Shock in animals

Shock in animals

Shock is an acute circulatory state in which the cardiac output is insufficient to maintain adequate blood flow to the tissues. Any condition causes a sudden decrease in circulating blood volume or increase in volume of the vascular system reduces the mean circulatory pressure results in shock.

Causes of shock

Shock may be caused either by decreased cardiac output or without decreased cardiac output-

1. Shock caused by decreased cardiac output
   • Cardiac abnormalities include myocardial infarction, severe heart valve dysfunction, and cardiac arrhythmias etc decrease cardiac ability to pump blood, called as cardiogenic shock.
   • Factors decreasing venous return due to reduced blood volume, decreased vascular tone and obstruction to blood flow.
2. Shock caused by without diminished cardiac output
   • Cardiac output may be normal or may even be greater than normal. This can result from
     • Excessive metabolism of the body so that the normal cardiac output is not adequate.
     • Abnormal tissue perfusion.
   • These conditions are present in septic shock (blood poisoning) due to massive bacterial infection.
3. Tissue deterioration is the end stage of circulatory shock

Classification of shock

Shock can be classified into-

1. Hemorrhagic shock due to blood loss (hypovolumic).
2. Cardiogenic shock due to myocardial damage.
3. Septic or endotoxic shock due to massive bacterial infection. The infection is carried by blood from tissue to tissue causing extensive damage. Septic shock may result in peritonitis, generalized infection by streptococci or staphylococci, generalized gangrenous infections etc.
4. Anaphylactic shock is due to immunological reaction. Antigen-antibody reaction provokes the release of harmful chemical substances like histamine, slow reacting substance (SRS), eosinphil chemotactic factor which causes cardiac failure and vascular fluid loss. Histamine and SRS increase capillary permeability and bronchospasm.

Stages of shock

1. Compensated or recovering shock

Blood volume and pressure are reduced. Compensatory mechanisms try to maintain adequate blood flow to vital organs and the patient can recover without transfusion.

2. Progressive or degenerative shock

Blood pressure progressively falls despite compensatory mechanisms like reflex tachycardia, vasoconstriction of skin, muscle and splanchnic areas and fluid shift from interstitial compartment to plasma. When B.P falls to 60 mm Hg or less, functions of vital organs heart, lungs, CNS etc. are impaired. Transfusion and other treatments can arrest this stage to restore normality.

3. Irreversible shock

Cardiac output and B.P. are very low; vital functions are depressed. Effects of transfusion are transient, B.P. falls and the patient dies of myocardial failure and cerebral depression.

Positive feedback in progressive shock

• Progressive shock if untreated, leads to irreversible shock because of a vicious cycle in which, with each degree of increase in shock, the cardiac output and B.P. further decreases and death occurs. Five feedback mechanisms account for this vicious cycle response.
• As shock is advancing, cerebral blood flow decreases progressively causing depression of the vasomotor and respiratory centers.
  1. Vasomotor failure: Vascular tone is reduced because of reduced blood supply to vasomotor centre. The sympathetic discharge is reduced; this lead to decrease in means circulatory pressure; venous return leads to reduced cardiac output.
  2. Increased capillary permeability: After many hours of capillary hypoxia and lack of nutrients leads to increased capillary permeability and fluid loss to ECF. Reduced blood volume further reduces cardiac output.
  3. Blockage of minute blood vessels: Addition of acids and deterioration products of tissues causes agglutination of local blood due to sluggish blood flow in capillaries; lead to clotting and further impeding blood flow.
  4. Cardiac depression: When arterial pressure falls, coronary blood flow is reduced leads to myocardial hypoxia and reduction in the cardiac output.
  5. At an arterial pressure of 50 mm Hg or less the positive feedback exceeds the negative feedback gains and the circulatory system collapses.

Negative feedback resistance to shock

Regulatory mechanisms of the circulatory system tend to maintain cardiac output and B.P. at a normal level operate as a negative feedback control. These mechanisms have very short response time in restoring the cardiac output and BP.

This include the baroreceprtor reflex, chemoreceptor reflex, central nervous ischemic responses. The renin-angiotensin-aldosterone and renal fluid balance mechanisms respond gradually over a period of several hours.

Within minutes to few hours, the renin-angiotensin mechanism shows vasoconstriction and shifting of fluids into blood to maintain the normal blood volume and BP.