Trauma ICU Rounds – Shock Talk I: Pathophysiology & Classification

Originally posted on Trauma ICU Rounds on April 24, 2020. Follow Trauma ICU Rounds (@traumaicurounds) and Dr. Kim (@dennisyongkim) to learn more on simplifying trauma critical care together.

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What is shock? A clear grasp of this concept is a MUST for any practitioner taking care of patients. Shock is many things. It is dynamic. It is elusive. It is lethal. A high index of suspicion is required to identify patients in shock. While lifesaving therapies are initiated, we must have an organized approach to to shock in order to identify the best diagnostic and therapeutic pathways for our patients. As such, we will review a simple classification system for shock. In a follow-up episode, we’ll further examine the clinical presentation and management of patients presenting with hypovolemic, cardiogenic, distributive, or obstructive shock.


In this episode, we review the pathophysiology and classification of a common and potentially fatal condition, shock. Starting with the concept of oxygen delivery, we’ll examine the relationship between supply and demand, in an effort to grasp the concept of critical DO2. Further, we’ll review a 4 category classification system for shock which we should all have at the front of our minds as we begin our initial resuscitaton, sift through the data, and examine our patients in an effort to determine the etiology for this common yet elusive condition.

Learning Objectives

1. Understand the common pathophysiologic mechanisms underlying shock (critical DO2)

2. Immediately recall the 4 key categories of shock, and finally

3. Provide a differential diagnosis for the common causes of shock across the 4 key categories

Take Home Points

  • Shock is dynamic and elusive. Therefore, a HIGH INDEX OF SUSPICION is necessary in order to identify shock in our patients.

  • Once shock is recognized IMMEDIATE intervention is required. These interventions are designed to INCREASE oxygen delivery and DECREASE oxygen demand.

  • Patients may be “sick” versus “not sick”. They may also be in compensated versus uncompensated shock. Try to avoid using terms like “stable” versus “unstable”.

Time Stamps

  • 00:12 Introduction

  • 01:40 Objectives

  • 02:01 Shock defined

  • 02:28 DO2 equation revisited

  • 03:27 Oxygen uptake or VO2

  • 04:20 Aerobic glycolysis

  • 05:03 Anaerobic glycolysis

  • 05:36 DO2 : VO2

  • 06:25 Oxygen extraction ratio or VO2 / DO2

  • 08:16 Critical DO2 & dysoxia

  • 09:25 Shock DOES NOT EQUAL blood pressure

  • 10:20 Importance of flow (Q)

  • 11:05 Stages of shock

  • 12:48 Why classify shock?

  • 14:25 Classification of shock

  • 15:10 Hypovolemic shock

  • 16:21 Cardiogenic shock

  • 17:14 Distributive shock

  • 18:42 Obstructive shock

  • 19:50 Traumatic, occult, and undifferentiated shock

  • 22:02 Outro

Core Concepts, Equations, and Other Stuff

DO2 = Q x [(1.34 x Hb x SaO2) + (paO2 x 0.0031)]

Q = HR x SV; determinant of SV are: preload, contractility, and afterload


VO2 = CO x (CaO2 – CvO2) x 10 mL / min

Fick equation: CO = VO2 / CaO2 – CvO2

Reverse Fick equation: VO2 = CO x CaO2 – CvO2

*because CaO2 and CvO2 share common term or 1.34 x Hb, can re-write as:

VO2 = CO x 1.34 x Hb x (SaO2 – SvO2) x 10

O2ER = VO2 / DO2

Also, because VO2 and DO2 share common terms (Q x 1.34 x [Hb] x 10) *factor of 10 is used to convert CaO2 from mL/dL to mL/L

O2ER = (SaO2 – SvO2) / (SaO2) or O2ER = 1 – SvO2

VO2 = DO2 x O2ER or VO2 = DO2 – (SaO2 -SvO2)

Critical DO2 or Maximal O2 Extraction in Hemorrhagic Shock

Crit Care . 2004 Oct;8(5):373-81. Epub 2004 Apr 2.

Crit Care. 2004 Oct;8(5):373-81. Epub 2004 Apr 2.

Classification of Shock Table

Coming Soon

Recommended Reading

Gutierrez G, Reines HD, Wulf-Gutierrez ME. Clinical review: hemorrhagic shock. Crit Care. 2004 Oct;8(5):373-81. Epub 2004 Apr 2.

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