A 55-year-old man with massive hemorrhage from a ruptured aortic aneurysm is in hypovolemic shock. A 48-year-old woman with anaphylaxis to penicillin is in distributive shock. Which feature most reliably distinguishes hypovolemic shock from anaphylactic shock?

## Distinguishing Hypovolemic from Anaphylactic (Distributive) Shock ### Core Pathophysiologic Difference **Key Point:** Hypovolemic shock is characterized by **absolute loss of intravascular volume**, while anaphylactic shock is a form of **distributive shock** caused by **massive vasodilation and increased capillary permeability**. SVR is the key discriminator. ### Hemodynamic and Clinical Comparison | Feature | Hypovolemic Shock | Anaphylactic Shock | |---------|-------------------|--------------------| | **Mechanism** | Absolute volume loss | Relative hypovolemia (vasodilation + capillary leak) | | **SVR** | **ELEVATED** (compensatory vasoconstriction) | **MARKEDLY LOW** (histamine, tryptase, bradykinin) | | **Skin** | Cold, clammy, mottled | Warm initially; may become mottled if severe | | **Urticaria/Angioedema** | **ABSENT** | **PRESENT** (pathognomonic) | | **Airway** | Normal | **Stridor, laryngeal edema** (life-threatening) | | **Bronchospasm** | No | Yes (common) | | **Onset** | Gradual (minutes to hours) | **Rapid** (seconds to minutes) | | **CVP** | Low | Low | ### Why SVR + Clinical Signs Distinguish Them 1. **Hypovolemic Shock**: Body compensates for blood loss by **sympathetic activation** → vasoconstriction → **HIGH SVR**. Skin is cold and clammy. No allergic manifestations. 2. **Anaphylactic Shock**: Mast cell degranulation releases histamine, tryptase, and bradykinin → **profound vasodilation** → **VERY LOW SVR**. Skin is warm (initially). **Urticaria, angioedema, stridor, bronchospasm** are hallmark signs. **High-Yield:** **Elevated SVR in hypovolemic shock vs. markedly low SVR in anaphylactic shock** is the hemodynamic gold standard. **Urticaria and angioedema are pathognomonic for anaphylaxis** and absent in hypovolemic shock. **Clinical Pearl:** In hypovolemic shock, the body is "fighting to maintain perfusion" (vasoconstriction, tachycardia). In anaphylactic shock, the body is "fighting the allergen" (mast cell activation, vasodilation, airway edema). **Mnemonic:** **SLUDGE** (Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis) — features of anaphylaxis and other distributive shocks due to parasympathomimetic effects; absent in hypovolemic shock. ### Why Other Features Are Non-Discriminatory - **Lactate & urine output**: Both are elevated/decreased in both shock states due to tissue hypoperfusion. - **HR & BP**: Both are low in both conditions; tachycardia is compensatory in both. - **Capillary refill & mottling**: Both can occur in both conditions, though mottling is more typical of hypovolemic shock.