SN7
SHORT NOTE
5M
★ High-yield
Define shock. Classify it and describe the compensatory mechanisms.
Definition
Shock = a state of generalised inadequate tissue perfusion, such that oxygen delivery fails to meet metabolic demand, leading to cellular hypoxia and organ dysfunction.
Classification
| Type | Mechanism | Causes | Cardiac output | Skin |
|---|---|---|---|---|
| Hypovolaemic | ↓ Blood volume | Haemorrhage, burns, vomiting, diarrhoea | ↓ | Cold, clammy |
| Cardiogenic | Pump failure | Myocardial infarction, arrhythmia, tamponade | ↓ | Cold, clammy |
| Distributive | ↓ Peripheral resistance | Septic, anaphylactic, neurogenic | ↑ or normal | Warm, flushed (early) |
| Obstructive | Mechanical obstruction to flow | Pulmonary embolism, tension pneumothorax | ↓ | Cold |
Stages
| Stage | Features |
|---|---|
| 1. Compensated (non-progressive) | Baroreceptor reflex, RAAS, ADH, capillary fluid shift maintain BP; tachycardia, cool peripheries, BP still normal |
| 2. Progressive (decompensated) | Compensation fails; hypotension, oliguria, acidosis, confusion |
| 3. Irreversible | Cell and organ damage established; death occurs despite restoring blood volume |
Compensatory Mechanisms
↓ BP → Baroreceptors unloaded → ↑ Sympathetic discharge → Tachycardia + vasoconstriction + venoconstriction → ↑ RAAS, ADH, capillary fluid reabsorption → BP maintained
- Blood is redistributed to brain and heart, away from skin, gut, muscle and kidney
- Cerebral and coronary vessels have few α receptors, so they escape vasoconstriction
Clinical Features
- Tachycardia with a weak thready pulse
- Hypotension (a late sign — up to 30% of blood volume may be lost before BP falls)
- Cold clammy skin, delayed capillary refill; warm skin in early septic shock
- Oliguria (< 0.5 mL/kg/h) — an early and sensitive index
- Restlessness, confusion, air hunger; metabolic acidosis with raised lactate
Applied Aspects
- Vicious cycles in irreversible shock — cardiac depression, vasomotor failure, DIC, and increased capillary permeability all worsen perfusion further (positive feedback)
- Treatment — restore volume (except in cardiogenic shock), treat the cause, oxygen, inotropes and vasopressors as needed
- Adrenaline is life-saving in anaphylactic shock
- MODS — multi-organ dysfunction; the usual mode of death, with acute tubular necrosis and ARDS
All questions in Cardiovascular System
- Q1Describe the cardiac cycle. Explain the pressure and volume changes in the left ventricle during one cycle. Add a note on the jugular venous pulse. ★
- Q2Define cardiac output. Describe its determinants and the factors regulating it. Add a note on the methods of its measurement. ★
- Q3Describe the short-term and long-term mechanisms regulating arterial blood pressure. Add a note on the renin–angiotensin system. ★
- Q4Describe the conducting system of the heart. Explain the genesis of the pacemaker potential and the properties of cardiac muscle. ★
- Q5Describe the normal electrocardiogram. Explain the leads, waves and intervals with their normal values, and give the uses of the ECG. ★
- SN1Enumerate the heart sounds. Give their causes, characteristics and areas of auscultation. ★
- SN2State Starling’s law of the heart. Explain its basis and significance. ★
- SN3Describe the baroreceptor reflex and give its significance. ★
- SN4Describe the coronary circulation and the factors regulating coronary blood flow. ★
- SN5Describe the triple response of Lewis. Add a note on the axon reflex.
- SN6Describe the fetal circulation and the changes that occur at birth. ★
- SN7Define shock. Classify it and describe the compensatory mechanisms. ★