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 **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.