Venular oscillatory flow during hemorrhagic shock and NO inhibition in hamster cheek pouch microcirculation

Hemorrhagic Shock and the Microvasculature

The microvasculature plays a central role in the pathophysiology of hemorrhagic shock and is also involved in arguably all therapeutic attempts to reverse or minimize the adverse consequences of shock. Microvascular studies specific to hemorrhagic shock were reviewed and broadly grouped depending on whether data were obtained on animal or human subjects. Dedicated sections were assigned to microcirculatory changes in specific organs, and major categories of pathophysiological alterations and mechanisms such as oxygen distribution, ischemia, inflammation, glycocalyx changes, vasomotion, endothelial dysfunction, and coagulopathy as well as biomarkers and some therapeutic strategies. Innovative experimental methods were also reviewed for quantitative microcirculatory assessment as it pertains to changes during hemorrhagic shock. The text and figures include representative quantitative microvascular data obtained in various organs and tissues such as skin, muscle, lung, liver, brain, heart, kidney, pancreas, intestines, and mesentery from various species including mice, rats, hamsters, sheep, swine, bats, and humans. Based on reviewed findings, a new integrative conceptual model is presented that includes about 100 systemic and local factors linked to microvessels in hemorrhagic shock. The combination of systemic measures with the understanding of these processes at the microvascular level is fundamental to further develop targeted and personalized interventions that will reduce tissue injury, organ dysfunction, and ultimately mortality due to hemorrhagic shock. Published 2018. Compr Physiol 8:61-101, 2018.

A new hypothesis for microvascular inflammation in shock and multiorgan failure: self-digestion by pancreatic enzymes

Shock is accompanied by a severe inflammatory cascade in the microcirculation, the origin of which has been hypothesized in the past to be associated with specific mediators such as endotoxin, oxygen free radicals, nitric oxide, cytokines, and lipid products. But no intervention with clinical effectiveness has been derived from these ideas to date. The authors propose here a new hypothesis suggesting that degradative enzymes, synthesized in the pancreas as part of normal digestion, may play a central role in shock and multiorgan failure. These powerful enzymes have the ability to digest almost every biological material. Self-digestion (i.e. autodegradation) is prevented by compartmentalizing the fully activated degradative enzymes in the intestinal lumen by the mucosal barrier. In shock, maintenance of the mucosal barrier is impaired and it becomes permeable to pancreatic enzymes. Digestive enzymes thereby gain access to the wall of the intestine and initiate self-digestion of submucosal extracellular matrix proteins and interstitial cells. The process leads to generation and release of a host of strong inflammatory mediators. The authors hypothesize that inhibition of pancreatic enzymes in the lumen of tile intestine can serve to attenuate formation of these inflammatory mediators in ischemic tissues following hemorrhagic shock, and consequently prevent cell and tissue injury as well as multiorgan failure.

Heterogeneous Microcirculation in the Rat Small Intestine during Hemorrhagic Shock: Quantification of the Effects of Hypertonic-Hyperoncotic Resuscitation

PURPOSE

In the event of a spatial or temporal microvascular perfusion heterogeneity conventional methods are often inadequate to describe the microcirculatory changes. Our aim was to use a new formula to characterize and compare the microcirculatory reactions in the mucosa and longitudinal muscle of the rat small intestine in response to hypertonic/hyperoncotic and normotonic resuscitation strategies.

METHODS

Intravital videomicroscopy with an orthogonal polarization spectral (OPS) imaging technique was utilized. Microcirculatory variables were recorded during hemorrhagic shock (HS; 50 mm Hg mean arterial pressure for 60 min) and fluid replacement with 0.9% saline or with 7.2% saline containing 10% hydroxyethylstarch 200/0.5 (Osmohes; 4 ml/kg). Due to the temporal perfusion variability, microcirculatory changes were described using the calculation of the average red blood cell velocity (A-RBCV), while the spatial changes were calculated as a function of the size of the perfused capillary network.

RESULTS

During HS and the late phase of resuscitation, perfusion was characterized by capillary flow motion (i.e. variability in time) in the villi, and by spatial flow heterogeneity in the longitudinal muscle layer. The approximately 40% decrease in the calculated villus A-RBCV during HS was only partially affected by 0.9% saline, whereas Osmohes completely restored A-RBCV by increasing both the red blood cell velocity and the duration of high-flow periods at the onset of resuscitation in the villi. The approximately 60% reduction in A-RBCV in the muscle layer during HS was not followed by an appreciable recovery in either group, but Osmohes significantly increased A-RBCV in the late resuscitation phase.

CONCLUSIONS

The hypertonic/hyperoncotic solution induces a considerable microcirculatory improvement in two distinct layers of the small intestine after HS. This positive effect is related to the amelioration of the intestinal microcirculatory heterogeneity.

Regulation of human cutaneous circulation evaluated by laser Doppler flowmetry, iontophoresis, and spectral analysis: importance of nitric oxide and prostaglandines

Nitric oxide (NO) and prostaglandines (PGs) are important in regulation of vascular tone and blood flow. Their contribution in human cutaneous circulation is still uncertain. We inhibited NO synthesis by infusing N(G)-monomethyl-L-arginine (L-NMMA) in the brachial artery (16 micromol/min for 5 min) and reversed it by intraarterial infusion of L-arginine (40 micromol/min for 7.5 min). PG synthesis was inhibited by the cyclooxygenase inhibitor aspirin (600 mg over 5 min intravenously). Basal cutaneous perfusion and perfusion responses during iontophoresis with the endothelium-dependent vasodilator acetylcholine (ACh) and the endothelium-independent vasodilator sodium nitroprusside (SNP) were recorded by laser Doppler flowmetry (LDF). We performed wavelet transforms of the measured signals. Mean spectral amplitude within the frequency interval from 0.0095 to 1.6 Hz and mean and normalized amplitudes of five intervals around 1, 0.3, 0.1, 0.04, and 0.01 Hz were analysed. The oscillations with frequencies around 1, 0.3, 0.1, and 0.04 Hz are influenced by the heartbeat, the respiration, the intrinsic myogenic activity of vascular smooth muscle, and the neurogenic activity of the vessel wall, respectively. We have previously shown that the oscillation with a frequency around 0.01 Hz is modulated by the vascular endothelium. L-NMMA reduced mean value of the LDF signal by approximately 20% (P = 0.0067). This reduction was reversed by L-arginine. Mean value of the LDF signals during ACh and SNP iontophoresis did not change after infusion of L-NMMA. Aspirin did not affect mean value of the LDF signal or the LDF signal during ACh or SNP iontophoresis. Before interventions the only significant difference between the effects of ACh and SNP was observed in the frequency around 0.01 Hz, where ACh increased normalized amplitude to a greater extent than SNP. L-NMMA abolished this difference, whereas it reappeared after infusion of L-arginine (P = 0.0084). Aspirin did not affect this difference (P = 0.006). We conclude that basal cutaneous blood flow and the endothelial dependency of the oscillation around 0.01 Hz are partly mediated by NO, but not by endogenous PGs. Other aspects of human cutaneous circulation studied are not regulated by NO or PGs.

Differential effects of intravenous anaesthetic agents on the response of rat mesenteric microcirculation in vivo after haemorrhage

BACKGROUND

The differential effects of i.v. anaesthesia on the response of the mesenteric microcirculation after haemorrhage in vivo are previously unexplored.

METHODS

Male Wistar rats (n=56) were anaesthetized intravenously either with propofol and fentanyl (propofol/fentanyl), ketamine or thiopental. A tracheostomy and carotid cannulation were performed and the mesentery surgically prepared for observation of the microcirculation using fluorescent in vivo microscopy. Animals were allocated to one of three groups: control, haemorrhage or haemorrhage re-infusion.

RESULTS

After haemorrhage, the response of the microcirculation differed during propofol/fentanyl, ketamine and thiopental anaesthesia. During propofol/fentanyl anaesthesia there was constriction of arterioles (-16.7 (3.9)%), venules (-5.9 (1.7)) and capillaries (-16.3 (2.8)) (n=12). During ketamine and thiopental anaesthesia both constriction and dilation was observed. After haemorrhage and re-infusion, macromolecular leak occurred from venules during propofol/fentanyl and thiopental anaesthesia (P<0.05), but not during ketamine anaesthesia.

CONCLUSION

In summary, i.v. anaesthetic agents differentially alter the response of the mesenteric microcirculation to haemorrhage.

Increased viscosity is protective for arteriolar endothelium and microvascular perfusion during severe hemodilution in hamster cheek pouch

We hypothesized that during severe hemodilution (SH), i.e., hemodilution beyond 50%, the reduced conditions of shear stress result in endothelium dysfunction and subsequent vasoconstriction. To evaluate the endothelial responses associated with the reduction of hematocrit we tested the responsiveness of arterioles to acetylcholine. Infusion with high-molecular-weight dextran 500,000 following SH was characterized by laser Doppler flowmetry (LDF), total perfused capillary length (TPCL), vessel diameter, and red blood cell (RBC) velocity as visualized by fluorescence microscopy in the hamster cheek pouch. Hemodilution was performed by blood removal and simultaneous infusion of 6% dextran 70. LDF increased significantly during hemodilution in arterioles and in venules (100 +/- 20 vs 37 +/- 11 and 34.2 +/- 3.5 vs 28.6 +/- 4.0 perfusion units, PU, respectively). During the final step of hemodilution LDF decreased significantly to 12 +/- 4.5 PU in arterioles and 6.2 +/- 1.5 PU in venules, which correlated with the decrease of arteriolar RBC velocity. Arterioles constricted and TPCL decreased significantly (-35 +/- 5.3%). Hyperviscosity infusion significantly raised arteriolar and venular LDF to 184 +/- 15 and 40.2 +/- 3.5 PU, arterioles dilated, RBC velocity, TPCL, and mean blood pressure. There was an impairment of endothelial-dependent dilation that was not present in the group with dextran 500, which suggests that viscosity was involved in the development of vasoconstriction during SH. In conclusion, an increase in plasma viscosity has beneficial effects on the microcirculation during SH that might preserve arteriolar endothelium and capillary perfusion.