What is the difference between mobitz 1 and mobitz 2

When you are learning EKG heart rhythms, you must be familiar with the differences between each heart block. In nursing school, I had difficultly at first figuring out the differences between each AV heart block, but now it is second nature to me. Whether you are in nursing school, a new nurse, seasoned nurse needing to brush-up on this material, or another type of healthcare professional learning heart blocks, this article will help refresh you on the important things you need to know about AV heart blocks.

In this article, I am going to cover the following heart blocks:

• AV 1st Degree
• 2nd Degree Type 1, also called Wenckeback, Mobitz I
• 2nd Degree Type 2, also called Mobitz II
• 3rd Degree which is known as a Complete Heart Block

Video Teaching Tutorial on Heart Blocks

After you read this article, I highly recommend you watch my teaching tutorial on these heart blocks to further help you understand the material. Then, take the EKG Heart Block Quiz to test your knowledge on what you have learned.

1st Degree AV Heart Block

The picture above will help illustrate to you what I am talking about. The big thing you need to take away from this rhythm is that it looks normal (like normal sinus rhythm) BUT it has a secret. Note the PR interval on the strip. It is much longer than a normal PR interval. A normal PR interval is 0.12-0.20 and here the PR interval is greater than 0.20. If you don’t know how to measure a PR interval see this article and video.

In addition, this is what will be present with a 1st Degree AV Heart Block:

• Regular P-waves and R-waves
• P-wave always accompanying the QRS complex
• QRS complex will measure normal
• PR INTERVAL WILL BE PROLONGED

2ND Degree Type 1  | Wenckebach | Mobitz I

This rhythm is so easy to remember once you figure out its “hallmark”. Note the PR interval on the EKG strip. See how the PR interval are progressively lengthening and then all of a sudden a QRS complex is missing and then the pattern starts all over? This is the key in understanding a Wenckebach.

This rhythm is CYCLIC and will always present with progressively lengthen PR intervals until a QRS complex disappears and then it will repeat itself. You will also have the following with this rhythm:

• PR intervals ABNORMAL
• Missing QRS complex
• CYCLIC

2nd Degree Type 2 | Mobitz II

Many people like to confuse this rhythm with a Wenckebach and third degree. However, there are some major differences. One being the rhythm is not cyclic, it does NOT have a pattern.  Second, its QRS complexes will be IRREGULAR and this is the opposite for a 3rd degree heart block. Third, it can have NORMAL or ABNORMAL PR Intervals, where a 3rd degree heart block does not contain any PR Intervals.

Notice the strip above: The p-waves are nice and regular while there are some missing QRS complexes which makes the R-wave irregular. In addition, there is no pattern of lengthening p-waves. So, the things to take away from this rhythm are the following:

• P-waves will be regular, however R-waves will NOT
• PR interval can measure normal or abnormal
• NO Pattern
• Missing QRS Complexes after p-waves randomly

3rd Degree Heart Block (Complete Heart Block)

Out of all the heart blocks for a patient, this is the worst one. It requires major interventions. In this rhythm, the atriums and ventricles are NOT beating together and are working independently of each other. Therefore, the important “hallmarks” to take away from this rhythm is the following:

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

Mike Cadogan

Associate Professor Curtin Medical School, Curtin University. Emergency physician MA (Oxon) MBChB (Edin) FACEM FFSEM Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

A form of 2nd degree AV block in which there is intermittent non-conducted P waves without progressive prolongation of the PR interval

Other features:

• The PR interval in the conducted beats remains constant
• The P waves ‘march through’ at a constant rate
• The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, triple for two dropped beats, etc)

Mechanism

• Mobitz II is usually due to failure of conduction at the level of the His-Purkinje system (i.e. below the AV node)
• While Mobitz I is usually due to a functional suppression of AV conduction (e.g. due to drugs, reversible ischaemia), Mobitz II is more likely to be due to structural damage to the conducting system (e.g. infarction, fibrosis, necrosis)
• Patients typically have a pre-existing LBBB or bifascicular block, and the 2nd degree AV block is produced by intermittent failure of the remaining fascicle (“bilateral bundle-branch block”)
• In around 75% of cases, the conduction block is located distal to the Bundle of His, producing broad QRS complexes.
• In the remaining 25% of cases, the conduction block is located within the His Bundle itself, producing narrow QRS complexes.
• Unlike Mobitz I, which is produced by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cells suddenly and unexpectedly fail to conduct a supraventricular impulse.
• There may be no pattern to the conduction blockade, or alternatively there may be a fixed relationship between the P waves and QRS complexes, e.g. 2:1 block, 3:1 block.

Causes of Mobitz II

• Anterior MI (due to septal infarction with necrosis of the bundle branches)
• Idiopathic fibrosis of the conducting system (Lenègre-Lev disease)
• Cardiac surgery, especially surgery occurring close to the septum e.g. mitral valve repair
• Inflammatory conditions (rheumatic fever, myocarditis, Lyme disease)
• Autoimmune (SLE, systemic sclerosis)
• Infiltrative myocardial disease (amyloidosis, haemochromatosis, sarcoidosis)
• Hyperkalaemia
• Drugs: beta-blockers, calcium channel blockers, digoxin, amiodarone

Clinical Significance

• Mobitz II is much more likely than Mobitz I to be associated with haemodynamic compromise, severe bradycardia and progression to 3rd degree heart block
• Onset of haemodynamic instability may be sudden and unexpected, causing syncope (Stokes-Adams attacks) or sudden cardiac death
• The risk of asystole is around 35% per year
• Mobitz II mandates immediate admission for cardiac monitoring, backup temporary pacing and ultimately insertion of a permanent pacemaker

Examples

ECG 1

Related Topics

• AV block: 1st degree
• AV block: 2nd degree, Mobitz I
• AV block: 2nd degree, “fixed ratio blocks” (2:1, 3:1)
• AV block: 2nd degree, “high grade AV block”
• AV block: 3rd degree (complete heart block)
• Eponymythology: History of Second-degree AV block. 2020
• Eponym: Karel Frederik Wenckebach (1864 -1940). LITFL 2020
• Eponym: Woldemar Mobitz (1889 – 1951). LITFL 2020
• Eponym: John Hay (1873 – 1959). LITFL 2020

Advanced Reading

Online

• Wiesbauer F, Kühn P. ECG Yellow Belt online course: Become an ECG expert. Medmastery
• Wiesbauer F, Kühn P. ECG Blue Belt online course: Learn to diagnose any rhythm problem. Medmastery
• Rawshani A. Clinical ECG Interpretation ECG Waves
• Smith SW. Dr Smith’s ECG blog.

Textbooks

• Mattu A, Tabas JA, Brady WJ. Electrocardiography in Emergency, Acute, and Critical Care. 2e, 2019
• Brady WJ, Lipinski MJ et al. Electrocardiogram in Clinical Medicine. 1e, 2020
• Straus DG, Schocken DD. Marriott’s Practical Electrocardiography 13e, 2021
• Hampton J. The ECG Made Practical 7e, 2019
• Grauer K. ECG Pocket Brain (Expanded) 6e, 2014
• Brady WJ, Truwit JD. Critical Decisions in Emergency and Acute Care Electrocardiography 1e, 2009
• Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric 6e, 2008
• Mattu A, Brady W. ECG’s for the Emergency Physician Part I 1e, 2003 and Part II
• Chan TC. ECG in Emergency Medicine and Acute Care 1e, 2004
• Smith SW. The ECG in Acute MI. 2002 [PDF]

LITFL Further Reading

• ECG Library Basics – Waves, Intervals, Segments and Clinical Interpretation
• ECG A to Z by diagnosis – ECG interpretation in clinical context
• ECG Exigency and Cardiovascular Curveball – ECG Clinical Cases
• 100 ECG Quiz – Self-assessment tool for examination practice
• ECG Reference SITES and BOOKS – the best of the rest

ECG LIBRARY

Electrocardiogram

EKG Library

Ed Burns

Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

Robert Buttner

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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