Shock is not a diagnosis. It is a physiologic state — inadequate tissue oxygen delivery relative to demand — and the treatment depends entirely on which mechanism is producing it. Getting the mechanism wrong means treating the wrong problem while the right one continues.
This is the framework for differentiating shock at the bedside and on the exam.
The common pathway, understood first.
Every type of shock produces the same end result: cells do not get enough oxygen to maintain aerobic metabolism. When aerobic metabolism fails, cells shift to anaerobic pathways, producing lactate. Elevated lactate is the biochemical footprint of shock regardless of type. It is not specific to any mechanism — it is the downstream consequence of all of them.
What differs between shock types is the upstream mechanism producing the oxygen delivery failure. Oxygen delivery (DO₂) is the product of cardiac output and arterial oxygen content — expressed as DO₂ = CO × CaO₂ × 10 (where the factor of 10 converts mL/dL to mL/min). Every type of shock either reduces cardiac output, reduces oxygen content, or creates a distribution problem where delivery is adequate globally but not locally. Knowing where the failure lives determines what you do next.
The four mechanisms.
Distributive shock is the most common type encountered in clinical practice and the most commonly tested. The defining physiology is inappropriately low systemic vascular resistance (SVR) — the vasculature is dilated and cannot maintain perfusion pressure. Cardiac output is typically elevated (the heart is working harder to compensate), but the distribution of that output is abnormal. Septic shock is the prototype. Anaphylaxis and neurogenic shock follow the same hemodynamic pattern: low SVR, elevated or normal CO, warm extremities in early stages, hypotension.
The clinical picture of distributive shock is a hypotensive patient who is warm and flushed early, with a wide pulse pressure and bounding pulses. This distinguishes it from the cold, clamped-down patient of cardiogenic or hypovolemic shock. Note that as distributive shock progresses and capillary beds decompensate, extremities may cool even without a vasoconstrictive mechanism.
Hypovolemic shock occurs when intravascular volume is insufficient to maintain preload and therefore cardiac output. The mechanism is reduced venous return — the heart cannot fill adequately and cannot eject what it does not receive. The compensatory response is predictable: tachycardia, increased SVR (cold, clamped extremities), reduced pulse pressure. Hemorrhagic and non-hemorrhagic causes produce the same hemodynamic pattern.
The exam distinction that matters: hypovolemic shock is the only type where the primary intervention is volume restoration. In cardiogenic shock, volume can be actively harmful. In distributive shock, volume is part of resuscitation but insufficient alone. In obstructive shock, volume buys time but does not treat the cause.
Cardiogenic shock occurs when the heart fails as a pump. Preload is adequate, volume is not the problem — the myocardium cannot convert that preload into effective forward flow. The result is elevated filling pressures (pulmonary edema, elevated JVP), low CO, compensatory peripheral vasoconstriction. The patient is cold, clamped, and typically in respiratory distress from pulmonary congestion. Aggressive fluid administration in cardiogenic shock worsens pulmonary edema without improving perfusion.
The classic hemodynamic profile: elevated PCWP, low CO/cardiac index, elevated SVR. On the exam, this profile maps to the cold-wet quadrant of the Forrester classification. Treatment targets are inotropy and afterload reduction, not volume.
Obstructive shock is mechanical obstruction to blood flow — the heart and volume are both adequate, but something external is preventing effective circulation. Tension pneumothorax compresses the mediastinum and impedes venous return. Massive PE creates right ventricular outflow obstruction and acute RV failure. Cardiac tamponade compresses all chambers, preventing filling.
The key exam and bedside skill for obstructive shock is pattern recognition: Beck’s triad (hypotension, JVD, muffled heart sounds) for tamponade; tracheal deviation, absent breath sounds, JVD for tension pneumothorax. These diagnoses are not confirmed by echo or CT before intervention — they are clinical diagnoses that require immediate action.
The hemodynamic matrix.
When advanced hemodynamic monitoring is available, these parameters rapidly localize the mechanism:
| Type | Preload (CVP/PCWP) | CO/CI | SVR |
|---|---|---|---|
| Distributive | Low to normal | High | Low |
| Hypovolemic | Low | Low | High |
| Cardiogenic | High | Low | High |
| Obstructive | High (variable) | Low | High |
Obstructive and cardiogenic shock can look similar on the matrix. The differentiating information is clinical: the distribution of elevated filling pressures, presence of pulmonary edema, and the clinical context (trauma, known malignancy, sudden onset pleuritic chest pain).
Mixed shock states.
Real patients rarely present with textbook single-mechanism shock. Septic shock produces distributive physiology but can also cause myocardial depression (septic cardiomyopathy), creating a mixed distributive-cardiogenic picture. Trauma patients may have hemorrhagic shock with concurrent tension pneumothorax. The resuscitation approach must account for both mechanisms simultaneously.
On the exam, mixed shock scenarios are identified by hemodynamic parameters that do not fit a single category cleanly, or by clinical information that suggests more than one mechanism is operating. The correct approach is to treat the most immediately reversible cause first while addressing the others in sequence.
The trajectory question.
Every shock patient requires an answer to the same trajectory question within the first minutes: is this getting better, staying the same, or getting worse? The answer drives the pace and escalation of intervention. A patient in distributive shock who responds to initial fluid resuscitation and vasopressors is on a different trajectory than one whose lactate is rising and whose MAP is not responding. The monitoring strategy, resource activation, and disposition decision all follow from that trajectory assessment.
The reasoning framework for shock — mechanism recognition, hemodynamic pattern, appropriate intervention sequence — is the same one applied at the bedside and tested on every certification lane. It is not a different skill for a different context.