Omental Circulation in Morphinized Dogs Subjected to Graded Hemorrhage

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.

Prolonged observation time reveals temporal fluctuations in the sublingual microcirculation in pigs given arginine vasopressin

Intravital videomicroscopy of sublingual microcirculation is used to monitor critically ill patients. Existing guidelines suggest averaging handheld video recordings of ∼20 s in duration from five areas. We assessed whether an extended observation time may provide additional information on the microcirculation. Pigs (n = 8) under general anesthesia were divided between two groups, one with manually held camera, in which microcirculation was assessed continuously for 1 min in five areas, and one with a fixed camera, in which the observation time was extended to 10 min in a single area. The microcirculation was challenged by infusing arginine vasopressin (AVP). In the fixed group, ischemic acute heart failure was induced by left coronary microembolization, and the AVP infusion was repeated. All recordings were divided into 20-s sequences, and the small-vessel microvascular flow index (MFI) was scored and averaged for each measurement point. When administering 0.003, 0.006, and 0.012 IU·kg(-1)·min(-1) of AVP, we observed that the small-vessel MFI in the fixed 10-min group was significantly reduced (2.03 ± 0.38, 0.98 ± 0.18, and 0.48 ± 0.11) compared with both the initial 20 s (2.77 ± 0.04, 2.06 ± 0.04, and 1.74 ± 0.06; P < 0.05) and the 1-min total (2.63 ± 0.09, 1.70 ± 0.07, and 1.33 ± 0.16; P < 0.05) in the handheld group. In acute heart failure, the cardiac output decreased to half of the preischemic values. Interestingly, the small-vessel MFI was more affected by the administration of 0.001 and 0.003 IU·kg(-1)·min(-1) of AVP in acute heart failure (1.62 ± 0.60 and 1.16 ± 0.38) compared with preischemic values (2.86 ± 0.09 and 2.03 ± 0.38; P < 0.05). In conclusion, a prolonged recording time reveals temporal heterogeneity that may impact the assessment of microcirculatory function.

Bang-bang model for regulation of local blood flow

The classical model of metabolic regulation of blood flow in muscle tissue implies the maintenance of basal tone in arterioles of resting muscle and their dilation in response to exercise and/or tissue hypoxia via the evoked production of vasodilator metabolites by myocytes. A century-long effort to identify specific metabolites responsible for explaining active and reactive hyperemia has not been successful. Furthermore, the metabolic theory is not compatible with new knowledge on the role of physiological radicals (e.g., nitric oxide, NO, and superoxide anion, O2 (-) ) in the regulation of microvascular tone. We propose a model of regulation in which muscle contraction and active hyperemia are considered the physiologically normal state. We employ the "bang-bang" or "on/off" regulatory model which makes use of a threshold and hysteresis; a float valve to control the water level in a tank is a common example of this type of regulation. Active bang-bang regulation comes into effect when the supply of oxygen and glucose exceeds the demand, leading to activation of membrane NADPH oxidase, release of O2 (-) into the interstitial space and subsequent neutralization of the interstitial NO. Switching arterioles on/off when local blood flow crosses the threshold is realized by a local cell circuit with the properties of a bang-bang controller, determined by its threshold, hysteresis, and dead-band. This model provides a clear and unambiguous interpretation of the mechanism to balance tissue demand with a sufficient supply of nutrients and oxygen.

Peripheral vascular reactions in anaphylaxis of the mouse

Pronounced vascular changes occurring in the ears and claws of mice during anaphylactic shock are described. Practically at once after a foreign serum (pig, horse, or rabbit) enters the blood stream of sensitized animals both the arterial and venous vessels undergo marked, local or generalized constriction in the organs mentioned. Usually spasm of the vessel walls occurs simultaneously in the arteries and veins, but it may appear first in the arteries, or occasionally in the veins. When venous spasm precedes arterial spasm, the true capillaries become distended with cells; if the reverse order holds, the ears appear bloodless. There is no active constriction or dilatation of capillaries; the capillary behavior follows passively the changes in the large vessels. Peripheral vascular spasm occurs while the carotid blood pressure is high, but a few minutes later, while this still holds true, the ear vessels begin to relax and the circulation is resumed. Shortly afterwards the blood pressure falls to levels far below normal, but the vessels remain open. If the circulation of one ear is obstructed while anaphylactic shock is produced, no vascular spasm occurs in it. Release of the obstruction during the animal's recovery results in belated constriction of the blood vessels of this ear although by now the vessels in the other ear are dilated and the general systolic blood pressure is very low. The vascular reactions in the ears appear to be uninfluenced by the blood pressure in the large vessels, and they are not a response to nervous stimuli. They are local in origin. The vascular changes are often not clearly perceptible in the gross but are plainly to be seen under a low power of the microscope. They occur in some sensitized mice exhibiting no manifest signs of shock, differing only in degree from the changes taking place when shock is severe or fatal.

Hepatorenal factors in circulatory homeostasis. IV. Tissue origins of the vasotropic principles, VEM and VDM, which appear during evolution of hemorrhagi and tourniquet shock

Studies were carried out in dogs and rabbits subjected to hemorrhagic and tourniquet shock to determine the tissue origins of two newly described vasotropic principles, VEM and VDM. The vasoexcitor principle, VEM, predominates in the blood during the initial compensatory stage and was traced to the kidney. The vasodepressor principle, VDM, prevails during the decompensatory phase and was found to originate in the liver, skeletal muscle and spleen. During the decompensatory phase, there was a progressive deterioration of the hepatic mechanisms for inactivating VDM. Sequential tissue hypoxia during shock is probably responsible for the formation of these vasotropic principles and for the deterioration of the hepatic VDM inactivation system. Special emphasis is placed on the effects of local concentrations of VDM within the liver and on the resultant diversion of blood from the general circulation into the splanchnic viscera.

Low frequency flowmotion/(vasomotion) during patho-physiological conditions

The role of low frequency flowmotion in physiological or pathophysiological settings is unclear. We performed various series of experiments in young anesthetized New Zealand white (NZW) rabbits. Many animals exhibited flowmotion during control conditions. However, they very often seemed to be in unstable physiological conditions, and our preset inclusion criteria (as to arterial pressure and blood gases) were frequently not met.Therefore, in a first series, we correlated these systematically with the incidence of flowmotion. Eleven of 35 anesthetized rabbits, subjected to extensive surgery, showed flowmotion with a median frequency of 1.5 cpm and a relative "amplitude" of 32%. Arterial pressure was 10 mmHg lower, bicarbonate, base-excess, and PCO(2) values and relative blood flow were also significantly lower compared to animals not exhibiting flowmotion. In a second series, we tested whether flowmotion could be induced by an isolated metabolic acidosis in animals meeting the inclusion criteria and not showing flowmotion at control. Here, flowmotion was induced in 9/10 cases (p < 0.01) 30 min after the start of an HCl-infusion. In a third study, we related the onset of flowmotion to the pressure/flow autoregulation curve. At locally reduced blood pressure all 23 rabbits exhibited flowmotion (p < 0,00001) in the gastrocnemius and the tenuissimus muscles, with maximum flowmotion at a locally reduced blood pressure of 30 mmHg; the LDF-flux level showing 67% of control flow.These results support the concept that low frequency periodic hemodynamics are a characteristic of pathophysiological conditions like hypoperfusion or acidosis rather than indicating a normal physiological state.

Contribution of vasomotion to vascular resistance: a comparison of arteries from virgin and pregnant rats

Intrinsic oscillatory activity, or vasomotion, within the microcirculation has many potential functions, including modulation of vascular resistance. Alterations in oscillatory activity during pregnancy may contribute to the marked reduction in vascular resistance. The purpose of this study was 1) to mathematically model the oscillatory changes in vessel diameter and determine the effect on vascular resistance and 2) to characterize the vasomotion in resistance arteries of pregnant and nonpregnant (virgin) rats. Mesenteric arteries were isolated from Sprague-Dawley rats and studied in a pressurized arteriograph. Mathematical modeling demonstrated that the resistance in a vessel with vasomotion was greater than that in a static vessel with the same mean radius. During constriction with the alpha1-adrenergic agonist phenylephrine, the amplitude of oscillation was less in the arteries from pregnant rats. We conclude that vasomotor activity may provide a mechanism to regulate vascular resistance and blood flow independent of static changes in arterial diameter. During pregnancy the decrease in vasomotor activity in resistance arteries may contribute to the reduction in peripheral vascular resistance.

Microvessel diameter changes during hemorrhagic shock in unanesthetized hamsters

The effects of hypovolemic shock on the time-dependent diameter changes of small arteries and arterioles were studied in the hamster skin fold window preparation. This experimental model permits the visualization of the microvasculature without the effects of acute surgery, anesthesia, and exposure. In these conditions, all the arterial microvessels showed vasomotion, while the venules and small veins, that were also studied, did not show rhythmic diameter changes. Hemorrhage was induced by the withdrawal of blood through a chronically implanted arterial catheter. The mean arterial blood pressure was reduced to 40 mm Hg in 20 min, and was maintained at this value for an additional 30-min period. Reinfusion of the withdrawn blood was made at 50 min. During the shock period, vasomotion disappeared in all arterial vessels. The small arteries and arterioles, A1 (70-100 micron, mean diameter), A2 (40-70 micron, md), and A3 (15-40 micron, md), contracted by 20 +/- 7, 33 +/- 10, and 34 +/- 11% of the control mean diameter, respectively. A4 terminal arterioles (less than 15 micron, md) dilated after the onset of bleeding; their rhythmic diameter changes subsequently stopped and their mean diameter increased by 75 +/- 7% of the original value. V1 small veins (150-200 micron, md) contracted during shock, while V2 (35-55 micron, md), V3 (25-35 micron, md), and V4 (15-25 micron, md) venous vessels did not show any significant change. Reinfusion of shed blood caused the reappearance of vasomotion; control vasomotion patterns recovered after reinfusion. Our results indicate that the microcirculatory responses to hypovolemic shock are dependent on the vessel type; this inhomogeneous reactivity may be due to the different responsiveness of microvessels to the mechanisms elicited by hemorrhage.

Intra-aortic balloon counterpulsation: potential for therapy in hemorrhagic shock with associated myocardial failure

After attaching appropriate monitoring devices enabling the measurement of the slope of the left ventricular function curve, left atrial pressure, mean aortic pressure, peak left ventricular pressure, and tension time index, three groups of ten dogs were subjected to varying periods of hemorrhagic shock until a slope of their ventricular function curve was reduced to either 75% (Group I), 50% (Group II), or 25% (Group III) of their baseline value. Resuscitation was attempted in all dogs by the intravenous infusion of shed blood plus additional balanced salt solution. This infusate was administered to maintain either the mean aortic pressure within 15 mm Hg of the baseline value or a left atrial pressure of 15 mm Hg, whichever occurred forst. One half of the dogs received, in addition, intra-aortic balloon counterpulsation. All dogs not receiving counterpulsation expired within two hours. There was no apparent effect of counterpulsation on Group I animals. Three of five animals (Group II) and four of five animals (Group III) receiving counterpulsation survived to the end of the experiment with significant (p smaller than .01) improvement in the parameters monitored. The utilization of counterpulsation as an adjunct to treatment in hemorrhagic shock is suggested.

Current concepts on the management of shock

In the handling of a patient with shock it is essential to identify the underlying cause in order to plan rational treatment. On the basis of information presently available, the specific causes of shock have been classified into 6 groups: hypovolemia, cardiac failure, bacteremia, hypersensitivity, neurogenic factors, and obstruction to blood flow. Treatment was discussed with reference to these groups.

Vasopressor agents are helpful in most instances of shock related to cardiac failure, bacteremia, and hypersensitivity. They usually are contraindicated in shock due to vascular obstruction and in hypovolemic shock until optimal replacement of fluid has been achieved. Recent studies have indicated that metaraminol may be the pressor amine of choice because it is therapeutically effective, simple to administer, without risk of injury to skin and subcutaneous tissues, and available for injection without additional fluid (thus especially suitable for patients with renal failure).

Rigorous attention to the fluid and electrolyte state is of special importance. In the presence of acidosis, the response to vasopressor agents is greatly diminished. The use of molar solution of sodium lactate to re-establish this responsiveness has met with limited success and seems worthy of trial in selected cases.

Adrenocortical hormones may be of striking benefit in shock due to bacteremia or hypersensitivity when an overwhelming response to inflammation threatens life. These drugs may be used also to augment the effectiveness of vasopressor drugs. Relatively little risk is involved, provided that the periods of employment are short and that antibiotics are used concurrently.

The indications for the use of digitalis glycosides in shock are the same as at other times, and their routine use is of no proved benefit and may be injurious. Atropine is of value when excessive vagal activity with bradycardia produces or complicates the hypotensive state.

Chlorpromazine is of no proved worth in the treatment of shock, and possible benefits achieved with anticoagulants are not established as yet. Preliminary observations suggest that hypothermia may be of some value. The head-down position provides only transient benefit in patients with shock, and its prolonged or routine use may delay recovery.

The relationship between the vascular manifestations of shock produced by endotoxin, trauma, and hemorrhage. I. Certain similarities between the reactions in normal and endotoxin-tolerant rats

The vascular effects of lethal doses of E. coli endotoxin, as observed in the mesentery of the rat, resemble the reactions of traumatic and hemorrhagic shock in the following respects: a profound inhibition of arteriolar and precapillary reactivity to topical epinephrine occurs after an initial stage of hyperreactivity; the small veins show failure to relax completely following constrictor doses of epinephrine; and the terminal vessels develop an unusual sensitivity to fluctuations in temperature of the fluid irrigating the tissue. Rats in which tolerance to bacterial endotoxin is induced, by repeated doses given daily, become highly resistant to the lethal effects of both drum trauma and hemorrhagic shock. However, rats in which the adaptation to traumatic shock is produced by repeated exposure to drum trauma, do not develop a significant degree of tolerance to lethal doses of endotoxin. The injection of small non-lethal doses of bacterial endotoxin during non-lethal episodes of trauma or hemorrhage, leads to the development of irreversible shock and death. The bearing of these findings on the problem of the relationship between endotoxin and traumatic shock is discussed.

The relationship between the vascular manifestations of shock produced by endotoxin, trauma, and hemorrhage. II. The possible role of the reticulo-endothelial system in resistance to each type of shock

In studies designed to establish the interrelationship between bacterial endotoxins and the vascular sequelae of hemorrhagic and traumatic shock, the effect of factors known to influence the phagocytic behavior of the reticulo-endothelial system (RES) were investigated. Measures which induced a so called "blockade" of the RES were uniformly associated with an exacerbation of the vascular effects of the endotoxin of E. coli. Such pretreatment also counteracted the cross-tolerance induced by endotoxins against the lethal effects of hemorrhage or drum trauma. The vascular reactions characteristic of irreversible hemorrhagic shock could be simulated by a combination of pretreatment with carbon or proferrin and the infusion of small doses of E. coli endotoxin. An increase in the phagocytic activity of the RES, induced by repeated injections of certain colloids, was associated with an enhanced tolerance of shock. Measurement of carbon clearance values indicated that although an augmented phagocytic capacity was present in rats with induced tolerance to bacterial endotoxins, the development of resistance to trauma was not associated with a comparable change in the phagocytic function of the RES.

The role of epinephrine in the reactions produced by the endotoxins of gram-negative bacteria. II. The changes produced by endotoxin in the vascular reactivity to epinephrine, in the rat mesoappendix and the isolated, perfused rabbit ear

The effects of endotoxin on the epinephrine reactivity of blood vessels in the rat mesoappendix have been studied. Following intravenous injection of a relatively small, sublethal dose of endotoxin, the terminal arterioles and venules exhibited greatly augmented and prolonged vasoconstrictor responses to epinephrine and norepinephrine. Hyperreactivity became evident within 30 minutes after injection of endotoxin, and persisted for as long as 6 hours. After larger doses of endotoxin, sufficient to cause illness or death, the vascular hyperreactivity to epinephrine was of briefer duration, and was followed by a stage of increasing hyporeactivity reaching levels much below normal. With lethal doses, the terminal arterioles and venules became completely refractory to epinephrine, while heightened reactivity persisted in the larger arteries and veins. The end result was pooling of stagnant blood in distended capillaries and venules, accompanied by the appearance of petechiae. Topical applications of epinephrine during this stage were followed promptly by an increase in petechial hemorrhage at the site of testing. Rats which were rendered tolerant to the lethal effect of endotoxin, by repeated daily injections of small doses, developed resistance to the effects of endotoxin on epinephrine reactivity. Neither hyperreactivity nor hyporeactivity to epinephrine were demonstrable in these animals, nor were spontaneous abnormalities of blood flow or petechial hemorrhages observed in the mesoappendix. Analogous results were obtained in perfusion studies of the vessels of the isolated rabbit ear. Perfusion of small amounts of endotoxin was followed within a few minutes by potentiation of epinephrine reactivity. Larger doses caused complete reversal of this effect, to such an extent that epinephrine now produced marked degrees of vasodilation. The possible meaning of these observations in the interpretation of the endotoxin-epinephrine skin lesions described in the preceding paper is discussed. It is suggested that abnormal reactions to epinephrine or norepinephrine in the tissues of intact animals may represent a basic mechanism in the intoxicating and tissue-damaging properties of endotoxin.

Impending Hemorrhagic Shock and the Course of Events following Administration of Dibenamine

By blocking sympathogenic vasoconstrictor mechanisms with dibenamine, a dog's tolerance to severe hemorrhagic hypotension states is definitely improved. New evidence is presented that partial release from vasoconstriction achieved by, giving small doses of dibenamine during the impending shock state seems to provide animals with even greater tolerance to this standardized form of stress. Although the use of dibenamine clinically for this purpose seems contraindicated, the dangers of at administering vasoconstrictor drugs to patients in impending shock are emphasized.