Shock: Cardiogenic, Septic, Hypovolemic, and Anaphylactic

Core Idea

Shock is inadequate tissue perfusion and oxygenation. Cardiogenic shock results from pump failure (heart attack, arrhythmia); hypovolemic shock from blood loss or dehydration; septic shock from vasodilation and increased capillary permeability; anaphylactic from histamine-mediated vasodilation. Compensation through increased sympathetic tone eventually fails, leading to irreversible tissue damage.

How It's Best Learned

Use hemodynamic parameters (MAP, cardiac output, SVR) to classify shock type. Understand the three phases: compensated (normal BP, tachycardia), decompensated (falling BP, oliguria), and irreversible (refractory hypotension, organ failure).

Common Misconceptions

Hypotension is a late sign of shock—tissue hypoperfusion can occur with normal blood pressure. Lactate normalization does not indicate recovery; persistent elevation predicts mortality despite apparent clinical improvement.

Explainer

From your study of cardiac output and blood pressure regulation, you know that tissue perfusion depends on two things: an adequate driving pressure (mean arterial pressure, MAP = cardiac output × systemic vascular resistance) and vessels that can distribute flow to where it is needed. Shock is the state in which this delivery system fails to meet the oxygen demands of the tissues — not simply low blood pressure, but inadequate cellular oxygenation. The four major types of shock represent four different mechanisms of failure in this delivery system, and understanding each requires asking: which part of the equation broke?

Hypovolemic shock is the simplest to conceptualize: the circuit loses volume. Hemorrhage, burns, or severe dehydration reduce venous return, which reduces end-diastolic volume and therefore stroke volume (the Frank-Starling mechanism you know from cardiac physiology). Cardiac output falls, and with it MAP. Compensatory responses — baroreceptor-driven sympathetic activation, tachycardia, vasoconstriction, and ADH release — attempt to restore pressure by squeezing the remaining volume into a narrower vascular bed. Cardiogenic shock has the opposite origin: the pump itself fails. A large myocardial infarction destroys enough contractile tissue that stroke volume collapses regardless of adequate filling pressure. The hemodynamic signature is characteristically different from hypovolemia: filling pressures are elevated (the failing ventricle backs up blood into the lungs, causing pulmonary edema), while cardiac output and MAP are low.

Septic shock introduces a fundamentally different mechanism — distributive failure. Bacterial endotoxins and inflammatory mediators (nitric oxide, histamine, cytokines) cause massive vasodilation, dropping systemic vascular resistance precipitously. Unlike hypovolemic and cardiogenic shock where the vasculature is constricted, in sepsis the vessels dilate inappropriately, pooling blood in the periphery. Cardiac output is often initially elevated (the heart is pumping faster to compensate), yet MAP is low because SVR has collapsed. Additionally, increased capillary permeability allows plasma to leak into the interstitium — a functional volume loss compounding the distributive problem. Anaphylactic shock operates through a similar vasodilatory mechanism but is immunologically triggered: IgE-mediated mast cell and basophil degranulation releases histamine, causing acute massive vasodilation and bronchoconstriction simultaneously.

Your prerequisite knowledge of homeostasis and feedback loops explains the progression through shock's three phases. In compensated shock, baroreceptors detect the fall in MAP and activate the sympathetic nervous system — heart rate rises, vessels constrict, and adrenal glands release catecholamines and cortisol. Blood pressure may remain normal or nearly so, but the signs of compensation are already detectable: tachycardia, cool clammy skin (vasoconstriction), and restlessness. In decompensated shock, the compensatory mechanisms are overwhelmed. Blood pressure falls, oliguria develops as renal perfusion drops below the autoregulatory threshold, and tissues switch to anaerobic glycolysis — producing the lactic acid that clinically manifests as an elevated lactate level. In irreversible shock, prolonged ischemia triggers cell death across multiple organ systems simultaneously, inflammatory cascades become self-amplifying, and restoration of perfusion precipitates further injury through reperfusion — this stage has a very high mortality even with aggressive resuscitation.

The critical clinical insight — and the reason hypotension is a late and dangerous sign — is that MAP can be maintained by compensation even as tissue perfusion is critically impaired. A patient in early septic shock may have a normal blood pressure, normal mentation, and only subtle tachycardia. But their lactate is rising, their microcirculation is maldistributed, and their cells are in oxygen debt. This is why lactate clearance — not blood pressure normalization — is the modern target of resuscitation: it directly measures the adequacy of cellular oxygen delivery in a way that blood pressure alone cannot.