Microvascular responses to (hyper-)gravitational stress by short-arm human centrifuge: arteriolar vasoconstriction and venous pooling.

PURPOSE: We hypothesized that lower body microvessels are particularly challenged during exposure to gravity and hypergravity leading to failure of resistance vessels to withstand excessive transmural pressure during hypergravitation and gravitation-dependent microvascular blood pooling. METHODS: Using a short-arm human centrifuge (SAHC), 12 subjects were exposed to +1Gz, +2Gz and +1Gz, all at foot level, for 4 min each. Laser Doppler imaging and near-infrared spectroscopy were used to measure skin perfusion and tissue haemoglobin concentrations, respectively. RESULTS: Pretibial skin perfusion decreased by 19% during +1Gz and remained at this level during +2Gz. In the dilated area, skin perfusion increased by 24 and 35% during +1Gz and +2Gz, respectively. In the upper arm, oxygenated haemoglobin (Hb) decreased, while deoxy Hb increased with little change in total Hb. In the calf muscle, O2Hb and deoxy Hb increased, resulting in total Hb increase by 7.5 ± 1.4 and 26.6 ± 2.6 µmol/L at +1Gz and +2Gz, respectively. The dynamics of Hb increase suggests a fast and a slow component. CONCLUSION: Despite transmural pressures well beyond the upper myogenic control limit, intact lower body resistance vessels withstand these pressures up to +2Gz, suggesting that myogenic control may contribute only little to increased vascular resistance. The fast component of increasing total Hb indicates microvascular blood pooling contributing to soft tissue capacitance. Future research will have to address possible alterations of these acute adaptations to gravity after deconditioning by exposure to micro-g.

Temporal and spatial dispersion of human body temperature during deep hypothermia.

BACKGROUND: Clinical temperature management remains challenging. Choosing the right sensor location to determine the core body temperature is a particular matter of academic and clinical debate. This study aimed to investigate the relationship of measured temperatures at different sites during surgery in deep hypothermic patients. METHODS: In this prospective single-centre study, we studied 24 patients undergoing cardiothoracic surgery: 12 in normothermia, 3 in mild, and 9 in deep hypothermia. Temperature recordings of a non-invasive heat flux sensor at the forehead were compared with the arterial outlet temperature of a heart-lung machine, with the temperature on a conventional vesical bladder thermistor and, for patients undergoing deep hypothermia, with oesophageal temperature. RESULTS: Using a linear model for sensor comparison, the arterial outlet sensor showed a difference among the other sensor positions between -0.54 and -1.12°C. The 95% confidence interval ranged between 7.06 and 8.82°C for the upper limit and -8.14 and -10.62°C for the lower limit. Because of the hysteretic shape, the curves were divided into phases and fitted into a non-linear model according to time and placement of the sensors. During cooling and warming phases, a quadratic relationship could be observed among arterial, oesophageal, vesical, and cranial temperature recordings, with coefficients of determination ranging between 0.95 and 0.98 (standard errors of the estimate 0.69-1.12°C). CONCLUSION: We suggest that measured surrogate temperatures as indices of the cerebral temperature (e.g. vesical bladder temperature) should be interpreted with respect to the temporal and spatial dispersion during cooling and rewarming phases.

Are there sex-related differences in responses to repetitive olfactory/trigeminal stimuli?

Sex differences in olfactory sensitivity have been reported since the late 1800's with women typically outperforming men on tests of odor detection, discrimination or identification. It is not known whether women adapt differently than men to olfactory and trigeminal stimuli. Seventeen healthy volunteers participated (9 female, 8 male, mean age 22 years) in the study. As established by an odor identification test (UPSIT, score > or =38) all subjects had normal olfactory function. Event-related potentials (ERPs) were recorded in response to olfactory (25% v/v phenyl ethyl alcohol) and trigeminal (44% v/v CO(2)) stimuli using a computer controlled olfactometer (flow 8 L/min; stimulus duration 200 ms). Stimuli were applied at four intervals (5, 10, 20, and 60 s). Amplitudes and latencies of ERP peaks P1, N1, and P2 were measured. Stimulus intensity also rated using visual analogue scales subjects. When compared to phenyl ethyl alcohol, the slightly more intense CO(2) produced larger amplitudes and shorter latencies. Both, ratings and ERP amplitudes P2 decreased with decreasing interval between stimuli. Responses to the trigeminal and olfactory stimuli changed similarly in relation to repetitive stimulation. Women had larger ERP amplitudes P2. No sex-related difference for ratings and ERP in relation to repeated stimulation amplitudes was observed. Although women exhibit larger ERP amplitudes to chemosensory stimuli compared to men, the present data indicate on both psychophysical and electrophysiological levels that there is no major difference between young, healthy men and women in relation to short-term adaptation to suprathreshold chemosensory stimulation.

Low-frequency short-time nociceptive stimulation of the greater occipital nerve does not modulate the trigeminal system.

Occipital stimulation in a small group of refractory chronic migraine and cluster headache patients has been suggested as a novel therapeutic approach with promising results. In an earlier study we have shown that a drug-induced block of the greater occipital nerve (GON) inhibits the nociceptive blink reflex (nBR). Now, we sought to examine the effects of low-frequency (3 Hz) short-time nociceptive stimulation of the GON on the trigeminal system. We recorded the nBR responses before and after stimulation in 34 healthy subjects. Selectivity of GON stimulation was confirmed by eliciting somatosensory evoked potentials of the GON upon stimulation. In contrast to an anaesthetic block of the occipital nerve, no significant changes of the R2-latencies and R2-response areas of the nBR can be elicited following GON stimulation. Various modes of electrical stimulation exist with differences in frequency, stimulus intensity, duration of stimulation and pulse width. One explanation for a missing modulatory effect in our study is the relatively short duration of the stimulation.