In hemorrhagic shock, which of the following physiologic responses occurs first as a result of sympathetic activation?

Question

Accepted Answer

D. Vasoconstriction and increased heart rate. ## Pathophysiology of Hemorrhagic Shock — Sequence of Compensation

**Key Point:** Hemorrhagic shock progresses through a predictable sequence of compensatory mechanisms. Sympathetic activation is the **first and earliest** response to blood volume loss, occurring before systemic hypotension develops.

### Timeline of Compensatory Mechanisms

1. **Immediate (within seconds):** Sympathetic nervous system activation
   - Increased catecholamine release (epinephrine, norepinephrine)
   - Arteriolar and venous vasoconstriction
   - Increased heart rate (tachycardia)
   - Increased myocardial contractility

2. **Early (minutes):** Fluid shifts and hormonal response
   - Interstitial fluid shifts into intravascular space
   - Activation of renin–angiotensin–aldosterone system (RAAS)
   - ADH (vasopressin) release
   - Increased respiratory rate (tachypnea)

3. **Late (if uncompensated):** Decompensation and organ hypoperfusion
   - Systolic BP falls (loss of compensation)
   - Altered mental status (cerebral hypoperfusion)
   - Oliguria (renal hypoperfusion)
   - Metabolic acidosis (tissue hypoxia)

### Mechanism of Sympathetic Compensation

**High-Yield:** The baroreceptor reflex is triggered by a **decrease in mean arterial pressure** detected by carotid sinus and aortic arch baroreceptors. This activates the sympathetic nervous system via the brainstem, resulting in:

- **Vasoconstriction:** Preferential perfusion of vital organs (brain, heart, kidneys)
- **Tachycardia:** Maintains cardiac output (CO = HR × SV) despite reduced stroke volume
- **Increased contractility:** Maximizes forward flow

**Clinical Pearl:** In **Class I and II hemorrhagic shock**, systolic blood pressure remains **normal or only mildly reduced** because sympathetic compensation is adequate. The presence of tachycardia and tachypnea in a normotensive patient is a sign of **early shock** and should prompt aggressive resuscitation before decompensation occurs.

**Mnemonic: "SAVE the vital organs"**
- **S**ympathetic activation (first response)
- **A**rteriolar vasoconstriction
- **V**enous constriction
- **E**levated heart rate and contractility

These mechanisms preserve perfusion to the **brain, heart, and kidneys** at the expense of skin, GI tract, and skeletal muscle.

Answer

A. Oliguria and reduced urine output

Answer

B. Altered mental status and confusion

Answer

C. Systolic blood pressure falls below 90 mmHg

In hemorrhagic shock, which of the following physiologic responses occurs first as a result of sympathetic activation?

Explanation

Pathophysiology of Hemorrhagic Shock — Sequence of Compensation

Timeline of Compensatory Mechanisms

Mechanism of Sympathetic Compensation

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