The effect of chemoreceptor stimulation on the centripetal transfer of somatosensory information in the urethane-anaesthetized rat

Revelations about carotid body function through its pathological role in resistant hypertension

Much recent attention has been given to the carotid body because of its potential role in cardiovascular disease states. One disease, neurogenic hypertension, characterised by excessive sympathetic activity, appears dependent on carotid body activity that may or may not be accompanied by sleep-disordered breathing. Herein, we review recent literature suggesting that the carotid body acquires tonicity in hypertension. We predict that carotid glomectomy will be a powerful way to temper excessive sympathetic discharge in diseases such as hypertension. We propose a model to explain that signalling from the 'hypertensive' carotid body is tonic, and hypothesise that there will be a sub-population of glomus cells that channel separately into reflex pathways controlling sympathetic motor outflows.

Early and late stimulus-evoked cortical hemodynamic responses provide insight into the neurogenic nature of neurovascular coupling

Understanding neurovascular coupling is a prerequisite for the interpretation of results obtained from modern neuroimaging techniques. This study investigated the hemodynamic and neural responses in rat somatosensory cortex elicited by 16 seconds electrical whisker stimuli. Hemodynamics were measured by optical imaging spectroscopy and neural activity by multichannel electrophysiology. Previous studies have suggested that the whisker-evoked hemodynamic response contains two mechanisms, a transient 'backwards' dilation of the middle cerebral artery, followed by an increase in blood volume localized to the site of neural activity. To distinguish between the mechanisms responsible for these aspects of the response, we presented whisker stimuli during normocapnia ('control'), and during a high level of hypercapnia. Hypercapnia was used to 'predilate' arteries and thus possibly 'inhibit' aspects of the response related to the 'early' mechanism. Indeed, hemodynamic data suggested that the transient stimulus-evoked response was absent under hypercapnia. However, evoked neural responses were also altered during hypercapnia and convolution of the neural responses from both the normocapnic and hypercapnic conditions with a canonical impulse response function, suggested that neurovascular coupling was similar in both conditions. Although data did not clearly dissociate early and late vascular responses, they suggest that the neurovascular coupling relationship is neurogenic in origin.

Influence of hypercapnia on cardiovascular responses to tracheal intubation

BACKGROUND

Laryngoscopy and tracheal intubation are often associated with tachycardia, hypertension, and arrhythmias. There is a risk of hypercapnia in the case of difficult mask ventilation. The circulatory response to hypercapnia is increases in arterial pressure and heart rate. We evaluated the difference of cardiovascular responses to tracheal intubation between normocapnia and hypercapnia during mask ventilation before tracheal intubation.

METHODS

We studied 40 ASA physical status I to II patients under general anesthesia. Induction of anesthesia was achieved with midazolam 0.05 mg/kg, propofol 1.5 mg/kg, alfentanil 10 microg/kg, and rocuronium 0.6 mg/kg IV. The lungs were mechanically ventilated with a tidal volume of 10 mL/kg and 6 to 10 bpm in the hypercapnia group (n = 20) or 12 to 15 bpm in the normocapnia group (n = 20) during the induction period. Intubation was performed 3 minutes after the induction, and anesthesia was maintained using 1.5% sevoflurane (inspired) and 75% N(2)O in oxygen. Heart rate, systolic arterial pressure (SAP), and diastolic arterial pressure were recorded every minute throughout the study.

RESULTS

The proportion of the patients whose increase of SAP between just before intubation and 1 minute after intubation was more than 30 mm Hg in the hypercapnia group (40%) was greater than that in the normocapnia group (9.5%) (P = .0325). There were no differences in heart rate and diastolic arterial pressure between hypercapnia and normocapnia groups. For the SAP of the patients, the trend of changes was increased (P = .024).

CONCLUSIONS

Hypercapnia during mask ventilation before tracheal intubation may exaggerate the increase of SAP during intubation compared to normocapnia. Ventilation was important in minimizing hemodynamic responses during induction regardless of using drugs.

The effect of hypercapnia on the neural and hemodynamic responses to somatosensory stimulation

Modern non-invasive imaging techniques utilize the coupling between neural activity and changes in blood flow, volume and oxygenation to map the functional architecture of the human brain. An understanding of how the hemodynamic response is influenced by pre-stimulus baseline perfusion is important for the interpretation of imaging data. To address this issue, the present study measured hemodynamics with optical imaging spectroscopy and laser Doppler flowmetry, while multi-channel electrophysiology was used to record local field potentials (LFP) and multi-unit activity (MUA). The response to whisker stimulation in rodent barrel cortex was recorded during baseline (normocapnia) and elevated perfusion rates produced by two levels of hypercapnia (5 and 10%). With the exception of the 'washout' of deoxyhemoglobin, which was attenuated, all aspects of the neural and hemodynamic response to whisker stimulation were similar during 5% hypercapnia to those evoked during normocapnia. In contrast, 10% hypercapnia produced cortical arousal and a reduction in both the current sink and MUA elicited by stimulation. Blood flow and volume responses were reduced by a similar magnitude to that observed in the current sink. The deoxyhemoglobin 'washout', however, was attenuated to a greater degree than could be expected from the neural activity. These data suggest that imaging techniques based on perfusion or blood volume changes may be more robust to shifts in baseline than those based on the dilution of deoxyhemoglobin, such as conventional BOLD fMRI.

Heart rate variability is encoded in the spontaneous discharge of thalamic somatosensory neurones in cat

1. We studied the spontaneous discharge variability of thalamocortical somatosensory neurones in the awake cat in order to disclose its possible information content. The presence of slow (0.09-1.39 Hz) regular fluctuations in the discharge rate of these cells during the waking state has been previously reported. Oscillations in a similar frequency range are known to characterize the activity of central and peripheral neurones pertaining to the autonomic nervous system and the variability of heart period (RR interval variability). 2. A surrogate data test, performed on our database, confirmed the presence of slow (0.05-1 Hz) non-random fluctuations in firing rate. 3. Linear regression detected the presence of an inverse relationship between the values of RR interval and the concurrent levels of neural discharge. 4. Frequency domain analysis indicated that a significant coupling between the two variability signals preferentially occurred in two frequency bands: in the frequency of the respiratory sinus arrhythmia and in correspondence with a slower rhythm (0.07-0.3 Hz), the two signals being in phase opposition in most of the cases. 5. Coherent fluctuations could also be observed when epochs of evoked activity were analysed, while coupling between the two variability signals appeared to be disrupted after sleep onset. 6. We conclude that RR interval variability, an internally generated dynamic related to basic visceral regulation, is encoded in the discharge of single somatosensory thalamocortical neurones during wakefulness. A possible interaction with the transmission of somatosensory information has to be evaluated.