Ischemia and Congestion

Transcription

1 1 MLU General Pathology Unit 3 Session 8 Ischemia and Congestion Session outline: 1.1 Introduction 1.2. Causes of tissue ischemia: 1.3 Arterial obstruction -Causes and effects 1.4. Venous obstruction.-causes and effects 1.5 Venous congestion 1.6. Hyperemia and congestion Revision activities References.

2 Introduction. The maintenance of adequate blood circulation is a highly complex process that depends on proper function of the heart, the integrity of the vasculature, and the maintenance of a delicate balance between coagulation and fibrinolytic systems. Failure of blood supply to tissue may be localized to an organ or may be generalized. Ischemia Definition: Failure of blood supply, to a tissue to maintain adequate tissue perfusion. This can be, A) Localized (confined to one organ, leads to infarction) B) Generalized (this leads to shock) 1.2. Causes of tissue ischemia: 1. Impaired tissue perfusion due to Arterial obstruction 2. Impaired venous drainage due to: A) Venous obstruction B) Venous congestion due to cardiac failure.

3 3 1.3 Arterial obstruction Causes and effects. Causes of arterial obstruction : 1.External compression. (tourniquet) 2. Vessel wall disease. (Atherosclerosis, Vasculitis) 3. Arterial spasm

4 4 4. Thrombosis and embolism Thrombosis in a vessel Effects of arterial obstruction Depend on the: 1. Rate of development of ischemia. In gradual onset ischemia there is time for compensatory mechanisms to develop. 2. Availability of collateral circulation.

5 5 Some organs have well developed collateral circulation while some organs do not. Organs such as heart and brain do not have a collateral circulation and are supplied by end arteries. Eg. Middle cerebral artery, retinal artery. Organs with well developed collateral circulation are less susceptible to ischemia than those that do not.

6 6 1 Integrity of the collateral circulation. Even if there is a collateral circulation if it is impaired by obstruction there is more susceptibility to ischemia. 2. Tissue susceptibility to ischemia. Organs like brain and heart are highly susceptible to ischemia whereas tissues like skeletal muscle and bone are less susceptible. 3. Tissue metabolic rate. Reduction in tissue temperature reduces the metabolic rate there by rendering it less Susceptible to ischemia. This is employed in organ transplantation by transporting organs under hypothermic conditions. Also organs such as brain and heart are more susceptible to ischemia as they have higher Metabolic rates. Effects. 1. Chronic ischemia resulting in atrophy 2. Compensatory development of collateral circulation 3. Uncompensated ischemia causes infarction. (Tissue death) 1.4Venous obstruction Causes and effects: Although this is common clinically less life threatening than arterial obstruction due to the presence of good collateral systems in the venous circulation. Venous obstruction (Impaired drainage) Causes:

7 7 1. External compression. 2. Inflammation 3. Poor muscle pump 4. Valvular incompetence of veins. 5. Right heart failure. Effects. 1Edema or effusion formation 2.Congestion 3.Capillary rupture and hemorrhage. 4.Venous infarction 1.5 Venous congestion Venous obstruction becomes symptomatic when it affects large vessels (Eg. Superior vena cava) and when there is inadequate venous circulation.(eg. Superior sagittal sinus, retina, renal vein.) It is a passive process.

8 8 Venous congestion in heart failure Failure of the heart to pump blood result in venous congestion. Pulmonary venous congestion. Pulmonary circulation gets congested in left heart failure. Clinically patients present with shortness of breath. Acute congestion : lead to dilatation of alveolar capillaries and transudation of fluid into alveoli.(pulmonary edema)intra alveolar hemorrhages may occur In chronic congestion escape of red blood cells into alveoli may occur resulting in the presence of hemosiderin laden macrophages within alveoli.(heart failure cells)

9 9 Hemosiderin laden macrophages within alveoli. Hepatic venous congestion. Systemic venous congestion occurs in right heart failure. Clinically patients present with bilateral ankle edema and hepatomegaly. Nutmeg liver Microscopy of nutmeg liver In the liver central vein is dilated and sinusoids in the central area are congested. There are

10 10 darker central areas and paler tissue in the peripheral areas giving rise to nutmeg appearance of liver. As congestion increases hypoxia due to reduced blood supply occur resulting in enhancement of mottled appearance. Central area eventually undergoes necrosis with subsequent fibrosis. Contraction of centrzonal fibrous tissue results in nodular liver. (cardiac cirrhosis.) 1.6. Hyperemia and congestion Hyperemia is an increase in the amount of blood within a vessel occurs due to active dilatation of micro circulation as in acute inflammation. In venous congestion passive dilatation of vessels occur due to impaired drainage of blood. Both hyperemic and congested tissue appears red on gross examination due to increased amounts of blood Revision activities. 1. Define ischemia and give the causes of ischemia. 2. What are the similarities and differences in ischemia and congestion? 1.8. References: 1.Concise Pathology. 3 rd edition. Parakrama Chandrasoma, Clive R. Taylor. Mc Graw hill.

11 11 2.Pathologic Basis of disease. Robbins and Cotran. 7 th edition. Saunders. 3 Muir s text book of pathology. 16 th edition. 4.Webpath. Internet pathology laboratory.