Effect of isoflurane on skin-pressure-induced vasodilation

A Systematic Review and Meta-Analysis of the Pressure-Induced Vasodilation Phenomenon and Its Role in the Pathophysiology of Ulcers

BACKGROUND

Physiologic studies show that tissue perfusion increases during moderate amounts of tissue compression. This is attributed to sensory nerves initiating a vasodilatory cascade referred to as pressure-induced vasodilation.

METHODS

PubMed, Embase, and the Cochrane Central Register of Controlled Trials were searched for studies investigating perfusion during pressure exposure longer than 10 minutes. Retrieved studies were assessed using the Office of Health Assessment and Translation Risk of Bias Rating Tool for Human and Animal Studies. Results were pooled with random effects models. The body of evidence was rated using the Office of Health Assessment and Translation approach.

RESULTS

Twenty-nine articles were included, of which 19 articles were included in meta-analyses. The evidence indicates that moderate amounts of tissue compression have the capacity to increase perfusion in healthy humans by 46 percent (95 percent CI, 30 to 62 percent). Using the Office of Health Assessment and Translation approach, the authors found a high level of confidence in the body of evidence. Pressure-induced vasodilation blockade was associated with increased pressure ulcer formation. Pressure-induced vasodilation was impaired by neuropathy and by the drugs diclofenac and amiloride.

CONCLUSIONS

This systematic review and meta-analysis indicates that healthy humans have the capacity to increase local perfusion in response to mechanical stress resulting from tissue compression. Because pressure-induced vasodilation is mediated by sensory nerves, pressure-induced vasodilation emphasizes the importance of sensory innervation for durable tissue integrity. Pressure-induced vasodilation impairment seems to provide a complementary explanation for the susceptibility of neuropathic tissues to pressure-induced lesions.

Impact of intraocular pressure on changes of blood flow in the retina, choroid, and optic nerve head in rats investigated by optical microangiography

In this paper, we demonstrate the use of optical coherence tomography/optical microangiography (OCT/OMAG) to image and measure the effects of acute intraocular pressure (IOP) elevation on retinal, choroidal and optic nerve head (ONH) perfusion in the rat eye. In the experiments, IOP was elevated from 10 to 100 mmHg in 10 mmHg increments. At each IOP level, three-dimensional data volumes were captured using an ultrahigh sensitive (UHS) OMAG scanning protocol for 3D volumetric perfusion imaging, followed by repeated B-scans for Doppler OMAG analysis to determine blood flow velocity. Velocity and vessel diameter measurements were used to calculate blood flow in selected retinal blood vessels. Choroidal perfusion was calculated by determining the peripapillary choroidal filling at each pressure level and calculating this as a percentage of area filling at baseline (10 mmHg). ONH blood perfusion was calculated as the percentage of blood flow area over a segmented ONH area to a depth 150 microns posterior to the choroidal opening. We show that volumetric blood flow reconstructions revealed detailed 3D maps, to the capillary level, of the retinal, choroidal and ONH microvasculature, revealing retinal arterioles, capillaries and veins, the choroidal opening and a consistent presence of the central retinal artery inferior to the ONH. While OCT structural images revealed a reversible compression of the ONH and vasculature with elevated IOP, OMAG successfully documented changes in retinal, choroidal and ONH blood perfusion and allowed quantitative measurements of these changes. Starting from 30 mm Hg, retinal blood flow (RBF) diminished linearly with increasing IOP and was nearly extinguished at 100 mm Hg, with full recovery after return of IOP to baseline. Choroidal filling was unaffected until IOP reached 60 mmHg, then decreased to 20% of baseline at IOP 100 mmHg, and normalized when IOP returned to baseline. A reduction in ONH blood perfusion at higher IOP's was also observed, but shadow from overlying retinal vessels at lower IOP's limited precise measurements of changes in ONH capillary perfusion compared to baseline. Therefore, OCT/OMAG can be a useful tool to image and measure blood flow in the retina, choroidal and ONH of the rat eye as well as document the effects of elevated IOP on blood flow in these vascular beds.

[11C]Acetate rest-stress protocol to assess myocardial perfusion and oxygen consumption reserve in a model of congestive heart failure in rats

This study describes an [(11)C]acetate rest-stress method to obtain an indirect estimate of myocardial blood flow (MBF) and myocardial oxygen consumption (MVO(2)) in rats. Doxorubicin cardiotoxicity was used to test the usefulness of this approach for the assessment of congestive heart failure.

METHODS

[(11)C]Acetate rest-stress studies have been used in clinical research to assess the capacity of the coronary arteries to respond to stress. In this article, we used this approach to assess the cardiotoxicity of doxorubicin in a rat model. The method was first validated in a group of healthy rats and then used to follow the effect of doxorubicin chemotherapy on cardiac function. The effect of doxorubicin on myocardial perfusion and oxygen consumption reserve was measured at rest and under dobutamine stimulation.

RESULTS

Validation of the protocol showed a good correlation between the MBF and MVO(2) (r(2)=.68). The doxorubicin-treated group showed a significant (P=.04) decrease in cardiovascular perfusion reserve at 1.3±0.2 compared with the control animals at 1.6±0.2. Similar results were obtained for the MVO(2) reserve (treated 1.8±0.4 vs. controls 2.3±0.3; P=.02).

CONCLUSIONS

We describe an [(11)C]acetate PET rest-stress protocol for the assessment of congestive heart failure in rats and its application to the follow-up of cardiotoxicity under doxorubicin chemotherapy. This is a rapid and reliable approach to the measurement of cardiac perfusion and oxygen consumption reserve that could be applied to the development of new strategies to reduce the cardiotoxicity of anthracycline.

Potential mechanisms by which a single drink of alcohol can increase transdermal absorption of topically applied chemicals

BACKGROUND

Both chronic and acute ethanol consumption increase transdermal penetration of topically applied xenobiotics. The mechanisms by which this enhancement occurs are unknown. We hypothesized that either the vasodilatory effects of ethanol or its ability to disrupt the lipid bilayer via lipid peroxidation, may be contributing to the increased transdermal absorption observed in alcohol consuming animals.

METHODS

Male Wistar rats were gavaged with 1.5, 3, 4.3, 6 or 10 g/kg ethanol or saline control or were treated with either the vasoconstrictor epinephrine or with the vasodilator prilocaine. Dermal blood flow, transepidermal water loss (TEWL), and skin moisture were non-invasively measured. Transdermal penetration was then determined for four xenobiotics (paraquat, dimethyl formamide (DMF), 2,4-dichlorophenoxyacetic acid (2,4-D) and N,N-diethyl-m-toluamide (DEET)). Lipid peroxidation was also determined by monitoring the formation of malondialdehyde.

RESULTS

Dermal blood flow increased by approximately 27% (p<0.05), TEWL increased 1.12+/-0.2-fold while skin lipid peroxidation increased 1.4-fold (p<0.05) 2h after gavage with 10 g/kg alcohol. Transdermal penetration of paraquat was increased by prilocaine (ER=2.1+/-0.4, p<0.05), but the absorption of DEET, 2,4-D and DMF were not influenced by greater blood flow. Reducing dermal blood flow with epinephrine did not cause any significant changes in transdermal penetration.

CONCLUSIONS

Vasodilation triggered by a single episode of ethanol ingestion is not responsible for the observed increase in transdermal absorption. Ethanol induced changes in lipid peroxidation and TEWL demonstrate that drinking alcohol induces transdermal absorption of xenobiotics.

Neuroendocrine pathway involvement in the loss of the cutaneous pressure-induced vasodilatation during acute pain in rats

Pain is regarded as a risk factor in pressure ulcer development by contributing to immobility. Pressure-induced vasodilatation (PIV) is a mechanism whereby cutaneous blood flow increases in response to progressive locally applied pressure, thereby delaying the occurrence of ischaemia and appearing to be a protective response to local pressure. When the interaction between nervous and vascular systems is deregulated, PIV, which relies on both systems, is absent. We thus hypothesized that acute pain could alter PIV. This study investigated the effects on PIV of acute pain triggered by noxious heat (50 degrees C) applied to the tail of anaesthetized rats. To address the mechanisms underlying these effects, chronic sympathectomy was performed using guanethidine, and the plasma concentrations of pituitary adrenocorticotrophin (ACTH) and catecholamines were measured. Our results show that acute pain induces a loss of PIV associated with an increase of ACTH. Direct involvement of hypertensive effects and peripheral sympathetic nervous system are excluded in the loss of PIV, whereas the activation of brain structures that have descending inhibitory control cannot be excluded. A low dose of systemic morphine prevented this loss of PIV and maintained the ability of the cutaneous microcirculation to adapt to the applied pressure. The loss of a protective response to local pressure (PIV) induced by acute pain lends physiological support to the direct involvement of pain in pressure ulcer development. Therefore, an adequate evaluation and treatment of pain is crucial.

Lyapunov exponents of laser Doppler flowmetry signals in healthy and type 1 diabetic subjects

The skin of diabetic subjects presents abnormalities in capillary blood flow and its regulation, often leading to the generation of plantar ulcers. In order to gain insight into this pathology for type 1 diabetic patients, Lyapunov exponents (LEs) of signals reflecting microvascular perfusion--laser Doppler flowmetry (LDF) signals--are calculated. The algorithm to compute LEs is first validated on simulated data and LDF surrogates. Then, LDF signals recorded at rest and during the application of local and progressive pressure of 11.1 Pa/s are processed. The exponents appear in pairs and are different for healthy and type 1 diabetic subjects at rest; P = 0.0556 for the 7th, 8th, and 9th LEs. Furthermore, progressive pressure has also a distinct effect on LEs. The difference is more pronounced for diabetic patients, for whom P = 0.0625 for the four LEs of highest absolute value. Because these differences arise from abnormalities in microvascular blood flow, they may help to explain the high prevalence of type 1 diabetic patients developing foot ulcers.

Comparison between Hilbert-Huang transform and scalogram methods on non-stationary biomedical signals: application to laser Doppler flowmetry recordings

A significant transient increase in laser Doppler flowmetry (LDF) signals is observed in response to a local and progressive cutaneous pressure application on healthy subjects. This reflex may be impaired in diabetic patients. The work presents a comparison between two signal processing methods that provide a clarification of this phenomenon. Analyses by the scalogram and the Hilbert-Huang transform (HHT) of LDF signals recorded at rest and during a local and progressive cutaneous pressure application are performed on healthy and type 1 diabetic subjects. Three frequency bands, corresponding to myogenic, neurogenic and endothelial related metabolic activities, are studied at different time intervals in order to take into account the dynamics of the phenomenon. The results show that both the scalogram and the HHT methods lead to the same conclusions concerning the comparisons of the myogenic, neurogenic and endothelial related metabolic activities-during the progressive pressure and at rest-in healthy and diabetic subjects. However, the HHT shows more details that may be obscured by the scalogram. Indeed, the non-locally adaptative limitations of the scalogram can remove some definition from the data. These results may improve knowledge on the above-mentioned reflex as well as on non-stationary biomedical signal processing methods.

Cellular mechanisms underlying cutaneous pressure-induced vasodilation: in vivo involvement of potassium channels

In the skin of humans and rodents, local pressure induces localized cutaneous vasodilation, which may be protective against pressure-induced microvascular dysfunction and lesion formation. Once activated by the local pressure application, capsaicin-sensitive nerve fibers release neuropeptides that act on the endothelium to synthesize and release nitric oxide (NO) and prostaglandins, leading to the development of the cutaneous pressure-induced vasodilation (PIV). The present study was undertaken to test in vivo the hypothesis that PIV is mediated or modulated by differential activation of K+ channels in anesthetized rats using pharmacological methods. Local pressure was applied at 11.1 Pa/s. Endothelium-independent and -dependent vasodilation were tested using iontophoretic delivery of sodium nitroprusside (SNP) and acetylcholine (ACh), respectively, and was correlated with PIV response. PIV was reduced after systemic administration of tetraethylammonium (a nonspecific K+ channel blocker), iberiotoxin [a specific large-conductance Ca2+-activated K+ (BKCa) channel blocker], and glibenclamide [a specific ATP-sensitive K+ (KATP) channel blocker], whereas PIV was unchanged by apamin (a specific small-conductance Ca2+-activated K+ channel blocker) and 4-aminopyridine (a specific voltage-sensitive K+ channel blocker). The responses to SNP and ACh were reduced by iberiotoxin but were unchanged by glibenclamide. We conclude that the cellular mechanism of PIV in skin involves BKCa and KATP channels. We suggest that the opening of BKCa and KATP channels contributes to the hyperpolarization of vascular smooth muscle cells to produce PIV development mainly via the NO and prostaglandin pathways, respectively.

Spectral components of laser Doppler flowmetry signals recorded in healthy and type 1 diabetic subjects at rest and during a local and progressive cutaneous pressure application: scalogram analyses

A significant transient increase in laser Doppler flowmetry (LDF) signals is observed in response to a local and progressive cutaneous pressure application in healthy subjects. This reflex may be impaired in diabetic patients. The work presents a signal processing providing the clarification of this phenomenon. Scalogram analyses of LDF signals recorded at rest and during a local and progressive cutaneous pressure application are performed on healthy and type 1 diabetic subjects. Three frequency bands, corresponding to myogenic, neurogenic and endothelial related metabolic activities, are studied. The results show that, at rest, the scalogram energy of each frequency band is significantly lower for diabetic patients than for healthy subjects, but the scalogram relative energies do not show any statistical difference between the two groups. Moreover, the neurogenic and endothelial related metabolic activities are significantly higher during the progressive pressure than at rest, in healthy and diabetic subjects. However, the relative contribution of the endothelial related metabolic activity is significantly higher during the progressive pressure than at rest, in the interval 200-400 s following the beginning of the pressure application, but only for healthy subjects. These results may improve knowledge on cutaneous microvascular responses to injuries or local pressures initiating diabetic complications.

Evidence for the involvement of VPAC1 and VPAC2 receptors in pressure-induced vasodilatation in rodents

A transient increase in skin blood flow in response to an innocuous local pressure application, defined as pressure-induced vasodilatation (PIV), delays the occurrence of ischaemia, suggesting a protective feature against applied pressure. The PIV response depends on capsaicin-sensitive nerve fibres and calcitonin gene-related peptide (CGRP) has been shown to be involved. In these fibres, CGRP coexists with pituitary adenylate cyclase-activating polypeptide (PACAP). Three distinct receptors mediate the biological effects of PACAP: VPAC1 and VPAC2 receptors binding with the same affinity for PACAP and vasoactive intestinal peptide and PAC1 receptors showing high selectivity for PACAP. Because the receptors are widely expressed in the nervous system and in the skin, we hypothesized that at least one of them is involved in PIV development. To verify this hypothesis, we used [D-p-Cl-Phe(6),Leu(17)]-VIP (nonspecific antagonist of VPAC1/VPAC2 receptors), PG 97-269 (antagonist of VPAC1 receptors), PACAP(6-38) (antagonist of VPAC2/PAC1 receptors) and Max.d.4 (antagonist of PAC1 receptors) in anaesthetized rodents. The blockade of VPAC1/VPAC2, VPAC1 or VPAC2/PAC1 receptors eliminated the PIV response, whereas PAC1 blockade had no effect, demonstrating an involvement of VPAC1/VPAC2 receptors in PIV development. Moreover, endothelium-independent and -dependent vasodilator responses were unchanged by the VPAC1/VPAC2 antagonist. Thus, the absence of a PIV response following VPAC1/VPAC2 blockade cannot be explained by any dysfunction of the vascular smooth muscle or endothelial vasodilator capacity. The involvement of VPAC1/VPAC2 receptors in the development of PIV seems to imply a series relationship in which each receptor type (CGRP, VPAC1, VPAC2) is necessary for the full transmission of the response.