Magnitude of skin vasomotor reflex represents the intensity of nociception under general anesthesia

Neuroanesthesiology Update

This review is intended to provide a summary of the literature pertaining to the perioperative care of neurosurgical patients and patients with neurological diseases. General topics addressed in this review include general neurosurgical considerations, stroke, neurological monitoring, and perioperative disorders of cognitive function.

The nociception level index (NOL) response to intubation and incision in patients undergoing video-assisted thoracoscopic surgery (VATS) with and without thoracic epidural analgesia. A pilot study

Background: The PMD100™ (Medasense Biometrics Ltd., Ramat Yishai, Israel) is a novel non-invasive nociception monitor that integrates physiological parameters to compute a real-time nociception level index (NOL) in the anesthetized patients. Thoracic epidural analgesia provides effective analgesia and improves surgical outcomes. Side effects include sympathectomy, hypotension, changes in skin temperature and a decreased cardiac accelerator fiber tone. The purpose of this pilot study was to evaluate changes in NOL values after incision in patients with and without epidural analgesia.   Methods: Half of the patients scheduled for Video-Assisted Thoracoscopic Surgery (VATS) received a thoracic epidural catheter, placed and tested 2h before surgery and activated prior to incision. The other half of the patients received i.v. fentanyl (1 mcg/kg) five minutes before incision. Anesthesia and analgesia were maintained in a standardized manner. NOL and heart rate (HR) were compared before and after the nociceptive stimuli intubation and skin incision. Results: NOL significantly increased in all patients after intubation by 10.2 points (CI: 4.5-16.0; p=0.002) as well as HR by 9 beats per minute after intubation in all patients (CI: 3.3-15.6; p=0.01). After incision, in patients without epidural analgesia the NOL increased by 13.9 points (CI: 7.4-20.3; p=0.0001), compared to 5.4 points (CI: -6.3-17.1; p=0.29) in patients with epidural analgesia. HR did not significantly vary after incision in both groups. The area under the curve of delta NOL and delta HR variations after incision were significantly different (p<0.05) between groups and delta NOL variations were significantly different from baseline values but not the delta HR variations. Conclusions: This pilot study suggests that the PMD100™ Monitor may be a useful tool to evaluate the efficacy of an intraoperative thoracic epidural analgesia. Clinical Trial Registry Number: ClinicalTrials.gov record ID: NCT01978379 registered 10/25/2014.

NGF-dependent neurons and neurobiology of emotions and feelings: Lessons from congenital insensitivity to pain with anhidrosis

NGF is a well-studied neurotrophic factor, and TrkA is a receptor tyrosine kinase for NGF. The NGF-TrkA system supports the survival and maintenance of NGF-dependent neurons during development. Congenital insensitivity to pain with anhidrosis (CIPA) is an autosomal recessive genetic disorder due to loss-of-function mutations in the NTRK1 gene encoding TrkA. Individuals with CIPA lack NGF-dependent neurons, including NGF-dependent primary afferents and sympathetic postganglionic neurons, in otherwise intact systems. Thus, the pathophysiology of CIPA can provide intriguing findings to elucidate the unique functions that NGF-dependent neurons serve in humans, which might be difficult to evaluate in animal studies. Preceding studies have shown that the NGF-TrkA system plays critical roles in pain, itching and inflammation. This review focuses on the clinical and neurobiological aspects of CIPA and explains that NGF-dependent neurons in the peripheral nervous system play pivotal roles in interoception and homeostasis of our body, as well as in the stress response. Furthermore, these NGF-dependent neurons are likely requisite for neurobiological processes of 'emotions and feelings' in our species.

Arousal electrical stimuli evoke sudomotor activity related to P300, and skin vasoconstrictor activity related to N140 in humans

OBJECTIVE

Arousal stimuli evoke bursts of skin sympathetic nerve activity (SSNA). SSNA usually contains sudomotor and vasoconstrictor neural spikes. The aim of this study was to elucidate which components of event-related potentials (ERPs) are related to sudomotor and vasoconstrictor responses comprising arousal SSNA bursts.

METHODS

We recorded SSNA from the tibial nerve by microneurography, with corresponding sympathetic skin response (SSR), sympathetic flow response (SFR), and ERPs in 10 healthy subjects. Electrical stimulation of the median nerve was used to induce arousal responses. ERPs were classified by the occurrence of SSR and SFR.

RESULTS

SSNA bursts followed by SSR were associated with larger P300 than SSNA bursts followed by no SSR. For N140, no difference in the amplitude was found between SSNA bursts with and without SSR. SSNA bursts followed by SFR were associated with larger N140 than SSNA bursts followed by no SFR. However, there were no differences in the amplitude of P300 between SSNA bursts with and without SFR.

CONCLUSIONS

Sudomotor and skin vasoconstrictor responses to arousal stimuli were differently associated with distinct ERP components.

SIGNIFICANCE

The possibility that sudomotor and skin vasoconstrictor activities comprising arousal SSNA reflect different stages of the cognitive process is suggested.

Photoplethysmography and nociception

Photoplethysmography (PPG), i.e. pulse oximetric wave, is a non-invasive technique that is used in anaesthesia monitoring primarily to monitor blood oxygenation. The PPG waveform resembles that of the arterial blood pressure but instead of pressure it is related to the volume changes in the measurement site and hence contains information related to the peripheral blood circulation, including skin vasomotion, which is controlled by the sympathetic nervous system. Because of this link, skin vasomotor response and PPG amplitude response have been associated with nociception under general anaesthesia. Recently, there has been interest in monitoring nociception during general anaesthesia. In many of the published studies, PPG waveform information has been included. The focus of this topical review is to provide an overview on the information embedded in the PPG waveform especially in the context of the autonomic nervous system and analgesia monitoring.

Mechanisms underlying development of spatially distributed chronic pain (fibromyalgia)

Chronic fibromyalgia (FM) pain is prevalent (estimated as high as 13%), predominantly affects women, and is associated with a variety of focal pain conditions. Ongoing FM pain is referred to deep tissues and is described as widespread but usually is maximally located within a restricted region such as the shoulders. Palpation of deep tissues reveals an enhanced nociceptive sensitivity that is not restricted to regions of clinical pain. Similarly, psychophysical testing reveals allodynia and hyperalgesia for cutaneous stimulation at locations beyond regions of clinical pain referral. The combination of widely distributed clinical pain and generalized hypersensitivity is highly disabling, but no satisfactory treatment is regularly prescribed. A thorough understanding of mechanisms will likely be required to develop and document adequate therapies. The generalized hypersensitivity associated with FM has focused considerable interest on central (CNS) mechanisms for the disorder. These include central sensitization, central disinhibition and a dysfunctional hypothalamic-pituitary-adrenal (HPA) axis. However, the central effects associated with FM can be produced by a peripheral source of pain. Chronic nociceptive input induces central sensitization, magnifying pain, and it activates the HPA and the sympathetic nervous system. Chronic sympathetic activation indirectly sensitizes peripheral nociceptors and sets up a vicious cycle. Thus, it appears that central mechanisms of FM pain are dependent on abnormal peripheral input(s) for development and maintenance of this condition. A substantial literature defines peripheral-CNS-peripheral interactions that are integral to FM pain. These reciprocal actions and related phenomena of relevance to FM pain are reviewed here, leading to suggestions for testing of therapeutic approaches.

Monitoring analgesia

Analgesia (pain relief) amnesia (loss of memory) and immobilisation are the three major components of anaesthesia. The perception of pain, and therefore, the need for analgesia, is individual, and the monitoring of analgesia is indirect and, in essence, of the moment. Under general anaesthesia, analgesia is continually influenced by external stimuli and the administration of analgesic drugs, and cannot be really separated from anaesthesia: the interaction between analgesia and anaesthesia is inescapable. Autonomic reactions, such as tachycardia, hypertension, sweating and lacrimation, although non-specific, are always regarded as signs of nociception or inadequate analgesia. Autonomic monitoring techniques, such as the analysis of heart rate variability, laser Doppler flowmetry, phlethysmographically derived indices and the pupillary light reflex, may help to quantitate reactions of the autonomic nervous system. For the past few years, automated electroencephalographic analysis has been of great interest in monitoring anaesthesia and could be useful in adapting the peroperative administration of opioids. A range of information collected from the electroencephalogram, haemodynamic readings and pulse plethysmography might be necessary for monitoring the level of nociception during anaesthesia. Information theory, multimodal monitoring, and signal processing and integration are the basis of future monitoring.

CRF1-receptor antagonist CP-154526 reduces alerting-related cutaneous vasoconstriction in conscious rabbits

Cutaneous vasoconstrictor responses elicited by salient stimuli in conscious rabbits may be a sensitive physiological index of emotional arousal/anxiety. Ear-pinna blood flow was measured by preimplanted laser Doppler probes, and animals were exposed to situations involving different types of potentially salient stimuli before and after i.v. administration of CP-154526 (15 mg/kg) or diazepam (4 mg/kg). At rest, ear-pinna blood flow was stable (coefficient of varition=11+/-2) and remained at high level 93+/-13% of test time. Exposure to novel environment elicited flow fluctuations (coefficient of variation=79+/-8) and reduced amount of time spent at high level to 25+/-6%. Defined unconditioned stimuli caused rapid falls in ear-pinna flow, with nociceptive stimulation producing more vigorous and consistent effects (flow response index 0.66+/-0.02) compared with non-nociceptive (flow response index 0.49+/-0.04). CP-154526 slightly raised mean arterial pressure (from 81+/-2 to 93+/-3 mmHg), increased heart rate (from 198+/-1 to 220+/-4 beats/min) and produced a mild vasoconstriction in the ear-pinna bed (flow fell from 46+/-10 to 25+/-6 cm/s). CP-154526 substantially reduced cutaneous vasoconstrictor responses elicited by the exposure to novel environment and by defined non-nociceptive stimuli, with flow-response index fall from 0.53+/-0.10 to 0.17+/-0.09 and from 0.47+/-0.04 to 0.24+/-0.04, respectively, without affecting responses to nociceptive stimuli. Diazepam reduced only vasoconstrictor responses elicited by the exposure to novel environment, with flow-response index fall from 0.40+/-0.12 to 0.27+/-0.07. Sensitivity of rapid changes in rabbit ear-pinna blood flow to anxiolytic drugs supports the idea that increased cutaneous vascular tone reflects enhanced arousal in rabbits.

The central autonomic nervous system: conscious visceral perception and autonomic pattern generation

The overall organization of the peripheral autonomic nervous system has been known for many decades, but the mechanisms by which it is controlled by the central nervous system are just now coming to light. In particular, two major issues have seen considerable progress in the past decade. First, the pathways that provide visceral sensation to conscious perception at a cortical level have been elucidated in both animals and humans. The nociceptive system runs in parallel to the pathways carrying visceral sensation from the cranial nerves and may be considered in itself a component of visceral sensation. Second, structures in the central nervous system that generate patterns of autonomic response have been identified. These pattern generators are located at multiple levels of the central nervous system, and they can be combined in temporal and spatial patterns to subserve a wide range of behavioral needs.

Effect of local application of cold or heat for relief of pricking pain

The present study was designed to determine the effect of the application of cold or heat on the sensation of pricking pain based on autonomic responses. Electrical stimulation was applied to the antebrachium or brachium of subjects as an artificial pricking pain, and skin blood flow (BF) and skin conductance level (SCL) at the fingertip were measured. Pain sensation was evaluated using the visual analog scale. Pain stimulation produced a significant increase in SCL and a significant decrease in BF at both the antebrachium and brachium. Application of cold to the stimulation site using an ice-water pack reduced BF and SCL responses and pain sensation. Application of heat using a hot water bottle caused a significant increase in pain sensation and enhancement of BF and SCL responses. These results suggest that application of cold promotes relief of pricking pain sensation and suppression of autonomic responses, and that application of heat has no such effect. It is important that nurses ascertain the type of pain or source of pain and take proper measures for its relief.

Raphe magnus/pallidus neurons regulate tail but not mesenteric arterial blood flow in rats

In urethane-anesthetized rats with body temperature maintained at 39-40 degrees C, electrical stimulation of raphe magnus/pallidus/parapyramidal region within 0.5 mm of the ventral medullary surface reduced arterial blood flow to the tail cutaneous bed (measured with a chronically implanted Doppler ultrasonic flowmeter) from 28+/-5 to 6+/-1 cm/s (P<0.01), without changing mesenteric arterial blood flow, and with only small, variable changes in arterial pressure. Injection of bicuculline (50 pmol in 50 nl) at the same site reduced tail flow from 19+/-2 to 3+/-1 cm/s (P<0.01), again without significantly changing mesenteric flow, but with a moderate increase in arterial pressure. When the rat was cooled to reduce basal tail blood flow, injection of muscimol (1 nmol in 100 nl) or GABA (100 nmol in 100 nl) into the raphe site restored tail blood flow to 93+/-4% of the pre-cooling level. These recordings are the first reported direct measurements of rat tail blood flow changes elicited by alteration of neuronal function in the brainstem. The rostral medullary raphe controls the tail cutaneous vascular bed in a relatively selective manner. Our findings add to evidence that raphe magnus/pallidus/parapyramidal neurons are involved in regulating cutaneous blood flow in response to changes in body temperature in the rat.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Routine intra-operative assessment of pain and/or depth of anaesthesia by nurse anaesthetists in clinical practice

Patient safety and comfort during general anaesthesia and surgery are to a considerable extent dependent on the capability of anaesthesia personnel to interpret directly monitored as well as indirect clinical signs of pain and/or depth of anaesthesia. The aim of the present study was to evaluate how nurse anaesthetists in their clinical routine work assess and interpret intra-operative responses evoked by pain stimuli and/or insufficient depth of anaesthesia. A questionnaire was designed to assess the perceived relevance and validity of cardiovascular, respiratory, mucocutaneous, eye-associated, and muscular responses for routine assessment of intra-operative pain and/or insufficient depth of anaesthesia in patients undergoing surgery under general anaesthesia. Data were obtained from 223 nurse anaesthetists working at nine different university anaesthesia departments in Sweden. A number of significant indicators for pain and depth of anaesthesia could be identified for spontaneously breathing as well as for mechanically ventilated patients. No variable was considered entirely specific for either intra-operative pain or depth of anaesthesia. Changes in breathing rate/volume, central haemodynamics (BP, HR), lacrimation, and presence of moist and sticky skin were given higher score values as indicators of pain than as indicators of depth of anaesthesia. Occurrence of grimaces, attempted movements, and presence of non-centred pupils were variables considered more indicative of insufficient depth of anaesthesia than intra-operative pain. In conclusion, it is obvious from the present data that indirect physiological signs of intra-operative pain and depth of anaesthesia are still considered of importance by Swedish anaesthesia nurses in the anaesthetic management of surgical patients.

Raphe region mediates changes in cutaneous vascular tone elicited by stimulation of amygdala and hypothalamus in rabbits

Raphe pallidus/parapyramidal neurons control cutaneous vasoconstriction induced by noxious stimuli. To determine whether they mediate forebrain-induced cutaneous vasoconstriction, we assessed changes in ear pinna blood flow elicited by electrical stimulation of amygdala and hypothalamus before and after injection of muscimol into the raphe/parapyramidal region. We compared ear flow with simultaneously recorded mesenteric flow. Experiments were performed in rabbits anesthetized with urethane (1.25-1.5 g/kg), paralysed and mechanically ventilated. Amygdala stimulation reduced skin conductance from 0.32+/-0.06 to 0.10+/-0.02 cm/s per mmHg (P<0.05, n=9), without effect on mesenteric conductance. Hypothalamic stimulation caused vasoconstriction in both cutaneous and mesenteric beds (conductances fell from 0.27+/-0.05 to 0.05+/-0.02 cm/s per mmHg and from 0.27+/-0.06 to 0.14+/-0.04 cm/s per mmHg (P<0.05, n=9), respectively). Muscimol microinjection (5 nmol in 100 nl) to raphe/parapyramidal region eliminated amygdala- and hypothalamus-induced skin vasoconstriction (pre-stimulus conductance 0.42+/-0.13 and 0.41+/-0.11 cm/s per mmHg, post-stimulus 0.41+/-0.12 and 0.39+/-0.10 cm/s per mmHg, respectively), but not hypothalamically-induced mesenteric vasoconstriction (pre-stimulus 0.29+/-0.06, post-stimulus 0.16+/-0.03 cm/s per mmHg, P<0.05, n=8). The latter was strongly attenuated by bilateral injection of muscimol to the rostral ventrolateral medulla. Data suggest that descending hypothalamo-spinal and amygdala-spinal pathways constricting the cutaneous vascular bed relay in the raphe/parapyramidal area. A relay in the rostral ventrolateral medulla contributes substantially to mesenteric vasoconstriction elicited from the hypothalamus.

Regional blood flow and nociceptive stimuli in rabbits: patterning by medullary raphe, not ventrolateral medulla

1. Regional blood flow was measured with Doppler ultrasonic probes in anaesthetized rabbits. We used focal microinjections of pharmacological agents to investigate medullary pathways mediating ear pinna vasoconstriction elicited by electrical stimulation of the spinal tract of the trigeminal nerve or by pinching the lip, and pathways mediating mesenteric vasoconstriction elicited by electrical stimulation of the afferent abdominal vagus nerve. 2. Bilateral injection of kynurenate into the rostral ventrolateral medulla reduced arterial pressure and prevented the mesenteric vasoconstriction and the rise in arterial pressure elicited by abdominal vagal stimulation. However, kynurenate did not prevent ear pinna vasoconstriction or the fall in pressure elicited by trigeminal tract stimulation. Similar injections of muscimol also failed to prevent the trigeminally elicited cardiovascular changes. 3. Injections of kynurenate into the raphe-parapyramidal area did not diminish trigeminally elicited ear vasoconstriction or the depressor response. However, injections of muscimol substantially reduced or abolished the trigeminally elicited ear vasoconstriction, without affecting the depressor response. Raphe-parapyramidal muscimol injections also entirely abolished ear vasoconstriction elicited by pinching the rabbit's lip. 4. The trigeminal depressor response does not depend on either the rostral ventrolateral medulla or the raphe-parapyramidal region. 5. Mesenteric vasoconstriction elicited by stimulation of the afferent abdominal vagus nerve is mediated via the rostral ventrolateral medulla, but ear vasoconstriction elicited by lip pinch or by stimulation of the trigeminal tract is mediated by the raphe-parapyramidal region. Our study is the first to suggest a brainstem pathway mediating cutaneous vasoconstriction elicited by nociceptive stimulation.

Synchronous changes in ear and tail blood flow following salient and noxious stimuli in rabbits

Simultaneous recordings were made of ear and tail blood flow during alerting responses to salient environmental stimuli in conscious rabbits, and during electrical stimulation of the spinal trigeminal tract and raphe pallidus in anesthetized rabbits. Blood flow fell in a highly correlated manner (Pearson coefficient ranging from 0.52 to 0.95) in these experimental situations. Salient stimuli in conscious rabbits, and noxious stimuli in anesthetized rabbits appear to cause a generalized vasoconstriction in cutaneous beds.