression and turbo-spin-echo acquisition. During MRI, subjects rested for 1 minute followed by 1 minute of handgrip, repeating three times, while maintaining stable eye fixation on a target with cued eye blinks at the end of each data acquisition (every 4.6 seconds).

RESULTS

Robust BF of the unanesthetized human retina/choroid was detected. Basal BF in the posterior retina/choroid was 149±48 mL/100 mL/min with a mean heart rate of 60±5 beats per minute, mean arterial pressure of 78±5 mm Hg, ocular perfusion pressure of 67±4 mm Hg at rest (mean±SD, n=4 subjects). Handgrip significantly increased retina/choroid BF by 25%±7%, heart rate by 19%±8%, mean arterial pressure by 22%±5% (measured at the middle of the handgrip task), and ocular perfusion pressure by 25%±6% (averaged across the entire handgrip task) (P<0.01), but did not change intraocular pressure, arterial oxygen saturation, end-tidal CO2, and respiration rate (P>0.05).

CONCLUSIONS

This study demonstrates a novel MRI application to image quantitative BF of the human retina/choroid during rest and isometric exercise. Retina/choroid BF increases during brief handgrip exercise, paralleling increases in mean arterial pressure. Handgrip exercise changes ocular perfusion pressure free of potential drug side effect and can be done in the MRI scanner. MRI offers quantitative BF with large field of view without depth limitation, potentially providing insights into retinal pathophysiology.

Chronic smoking and the BOLD response to a visual activation task and a breath hold task in patients with schizophrenia and healthy controls

Many psychiatric patient groups smoke heavily, but little is known regarding the effects of this habit on functional brain imaging results. The present report assesses the effect of chronic smoking on the blood oxygen level-dependent (BOLD) response to a simple visual activation (VA) task and a breath hold (BH) task in patients with schizophrenia. Eight healthy controls and twelve patients with schizophrenia were studied. Half of each group had never smoked and the other half of each group had smoked for more than 10 pack years. Responses to the VA task were assessed in the visual cortex and responses to the BH task were assessed in gray matter generally. There were four fMRI-dependent measures: (1) median percent signal change; (2) activation volume (in voxels); (3) time-to-peak of the impulse response function (IRF); and (4) time-to-trough of the IRF. All measures were tested as dependent variables in an ANCOVA with diagnosis and smoking status as crossed factors and age as a covariate. Heavy smokers had 22% larger percent signal change for the VA task and 50% larger percent signal change for the BH task. Patients had a 40% larger percent signal change for the breath hold task. Other statistically significant effects of smoking history on activation volume and the timing of the brain responses were noted. If replicated, the results may have important implications for fMRI studies comparing groups with markedly different smoking habits, such as studies comparing patients with schizophrenia, 60-90% of whom smoke, and healthy controls, who smoke with a much lower frequency.

Multimodal investigation of fMRI and fNIRS derived breath hold BOLD signals with an expanded balloon model

Multimodal investigation of blood oxygenation level-dependent (BOLD) signals, using both functional near-infrared spectroscopy (fNIRS) and functional magnetic resonance imaging (fMRI), may give further insight to the underlying physiological principles and the detailed transient dynamics of the vascular response. Utilizing a breath hold task (BHT), we measured deoxy-hemoglobin (HbR) and oxy-hemoglobin (HbO) changes via fNIRS and blood oxygen level dependent (BOLD) changes by fMRI. Measurements were taken in four volunteers asynchronously and carefully aligned for comparative analysis. In order to describe the main stimulus in BHT, partial pressure of carbon dioxide (PaCO(2)) parameter was integrated into the balloon model as the driving function of cerebral blood flow (CBF) which led to the development of an expanded balloon model (EBM). During BHT, the increase in HbR was observed later than the BOLD peak and coincided temporally with its post-stimulus undershoot. Further investigation of these transients with a PaCO(2) integrated balloon model suggests that post-stimulus undershoot measured by fMRI is dominated by slow return of cerebral blood volume (CBV). This was confirmed by fNIRS measurements. In addition, the BOLD signal decreased with the increase of the initial level of PaCO(2) derived from EBM, indicating an effect of basal CBF level on the BOLD signal. In conclusion, a multimodal approach with an appropriate biophysical model gave a comprehensive description of the hemodynamic response during BHT.

MR susceptometry for measuring global brain oxygen extraction

Monitoring of oxygen saturation in jugular venous blood gives an estimate of the balance of global oxygen delivery and cerebral oxygen consumption. We present a noninvasive approach to measure oxygen saturation in vivo in the internal jugular vein using MR susceptometry by exploiting the characteristic susceptibility of deoxyhemoglobin, and demonstrate the feasibility of performing such measurements in a group of subjects. We assessed the sensitivity of the method for detecting small changes in oxygen saturation by monitoring the variations observed during breath-holding and hypoventilation experiments. Unlike alternative methods, the susceptometric technique does not require calibration.

Cluster headache: orbital hemodynamic changes during Valsalva maneuver

BACKGROUND

The clinical features of cluster headache (CH) disclose some vascular changes in the symptomatic region, but few instrumental studies have assessed orbital hemodynamics in patients with this disorder.

METHODS

Orbital blood flow reactivity elicited by Valsalva maneuver (VM) was studied with ophthalmic artery eco-Doppler in 16 patients (14 men and 2 women; mean age: 41.2) suffering from episodic CH and in 18 healthy controls. Patients were examined twice: first, in a cluster period (between pain attacks), and second, in a remission period. Each time peak-systolic and end-diastolic flow velocities were recorded in both ophthalmic arteries at rest and during all phases of VM.

RESULTS

Valsalva phase IV was consistently associated with an increment of blood flow velocities through the ophthalmic arteries. Unlike controls, patients showed an asymmetric vascular reactivity. In the cluster period peak-systolic flow velocity increments were lower on the symptomatic side than on the asymptomatic side (14.1% vs. 34.4%; P < .001), while in remission end-diastolic flow velocity increments were higher in the previously symptomatic orbit (129% vs. 72.9%; P < .05). Vascular reactivity on the asymptomatic side was always similar to that of healthy controls.

CONCLUSIONS

In episodic CH, the symptomatic orbit shows an abnormal vascular reactivity. During the cluster period, basal vasodilation and hyperemia could preclude it from admitting a much greater amount of blood at the end of Valsalva. During remission, there might be some latent vascular changes that lead to supersensitive vasodilator responses and/or opening of arteriovenous shunts under certain circumstances such as Valsalva. These phenomena could be relevant in the pathophysiology of CH.

Middle cerebral artery blood velocity during intense static exercise is dominated by a Valsalva maneuver

Lifting of a heavy weight may lead to "blackout" and occasionally also to cerebral hemorrhage, indicating pronounced consequences for the blood flow through the brain. We hypothesized that especially strenuous respiratory straining (a Valsalva-like maneuver) associated with intense static exercise would lead to a precipitous rise in mean arterial and central venous pressures and, in turn, influence the middle cerebral artery blood velocity (MCA V(mean)) as a noninvasive indicator of changes in cerebral blood flow. In 10 healthy subjects, MCA V(mean) was evaluated in response to maximal static two-legged exercise performed either with a concomitantly performed Valsalva maneuver or with continued ventilation and also during a Valsalva maneuver without associated exercise (n = 6). During static two-legged exercise, the largest rise for mean arterial pressure and MCA V(mean) was established at the onset of exercise performed with a Valsalva-like maneuver (by 42 +/- 5 mmHg and 31 +/- 3% vs. 22 +/- 6 mmHg and 25 +/- 6% with continued ventilation; P < 0.05). Profound reductions in MCA V(mean) were observed both after exercise with continued ventilation (-29 +/- 4% together with a reduction in the arterial CO(2) tension by -5 +/- 1 Torr) and during the maintained Valsalva maneuver (-21 +/- 3% together with an elevation in central venous pressure to 40 +/- 7 mmHg). Responses to performance of the Valsalva maneuver with and without exercise were similar, reflecting the deterministic importance of the Valsalva maneuver for the central and cerebral hemodynamic response to intense static exercise. Continued ventilation during intense static exercise may limit the initial rise in arterial pressure and may in turn reduce the risk of hemorrhage. On the other hand, blackout during and after intense static exercise may reflect a reduction in cerebral blood flow due to expiratory straining and/or hyperventilation.

Differential fMRI-BOLD signal response to apnea in humans and anesthetized rats

Blood oxygenation level dependent (BOLD) signal intensity (SI) and regional cerebral blood flow (CBF) during a 20-s apnea stimulus in awake humans and pentobarbital-anesthetized rats were measured to assess the usefulness of apnea in estimating cerebral vasodilatory capacity for functional MRI (fMRI) experiments. Rats were ventilated with either room air or 100% O(2.) While breathing room air, apnea for 20 s increased the BOLD SI in humans but decreased it in rats. However, in rats ventilated with 100% O(2), BOLD SI increased upon apnea for 20 s. CBF measurements in rats using laser Doppler flowmetry (LDF) showed a 45% +/- 8% increase during apnea with room air ventilation, and a 10% +/- 3% increase with 100% O(2). Arterial blood oxygen saturation fell from 96% +/- 1% to 29% +/- 5%, and cerebral tissue PO(2) decreased from 15 +/- 3 mmHg to 6 +/- 2 mmHg by the end of 20-s apnea in rats breathing room air. However, with 100% O(2) respiration, apnea produced no change in the arterial blood oxygen saturation, which remained at 99%, but increased tissue PO(2) from 35 +/- 9 mmHg to 39 +/- 10 mmHg. From the results obtained in rats ventilated with room air, it is concluded that apnea induces hypoxia that results in a decrease in fMRI-BOLD signal. The signal decrease occurred despite an increase in P(a)CO(2) and CBF. This BOLD response is the opposite of that observed in humans, who presumably do not develop hypoxia within the applied apnea period. These studies highlight the importance of the choice of ventilating gas mixture on the outcome of BOLD experiments during systemic perturbations.

Chronological analysis of physiological T2* signal change in the cerebrum during breath holding

The purpose of this study was to examine which physiological factors affect cerebral T2* signal intensity (SI) during breath holding (BH) (apnea after inspiration and breathing after expiration) in normal volunteers. We examined SI changes caused by anoxic gas inhalation, by respiratory movements, and by BH. High-speed echo planar images (EPI) showed changes in SI that could be divided into five phases. Reports indicate that SI changes induced by BH are due to the effects on the magnetic susceptibility of deoxygenated hemoglobin (deoxyhemoglobin (dHb)) and to hypercapnia, but these reports could not fully explain the observed five phases. In addition to deoxyhemoglobin susceptibility and hypercapnia, we found that respiratory movements play a third critical role in modifying SI by affecting blood flow into the region of interest (ROI), as judged from right carotid artery flow. Consequently, we propose that the physiological SI changes induced by BH are derived from blood oxygenation, hypercapnia, and respiratory movements.

Physiologic variations in dural venous sinus flow on phase-contrast MR imaging

OBJECTIVE

Our study quantifies normal physiologic variations of dural sinus flow using phase-contrast MR imaging.

SUBJECTS AND METHODS

Fifteen volunteers were imaged using nontriggered and triggered phase-contrast MR venography of the superior sagittal and transverse sinuses. Triggered scans were obtained during regular breathing; nontriggered scans were obtained during regular breathing, breath-holding, deep inspiratory breath-holding, and deep expiratory breath-holding. Analysis of variance, Bonferroni method, and Dunn post hoc analysis were used to determine any significant differences in the mean flow and velocity between the different breathing maneuvers. A paired t test was used to compare flow between sinuses during regular breathing.

RESULTS

Deep inspiratory breath-holding and deep expiratory breath-holding resulted in a significant decrease in blood flow and velocity in all dural sinuses compared with regular breathing. During deep inspiratory breath-holding, blood flow decreased 30.8% in the superior sagittal sinus, 19.7% in the left transverse sinus, and 19.1% in the right transverse sinus. Similarly, during deep expiratory breath-holding, blood flow decreased 30.2% in the superior sagittal sinus, 20.8% in the left transverse sinus, and 20.3% in the right transverse sinus. The sum of the flow in the transverse sinuses was significantly greater than in the sagittal sinus. Normal pulsatility of dural sinus blood velocity was also characterized for all measured sinuses.

CONCLUSION

Characterization of variations in dural sinus velocity and flow as a function of the cardiac cycle and breathing maneuvers, using phase-contrast MR imaging, may help separate physiologic from pathologic changes of flow resulting from conditions that influence the cerebrovascular circulation.

Middle cerebral artery blood velocity during a valsalva maneuver in the standing position

Occasionally, lifting of a heavy weight leads to dizziness and even to fainting, suggesting that, especially in the standing position, expiratory straining compromises cerebral perfusion. In 10 subjects, the middle cerebral artery mean blood velocity (V(mean)) was evaluated during a Valsalva maneuver (mouth pressure 40 mmHg for 15 s) both in the supine and in the standing position. During standing, cardiac output decreased by 16 +/- 4 (SE) % (P < 0.05), and at the level of the brain mean arterial pressure (MAP) decreased from 89 +/- 2 to 78 +/- 3 mmHg (P < 0.05), as did V(mean) from 73 +/- 4 to 62 +/- 5 cm/s (P < 0.05). In both postures, the Valsalva maneuver increased central venous pressure by approximately 40 mmHg with a nadir in MAP and cardiac output that was most pronounced during standing (MAP: 65 +/- 6 vs. 87 +/- 3 mmHg; cardiac output: 37 +/- 3 vs. 57 +/- 4% of the resting value; P < 0.05). Also, V(mean) was lowest during the standing Valsalva maneuver (39 +/- 5 vs. 47 +/- 4 cm/s; P < 0.05). In healthy individuals, orthostasis induces an approximately 15% reduction in middle cerebral artery V(mean) that is exaggerated by a Valsalva maneuver performed with 40-mmHg mouth pressure to approximately 50% of supine rest.

Assessment of cerebral oxidative metabolism with breath holding and fMRI

Carbon dioxide inhalation can be used to map changes in cerebral metabolic rate of oxygen (CMRO(2)) during neuronal activation with functional MRI (fMRI). A hypercapnic stress also can be achieved with a simple breath-holding test. Using this test as means of manipulating cerebral blood flow (CBF) independent of CMRO(2), we assessed changes in CMRO(2) during visual stimulation. With this task, CBF increased by 61 +/- 7%, whereas CMRO(2) changed by 2.43 +/- 4.97%. These results are in good agreement with previous positron emission tomographic (PET) data, indicating that changes in oxidative metabolism during focal neuronal activity can potentially be determined with the breath-holding test. This test could easily be performed during a routine MRI examination. Magn Reson Med 42:608-611, 1999.

Functional magnetic resonance imaging of regional cerebral blood oxygenation changes during breath holding

BACKGROUND AND PURPOSE

Recently, noninvasive MRI methods have been developed that are now capable of detecting and mapping regional hemodynamic responses to various stress tests, which involve the use of vasoactive substances such as acetazolamide or inhalation of carbon dioxide. The aim of this study was to assess regional cerebral blood oxygenation changes during breath holding at 1.5 T.

METHODS

In 6 healthy volunteers, T2*-weighted gradient echo images were acquired for a total dynamic scanning time of 10 minutes during alternating periods of breath holding and normal breathing at 40-second intervals after inspiration, at 30-second intervals after expiration, and at 18 seconds after expiration. To quantify the relative signal changes, 2.5-minute baseline image sampling with normal breathing was carried out.

RESULTS

Repeated challenges of breath holding of various durations induced an overall rise in blood oxygen level-dependent (BOLD) signal intensities. In general, BOLD signal intensity increases were greatest in gray matter and nonsignificant in white matter. Depending on the breath-holding duration and techniques, BOLD signal intensity increases of all activated pixels varied from 0.8% to 3.5%.

CONCLUSIONS

The present study demonstrates that cerebral blood oxygenation changes during breath holding can be detected by means of fMRI at 1.5 T. The breath-holding test, a short and noninvasive method to study cerebral hemodynamics with fMRI, could become a useful alternative to the acetazolamide or CO2 test.

Evaluation of impaired cerebral autoregulation by the Valsalva maneuver

BACKGROUND AND PURPOSE

Transcranial Doppler sonography has recently been used to describe cerebral hemodynamics during the Valsalva maneuver in normal human subjects. Since some changes in flow velocity during the Valsalva maneuver seem to reflect the brain's autoregulatory response to a decrease in cerebral perfusion pressure during the strain, we hypothesized that this method could identify vascular territories with impaired autoregulatory capacity.

METHODS

Eight patients with unilateral (n=7) or bilateral (n=1) severe obstruction of the internal carotid artery and impaired vascular responses to the CO2 reactivity test and to dynamic autoregulation testing were studied. We compared changes in flow velocities and blood pressures during defined phases of the Valsalva maneuver in the patients with the results in a group of 17 normal volunteers. We defined two indices to evaluate autoregulatory capacity based on the response to the Valsalva maneuver.

RESULTS

During the Valsalva maneuver, changes in flow velocity in the middle cerebral arteries ipsilateral to the lesions showed characteristic abnormalities compared with the normal pattern. Autoregulatory indices of these vessels as defined by the Valsalva maneuver were significantly different from those with normal vascular reactivity to CO2 (P<.0001). There were good correlations between an index based on the changes in flow velocity and blood pressure in phase II and the results of the CO2 test (r=.78; P<.0001) or those of dynamic autoregulatory testing (r=.6; P<.0001).

CONCLUSIONS

Vascular territories with severely impaired vasomotor reactivity due to carotid obstruction can be identified by transcranial Doppler sonography by their pattern of flow velocity changes if their autoregulatory capacity is challenged during the Valsalva maneuver.

Effects of the valsalva maneuver on cerebral circulation in healthy adults. A transcranial Doppler Study

BACKGROUND AND PURPOSE

Knowledge is limited about the effects of the Valsalva maneuver on cerebral circulation because of the poor temporal resolution of traditional cerebral blood flow measurements. The purpose of this study was to investigate changes in cerebral blood flow during the Valsalva maneuver and to explore its potential use for the evaluation of cerebral autoregulation.

METHODS

Using transcranial Doppler ultrasonography, we simultaneously recorded systemic arterial blood pressure in the radial artery and flow velocities in both middle cerebral arteries in 10 healthy adults during the Valsalva maneuver. Gosling's pulsatility index was calculated for all phases of the Valsalva maneuver. Autoregulatory capacities were estimated from the change in cerebrovascular resistance (flow velocity in relationship to blood pressure) during phase II and changes in the velocity-pressure relationship in phase IV relative to phase I.

RESULTS

The characteristic changes in blood pressure (phases I to IV) were seen in all subjects, accompanying distinct changes in cerebral blood flow velocity. The relative changes in mean velocity during phases II and IV were significantly greater than those in mean blood pressure. Compared with the baseline value, velocity decreased by 35% in phase IIa, then rose by 56.5% in phase IV (corresponding changes in blood pressure were -10.2% and +29.8%, respectively). During phase II, the pulsatility and cerebrovascular resistance decreased by 19.9%. The increase in cerebral blood flow velocity in phase IV was significantly higher than in phase I (P < .0004), and there was no corresponding significant difference in blood pressure.

CONCLUSIONS

These results demonstrated that in healthy humans the Valsalva maneuver causes characteristic changes in systemic blood pressure as well as in flow velocity in the middle cerebral artery, reflecting the sympathetic and cerebral autoregulatory responses, respectively. Analysis of these changes may provide an estimate of autoregulatory capacity.

Positron imaging of cerebral blood flow during continuous inhalation of C15O2

This investigation tests the hypothesis that the normal cerebral image obtained non-invasively during continuous inhalation of C15O2 is related to cerebral blood flow. Trace amounts of CO2 labeled with the positron-emitting radionuclide 15O were administered to 4 normal subjects at normo- and hypocapnia and to 2 of these subjects at hypercapnia. Hypocapnia typically caused a marked decrease in cerebral 15O activity, and hypercapnia a small increase in activity. The relative difference in the change in count rate in response to hypo- and hypercapnia is what one would expect if the activity represented bloow flow, according to a mathematical model which assumes the 15O label enters the brain as water of perfusion. The findings in this study suggest that the normal cerebral image obtained during continuous inhalation of C15O2 is related to cerebral blood flow, but in a non-linear fashion, and that the technique would be more sensitive to ischemic events than to hyperemic phenomena.

Circulatory effect of respiratory maneuvers

The circulatory effects of three kinds of respiratory maneuvers--maximum breathing, voluntary hyperventilation, and the Valsalva maneuver--on cerebral blood flow, brachial blood flow, heart rate, and systolic blood pressure were investigated by means of the on-line Doppler ultrasonic technique in 20 men. Arterial gas contents (PO2, PCO2, pH) were also examined. Cerebral blood flow was increased or showed the biphasic response (increase in the former and decrease in the latter part of the maneuver) with maximum breathing. Cerebral blood flow was decreased with voluntary hyperventilation. The cerebral blood flow was maintained at a same level during the maneuver or tended to return to the control level. Cerebral blood flow was decreased in the Valsalva maneuver. There was a transient but conspicious increase of the cerebral blood flow immediately after the maneuver.