What are the types of shock and which vasopressor (vasoconstrictor) to use for each?

Types of Shock and Vasopressor Selection

Classification of Shock

Shock is classified into four main types based on underlying pathophysiology:

1. Hypovolemic Shock

• Results from inadequate intravascular volume due to hemorrhage, fluid losses, or severe dehydration 1
• Vasopressors are NOT the primary treatment—aggressive fluid resuscitation with balanced crystalloids is the definitive therapy 1
• Vasopressors may be used only transiently in life-threatening hypotension while simultaneously achieving hemorrhage control and volume restoration 2, 1
• The primary goal is restoration of intravascular volume—vasopressors serve only as a temporizing bridge 1

2. Cardiogenic Shock

• Characterized by inadequate cardiac output due to myocardial dysfunction 1
• Presents with hypotension, elevated filling pressures, and signs of end-organ hypoperfusion 1

3. Distributive Shock

• Characterized by pathological vasodilation resulting in decreased systemic vascular resistance despite normal or elevated cardiac output 3
• Includes septic shock, neurogenic shock, anaphylactic shock, pancreatitis, and burns 3
• Hypotension refractory to fluid administration is the cardinal feature 3

4. Obstructive Shock

• Results from mechanical obstruction to cardiac output (e.g., pulmonary embolism, cardiac tamponade, tension pneumothorax)
• Treatment focuses on relieving the obstruction rather than vasopressor therapy

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Vasopressor Selection by Shock Type

Hypovolemic Shock: Volume First, Vasopressors Last

Immediate fluid resuscitation with isotonic crystalloids is the cornerstone of treatment 1. Vasopressors should only be considered as an emergency measure during life-threatening hypotension while volume restoration is ongoing 2, 1.

• If vasopressors are absolutely necessary: Norepinephrine may be used transiently, but must be weaned aggressively as fluid resuscitation progresses 2
• Critical caveat: Using vasopressors without addressing the underlying volume deficit can worsen tissue perfusion and increase mortality 2

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Cardiogenic Shock: Match the Agent to Hemodynamics

Norepinephrine is the vasopressor of choice for most patients with cardiogenic shock, particularly those with tachycardia 1. This recommendation is based on randomized trial data showing improved survival and fewer arrhythmias compared to dopamine 1.

Algorithm for Cardiogenic Shock:

• Low cardiac output with adequate preload: Add dobutamine (up to 20 μg/kg/min) to norepinephrine 1, 4
• Persistently hypotensive with tachycardia: Use norepinephrine 1
• Bradycardia present: Consider dopamine 1
• Afterload-dependent states (aortic stenosis, mitral stenosis): Use phenylephrine or vasopressin 1

Inotropes (dobutamine or milrinone) are first-line when hypotension occurs with low cardiac output in acute heart failure 1, 4. The combination of dobutamine and norepinephrine is recommended as first-line treatment in patients with low cardiac output and hypotension 4.

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Distributive Shock: Norepinephrine First, Add Vasopressin Second

Norepinephrine is the first-line vasopressor for distributive shock after adequate fluid resuscitation 2, 1, 3. This is the strongest recommendation across all major guidelines, with Grade 1B evidence from the Surviving Sepsis Campaign 2.

Stepwise Algorithm for Distributive Shock:

1. Initial resuscitation: Balanced crystalloids 3
2. First-line vasopressor: Norepinephrine, targeting MAP ≥65 mmHg 2, 1, 3
3. Second-line agent: Add vasopressin (up to 0.03 units/min) if hypotension persists despite norepinephrine 2, 1, 3
4. Myocardial dysfunction: Add dobutamine to norepinephrine when there is evidence of elevated filling pressures and low cardiac output 1, 3

Vasopressin should be added to reduce norepinephrine requirements rather than escalating norepinephrine doses 2, 3. Vasopressin doses higher than 0.03-0.04 U/min should be reserved for salvage therapy 2.

Alternative Agents:

• Epinephrine: Added to or substituted for norepinephrine when an additional agent is needed, though it causes more metabolic disturbances (hyperglycemia, hyperlactatemia) 2, 5
• Dopamine: Only in highly selected patients with low risk of tachyarrhythmias and absolute or relative bradycardia 2—dopamine is associated with higher arrhythmia rates and should generally be avoided 6
• Phenylephrine: Reserved for specific circumstances: (a) norepinephrine causes serious arrhythmias, (b) cardiac output is high but blood pressure remains low, or (c) salvage therapy 2, 4

Meta-analysis data strongly support norepinephrine over dopamine: Norepinephrine was associated with decreased all-cause mortality (RR 0.89,95% CI 0.81-0.98), corresponding to an absolute risk reduction of 11% and number needed to treat of 9 6.

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Neurogenic Shock: Same as Distributive Shock

Norepinephrine is the first-choice vasopressor for neurogenic shock, with an initial target MAP of 65 mmHg 4. The management algorithm mirrors distributive shock:

1. First-line: Norepinephrine 4
2. Second-line: Add vasopressin to raise MAP or decrease norepinephrine requirements 4
3. Alternative: Epinephrine can be added or substituted 4
4. Inotropic support: Dobutamine when there is myocardial dysfunction with elevated filling pressures and low cardiac output 4

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General Principles Across All Shock Types

• Target MAP ≥65 mmHg as the standard goal 2, 1, 3, though this should be higher in patients with chronic hypertension or atherosclerosis 2
• Early vasopressor use reduces organ failure incidence 1
• Arterial catheter placement should occur as soon as practical in all patients requiring vasopressors 1, 4
• Vasopressors may be initiated during fluid resuscitation and weaned as tolerated 1
• Adequate fluid resuscitation must precede or accompany vasopressor therapy 1

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Critical Pitfalls to Avoid

• Never use vasopressors as a substitute for volume resuscitation in hypovolemic shock 1—this is a common error that worsens outcomes
• Avoid dopamine in septic shock 2, 6—it increases arrhythmias without mortality benefit
• Do not use vasopressin as monotherapy 2—it should always be added to norepinephrine, not used alone
• Phenylephrine has detrimental effects on microcirculatory perfusion 2—reserve for specific indications only
• Failure to improve with appropriate therapy should prompt reassessment for alternative or additional shock types 3