Four Types of Shock: Flow, Filling and Resistance
How do the four types of shock differ?
Hypovolemic shock begins with lost circulating volume, cardiogenic shock with inadequate cardiac pumping, distributive shock with abnormal vascular tone and distribution, and obstructive shock with mechanical interference to filling or forward flow. Their expected hemodynamic patterns overlap. Interpret flow, pressure and tissue findings together because patients sometimes have mixed or changing mechanisms.
| Shock mechanism | Starting problem | Pattern qualification |
|---|---|---|
| Hypovolemic | Loss of circulating volume reduces filling | Low filling/output and compensatory high resistance are common, not required |
| Cardiogenic | Primary cardiac impairment limits forward output | Filling pressures depend on the involved chamber and loading |
| Distributive | Abnormal tone and blood distribution | Output varies with filling, cardiac function and timing |
| Obstructive | Mechanical interference with filling or flow | Pressure patterns depend on the obstruction site |
Key takeaways
- Shock is circulatory failure affecting tissue function. Blood pressure alone does not establish or exclude it.
- Expected pressure and flow patterns describe mechanisms without independently diagnosing a disease.
- Right-sided and left-sided filling pressures do not move identically in every pump or obstructive problem.
- Mixed physiology and treatment effects change the pattern over time.
In this video
- When the blood pressure stays the same
- What shock means
- Pressure, flow and resistance
- Hypovolemic shock
- Cardiogenic shock
- Distributive shock
- Obstructive shock
- Comparing the four shock patterns
- The same pressure can hide a different flow
- Interpreting the change
- Hemodynamic tradeoffs
- Explaining shock in an interview
- Practice questions
- Recap: classify the mechanism
- Keep learning with The CRNA Club
What changes in hypovolemic shock?
Blood or fluid loss reduces the volume available for venous return and ventricular filling. Reduced preload then lowers stroke volume when the ventricle is operating on a preload-dependent portion of its function relationship. If the heart-rate response does not compensate, cardiac output falls.
An uncomplicated predominantly hypovolemic pattern often includes low filling pressures, reduced output and compensatory increased systemic resistance. The actual pattern changes with coexisting disease and treatment. Tachycardia does not independently diagnose blood loss, and a filling pressure is not a direct measurement of total circulating volume.
How does cardiogenic shock produce congestion and low flow?
A primary cardiac problem limits effective forward output. Impaired contraction, a major rhythm disturbance or structural disease contributes. In predominantly left ventricular systolic failure, reduced ejection raises left-sided filling pressure and congestion while systemic forward flow declines.
The familiar low-output, high-left-filling-pressure pattern does not mean every filling pressure must rise. Isolated right ventricular failure, left-sided dysfunction and combined ventricular failure create different relationships. Concurrent vasodilation also changes resistance, so the high-SVR teaching pattern is not mandatory in every cardiogenic presentation.
What makes distributive and obstructive shock different?
Distributive shock involves vascular tone and blood distribution. Arterial dilation lowers resistance, and venous dilation changes effective filling. In inflammatory disease, permeability injury adds extravascular fluid movement. Output is sometimes maintained or high when filling and pump function allow, but inadequate filling or cardiac impairment also produces low output.
Obstructive shock involves a mechanical barrier. Pulmonary vascular obstruction loads the right ventricle, tamponade limits chamber expansion and tension pneumothorax impedes return and filling through intrathoracic pressure. These mechanisms do not produce identical pressure patterns. High upstream pressure sometimes coexists with inadequate effective chamber filling.
How can pressure stay unchanged while cardiac output falls?
Pressure depends on both flow and resistance. Using conventional SVR units, SVR equals 80 times the difference between MAP and central venous pressure divided by cardiac output. Increased resistance sometimes maintains MAP despite falling output. The calculated value describes a large-scale relationship rather than tissue perfusion.
In the recording’s hypothetical example, MAP remains 74 mmHg. CVP rises from 8 to 14 mmHg while output falls from 6 to 3 L/min. Calculated SVR rises from 880 to 1,600 dyn·s/cm5. New confusion and declining urine output add concern. The unchanged arterial pressure does not establish an unchanged circulation, and none of these invented values is a treatment target.
Why does the initial shock label need reassessment?
One patient sometimes has several mechanisms. An infection-associated illness does not exclude a new cardiac problem, volume loss or obstruction. Compare serial flow, upstream and downstream pressures, organ findings and measurement quality rather than forcing each later result into the original label.
Interventions also have competing effects. More filling sometimes improves stroke volume while adding congestion. Greater arterial tone sometimes supports pressure while increasing ejection load. Improving contractility does not remove an external obstruction. The relevant question is which limitation the current assessment supports and how the patient responded afterward.
