EFFECTS OF HYPOXIA AND HYPERCAPNIA ON PERCEPTION OF THERMAL CUTANEOUS PAIN

The effect of steady-state CO2 on regional brain blood flow responses to increases in blood pressure via the cold pressor test

The pressure-passive cerebrovasculature is affected by alterations in cerebral perfusion pressure (CPP) and arterial blood gases (e.g., pressure of arterial [Pa]CO₂), where acute changes in either stimulus can influence cerebral blood flow (CBF). The effect of superimposed increases in CPP at different levels of steady-state PaCO₂ on regional CBF regulation is unclear. In 17 healthy participants, we simultaneously recorded continuous heart rate (electrocardiogram), blood pressure (finometer), pressure of end-tidal CO₂ (P_ETCO₂; gas analyzer), and middle (MCA) and posterior (PCA) cerebral artery blood velocity (CBV; transcranial Doppler ultrasound). Three separate CPTs were administered by passive immersion of both feet into 0-1 °C of ice water for 3-min under three randomized and coached steady-state P_ETCO₂ conditions: normocapnia (room air), hypocapnia (-10 Torr; hyperventilation) and hypercapnia (+9 Torr; 5% inspired CO₂;). CBV responses were calculated as the absolute difference (∆) between baseline and mean MCAv and PCAv during the 3-min CPT. Both the ∆MCAv and ∆PCAv responses to the CPT were larger under hypercapnic conditions. The absolute ∆MCAv response was larger than the ∆PCAv during the CPT across all three CO₂ trials. Cerebrovascular CO₂ reactivity (CVR) was larger in the MCA than PCA in both CPT and baseline conditions, but there were no differences in CVR between CPT and baseline conditions. Our data indicate that (a) increases in CO₂ increases the CBV responses to a CPT, (b) the anterior cerebrovasculature is more responsive to a CPT-induced increases in MAP, and (c) although unchanged during a CPT, CVR is larger in the anterior cerebral circulation.

Interactions Between Dyspnea and the Brain Processing of Nociceptive Stimuli: Experimental Air Hunger Attenuates Laser-Evoked Brain Potentials in Humans

Dyspnea and pain share several characteristics and certain neural networks and interact with each other. Dyspnea-pain counter-irritation consists of attenuation of preexisting pain by intercurrent dyspnea and has been shown to have neurophysiological correlates in the form of inhibition of the nociceptive spinal reflex RIII and laser-evoked potentials (LEPs). Experimentally induced exertional dyspnea inhibits RIII and LEPs, while “air hunger” dyspnea does not inhibit RIII despite its documented analgesic effects. We hypothesized that air hunger may act centrally and inhibit LEPs. LEPs were obtained in 12 healthy volunteers (age: 21–29) during spontaneous breathing (FB), ventilator-controlled breathing (VC) tailored to FB, after inducing air hunger by increasing the inspired fraction of carbon dioxide -FiCO₂- (VCCO₂), and during ventilator-controlled breathing recovery (VCR). VCCO₂ induced intense dyspnea (visual analog scale = 63% ± 6% of full scale, p < 0.001 vs. VC), predominantly of the air hunger type. VC alone reduced the amplitude of the N2-P2 component of LEPs (Δ = 24.0% ± 21.1%, p < 0.05, effect-size = 0.74) predominantly through a reduction in P2, and the amplitude of this inhibition was further reduced by inducting air hunger (Δ = 22.6% ± 17.9%, p < 0.05, effect-size = 0.53), predominantly through a reduction in N2. Somatosensory-evoked potentials (SEPs) were not affected by VC or VCCO₂, suggesting that the observed effects are specific to pain transmission. We conclude that air hunger interferes with the cortical mechanisms responsible for the cortical response to painful laser skin stimulation, which provides a neurophysiological substrate to the central nature of its otherwise documented analgesic effects.

Thermal performance trials on the habitability of private bushfire shelters: part 2

In the preceding communication, an investigation was described in which the thermal specifications for the design of private bushfire shelters were evaluated. Since those trials were undertaken with the thermal characteristics of the air clamped, survival uncertainty persisted if the internal ambient conditions were progressively changing, as would occur within an air-tight shelter. Therefore, two further investigations were performed. In the first, changes in the physical properties of air within an air-tight shelter simulator (1.2 m(3)), initially equilibrated to 43.7 °C and 42.3 % relative humidity, were studied when pre-heated, well-hydrated males were sealed inside (N = 16; 60 min; experimental series 2). Air temperature and humidity moved sigmoidally to 40.5 °C (standard deviation (SD), 0.5) and 90.1 % (SD, 2.1). Oxygen and carbon dioxide fractional concentrations changed reciprocally, with respective terminal averages of 16.7 % (SD, 0.8) and 3.94 % (SD, 0.72). Deep-body temperature rose beyond the tenth minute to a terminal mean of 39.3 °C (SD, 0.2). In the third experimental series, these air temperature and humidity changes were reproduced in trials commencing at two different thermal states (40 °C and 70 % relative humidity; 45 °C and 50 % relative humidity). Sixteen pre-heated and slightly dehydrated men and women were investigated. In neither condition did the auditory canal temperature of any individual change by more than 2 °C or exceed 40 °C. It may be concluded, within the limits of these experiments, that the recommended thermal and dimensional specifications for bushfire shelters can provide tenable conditions for healthy, young adults.

Do cardiorespiratory variables predict the antinociceptive effects of deep and slow breathing?

Deep and slow breathing (DSB) is a central part of behavioral exercises used for acute and chronic pain management. Its mechanisms of action are incompletely understood.

OBJECTIVES

1) To test the effects of breathing frequency on experimental pain perception in a dose dependent fashion. 2) To test the effects of breathing frequency on cardiorespiratory variables hypothesized to mediate DSB analgesia. 3) To determine the potential of the cardiorespiratory variables to mediate antinociceptive DSB effects by regression analysis.

DESIGN

Single-blind, randomized, crossover trial.

SUBJECTS

Twenty healthy participants.

INTERVENTIONS

Visually paced breathing at 0.14 Hz, 0.10 Hz, 0.06 Hz, and resting frequency.

OUTCOME MEASURES

Cardiorespiratory variables: RR-interval (= 60 seconds/heart rate), standard deviation of the RR-interval (SDRR), and respiratory CO2 . Experimental pain measures: heat pain thresholds, cold pain thresholds, pain intensity ratings, and pain unpleasantness ratings.

RESULTS

1) There was no effect of DSB frequency on experimental pain perception. 2) SDRR and respiratory CO2 were significantly modulated by DSB frequency, while RR-interval was not. 3) Baseline-to-DSB and session-to-session differences in RR-interval significantly predicted pain perception within participants: Prolonged RR-intervals predicted lower pain ratings, while shortened RR-intervals predicted higher pain ratings. SDRR and respiratory CO2 were not found to predict pain perception.

CONCLUSIONS

The present study could not confirm hypotheses that the antinociceptive effects of DSB are related to changes in breathing frequency, heart rate variability, or hypoventilation/hyperventilation when applied as a short-term intervention. It could confirm the notion that increased cardiac parasympathetic activity is associated with reduced pain perception.

[Critical analysis of animal models of acute pain. II]

OBJECTIVE

To analyse models of acute pain in experimental animals.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

The majority of tests permit only a measurement of threshold, whereas clinical pain is almost always prolonged. The relationships between tests of acute pain and motor activity are reviewed from a number of standpoints; in particular we consider the influence, which postural adjustments of the animal may exert on motor responses in the limbs and the significance of flexor and extensor reflexes. In analysing the problem of the sensitivity of tests, we raise the following questions: 1) what type(s) of fibres underlie the observed responses and might these be different depending on whether one is stimulating a healthy or an inflamed tissue; 2) what significance do measurements of "latency" have when a stimulus is increasing; 3) how valid are the methods of analysing the results? The influence of species and the genetic line used in tests and the specificity and predictivity of tests are considered. Finally, we review those factors, which may distort behavioural measurements in animals, notably--pharmacokinetics, interactions between heterotopic stimuli, environmental factors and related psychophysiological/psychological considerations (subjective "undesirable" phenomena, learning phenomena). We pay particular attention to related physiological functions (thermoregulation, vasomotricity, blood pressure). These considerations lead us to re-position nociception within a much larger homeostatic framework which in addition to pain, includes phenomena such as anxiety and vegetative functions. They also suggest that we should define an "effective stimulus" as one, which activates nociceptive nerve terminals after a physical stimulus, has passed through a "peripheral lens" which regulates its intensity for reasons, which are physical, albeit of biological origin. Finally they remind us that the "system of pain" forms part of a whole set of subsystems--sensory, motor, vegetative, emotional, motivational--which scientific method, being reductionist by nature, cannot study in its entirety. However one must consider results of nociceptive tests within this general context.

CONCLUSION

It is only by taking the approach described in this review, that fundamental and clinical research can interact usefully.

A selective suppression of human pain sensitivity by carbon dioxide: central mechanisms implicated

The effect of breathing CO2 on somatic sensitivity was studied in human subjects. Healthy humans breathed room air, 100% O2, or CO2 (5% or 8% in O2). Thresholds to heat pain, mechanical pain, electrically evoked pain, and electrically evoked perception were measured using psychophysical techniques. Also, the effects on sensory and affective components of experimental ischaemic pain, alertness and on cardiovascular parameters were observed. In addition, the effect of CO2 on heat pain threshold was determined in the ischaemic limb to exclude the possibility that the threshold elevation was due to an action on primary afferent fibres. Naloxone (0.8 mg), dexamethasone (0.1 mg) or placebo (0.9% NaCl) were applied intravenously in double blind tests to reverse the threshold elevations. In an electrophysiological experiment the effect of CO2 on a spinal nociceptive flexion reflex evoked by an electric stimulus was measured. The CO2 produced a dose-dependent elevation of the heat pain threshold. Similarly, the sensory and affective components of experimental ischaemic pain were attenuated by CO2. The heat pain threshold was significantly elevated by CO2 in the ischaemic limb, also. However, there was no effect on thresholds to mechanically or electrically induced pain or perception thresholds to electrically evoked sensations. One hundred percent O, did not elevate the heat pain threshold. In double blind tests the heat pain threshold elevation was not significant when the naloxone or dexamethasone was administered. The threshold to electrically evoked spinal flexion reflex was not elevated by CO2. The CO2 at the current dose produced sedation, an increase in blood pressure but no change in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypercapnia depresses nociception: endogenous opioids implicated

Hypoventilation produces hypercapnia which can elevate pain thresholds. Hypercapnia is a potent stressor which releases catecholamines and activates the sympathetic nervous system. Some stressors produce analgesia by releasing endogenous opioids. To determine the roles of endogenous opioids and catecholamines in hypercapnic analgesia, we administered CO2 in the inspired gas mixture to conscious rats. CO2 in the range 5-10% elevated tail flick and leg flexion latencies 2- to 3-fold in both intact and spinalised animals. The effects on reflex latencies but not on paCO2 or pHa were blocked by naloxone (2 mg/kg), and were not present in morphine-tolerant animals. The effects were reduced by dexamethasone but were not changed either by adrenalectomy or by systemic guanethidine, propanolol or phentolamine. Hypercapnia delayed the onset of the late phase of behavioural responses to formalin injected into the plantar surface of the hindpaw. We conclude that moderate hypercapnia powerfully depresses flexor withdrawal responses to noxious stimuli, by a mechanism involving release of endogenous opioids but not systemic catecholamines. This effect may account in part for the elevation in pain threshold during hypoventilation.

Painful sensation induced by a thermal cutaneous stimulus

This review emphasizes how little we know about pain induced by a thermal stimulus. The study of the intensity of pain evoked by heat is relatively exhaustive: the influence of various local, stimulus-dependent or general factors upon threshold values has been well studied, as has the relation between pain and stimulus intensities. On the contrary, few studies have used very cold stimuli, since highly efficient stimulators allowing accurate control of the stimulus parameters have been obtainable only recently. Only the influence of stimulation area and stimulation rate on cold pain thresholds have been studied. Moreover, old results obtained on pain quality cannot be used since the conditions of stimulation were not specified or not controlled accurately. It is only known that stimulus duration and stimulation area are determinant for thermal pain quality. There is still much work to be done in this field. All the more so as this type of study is absolutely necessary for the understanding of pain mechanisms--it describes what must be explained by the function of the nervous system. We have seen that at the periphery the intensity of heat pain is coded by the response of polymodal nociceptors, mechanothermal nociceptors, thermal nociceptors and possibly by the paradoxical discharge of cold receptors. If the stimulus is lower than 45 degrees C the activity of certain heat receptors comes into play. Although we lack information which would allow confirmation of this as a fact it seems likely that the activity of polymodal nociceptors, cold mechanothermal nociceptors and possibly certain cold receptors sensitive to very low temperatures code cold pain. These nociceptive impulses carried by A delta and C fibers reach the dorsal horn of the spinal cord through the dorsal roots. They are notably at the origin of the activation of the neurons in Rexed's layers I, V and VIII which are to a large extent at the origin of the spinothalamic and spinoreticulothalamic tracks [21,115,168] moving in the anterolateral quadrant of the spinal cord. At supraspinal level, the thermal information reappears in the reticular formation; there it appears to be solely relative to the pain threshold and not to the intensity of a supraliminary stimulus [55]. In the posterior group of nuclei [134] and the ventroposterolateral nucleus of the thalamus [103], on the contrary, the activity of the neurons reflects the intensity of the stimulation. It has been proved that the neurons of the ventroposterolateral nucleus project onto the SI cortex [103].(ABSTRACT TRUNCATED AT 400 WORDS)

[Comparison of the effects of acute respiratory acidosis and acute metabolic acidosis on the jaw-opening reflex in the anesthetized dog]

A few effects of carbon dioxide on pain threshold and acid-base balance are known. The purpose of this study was to investigate specifically the variations of analgesia in relation to hypercapnia during general anaesthesia and the respective roles played by carbon dioxide and [H+]. The nociceptive jaw opening reflex was studied on five beagle dogs anaesthetized with alfathesin administered at constant rate under acute hypercapnic conditions and acute metabolic acidosis. Acute hypercapnia did not decrease the jaw opening reflex significantly until a level was reached where PaCO2 values modified blood [H+] (pH) significantly (10 +/- 1.04 kPa corresponding to [H+] 91.5 +/- 13.24 nmol/l (pH 7.04 +/- 0.06) p less than 0.05)). At [H+] 176.2 +/- 42.77 nmol/l (pH 6.7 +/- 0.13) (p less than 0.01) the reflex was only 9.3 +/- 3.9 per cent (p less than 0.001) of its initial value. The infusion of decinormal solution of HCl during constant capnia caused an abrupt drop of the reflex. There was a correlation between reflex and metabolic acidosis (p less than 0.05). The authors conclude that modification of the jaw opening reflex occurs with extreme values of arterial [H+] incompatible with safe anaesthesia and they discuss the mechanisms involved.

The action of carbon dioxide on afferent transmission in the dorsal column-lemniscal system

1. Transmission in the lemniscal afferent pathway was studied in thirteen decerebrate, unanaesthetized cats while changing the concentration of inspired P(CO2).2. 2-20% CO(2), when inhaled for </= 10 min, raised the mean tissue P(CO2), recorded from the surface of the medulla or cerebellum, from 22 torr to between 26 and 93 torr.3. Medial lemniscal potentials were evoked by stimulation of a forelimb nerve and recorded from the transected surface of the contralateral mid-brain. The transmission of supramaximal responses was progressively and reversibly depressed as tissue levels of P(CO2) were raised and lowered. The time course of the changes in transmission corresponded closely to changes in the tissue P(CO2).4. The amplitude of the main early lemniscal peak was decreased to 80% by the inhalation of 20% CO(2). A late component of the lemniscal response, presumably due to repetitive firing and conduction in smaller fibres and polysynaptic pathways, was more affected than the early main response.5. The failure of transmission was unrelated to threshold changes in the peripheral nerve, since potentials recorded close to the site of stimulation showed no changes.6. Increases in the transmission time of the main lemniscal potentials were uniform and < 10% during the administration of CO(2) and did not appear to contribute to amplitude changes.7. The inhibition of afferent transmission from one nerve by a preceding conditioning volley in a second nerve was not altered by hypercarbia.8. It is concluded that CO(2) has a blocking action on afferent transmission in the pre-thalamic lemniscal system. The site of this block may be at the synapses and/or other regions of low safety factor in the afferent fibres.