Trauma ICU Rounds – Shock Talk I: Pathophysiology & Classification
- Aug 7th, 2020
- Dennis Kim
Listen to the accompanying podcast HERE
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.
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”.
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
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
MAP = Q x SVR
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
Classification of Shock Table
Gutierrez G, Reines HD, Wulf-Gutierrez ME. Clinical review: hemorrhagic shock. Crit Care. 2004 Oct;8(5):373-81. Epub 2004 Apr 2.