Hypocapnia related changes in pain-induced brain activation as measured by functional MRI

Hypocapnia attenuates local skin thermal perception to innocuous warm and cool stimuli in normothermic resting humans

When one is exposed to a stressful situation in their daily life, a common response is hyperventilation. Although the physiological significance of stress-induced hyperventilation remains uncertain, this response may blunt perception of the stress-inducing stimulus. This study examined the effects of voluntary hyperventilation and resultant hypocapnia on the local skin thermal detection threshold in normothermic resting humans. Local skin thermal detection thresholds were measured in 15 young adults (three females) under three breathing conditions: 1) spontaneous breathing (Control trial), 2) voluntary hypocapnic hyperventilation (HH trial), and 3) voluntary normocapnic hyperventilation (NH trial). Local skin thermal detection thresholds were measured using thermostimulators containing a Peltier element that were attached to the forearm and forehead. The temperature of the probe was initially equilibrated to the skin temperature, then gradually increased or decreased at a constant rate (±0.1 °C/s) until the participants felt warmth or coolness. The difference between the initial skin temperature and the local skin temperature at which the participant noticed warmth/coolness was assessed as an index of the local skin warm/cool detection threshold. Local detection of warm and cool stimuli did not differ between the Control and NH trials, but it was blunted in the HH trial as compared with the Control and NH trials, except for detection of warm stimuli on the forearm. These findings suggest that hyperventilation-induced hypocapnia, not hyperventilation per se, attenuates local skin thermal perception, though changes in responses to warm stimuli may not be clearly perceived at some skin areas (e.g., forearm).

Neck Pain and Disability: Are They Related to Dysfunctional Breathing and Stress?

Purpose: People with neck pain are likely to have negative respiratory findings. The purpose of this study was to investigate the relationship between neck pain and dysfunctional breathing and to examine their relationship to stress. Method: This cross-sectional study included 49 participants with neck pain and 49 age- and sex-matched controls. We measured neck pain using the numeric rating scale (NRS); neck disability using the Neck Disability Index (NDI); dysfunctional breathing using the Nijmegen Questionnaire (NQ), Self-Evaluation of Breathing Questionnaire (SEBQ), breath hold time, and respiratory rate (RR); and stress using the Perceived Stress Scale (PSS). Results:Participants with neck pain scored higher on the NQ ( p < 0.001) and the SEBQ ( p < 0.001) than controls. NQ and SEBQ scores correlated moderately with NDI scores ( r > 0.50; 95% CI: 0.25, 0.68 and 0.33, 0.73, respectively) and PSS scores ( r > 0.50; 95% CI: 0.29, 0.78 and 0.31, 0.73, respectively). SEBQ scores showed a fair correlation with NRS scores and RR a fair correlation with NDI scores. Conclusions: Participants with neck pain had more dysfunctional breathing symptoms than participants without neck pain, and dysfunctional breathing was correlated with increased neck disability and increased stress. The NQ and SEBQ can be useful in assessing dysfunctional breathing in patients with neck pain.

End-tidal CO2: an important parameter for a correct interpretation in functional brain studies using speech tasks

The aim was to investigate the effect of different speech tasks, i.e. recitation of prose (PR), alliteration (AR) and hexameter (HR) verses and a control task (mental arithmetic (MA) with voicing of the result on end-tidal CO2 (PETCO2), cerebral hemodynamics and oxygenation. CO2 levels in the blood are known to strongly affect cerebral blood flow. Speech changes breathing pattern and may affect CO2 levels. Measurements were performed on 24 healthy adult volunteers during the performance of the 4 tasks. Tissue oxygen saturation (StO2) and absolute concentrations of oxyhemoglobin ([O2Hb]), deoxyhemoglobin ([HHb]) and total hemoglobin ([tHb]) were measured by functional near-infrared spectroscopy (fNIRS) and PETCO2 by a gas analyzer. Statistical analysis was applied to the difference between baseline before the task, 2 recitation and 5 baseline periods after the task. The 2 brain hemispheres and 4 tasks were tested separately. A significant decrease in PETCO2 was found during all 4 tasks with the smallest decrease during the MA task. During the recitation tasks (PR, AR and HR) a statistically significant (p<0.05) decrease occurred for StO2 during PR and AR in the right prefrontal cortex (PFC) and during AR and HR in the left PFC. [O2Hb] decreased significantly during PR, AR and HR in both hemispheres. [HHb] increased significantly during the AR task in the right PFC. [tHb] decreased significantly during HR in the right PFC and during PR, AR and HR in the left PFC. During the MA task, StO2 increased and [HHb] decreased significantly during the MA task. We conclude that changes in breathing (hyperventilation) during the tasks led to lower CO2 pressure in the blood (hypocapnia), predominantly responsible for the measured changes in cerebral hemodynamics and oxygenation. In conclusion, our findings demonstrate that PETCO2 should be monitored during functional brain studies investigating speech using neuroimaging modalities, such as fNIRS, fMRI to ensure a correct interpretation of changes in hemodynamics and oxygenation.

Localization of pain-related brain activation: a meta-analysis of neuroimaging data

A meta-analysis of 140 neuroimaging studies was performed using the activation-likelihood-estimate (ALE) method to explore the location and extent of activation in the brain in response to noxious stimuli in healthy volunteers. The first analysis involved the creation of a likelihood map illustrating brain activation common across studies using noxious stimuli. The left thalamus, right anterior cingulate cortex (ACC), bilateral anterior insulae, and left dorsal posterior insula had the highest likelihood of being activated. The second analysis contrasted noxious cold with noxious heat stimulation and revealed higher likelihood of activation to noxious cold in the subgenual ACC and the amygdala. The third analysis assessed the implications of using either a warm stimulus or a resting baseline as the control condition to reveal activation attributed to noxious heat. Comparing noxious heat to warm stimulation led to peak ALE values that were restricted to cortical regions with known nociceptive input. The fourth analysis tested for a hemispheric dominance in pain processing and showed the importance of the right hemisphere, with the strongest ALE peaks and clusters found in the right insula and ACC. The fifth analysis compared noxious muscle with cutaneous stimuli and the former type was more likely to evoke activation in the posterior and anterior cingulate cortices, precuneus, dorsolateral prefrontal cortex, and cerebellum. In general, results indicate that some brain regions such as the thalamus, insula and ACC have a significant likelihood of activation regardless of the type of noxious stimuli, while other brain regions show a stimulus-specific likelihood of being activated.

Comparison between end-tidal CO₂ and respiration volume per time for detecting BOLD signal fluctuations during paced hyperventilation

Respiratory motion and capnometry monitoring were performed during blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) of the brain while a series of paced hyperventilation tasks were performed that caused significant hypocapnia. Respiration volume per time (RVT) and end-tidal carbon dioxide (ETCO(2)) were determined and compared for their ability to explain BOLD contrast changes in the data. A 35% decrease in ETCO(2) was observed along with corresponding changes in RVT. A best-fit ETCO(2) response function, with an average initial peak delay time of 12 s, was empirically determined. ETCO(2) data convolved with this response function was more strongly and prevalently correlated to BOLD signal changes than RVT data convolved with the corresponding respiration response function. The results suggest that ETCO(2) better models BOLD signal fluctuations in fMRI experiments with significant transient hypocapnia. This is due to hysteresis in the ETCO(2) response when moving from hypocapnia to normocapnia, compared to moving from normocapnia to hypocapnia.

Assessment of brain responses to innocuous and noxious electrical forepaw stimulation in mice using BOLD fMRI

Functional magnetic resonance imaging (fMRI) using the blood oxygen level-dependent (BOLD) contrast was used to study sensory processing in the brain of isoflurane-anesthetized mice. The use of a cryogenic surface coil in a small animal 9.4T system provided the sensitivity required for detection and quantitative analysis of hemodynamic changes caused by neural activity in the mouse brain in response to electrical forepaw stimulation at different amplitudes. A gradient echo-echo planar imaging (GE-EPI) sequence was used to acquire five coronal brain slices of 0.5mm thickness. BOLD signal changes were observed in primary and secondary somatosensory cortices, the thalamus and the insular cortex, important regions involved in sensory and nociceptive processing. Activation was observed consistently bilateral despite unilateral stimulation of the forepaw. The temporal BOLD profile was segregated into two signal components with different temporal characteristics. The maximum BOLD amplitude of both signal components correlated strongly with the stimulation amplitude. Analysis of the dynamic behavior of the somatosensory 'fast' BOLD component revealed a decreasing signal decay rate constant k(off) with increasing maximum BOLD amplitude (and stimulation amplitude). This study demonstrates the feasibility of a robust BOLD fMRI protocol to study nociceptive processing in isoflurane-anesthetized mice. The reliability of the method allows for detailed analysis of the temporal BOLD profile and for investigation of somatosensory and noxious signal processing in the brain, which is attractive for characterizing genetically engineered mouse models.