February 2, 2026

Shock is the “final common pathway to death.” This month’s message focuses on various topics related to this lethal condition. Today, we will focus on introductory concepts necessary for establishing a foundation to understand clinical manifestations of various shock types and treatment approaches. The four main classifications of shock are:  

1. Hypovolemic 

2. Distributive (the overall most common type of shock) 

3. Obstructive 

4. Cardiogenic  

To best understand shock, we must understand normal perfusion at the cellular level. Perfusion involves the transfer of oxygen (from the lungs) and glucose (eaten daily or broken down from glycogen stores in the liver during times of fasting or starvation) via the bloodstream to all our body cells. Cells take in oxygen and glucose and, through three complex processes (glycolysis, the Krebs Cycle, and the Electron Transport Chain), convert oxygen and glucose to carbon dioxide (CO2) and energy in the form of ATP. Energy is necessary for our cells to function (cellular reproduction, protein synthesis, immunity, repair damages, etc.). Without energy, cells die, and eventually the entire organism dies.  

Notice that CO2 is a byproduct of normal metabolism. Oxygen enters the cell, metabolism occurs, and CO2 is released from the cell and transported via the bloodstream to the lungs, where it is released during exhalation. It can be seen why and how CO2 production is an index of perfusion! No perfusion means no oxygen to the cells. No oxygen into the cells means no CO2 out of the cells! This is one reason why we measure and monitor CO2 levels in low perfusion states such as cardiac arrest. When return of spontaneous circulation (ROSC) occurs, CO2 levels rise—often even before the blood pressure rises (this is why we monitor CO2 capnography)! Metabolism under normal conditions is known as aerobic metabolism, meaning that oxygen is required. Aerobic metabolism is efficient and productive, yielding 36 molecules of ATP for every molecule of glucose burned. When oxygen is insufficient in quantity, the body switches to anaerobic metabolism, which only produces two molecules of ATP per molecule of glucose burned and produces lactic acid.  

Normal perfusion requires a functioning heart to pump blood into the cells. The heart must be able to contract effectively and therefore pump the blood with enough vigor; the heart rate must not be too fast or too slow. Heart valves must be intact to allow the one-way flow of blood into and out of the heart chambers. When the pump fails, this causes cardiogenic shock.  

Normal perfusion requires healthy blood vessels – the containers of the blood supply. Both arteries and veins are important in the normal perfusion cycle. Arteries carry blood from the heart to all cells, and veins return blood to the heart from the cells. Arteries are thick-walled vessels that can constrict, when necessary, as a compensatory mechanism to increase blood pressure to ensure perfusion during times of illness or trauma. Veins are essential to the perfusion process; if blood is not returned to the heart in adequate amounts, the heart cannot fill. If the heart cannot fill, the heart cannot expel or eject appropriate amounts of blood. Damaged veins remain dilated, therefore, causing blood to pool in the lower extremities. Damaged veins also become “leaky,” resulting in peripheral edema. Damaged or dysfunctional blood vessels (so-called container problem) are the mechanisms involved in distributive shock, which includes anaphylactic, septic, and neurogenic shock subcategories. Next week, we will discuss distributive shock, the most common type among the four main shock categories.  

Thanks for your time, 

Andrew P. Garlisi, MD, MPH, MBA, VAQSF