There are several kinds of shock, but the final mechanism in all of them is a fall in a patient’s cardiac output which reduces the supply of blood to his brain, kidneys, gut, liver, lungs, and muscles. His brain is the most sensitive of these organs.
Hypovolaemic shock is our main concern here. A patient’s blood volume can fall because of. (1) Sudden bleeding, either external or internal. (2) The slow loss of plasma from his circulation due to burns or peritonitis. (3) The loss of extracellular fluid as the result of vomiting, diarrhoea, intestinal obstruction or fistulae. In theory, the treatment of all these kinds of shock is straightforward—restore his blood volume with balanced electrolyte solutions or whole blood as appropriate. Section 14.3 of Primary Anaesthesia describes the treatment of surgical dehydration.
Vasovagal shock or neurogenic shock is the result of a strong sensory or emotional stimulus which causes widespread vasodilation and bradycardia. Trauma, either severe or trivial, is one such stimulus. The patient may yawn, feel hot, sweat, and then lose consciousness. He breathes with slow deep sighs, he becomes cold and pale, his blood pressure falls, and he has a slow pulse. If he lowers his head or lies down, he rapidly recovers.
Vasovagal attacks are common and normally harmless, but they can be important because: (1) Their symptoms can be added to those of hypovolaemic shock and make a patient seem worse than he really is. The critical sign is his pulse. If this is slow, suspect that there is a strong vasovagal component to his symptoms. (2) If vasovagal shock is added to hypovolaemia, he may collapse suddenly during the induction of anaesthesia. (3) Vasovagal shock can complicate such procedures as the manipulation of fractures. (4) If he remains seated upright during a vasovagal attack, his brain becomes anoxic and he may die.
Septic shock is caused by bacteria releasing endotoxins which cause circulatory collapse, especially when antibiotics kill them in large numbers in a patient’s circulation when he is septicaemic. Although this might logically be considered in Volume One, with the surgery of sepsis, it is more conveniently included here.
Cardiogenic shock has many medical causes, the most important one being cardiac infarction. There are also two important surgical causes—bruising of the heart and cardiac tamponade (65.9), due to blood in the pericardial cavity. This is rare but it is important because you can treat it. Don’t transfuse a patient if he is in cardiogenic shock, unless there are other reasons for doing so. More fluid in his circulation can weaken myocardial contractility and add to the work of his heart.
There are difficulties in diagnosing and managing hypovolaemic shock: (1) Diagnosing internal bleeding may not be easy, so (a) remember the possibility of an ectopic pregnancy, and (b) don’t forget that shock developing after trauma may be due to bleeding into a patient’s peritoneal cavity or behind it, into his pleural cavities (65.4), or into the muscles round frac tures, particularly those of his pelvis or femur. He can die from bleeding into any of these places, without any blood appearing on the surface. (2) Shock is not the only cause of reduced consciousness in an injured patient—he may be drunk, drugged, concussed, hypoxic or hypoglycaemic; he may also be suffering from a head injury. Sometimes he is unconscious for more than one of these reasons-the combination of a head injury and abdominal bleeding is common. Finally, (3) don’t diagnose an infarct without some positive evidence for it, such as precordial pain and no signs of any other cause of shock.