Haemodynamic disorders
Introduced| English | Meaning |
|---|---|
| shock | shock |
| capillary hydrostatic pressure | capillary hydrostatic pressure |
| plasma oncotic pressure | plasma oncotic pressure |
| oedema | Excess fluid in the interstitial spaces. |
| Virchow's triad | The three causes of thrombosis: stasis, endothelial injury and hypercoagulability. |
| thrombosis | thrombosis |
| embolus | embolus |
| infarct | infarct |
Swollen ankles, two different causes
- Two patients have swollen ankles. One has a failing heart; the other is losing protein in the urine. The swelling looks the same, but the causes are opposite.
- Fluid, clots and shock all follow simple pressure arithmetic, and the arithmetic points to the cause.
Starling forces
- Capillary hydrostatic pressure 毛细血管静水压 pushes fluid out: about 30 mmHg at the arterial end and 15 mmHg at the venous end.
- Plasma oncotic pressure 血浆胶体渗透压, mostly from albumin, pulls about 25 mmHg back. The small surplus that stays in the tissue returns as lymph.
At a capillary's arterial end, hydrostatic pressure is 32 mmHg and plasma oncotic pressure is 25 mmHg (ignore tissue pressures). What is the net outward pressure?
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32 − 25 = 7 mmHg pushing fluid out.
Four main causes of oedema
- Oedema 水肿 collects when filtration outruns lymph drainage. Hydrostatic pressure rises (heart failure) or oncotic pressure falls (low albumin from liver or kidney disease, or malnutrition).
- Permeability rises (inflammation, giving a protein-rich exudate), or lymph is blocked (after node surgery, or filarial worms).
Match each cause of oedema to its mechanism.
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Each clinical oedema is one of the four.
Virchow's triad
- Virchow's triad 魏尔啸三联征: endothelial injury, stasis of blood and hypercoagulability. Any one raises the risk of thrombosis 血栓形成; together they multiply it.
- A thrombus can break off as an embolus 栓子. A leg vein clot travels to the lung: pulmonary embolism.
A long flight raises the risk of a leg vein clot mainly through which part of Virchow's triad?
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Sitting still slows blood in the leg veins.
Infarcts, red and pale
- An infarct 梗死 is tissue killed by lost blood supply. Solid organs with one artery (heart, kidney, spleen) give pale infarcts.
- Loose tissue with two blood supplies or blocked veins (lung, gut, a twisted testis) fills with blood: red infarcts.
Lung infarcts are usually pale.
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The lung has two blood supplies and loose tissue, so its infarcts fill with blood and are red.
Shock
- Shock 休克 is failing tissue perfusion. Cardiogenic: the pump fails. Hypovolaemic: volume is lost (class III is 30–40% of blood volume). Distributive: vessels dilate (septic or anaphylactic). Obstructive: flow is blocked (a large embolus).
- Septic shock is often warm and flushed at first, so low pressure alone does not define shock.
Match each case to its type of shock.
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Pump, volume, vessel tone or a blocked path.
Worked example: the arterial end
- (Fictional values) Arterial end: hydrostatic 30 mmHg out, oncotic 25 mmHg in. Net = 30 − 25 = 5 mmHg out.
- In nephrotic syndrome albumin falls and oncotic pressure drops to 15 mmHg: net = 30 − 15 = 15 mmHg out, so far more fluid leaves and oedema forms.
Worked example: how much blood is lost?
- (Fictional) Blood volume is about 70 mL per kg. A 70 kg adult has 70 × 70 = 4,900 mL, about 4.9 L.
- Losing 1.7 L is 1.7 ÷ 4.9 ≈ 35%: class III shock, with a fast pulse, falling blood pressure and confusion.
A 70 kg adult (blood volume 70 mL/kg, so 4.9 L) loses 2 L of blood. What percentage of the blood volume is that? Give one decimal place.
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2 ÷ 4.9 × 100 ≈ 40.8%, just past class III into class IV (over 40%).
Put the chain from leg clot to lung damage in order.
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Each link has its own test (D-dimer, ultrasound, CT).
Three common slips
- Treating all oedema as one disease. There are four main mechanisms, and each needs a different treatment.
- Calling every infarct pale. Lung, gut and testis infarcts are red.
- Defining shock as low blood pressure. Shock is failing perfusion; septic shock can start warm.
Recap
- Starling: hydrostatic pushes out (about 30 then 15 mmHg), oncotic pulls in (about 25), lymph takes the rest.
- Oedema: raised hydrostatic pressure, low oncotic pressure, raised permeability or blocked lymph.
- Virchow: injury, stasis, hypercoagulability → thrombus → embolus → infarct (pale in solid organs, red in lung, gut and testis).