5.2 Shock States & Vasoactive Support

Pathophysiology and Classification of Shock States

Shock is a state of systemic tissue hypoperfusion resulting in cellular hypoxia, anaerobic metabolism, and progressive multi-organ dysfunction syndrome (MODS). Understanding the unique hemodynamic profile of each shock state is critical for directing appropriate therapy.

Hypovolemic Shock

Hypovolemic shock results from a critical depletion of intravascular volume, caused by either hemorrhagic (e.g., trauma, GI bleed) or non-hemorrhagic (e.g., severe burns, dehydration) fluid loss.

• Hemodynamics: Preload is severely reduced, leading to low central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP). Consequently, cardiac output (CO) falls. SVR rises dramatically as a compensatory mechanism (sympathetic vasoconstriction) to maintain MAP.
• Therapy: Resuscitation focuses on volume replacement with crystalloids (Lactated Ringer's) or blood products (PRBCs, FFP, platelets). Vasopressors are contraindicated until volume is restored.

Cardiogenic Shock

Cardiogenic shock is characterized by primary pump failure, most commonly due to acute myocardial infarction, severe cardiomyopathy, or valvular dysfunction.

• Hemodynamics: The heart is unable to eject blood, resulting in a low cardiac output (CO) and cardiac index (CI). Blood backs up into the pulmonary and systemic circulation, leading to elevated PCWP (left heart preload) and CVP (right heart preload). SVR is high to compensate.
• Therapy: Focused on improving contractility with inotropes (dobutamine) and reducing afterload. Mechanical support (e.g., Intra-Aortic Balloon Pump, Impella) may be required.

Distributive Shock (Septic Shock)

Distributive shock is characterized by profound, pathological systemic vasodilation. Sepsis is the most common cause, where an overwhelming host immune response to infection triggers nitric oxide release, relaxing vascular smooth muscle.

• Hemodynamics: SVR is profoundly low. The vascular space is dilated, causing a relative hypovolemia (low or normal CVP and PCWP). In the early phase, cardiac output (CO) is elevated (hyperdynamic or "warm" shock) due to low vascular resistance.
• Therapy: Early fluid resuscitation (30 mL/kg) followed by vasopressors to restore vascular tone.

Obstructive Shock

Obstructive shock occurs when physical obstruction to blood flow impairs cardiac filling or emptying (e.g., massive pulmonary embolism, cardiac tamponade, tension pneumothorax).

• Hemodynamics: A pulmonary embolism obstructs RV outflow, causing high CVP, high PAP, but a low PCWP (since blood cannot reach the left heart) and low CO. In cardiac tamponade, pressures equalize, showing high CVP and high PCWP. SVR is high.
• Therapy: Directly address the obstruction (e.g., thrombolytics for PE, pericardiocentesis for tamponade, needle decompression for tension pneumothorax).

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Vasoactive Support: Receptor Pharmacology and Dosages

When fluid resuscitation is insufficient, vasoactive medications are titrated to maintain organ perfusion.

Adrenergic Receptor Activity

• Alpha-1 ($\alpha_1$): Located on vascular smooth muscle. Stimulation causes vasoconstriction, increasing SVR and blood pressure.
• Beta-1 ($\beta_1$): Located in the myocardium. Stimulation increases heart rate (chronotropy) and contractility (inotropy).
• Beta-2 ($\beta_2$): Located on bronchial and vascular smooth muscle. Stimulation causes bronchodilation and vasodilation.
• Vasopressin-1 ($V_1$): Non-adrenergic receptors on vascular smooth muscle. Stimulation causes direct vasoconstriction.

Key Vasoactive Agents in the ICU

1. Norepinephrine (Levophed): A potent $\alpha_1$ agonist with minor $\beta_1$ activity. It is the first-line vasopressor for septic shock and is titrated from 2 to 30 mcg/minute. It increases SVR and blood pressure with minimal increase in heart rate.
2. Dobutamine (Dobutrex): A powerful $\beta_1$ agonist with mild $\beta_2$ activity. It is the first-line inotrope for cardiogenic shock. Titrated from 2 to 20 mcg/kg/minute. It increases cardiac output but can cause vasodilation and hypotension due to its $\beta_2$ effects, often requiring co-administration of norepinephrine.
3. Epinephrine (Adrenalin): A potent agonist of $\alpha_1$, $\beta_1$, and $\beta_2$ receptors. Used in anaphylaxis (0.3 mg IM or IV infusion) and refractory septic shock (1 to 10 mcg/minute).
   • Exam Trap: Epinephrine increases blood lactate levels by stimulating $\beta_2$-mediated aerobic glycolysis in skeletal muscle. This rise in lactate does not necessarily indicate tissue hypoxia.
4. Vasopressin (Pitressin): A pure $V_1$ agonist. Administered at a fixed, non-titratable dose of 0.03 to 0.04 units/minute as an adjunct to norepinephrine in septic shock to spare catecholamine requirements.
5. Phenylephrine (Neo-Synephrine): A pure $\alpha_1$ agonist. Titrated from 10 to 200 mcg/minute. It is indicated for distributive shock only when the patient has severe tachyarrhythmias and cannot tolerate the $\beta_1$ effects of norepinephrine.