Shock

Shock, in a nutshell, is inadequate peripheral perfusion that can lead to cell death and, well, death of the rest of the body if left untreated. Of course, there's more to say about shock than that: enough to fill up a whole 45-minute lecture, in fact!

Understand the different types and causes of shock

There are many different types of shock:

• Hypovolemic shock: Shock due to loss of blood volume (could be due to trauma, burns, vomiting, diarrhoea and so on). I will be mainly discussing hypovolemic shock (particularly haemorrhagic shock) in this post.
• Neurogenic shock: Shock due to sudden dilation of the blood vessels, which in turn may be due to CNS damage and loss of autonomic nervous system signals to the vascular smooth muscle.
• Psychogenic shock: Shock in response to stress, pain or fright. Like neurogenic shock, there is sudden dilation of the blood vessels, but this is transient.
• Septic shock: Shock in response to a bacterial infection. Bacteria release exotoxins and endotoxins, which result in vasodilation and an increase in capillary permeability.
• Anaphylactic shock: Shock due to a severe allergic reaction. You can read more about anaphylaxis here.
• Cardiogenic shock: Shock due to some problem with the heart resulting in decreased stroke volume and decreased cardiac output. This may be due to left heart failure (which also causes pulmonary oedema) or right heart failure (which causes systemic oedema). See here for more information on heart failure.
• Obstructive shock: Shock due to obstruction of important blood vessels, causing a decrease in cardiac output. This can be due to cardiac tamponade (fluid filling the pericardial cavity), pneumothorax (air in the pleural cavity) or pulmonary embolism.

Discuss the different stages of shock

Here are the stages of haemorrhagic shock:

1. Class I haemorrhage (loss of 0-15% of blood volume): Little tachycardia. Usually no significant change in blood pressure, pulse pressure or respiratory rate.
2. Class II (15-30%): Elevated heart rate (>100bpm), tachypnea, decreased pulse pressure
3. Class III (30-40%): Tachycardia, tachypnea, decreased systolic blood pressure, oliguria
4. Class IV (>40%): Tachycardia, decreased systolic blood pressure, decreased pulse pressure, little (or no) urinary output. This stage is immediately life-threatening.

Discuss the compensatory mechanisms

Mean arterial pressure actually stays pretty constant until a fairly large amount of blood is lost (>20%), largely due to the baroreceptor reflex. The baroreceptors are maximally sensitive when mean arterial pressure is normal, which means they can easily detect even a small change in blood pressure. When blood pressure drops, the firing rate of the baroreceptors decreases. This is picked up by the nucleus tractus solitarius in the medulla, leading to sympathetic activation and parasympathetic inhibition.

Aside from the baroreceptor reflex, there are several other compensatory responses for hypovolemia. Peripheral chemoreceptors can play a role, especially when mean arterial pressure drops below 60mmHg- I suppose that less blood means less oxygen to go around. Chemoreceptor stimulation also activates the sympathetic nervous system. Due to sympathetic nervous system activation, the respiratory system may also be activated, which may enhance venous return (this all has to do with something called the "abdominothoracic pump"- maybe inflation of the lungs squishes the veins?). Yet another compensatory mechanism is an increase in circulating vasoconstrictors, such as ADH, aldosterone and catecholamines (i.e. adrenaline and noradrenaline).

Aaaaand I'm not done yet! When mean arterial pressure drops below 60mmHg, not only do peripheral chemoreceptors get activated, but the CNS ischaemic response may also be activated! The CNS ischaemic response is a very intense sympathetic response (from both sympathetic nerves and adrenal glands) that mainly exists to maintain perfusion to the brain.

Aaaaaaaand I'm still not done! Other compensatory mechanisms include redistribution of interstitial fluid (return of fluid back to the blood), stimulation of thirst (so that you drink more and get your blood volume back up) and haematopoiesis (via stimulation of EPO).

Unfortunately, in severe haemorrhage, these mechanisms may not be sufficient. A large drop in arterial pressure can create not one but two vicious cycles. In the first vicious cycle, a drop in arterial pressure causes a drop in coronary perfusion, which decreases inotropy, which decreases cardiac output, which decreases arterial pressure. In the second vicious cycle, a drop in arterial pressure decreases blood flow to the organs, which causes hypoxia, which causes release of vasodilating mediators as well as a phenomenon called "sympathetic escape" (desensitisation to sympathetic activity so that it doesn't cause as much constriction), which causes a decrease in arterial pressure.

Decompensated shock can cause cardiac failure, acidosis, CNS depression, an increase in capillary permeability, an increase in toxins (from dead cells) and blockage of small blood vessels (stagnation in blood flow can lead to clots). So, in short: you're f***ed.

Describe signs and symptoms

Signs and symptoms of shock include restlessness, anxiety, a decreased level of consciousness, dull eyes, rapid shallow respiration, nausea, vomiting, thirst, diminished urine output and usually some tachycardia. The skin may be pale, cool and clammy (in the case of hypovolemic and cardiogenic shock) or dry and flushed (in the case of septic, anaphylactic and neurogenic shock). In the case of anaphylactic shock, there may be other signs of an allergic reaction, such as hives, itching, wheezing and difficulty breathing.

Discuss treatment of shock

Treatment of shock generally comes down to treating the causes. For example, if shock is due to hypovolemia, then you need to restore blood volume by using crystalloids, colloids or blood transfusions. Furthermore, if there is bleeding, then you may need surgery to stop it. Using the same principle of "treat the cause," septic shock can be treated with antibiotics, cardiogenic shock can be treated with supports such as inotropic and chronotropic agents and anaphylactic shock can be treated with EpiPens. Vasoactive substances can be used to treat vasodilation and bicarbonate can be used to treat the acidosis that often occurs as a result of shock.