Circulatory Shock

Definition

Circulatory shock is a state of inadequate tissue perfusion needed for the normal cellular respiration. In other words shock results from inadequate perfusion of the body cells with oxygenated blood.

Pathophysiology of Shock

When the delivery of oxygen and glucose is low, cellular metabolism changes from aerobic to anaerobic type. The product of aerobic respiration is carbon dioxide and that of anaerobic respiration is lactic acid. Accumulation of lactic acid due to anaerobic respiration results in metabolic acidosis.

Whenever there is metabolic acidosis, body tries to compensate it by hyperventilation producing respiratory alkalosis. Inadequate perfusion results in baroreceptor stimulation in the cardiovascular system.  This causes release of catecholamines and thereby sympathetic over activity. It is the cause of rapid pulse in shock.

Decreased renal perfusion leads to decreased glomerular filtration and low urine output.Anti Diuretic Hormone production is  also increased due to baroreceptor stimulation . ADH causes increased sodium and water re absorption which further reduces urine output.   

Activation of the Renin–angiotensin–aldosterone axis causes peripheral vaso- constriction.  This is the reason for the clinical manifestation of cold extremities in shock.

Ischemia of the intestine results in destruction of the mucosal barrier leading to bacterial translocation from the lumen of the intestine to the systemic circulation.

Hypoxia and acidosis in turn activates the inflammatory and complement cascade in the body resulting in the generation of oxygen free radicals and inflammatory mediators. Due to the action of these mediators vascular endothelial damage occurs and tissue oedema forms. Tissue oedema in turn causes tissue hypoxia.

Cessation of anaerobic metabolism causes failure of the Na/ K⁺ pumps and cell lysis. As a result intracellular K+ is released into the bloodstream producing the electrolyte abnormality called hyperkalemia.

When the normal circulation is regained the inflammatory mediators will be flushed away and can cause endothelial injury to lungs and kidneys. Ultimately this can result in multi organ failure. Such an injury is known as Re-perfusion Injury.

Types of Shock

¨ Hypovolaemic

¨ Cardiogenic

¨ Obstructive

¨ Distributive

¨ Endocrine

Hypovolaemic Shock

Caused by a reduced circulating volume and is the most common form of Shock. Two types of Hypovolaemic Shock are Haemorrhagic or Non-haemorrhagic. Haemorrhagic shock is due to the excessive blood loss.

Non-haemorrhagic causes of Hypovolaemic Shock

Dehydration –

Poor fluid intake, Vomiting, Diarrhoea                                             

Urinary loss (Polyuria -diabetes)

Evaporation- Heat exhaustion

              Third-space loss

Third space compartment is extra cellular space which is not normally perfused with fluids. E.g. Peritoneal cavity, Pleural space etc.The fluid is re distributed from ECF and is not available to the intravascular space. Hence fluid sequestration in Third Space can cause Hypovolemia. Third space loss occurs in –Peritonitis, Intestinal Obstruction, Pancreatitis and Burns.

Cardiogenic Shock

Is due to primary failure of the heart to pump blood

Causes of cardiogenic shock –

                                                  Myocardial infarction

                                                  Cardiac dysrhythmias

                                                  Valvular heart disease

                                                  Blunt myocardial injury

                                                  Cardiomyopathy

                                                  

Obstructive Shock

 Due to mechanical obstruction of cardiac filling

 Resulting in a fall in Cardiac Output

 Common causes

            Cardiac tamponade

            Tension pneumothorax

            Massive pulmonary embolus

Distributive shock/ Septic Shock

Is due to generalized vasodilatation and low systemic vascular resistance

Can occur in

              Anaphylaxis-histamine release

              Septic shock-endotoxins

              High spinal cord injury

Endocrine Shock

Is a combination of hypovolaemic, cardiogenic and distributive shock.

Causes are

Hypothyroidism – due to decreased responsiveness to catecholamines

Hyperthyroidism - due to highoutput cardiac failure.

Adrenal insufficiency - due to hypovolaemia

Phases of Shock

Phase of Compensation: Body tries to preserve blood supply to brain, kidney and lungs by reducing the blood flow to skin, muscles and gastro intestinal tract. Body can compensate a sudden loss of up to 15% of total blood volume. Blood pressure is also maintained within the normal limits even up to the loss of 40% of circulatory blood volume.

Phase of Decomposition: If the total loss is more than 15% of total blood volume, compensatory measures fails and there will be progressive reduction in the function of lungs, heart and brain.

Severity of Shock

Based on the severity, decompensated shock is further classified in to mild, moderate and severe.

Mild Shock: Mild tachycardia, tachypnoea and mild reduction in urine output. Blood pressure is maintained.

Moderate Shock: More tachycardia and tachypnoea. Urine output falls less than 0.5ml/ kg/ hr. Blood pressure falls and patient becomes drowsy.

Severe Shock: Severe tachycardia and laboured breathing with profound hypotension. Urine output becomes nil and patient becomes unconscious.

Monitoring of a patient in Shock

1.    Pulse rate

2.    Blood Pressure/ Central Venous Pressure Monitoing

3.    Respiratory Rate

4.    Level of Consciousness

5.    Urine Output

6.    Pulse Oxymetry for Oxygen Saturation

7.    Arterial Blood Gas Analysis

8.    Serum Lactate

Treatment of Shock

1.    Maintain Airway Patency

2.    Ensure proper Breathing

3.    Regain Circulation and control of the bleeding

4.    Intravenous Fluids – Crystalloids/ Colloids

5.    Grouped and Cross Matched Blood Transfusion

6.    Ionotropic agents like  Dopamine/ Dobutamine

7.    Nor adrenaline (Vasopressor for  peripheral vaso dilatation)

8.    Antibiotics in septic shock