J. clin. Path., 23, Suppl. (Roy. Coil. Path.), 4, 10-15

The microcirculation in shock J Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from

A. L. STALKER From the Department ofPathology, University of Aberdeen

It is right that attention should be directed early the Society's 'Techniques used in the study of in this symposium to the microcirculation in microcirculation' (Chambers and Monro, 1969). shock for it is at this level of the circulation that A note on the problems encountered in this field we must look for an explanation of this complex is, however, necessary. and sometimes irreversible clinical condition. In man the magnification achieved by the The word 'shock' is convenient and firmly available methods of study (nail-fold, skin capil- established in medical usage but gives no indica- lary, retinal and conjunctival microscopy) is tion of the multifactorial nature of a process inadequate for the visualization of individualcopyright. probably best defined as loss of microcirculatory blood cells that is a prerequisite for analysis of control with hypoperfusion. From the con- flow in capillaries (Stalker, 1964). High magnifica- sequences of this low flow state stem the clinical tion recordings have, however, recently been and biochemical features of shock. made in the transilluminated human omentum We know much about the cardiovascular (Bond, Guest, and Derrick, 1968), and Brane- responses to shock at the major circulatory levels mark (1966), using special titanium chambers but in the microcirculation the changes are more inserted into the arms of volunteers, has obtained difficult to explain, although relatively simple to excellent records of capillary flow. Inevitably forhttp://jcp.bmj.com/ describe. The mechanisms for homeostatic adjust- high magnification work one has to turn to the ment are numerous, interlinked, and far from experimental animal and a bewildering range of completely established; they are well reviewed in techniques. The main problems here include 'Microcirculation as related to shock' (Shepro species and organ variations in flow patterns; and Fulton, 1968). It is impossible in this com- individual variations in vasomotor activity; the munication to review the subject compre- ever-present risk that injury, however trivial, hensively, and I shall, therefore, give a short during preparation of the tissue or organ will on October 4, 2021 by guest. Protected account of microcirculatory techniques and the affect the observations; the impossibility as yet normal microcirculation, indicate the main of identifying and recording random sequential changes seen in shock, and select a few topics of events occurring at different areas of a large current importance and interest. vascular field; the difficulty in seeing deeply into the vasculature of a solid organ; and the need for extensive cine recording, much of it at speeds of 200 pictures per second or faster. Because of these Methods for Study of the Microcirculation limitations much of the literature on micro- circulation is still descriptive and accurate Microcirculatory methods abound-and there is measurement is often impossible. still ample scope for ingenuity in devising further An obvious advantage of the microcirculatory techniques. A resume of existing methods will approach is its very dynamism. It can transform not be attempted here. The British Micro- concepts based solely on study of tissue morphol- circulation Society, formed in 1963, exists to ogy. A further benefit is its ready correlation promote awareness of the techniques and hence with ultrastructural studies: the well known illus- their further use and improvement: a useful trations of leucocyte emigration (Florey and introduction, with bibliography, to some of the Grant, 1961) are early examples of an approach more commonly used methods will be found in that is being increasingly adopted. The microcirculation in shock 11 J Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from

P02 / pco2 glucocorticoids, bradykinin, amines

ARTERIOLE 50- 100/i

TERMINAL ARTERIOLE <50,u

PRECAPILLARY SPHI NCTER 7-15,u

CAPILLARY copyright. to 8,

POSTCAPILLARY

VENULE http://jcp.bmj.com/ 8-30p on October 4, 2021 by guest. Protected

MUSCULAR VENULES 30-100,

Figure Diagrammatic representation of the normal microcirculation and its control. A. L. Stalker 12

Most microcirculatory problems are clearly control. Examples are hormones, such as gluco-J Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from interdisciplinary and will continue to be tackled corticoids, and pharmacologically active agents, in the experimental animal, and the choice of such, as histamine, bradykinin, and catecho- species, organ, or tissue, and method is all- lamines. They act either on vasomotor activity or important. Whenever possible results should te capillary permeability and adjust flow to local confirmed in several species. requirements. Rheological control is best considered together with the changes in shock. The Normal Microcirculation and Its Control The detailed anatomy of the microcirculation The Microcirculation in Shock (Fig.) is still under investigation. A terminology derived from the classical early studies by optical Whether the cause be haemorrhage, trauma, microscopy and the recent careful ultrastructural burns, or other stimulus, the loss of intravascular work of Rhodin (1967, 1968) would find reason- fluid determines a distinct sequence of events in able acceptance today. Thevessels (with diameters) the experimental animal. There is an immediate are, in sequence, arterioles (50-100 [u), terminal homeostatic vasoconstrictor response with arteriole (less than 50 gL); precapillary sphincter arteriolar narrowing, reduction in the number of (7-15 g); capillary (up to 8 ,u); postcapillary open capillaries, and augmented venous return venule (8-30 ,u); collecting and muscular venules due to venospasm. Tissue anaerobiosis develops. (30-100 g). Smooth muscle is present in the The intensity of this circulatory adaptation varies arteriolar wall and forms a flow-regulating mech- in different organs; it can be maintained for only anism in the precapillary sphincter; it is absent a limited time. from capillaries and postcapillary venules but The arteriolar and precapillary sphincter gradually reappears in the larger venules. The control fails as a result of hypoxia and the effect arterioles and muscular venu1es are innervated. of vasodilating agents circulating generally or, The capillaries and postcapillary venules are more probably, accumulating locally as a result simple endothelial tubes, capillary fenestration of anaerobic metabolism. Vasomotor paralysiscopyright. increasing to a maximum at the venous end of may ensue. The venular channels are less sensitive the capillary just before the postcapillary venule to hypoxia and, surprisingly, seem to resist to receives its characteristic additional connective some degree the dilating effect oflocal metabolites. tissue and pericyte support. The micro-archi- Capillary engorgement follows and flow is slug- tecture of the capillary and postcapillary network gish or stagnant; outward movement of fluid is highly variable as is the length of this segment. occurs; the red cells in the now more concen- Control of the normal microcirculation (Fig.) is trated plasma aggregate and the leucocytes of paramount importance in considering the marginate and show increased adherence to thehttp://jcp.bmj.com/ breakdown that occurs in shock. It is neurogenic, endothelium. Blood flow in capillaries, and par- humoral, and rheological. Although other pos- ticularly in postcapillary venules, is at this stage sibilities exist, the main certain neurogenic characteristically viscous. This is even more control is effected by a baroreceptor reflex marked if the shock is of haemoconcentrated through adrenergic vasoconstrictor nerves supply- type as in burns. Occasionally microthrombus ing arterioles, sphincters, and muscular venules. formation, or the passage of small white micro- This response operates at can be seen in the differing intensities in emboli, postcapillary venules. on October 4, 2021 by guest. Protected the various regions, and serves, in hypotension, If this degree of microcirculatory hypo- to mnaintain cerebral and cardiac flow. The perfusion is treated by fluid replacement these arteriolar and sphincter control mechanisms are changes can be completely and rapidly reversed. especially sensitive to inadequate blood supply. If not they worsen and the condition may resist The major part of the peripheral resistance is treatment. Masses of aggregated red cells remain arteriolar, the sphincters determine the extent trapped in distended capillaries and in post- of opening of capillary networks, and the venules capillary venules. The final stage is necrosis of have a valuable capacitance role. The capillaries the tissue and its vessels, with haemorrhage. As and postcapillary venules are best regarded as an indication of the well known inability of passive tubes, with maximum permeability at the shocked tissues to resist infection, it is of interest venous end of the capillary. The concept of active to note that ear chambers in shocked rabbits capillary contractility has little to support it invariably become infected and fall out if the and most of the movement that one can observe degree of shock and hypoperfusion has been at in a capillary wall seems related to emptying or all severe. filling of the tube and not to active changes in These severe grades of hypoperfusion vary in the endothelial cell. the speed of their development in the various Many humoral factors, either circulating organs, the vital organs remaining the best generally or, more often, locally produced, are perfused for the longest time. superimposed on and modify this neurogenic The observation that this severe micro- The microcirculation in shock 13 circulatory failure cannot invariably be remedied administration of molecular high weight dextrans, J Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from by fluid replacement has led to extensive search and I have found (Stalker, 1967) that aggrega- for 'toxins' in shock. We must, however, be care- tion causes micro-infarction which, interestingly ful to be sure, before postulating a toxic factor, enough, is most prominent in the myocardium that physical mechanisms could not equally well and liver. explain the facts. The most immediate question Much research continues in many disciplines for consideration is the altered rheology of the on blood viscosity and flow and the last words on blood. This is an exceedingly complex subject. red cell aggregation and blood rheology have First, blood is a suspension of deformable cells certainly not been said. It is my present belief in a varying fluid protein matrix. Second, in flow that simple aggregation is normal and harmless, through the capillaries there is often deformation if not in fact to some degree regulatory, but that of the highly plastic red cells and the inherent severe aggregation in shock adds a lethal element viscosity of the red cell content must be taken into of capillary obstruction to the low flow state. account. Third, red cells remain discrete until Little need be said, of the general histo- their flow rate is reduced, but whenever this pathology of shock. Vascular congestion is non- occurs they tend to aggregate and form simple specific and the only other feature of note is the rouleaux, which in turn can join to form complex occurrence of micro-infarction similar to the aggregates. This aggregation, or sludging, as it lesions described following induced red cell has been termed by Knisely, Bloch, Eliot, and aggregation. It is of some interest that they tend Warner (1957), is also dependent on the plasma to occur in organs with sinusoidal blood supplies macromolecular content, and for each colloid in which flow is normally low, and at the edges or there is a critical value above which aggregation surfaces of organs where the possibility of col- occurs (Thorsen and Hint, 1950). The flow of lateral supply is restricted, eg, the surface of liver blood is anomalous in respect of its viscosity. and endocardium. It should be remembered that At high shear rates (high velocity flow) blood is a preexisting arterial disease may well intensify Newtonian fluid and behaves like water, but at hypoperfusion in shock and we are all well aware low shear rates it shows plasticity, ie, its viscosity of the disastrous effects of, say, gastric haemor- increases. This increased viscosity at low flow rhage in patients with atherosclerotic coronary rates is, understandably, further complicated by disease. the aggregation phenomenon, and, in the field of An occasional finding in fatal shock is fibrincopyright. blood viscometry, at the capillary and post- microthrombi in small vessels. This is considered capillary level there is, currently, much debate. in the next section. There has long been argument as to whether red cell aggregation is a normal or pathological event. It is seen in capillaries and postcapillary venules where flow is slowest and plasma macro- Disseminated Intravascular Coagulation and molecular concentrations can be at their highest. Endotoxic Shock

It is, in my opinion, best to regard it as a normal http://jcp.bmj.com/ event which is generally of slight degree. The Increasing attention is now being paid to dis- small aggregates in a sense may help to regulate seminated intravascular coagulation as a possible capillary flow by a transient obstructive mech- intermediary mechanism in producing or per- anism; capillary filling may in part be determined petuating shock (McKay, 1964; Hardaway, 1966). by the extent of red cell aggregation in the drain- There are now in the literature many reports ing venules. When aggregation is severe in the describing this process and some indicate striking hypoperfusion of shock we can view the picture depression of coagulation factors. Fibrin micro- on October 4, 2021 by guest. Protected differently. Complex aggregates are often im- thrombi can be found in many fatal cases of pacted and trapped in postcapillary venules. The shock of diverse aetiology; they are often in- larger venules show a sluggish flow of distinctly frequent and haphazardly distributed. Demon- granular blood. This can be seen in the con- stration of their fibrin nature may be difficult as junctival vessels in man, and it was the classical staining methods for fibrin are still not completely work of Gelin (1956) in this field that revived satisfactory, and perhaps some pathologists have interest in blood viscosity. This trapping, or been too ready to dismiss such microthrombi sequestration, of blood cells may persist after lightly, forgetting that postmortem fibrinolysis correction of blood volume in the experimental can greatly reduce the amount of deposited fibrin. animal. I have seen such aggregation break up The basic concept is that a process of intra- spontaneously after 48 hours' impaction in a post- vascular coagulation of from slow to rapid tempo capillary venule. On the other hand, the red cells can be initiated by a wide range of factors. These in an aggregate may lose their outlines, fibrin is include certain chemical agents, red cell break- deposited and a thrombus forms: the process is down products, exemplified by intravascular then irreversible. haemolysis or transfusion of incompatible blood, It is of importance to know whether or not this platelet aggregation and viscous metamorphosis severe aggregation is, of itself, harmful. Aggrega- induced by tissue injury or red cell damage, the tion can be induced experimentally by the circulation of particulate material, bacteriaemia A. L. Stalker 14 being a particularly good example, or of antigen- the other hand, cause failure of fibrinogenJ Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from antibody complexes, and the entry of thrombo- replenishment. Here again we have to deal with plastic material into the circulation from a site a balance of factors. of injury. The latter is especially likely to happen Disseminated intravascular coagulation, there- when there is haemorrhage and a rise in tissue fore, adds to the shock picture the possibility of pressure sufficient to induce parenchymatous further microcirculatory obstruction and a embolism (Young and Griffith, 1950). A well haemostatic defect. known example of this is obstetric accidental Closely related to disseminated intravascular haemorrhage which frequently causes hypo- coagulation, if not indeed the same, is endotoxic fibrinogenaemia. shock. This may be due to primary infection by Whenever fibrinogen breakdown products, endotoxin-producing organisms, mainly Gram- including fibrin, are formed there will be clear- negative bacilli. It may, however, be secondary to ance responses by the fibrinolytic enzyme system absorption of endotoxin from the gut devitalized and the reticuloendothelial system. The extent by the shock state (Fine, 1965). Endotoxin in to which fibrin is actually deposited on endo- small amounts is normally absorbed and is then thelial surfaces will depend upon the balance detoxified by the reticuloendothelial system, between the rates of fibrin formation and its especially in the liver. In shock absorption can clearance. Parallel studies of fibrin degradation be greater and detoxification reduced. How the products, fibrinolytic activity, and plasma endotoxin operates is far from clear. It may have fibrinogen are thus of special value in studying effects on vascular smooth muscle directly or via this phenomenon. the nervous system, but in view of the striking It is possible in experimental defibrination to similarities between the generalized Shwartzman see fibrin deposition in vivo in the microcircula- reaction experimentally induced by carefully tion (Irwin, Chiu, and Nims, 1968; Stalker, spaced doses of endotoxin and disseminated Brown, Hall, and Blench, 1969) but the more intravascular coagulation, it seems reasonable to usual methods for studying disseminated intra- believe that the special features of endotoxic vascular coagulation are histological, haemato- shock are largely determined by an intravascular logical, and ultrastructural. The intensifying effects coagulation mechanism. The term 'generalized

of reticulo-endothelial blockade on disseminated Shwartzman reaction', useful though it has beencopyright. intravascular coagulation are well established in directing research to the field of defibrination (Lee, 1962). and the role of the reticulo-endothelial system, Disseminated intravascular coagulation can should, in my view, be discarded as too restrictive. affect the microcirculation in several ways. It can cause capillary or postcapillary obstruction purely by deposition of fibrin. Circulation of fibrinogen-fibrin complexes may determine a Conclusions greater degree of red cell aggregation than fibrinogen itself and sludging is then worsened. The human body reacts to injury in a remarkablehttp://jcp.bmj.com/ The non-dissociable aggregates described in way. If the principles of microcirculatory break- severe burns may be due to fibrin entanglement down can be applied to each of the systems in an of the aggregates. One cannot but think in this analysis of the clinical and biochemical features connexion that it is fortunate that the endo- of shock we may hope increasingly to understand thelium of the venous side of the circulation, shock and how to treat it. Such a systems ap- where aggregation is greater, is rich in plas- proach is especially applicable in considering minogen activator. There is also evidence (Bull, so-called 'shock organs'. Microcirculatory hypo- on October 4, 2021 by guest. Protected Rubenberg, Daci,, and Brain, 1968; Rubenberg, perfusion of the myocardium, whether primary in Regoeczi, Bull, Dacie, and Brain, 1968) that red coronary inadequacy or secondary to shock, can cells can be deformed and fragmented during determine central circulatory failure; pulmonary passage through fibrin meshworks. Here we have hypoperfusion may initiate the fatal pulmonary a potential vicious circle of defibrination, red insufficiency we see today in some patients who cell injury starting a self-perpetuating process; have been initially resuscitated with success; in microangiopathic haemolytic anaemia might be other patients renal or adrenal features dominate the result of such a mechanism. Finally, with or there is a coagulation defect. We are only severe disseminated intravascular coagulation, beginning to understand some of these inter- plasma fibrinogen levels can be greatly depleted relationships. and a haemorrhagic tendency can develop-a 'consumption coagulopathy'. However, the rate of replenishment of fibrinogen is rapid (Hard- References away, Johnson, Elovitz, Houchin, Jenkins, Burns, Branemark, P-I. (1966). Intravital Microscopy. Its present status and Jackson, 1964) and a single restricted episode and its potentialities. Med. biol. Ill., 16, 100-108. of disseminated intravascular coagulation may Bond, T. P., Guest, M. M., and Derrick, J. R. (1968). High speed cinephotomicrography of the microcirculation in patients not, unless severe and rapid, produce bleeding; with haemolytic anaemia. In Proceedings of International but diminished liver function in shock may, on Conference on Microcirculation, Gothenburg. (Abstr.) The microcirculation in shock 15

Bull, B. S., Rubenberg, M. L., Dacie, J. V., and Brain, M. C. McKay, D. G. (1964). Disseminated Intravascular Coagulation. J Clin Pathol: first published as 10.1136/jcp.s3-4.1.10 on 1 January 1970. Downloaded from (1968). Microangiopathic haemolytic anaemia: mechanisms An Intermediary Mechanism ofDisease. Hoeber, New York. of red-cell fragmentation: in vitro studies. Brit. J. Haemat., Rhodin, J. A. G. (1967). The ultrastructure of mammalian 14, 643-652. arterioles and precapillary sphincters. J. Ultrastruct. Res., Chambers, D. R., and Monro, P. A. G. eds. (1969). Symposium 18, 181-223. on Techniques used in the Study of Microcirculation. British Rhodin, J. A. G. (1968). Ultrastructure of mammalian venous Microcirculation Society, Cambridge. capillaries, venules, and small collecting veins. J. Ultra- Fine, J. (1965). Shock and peripheral circulatory insufficiency. struct. Res., 25, 452-500. In Handbook of Physiology, Section 2, Circulation, edited Rubenberg, M. L., Regoeczi, E., Bull, B. S., Dacie, J. V., and by W. F. Hamilton, vol. 4, pp. 2037-2069. American Physio- Brain, M. C. (1968). Microangiopathic haemolytic anaemia; logical Society, Washington, DC. the experimental production of haemolysis and red-cell Florey, H. W., and Grant, L. H. (1961). Leucocyte migration fragmentation by defibrination in vivo. Brit. J. Haemat., 14, from small blood vessels stimulated with ultraviolet light: 627. an electron-microscope study. J. Path. Bact., 82, 13-17. Shepro, D., and Fulton, G. P. (eds). (1968). Microcirculation as Gelin, L-E. (1956). Studies in anemia of injury. Acta chir. scand., Related to Shock. Academic Press, New York and London. Suppl. 210. Stalker, A. L. (1964). Intravascular Erythrocyte Aggregation. Hardaway, R. M., Johnson, D. G., Elovitz, M. J., Houchin, D. N., Bibl. anat. (Basel), 4, 108-11 1. Jenkins, E. B., Burns, J. W., and Jackson, D. R. (1964). Stalker, A. L. (1967). Histological changes produced by experi- Studies on the fibrinogen replacement rate in dogs. Ann. mental erythrocyte aggregation. J. Path. Bact., 93, 203-212. Surg., 160, 825-838. Stalker, A. L., Brown, L. J., Hall, J., and Blench, S. M. (1969). Hardaway, R. M. (1966). Syndromes of Disseminated Intra- Studies on Experimental defibrination. I. Microcirculatory vascular Coagulation. Thomas, Springfield, Ill. observations. Microvasc. Res., 1, 287-294. Irwin, J. W., Chiu, H., and Nims, J. C. (1968). Biological emboli Thorsen, G., and Hint, H. (1950). Aggregation, sedimentation, in passive anaphylaxis. Proceedings of International Con- and intravascular sludging of erythrocytes. Acta chir. ference on Microcirculation, Gothenburg. (Abstr.) scand., Suppl. 154. Knisely, M. H., Bloch, E. H., Eliot, T. S., and Warner, L. (1947). Young, J. S., and Griffith, H. D. (1950). The dynamics of paren- Sludged blood. Science, 106, 431-440. chymatous embolism in relation to the dissemination of Lee, L. (1962). Reticuloendothelial clearance of circulating fibrin malignant tumours. J. Path. Bact., 62, 293-311. in the pathogenesis of the generalised Shwartzman reaction. J. exp. Med., 115, 1065-1082. copyright. http://jcp.bmj.com/ on October 4, 2021 by guest. Protected