Cardiac Arrest and What Happens When There Is No Pulse
Understanding CPR, Defibrillation, and the Science Behind Restarting the Heart
The Collapse
A man in his early fifties is standing in line at a food court. He is holding a tray. He suddenly drops it. Then he drops.
At first people think he fainted. Someone kneels beside him. No response. No pulse. Someone shouts for help. Another person runs for the automated external defibrillator, an AED. Compressions begin before the ambulance ever arrives. That early action may be the only reason he has a chance.
Cardiac arrest is not a heart attack. It is not chest pain. It is the abrupt cessation of meaningful heart function. There is no pulse. No effective circulation. The clock starts immediately.
What Is Cardiac Arrest?
Cardiac arrest means the heart is no longer pumping blood effectively. Blood flow to the brain stops. Within seconds, consciousness is lost. Within minutes, brain injury begins.
The heart normally contracts in a coordinated electrical pattern. That electrical system generates rhythmic contraction. In cardiac arrest, either there is no electrical activity at all, called asystole, or there is chaotic electrical activity such as ventricular fibrillation. Ventricular fibrillation is when the heart muscle quivers rapidly instead of pumping. Blood does not move in any meaningful way. Electrical activity without effective contraction is not enough. Circulation is what matters.
Why CPR Matters
Cardiopulmonary resuscitation, CPR, is controlled chest compression. It is not symbolic. It is mechanical circulation. When you compress the chest, you are manually squeezing the heart between the sternum and the spine. That compression forces blood out of the heart. When you release, the chest recoils and blood refills the chambers.
Good CPR generates a fraction of normal cardiac output. It is not perfect. It is not elegant. But it is enough to preserve some blood flow to the brain and heart until the underlying rhythm can be corrected. Depth, rate, and minimizing interruptions are critical. This is why trained teams rotate compressors frequently.
What Does the Defibrillator Do?
An AED analyzes the heart rhythm. If the rhythm is ventricular fibrillation or pulseless ventricular tachycardia, it delivers an electrical shock. The shock does not restart the heart in the way movies portray it. It resets the chaotic electrical activity, allowing the heart’s natural pacemaker to regain control.
If the rhythm is asystole, meaning flatline, a shock does not help. In those cases we focus on high quality CPR, medications such as epinephrine, and identifying reversible causes. Advanced Cardiac Life Support, or ACLS, protocols guide these steps in a standardized way.
The Five Hs and Ts
When someone is in cardiac arrest, we immediately think about reversible causes. The mnemonic often taught includes the Five Hs and Five Ts.
The Hs include hypoxia, meaning lack of oxygen; hypovolemia, meaning low blood volume; hydrogen ion excess, which is severe acidosis; hyperkalemia or hypokalemia, which are dangerous potassium abnormalities; and hypothermia.
The Ts include tension pneumothorax, which is trapped air compressing the lungs and heart; cardiac tamponade, which is fluid compressing the heart; toxins such as drug overdose; thrombosis of the coronary arteries, meaning heart attack; and thrombosis of the pulmonary arteries, meaning massive pulmonary embolism.
Each has a specific treatment. Tension pneumothorax requires needle decompression. Tamponade requires pericardiocentesis. Massive pulmonary embolism may require clot busting medication. Identifying and reversing the cause is how we restore sustainable circulation.
After the Pulse Returns
Return of spontaneous circulation, often abbreviated ROSC, means the heart is beating effectively again. That is not the end. It is the beginning of the next phase.
The patient often requires airway support, blood pressure stabilization, intensive care monitoring, and treatment of the underlying cause. Brain protection strategies may be initiated. Sometimes despite perfect execution, the outcome is not what we hope. Cardiac arrest is unforgiving. But early CPR and early defibrillation dramatically improve survival. Bystander action matters. Seconds matter. Systems matter.
THE BOTTOM LINE
• Cardiac arrest means the heart has stopped pumping effectively and there is no pulse.
• Immediate CPR preserves blood flow until defibrillation or treatment of the underlying cause can restore circulation
• Identifying reversible causes such as hypoxia, tamponade, or massive clot is essential to survival
Written by a Board-Certified Emergency Medicine Physician