Cells, compensation and shock

1. You findThe heart rate and breathing rate are up, the skin is pale, cool and sweaty, the patient is mildly anxious and capillary refill is delayed. The blood pressure is still normal.You doCall it compensated shock. The body is still managing by speeding the heart and squeezing the vessels. This is the stage to recognize.
2. You findThe blood pressure falls and the mental status changes.You doThat is decompensated shock: compensation has failed. If it is not reversed it becomes irreversible shock, and the patient dies.
3. You findYou ask yourself why this patient is in shock.You doSort it by mechanism. Hypovolemic: too little volume (bleeding, dehydration). Distributive: the vessels dilate (anaphylaxis, sepsis). Cardiogenic: the pump fails (heart attack, dysrhythmia). Obstructive: blood flow is physically blocked (tension pneumothorax, pericardial tamponade, pulmonary embolism).
4. You findA patient is breathing, but either very slowly or with very shallow breaths.You doThink minute volume: tidal volume times breathing rate. It falls if either one falls. About 150 mL of every breath is dead air space that never reaches the alveoli, so shallow breaths move very little useful air.
5. You findAn injured or sick child has a normal blood pressure.You doDo not be reassured. Children constrict their vessels so strongly that the blood pressure can stay normal after a massive volume loss. Capillary refill (normal is under 2 seconds) and the heart rate tell you more, and a fast heart rate in a child is a serious red flag.
6. You findA patient who should look like they are compensating does not.You doRemember what blunts the response. Beta blockers reduce the effect of epinephrine, so the heart rate may not rise. A patient in cardiogenic shock may be unable to raise the heart rate, and a patient in septic shock may be unable to constrict the vessels.

• Homeostasis is the body's stable internal state; compensation is the constant adjustment that keeps it there.
• A cell needs water, glucose and oxygen. Glucose is turned into ATP, the cell's fuel, and most cells need insulin to bring glucose in from the blood.
• Aerobic metabolism (with oxygen) makes far more energy and leaves carbon dioxide and water. Anaerobic metabolism (without enough oxygen) makes little energy and leaves lactic acid, which causes acidosis and damages cells.
• Room air is about 21% oxygen. That share is the fraction of inspired oxygen (FiO₂). A patent airway is one that is open and clear.
• Breathing fails in three ways: the control fails (stroke, narcotics, brain or cord injury), the pressure fails (a hole in the chest wall or pleural space collapses the lung), or the lung tissue fails (trauma, heart failure, sepsis).
• Cardiac output is stroke volume times heart rate. It falls with a slow heart, a low stroke volume (as in blood loss) or a rate so fast the ventricles cannot fill.
• In infants, a slow heart rate is often a sign of hypoxia, a breathing problem, rather than a heart problem.
• Two pressures move water across vessel walls: plasma oncotic pressure (proteins pull water in) and hydrostatic pressure (the heart pushes water out). Upset either and you get edema or dehydration.
• In anaphylaxis histamine makes capillaries leak and vessels dilate: swelling, a narrowed airway and a falling blood pressure. That is distributive shock.
• The skull is a closed box. Bleeding or swelling inside it has nowhere to go, so the pressure inside rises.

The wrong idea: Shock means low blood pressure, so a patient with a normal blood pressure is not in shock.

What is actually true: Shock is inadequate perfusion: the cells are not getting enough oxygen. In compensated shock the blood pressure may still be normal while the heart rate and breathing rate climb and the skin turns pale, cool and sweaty. A falling blood pressure means compensation has already failed.

The body holds a stable state by compensating, and pathophysiology is what happens when compensation runs out. Cells need water, glucose and oxygen; without oxygen they turn to anaerobic metabolism, lactic acid and acidosis. Breathing depends on minute volume (tidal volume times rate) and circulation on cardiac output (stroke volume times heart rate). Shock is inadequate perfusion with four mechanisms: hypovolemic, distributive, cardiogenic and obstructive. Compensated shock shows a fast pulse, fast breathing and pale, cool, sweaty skin with a blood pressure that may still be normal; a falling pressure and altered mental status mean decompensation. Children hold their pressure until late, and beta blockers, cardiogenic shock and septic shock can hide the usual signs.

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