Decoding ECG Results: Distinguishing V-Tach from V-Fib

Ventricular tachycardia (Vtach) and ventricular fibrillation (Vfib) are two potentially life-threatening heart arrhythmias that originate in the ventricles of the heart. The electrical activity of the heart varies with Vtach vs. Vfib and different treatments might be indicated. Differentiating between these two arrhythmias begins with a comprehensive understanding of how to decode ECG results.

What is VTach?

Normally, electrical signals in the heart originate in the sinoatrial (SA) node. Known as the heart’s natural pacemaker, the SA node begins each heartbeat and moves it through the heart in a coordinated manner.

During Vtach, one of the heart’s ventricles takes over the pacemaker duty and causes the heartbeat to speed up. In addition to a sensation that the heart is beating too fast, a patient might experience:

  • Shortness of breath
  • Light-headedness or fainting
  • Chest pain

Some patients do not experience any symptoms with Vtach, particularly if the episode is brief (lasting under 30 seconds), known as non-sustained Vtach. Non-sustained Vtach with symptoms may be treated with medication to control the heart rate.

Vtach that lasts more than 30 seconds is known as sustained Vtach and requires immediate medical attention. Patients that are stable may be given immediate intravenous medication to restore the heart’s rhythm. If the patient is not stable, electrical cardioversion is usually the next step. If no pulse is detected with Vtach, defibrillation can be used to shock the heart back to a normal rhythm.

Vtach that is not treated quickly may progress to Vfib, which is an even more dangerous arrhythmia that can lead to cardiac arrest within seconds.

Key Characteristics of VTach on ECG

  • Wide QRS complexes greater than 0.14 seconds
  • QRS complexes are not preceded by the usual P waves
  • R intervals are typically regular (not always, however)
  • Ventricular rate is usually between 150 and 250 beats per minute
  • No atrial rate can be determined
Ventricular Tachycardia ECG Rhythm
This rhythm occurs when the ventricle takes over control of the heart, increasing its firing rate. It should be treated like ventricular fibrillation if no pulse is detected.

Unraveling the Mystery of Vfib

Vfib is the most deadly of all heart arrhythmias. It is characterized by a fast, abnormal heart rhythm caused by ventricle walls quivering or twitching instead of pumping blood. It can lead to sudden cardiac arrest and sudden cardiac death. Prior to Vfib, a patient may exhibit the following symptoms:

  • Shortness of breath, gasping for air
  • Chest pain
  • Lightheadedness
  • Nausea
  • Sudden collapse with loss of consciousness

Vfib can come on suddenly and without warning. Without immediate medical attention, including CPR and defibrillation, the victim is unlikely to survive.

Key Characteristics of VFib on ECG

  • No clear QRS complexes, P waves, or T waves
  • Electrical activity is disorganized
  • The rate appears very rapid – up to 600 beats per minute
  • Coarse Vfib resembles a squiggly line, waves are 3 mm or larger
  • Fine Vfib features an almost straight line, waves are less than 3 mm
Ventricular Fibrillation ECG Rhythm
The most common rhythm seen immediately following cardiac arrest, ventricular fibrillation occurs when the ventricles cannot pump blood to the rest of the body. Immediate defibrillation is critical for any chance of survival.

VTach vs VFib ECG: Spot the Difference

The first ECG is a normal sinus rhythm, with an organized pattern. P waves, QRS complexes, and T waves show up as 3 distinct waves, equally spaced and similar in size.

Ventricular Tachycardia ECG Rhythm

The second ECG shows Vtach, with wide, irregular QRS complexes and occasional P waves. It is difficult to see separation between the QRS complex and the T wave.

The final ECG shows Vfib, with no clear P waves, T waves, or QRS complexes. The rate may be as fast as 600 beats per minute and the deflections will vary and often decrease as time goes on.

How to Read ECG Results Like a Pro

The ECG features these essential components:

  • P-Wave – records electrical activity in the atria
  • PR Interval – records atrial polarization to beginning of ventricular depolarization
  • QRS Complex – records electrical activity in the ventricles, features a Q wave (first downstroke), R wave (first upward deflection), and S wave (first downward deflection)
  • T-Wave – records relaxation of atria


The first step should always be to check the P wave, since abnormalities in the P wave indicate a heart arrhythmia. Next, measure the PR interval and the QRS complex. Examine the overall rhythm to see if it is in an organized or disorganized pattern. Finally, determine heart rate by counting the large boxes between R waves and dividing that number by 300.

ECG Results Explained

You started learning to read by learning your ABCs – to learn how to read ECG results, you need to know your ADDs:

  • A is for Amplitude: Wave height measures the voltage of the beat
  • D is for Deflection: Determine the lead the deflection is coming from
  • D is also for Duration: Wave and interval duration

The Clinical Implications of VTach vs. VFib

Vtach and Vfib are both medical emergencies that need to be identified and treated immediately. The ability to distinguish between the two is necessary to ensure appropriate treatment. Non-sustained Vtach may be addressed with medication in some cases, while Vfib and pulseless Vtach require immediate CPR and defibrillation.

Making the Right Call

When seconds count, your ability to quickly decode an ECG to diagnose Vfib vs. Vtach can make the difference between life and death for your patient. Getting training and certification now can prepare you for cardiac arrest emergencies in the future. Make sure you are equipped with the knowledge and practice you need to give your patients the highest level of care.

SureFire CPR’s Approach to EKG Mastery

An EKG certification course from SureFire CPR will teach you skills and give you the confidence you need to assess cardiac emergencies. Our instructors are all healthcare professionals who understand reading an ECG doesn’t have to be as hard as it may seem. By breaking down complex information into digestible steps, we can help you get EKG certified, so join now. We offer same-day completion certificates and CEs are available for participants.

Enroll in SureFire CPR’s EKG Certification

EKG certification can help you take the next step in your career and raise the bar on your ability to care for your patients. With instructors that are committed to making classes interesting and fun, you will find it easy to focus during training and retain the information long after your course is complete. We offer courses online, in-person and on-site. Check out our class schedule today to find the course that works best for you.

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About the author

I spent 15 years as a firefighter and paramedic...

And too often I would arrive on the scene of someone unconscious, surrounded by a circle of people feeling helpless. Sometimes those people would even have CPR training but lacked the confidence and experience to act.

That’s why I started SureFire CPR. Our classes are practical and engaging – teaching you the crucial skills you need to know what to do and feel empowered to take action.

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Zack Zarrilli, Founder

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