Pathology of Shock
Start from one fact: every cell needs a continuous supply of oxygen to make ATP, and almost no tissue stores more than a few minutes' worth. The whole circulation exists to keep oxygen flowing to mitochondria faster than the tissues consume it. Shock is the breakdown of that contract — an acute failure of circulation and cellular energy supply, so that tissues cannot receive or use enough oxygen and substrate to maintain aerobic metabolism, membrane ion pumps, mitochondrial function and organ integrity.
Notice what shock is not. It is not "a low blood pressure". Blood pressure is one late and imperfect sign, downstream of the real lesion. The real lesion is a cell starved of energy. Hold that idea throughout this chapter: every sign, stage, type and investigation below is just a different vantage point on the same cellular emergency.
From the first principle we can derive the master equation. If shock is an oxygen-supply problem, then we need to know what determines oxygen supply to the tissues:
oxygen delivery = cardiac output x arterial oxygen content.
Cardiac output depends on preload, contractility, heart rate and afterload. Arterial oxygen content depends mainly on haemoglobin concentration and oxygen saturation. Oxygen delivery therefore fails if the pump output drops, if the tank is empty, if the vessels are too dilated to hold useful pressure, if a mechanical block stops flow, or if the blood cannot carry enough oxygen. Shock develops when any part of this chain fails — or, importantly, when delivery is adequate but the microcirculation and mitochondria still cannot use the oxygen that arrives. Those two failure modes — delivery versus utilisation — are the spine of everything that follows.
| Shock feature | Pathology meaning |
|---|---|
| Tachycardia | Sympathetic compensation for low stroke volume, vasodilatation, pain, fever or hypoxia |
| Cool peripheries | Peripheral vasoconstriction and shunting to brain/heart, typical in hypovolaemic shock |
| Warm peripheries early in sepsis | Vasodilatation and maldistributed flow |
| Tachypnoea | Acidosis compensation, hypoxia, sepsis or PE; often an early danger sign |
| Oliguria | Renal hypoperfusion or acute kidney injury |
| Confusion/agitation | Cerebral hypoperfusion, hypoxia, inflammation or metabolic disturbance |
| Raised lactate | Cellular stress from hypoperfusion, adrenergic glycolysis and impaired clearance; a severity marker, not a diagnosis |
This chapter teaches the pathology. The resuscitation algorithms are covered in Intermediateresuscitation in pregnancy, Intermediateshock management, FinalPPH and Finalmaternal sepsis.
The Common Final Pathway
Regardless of cause, shock progresses through linked pathology:
- Reduced effective tissue perfusion or oxygen use.
- Cellular ATP depletion, sodium-potassium pump failure, calcium influx and mitochondrial dysfunction.
- Anaerobic and stress metabolism, with lactate and acidosis.
- Endothelial activation, capillary leak, leukocyte adhesion and microvascular thrombosis.
- Coagulation disturbance, from consumption, dilution, acidosis, hypothermia or inflammatory DIC.
- Organ dysfunction, commonly kidney, brain, lung, liver, placenta/fetus and coagulation system.
Compensated shock can exist with an apparently acceptable systolic pressure. Pregnant and recently postpartum patients are often young, vasodilated, tachycardic at baseline, and expanded in plasma volume. They can maintain blood pressure until a large proportion of circulating volume or vascular tone has already been lost.
Compensation Buys Time but Creates Clues
Compensatory physiology is protective at first, but each response has a bedside signature. The candidate should read compensation as evidence of stress, not reassurance.
| Compensation | Trigger | Useful clue | Failure point |
|---|---|---|---|
| Sympathetic tachycardia | Falling stroke volume, fever, pain or hypoxia | Rising shock index before hypotension | Myocardial oxygen demand rises and diastolic filling falls |
| Peripheral vasoconstriction | Baroreceptor response to low effective volume | Cool skin, delayed capillary refill, narrow pulse pressure | Kidney, gut, uterus and skin become underperfused |
| Increased extraction | Reduced oxygen delivery | Venous desaturation and lactate trend where measured | Extraction reserve is finite |
| RAAS/ADH activation | Renal hypoperfusion and low arterial stretch | Oliguria and concentrated urine | Fluid retention cannot replace blood cells or clotting factors |
| Tachypnoea | Acidosis, hypoxia or sepsis | Often earlier than hypotension | Fatigue and respiratory failure worsen acidosis |
In obstetric haemorrhage this is why a woman can be speaking with a "normal" systolic pressure while already building oxygen debt and coagulopathy.
Stages of Shock
The stages are not strict boxes, but they help explain clinical progression.
| Stage | Cellular and vascular pathology | Clinical expression |
|---|---|---|
| Compensated | Sympathetic tone, RAAS and ADH preserve central perfusion while peripheral beds are sacrificed | Tachycardia, anxiety, cool peripheries, delayed capillary refill, normal or near-normal BP |
| Progressive | Oxygen delivery fails; lactate and acidosis rise; endothelial leak and coagulation activation begin | Oliguria, tachypnoea, confusion, hypotension, rising lactate, worsening base deficit |
| Decompensated | Compensation fails; microcirculation and myocardium deteriorate | Persistent hypotension, hypoxia, coagulopathy, reduced consciousness |
| Refractory | Mitochondrial failure, severe endothelial injury and organ necrosis persist despite resuscitation | Multi-organ failure, DIC, death risk despite apparent restoration of macrocirculation |
The exam trap is to call the first stage "not shock" because blood pressure is preserved. If cells are extracting more oxygen, kidneys are shutting down and lactate is rising, shock physiology is already present.
Oxygen Debt and Cellular Injury
We promised the cell was the real lesion; this section is where shock returns to the cell biology it started from. When oxygen delivery falls below what the tissues consume, the deficit accumulates as an oxygen debt — the gap between the oxygen the cells needed and the oxygen they actually received. The deeper and longer that debt, the worse the eventual outcome, which is why duration of hypoperfusion matters as much as its depth.