Electrical Axis (Normal, Right Axis Deviation, and Left Axis Deviation)


Article Author:
Anthony Kashou
Hajira Basit


Article Editor:
Lovely Chhabra


Editors In Chief:
Kranthi Sitammagari
Mayank Singhal


Managing Editors:
Avais Raja
Orawan Chaigasame
Carrie Smith
Abdul Waheed
Khalid Alsayouri
Trevor Nezwek
Radia Jamil
Patrick Le
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Saad Nazir
William Gossman
Hassam Zulfiqar
Hussain Sajjad
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Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beata Beatty
Beenish Sohail
Nazia Sadiq
Hajira Basit
Phillip Hynes


Updated:
6/29/2019 8:01:45 AM

Introduction

One of the key steps in interpreting an electrocardiogram (EKG) is determining the electrical axis of the heart. Being able to determine the electrical axis can give insight into underlying disease states and help steer the differential diagnosis towards or away from certain diagnoses. Herein, we will discuss what makes up the electrical axis, ventricular (QRS) axis, axis classifications, various approaches to determining the electrical axis, and causes of axis deviation.[1][2][3]

Function

Electrical Axis

In electrophysiology, a vector represents both the magnitude and direction of the action potential generated by an individual myocyte. The sum of all the individual vectors generated by the depolarization waves makes up the electrical axis. Because each myocyte can produce an action potential, an axis for each wave and interval of the cardiac cycle can be determined. Knowing the axis of each and how they interact can reflect certain pathology.

When the electrical axis is discussed and taught, the ventricular axis is typically used. This is because the left ventricle makes up most of the heart muscle under normal circumstances; thus, it generates the most electrical force visible on the EKG. The normal ventricular axis is directed downward and to the left.

The ventricular axis can be determined by looking at the QRS complex, which represents ventricular depolarization. Because the QRS complex is used to determine the ventricular axis, it is also referred to as the QRS axis. The ventricular (QRS) axis signifies the sum of all individual vectors generated by the depolarization waves of ventricular myocytes.

Ventricular (QRS) Axis

The ventricular (QRS) axis is determined indirectly by evaluating the vectors produced under the electrodes. This is done by interpreting the electrical signal (QRS complex) recorded at each electrode as positive, negative, or isoelectric and then considering their relationship to each other.

In general, a positive QRS complex in a lead has a ventricular axis that is approximately in the same direction to that lead. Whereas a negative QRS complex in a lead has a ventricular axis that is approximately in the opposite direction to that lead. If the QRS complex is isoelectric in a lead, then the ventricular axis is perpendicular (90 degrees) to that lead. This is summarized in Figure 1.

Issues of Concern

Electrical Axis Classification

There are five main electrical axis classifications: 

  1. normal axis
  2. left axis deviation (LAD)
  3. right axis deviation (RAD)
  4. extreme axis deviation, and 
  5. indeterminate axis 

There is some disagreement on the exact degrees that define each type, but there are some general cutoffs that can be used for the QRS axis.

The QRS axis moves leftward throughout childhood and adolescence and into adulthood. At birth, the normal QRS axis lies between +30 degrees and +190 degrees. Between the ages of 8 to 16 years, the axis moves leftward with normal lying between 0° degrees to +120 degrees. The normal adult QRS axis is between -30 degrees and +90 degrees, which is directed downward and to the left. This adult range is sometimes extended from -30 degrees to +100 degrees.

The following axis classifications described are based on adults. If the QRS axis falls between -30 degrees and -90 degrees, it is considered LAD. In this case, the QRS vector is directed upward and to the left. If the QRS axis falls between +90 degrees and 180 degrees, or beyond +100 degrees if the adult range is used, then RAD is present. The QRS vector would be directed downward and to the right. If the QRS axis happens to fall between -90 degrees and 180 degrees, this would be referred to as extreme axis deviation, whereby the ventricular vector is directed upward and to the right. Lastly, if the QRS complex is isoelectric or equiphasic in all leads with no dominant QRS deflection, it is considered indeterminate axis. The electrical axis classifications are summarized in Figure 2.

Approach to Determining Axis

There are multiple methods to determine the electrical axis. The following are a few of these simple and adequate approaches to assess the ventricular (QRS) axis. Hence, the focus will be on the QRS complexes in specific leads.

The main QRS complexes to evaluate are those in leads I, II, and aVF. The positive ends of these three leads fall within the normal axis region. The positive ends of leads I, II, and aVF are 0 degrees, +60 degrees, and +90 degrees, respectively. Therefore, if all three of these leads have positive QRS complexes, the axis is normal.

Method 1. One simple way to learn how to determine electrical axis is to inspect limb leads I and aVF. This is referred to as the quadrant approach or two-lead method. Each of the four quadrants represents 90 degrees and an axis type. In other words, 0 degrees to +90 degrees is normal axis, +90 degrees to 180 degrees is RAD, 0 degrees to -90 degrees is LAD, and -90 degrees to 180 degrees is extreme axis. Therefore, if leads I and aVF are both positive, then the axis falls within the normal axis range. If lead I is positive and lead aVF is negative, then there is LAD. If lead I is negative and lead aVF is positive, then there is RAD. And, if both leads I and aVF are negative, then the axis falls within the extreme axis range. This quadrant approach is summarized in Figure 3.

One issue with this method is that it only gives a close approximation to the true axis. In addition, it narrows the normal axis range. This can result in an inaccurate interpretation of the true electrical axis. For instance, if using this approach with a positive lead I and negative lead aVF, the axis would be interpreted as LAD. However, if the true axis were -20 degrees, which lies in the LAD quadrant using this method, it would still be within the normal axis range. Nevertheless, this method is easy to learn and sufficient in most cases.

One way to resolve these issues is by locating the most isoelectric limb lead and knowing that the true axis lies nearly perpendicular to it. Using this can help narrow the axis down to within 10 degrees of the normal axis.

Method 2. A more accurate approach that the simple quadrant approach takes into account leads I and aVF, as well as lead II. This is referred to as the three-lead method. If the net QRS deflection is positive in both leads I and II, the QRS axis is normal. If the net QRS deflection is positive in lead I, but negative lead II, then there is LAD. Notice that in both cases lead aVF was not needed. In other words, if lead I is positive, look next to lead II. Now, if lead I is negative, look next to lead aVF. If lead aVF is positive, then the axis is rightward; however, if lead aVF is also negative, then there is the extreme axis. This approach is summarized in Figure 4 and Table 1.

Method 3. Another simple way to estimate the ventricular (QRS) axis is to locate the most isoelectric limb lead along the frontal plane. The isoelectric (equiphasic) lead represents the lead with a net amplitude of zero and smallest overall amplitude. The QRS axis is approximately perpendicular (90 degrees) from the positive pole of that lead.

In order to determine which direction to move 90 degrees from that positive pole, look at the net deflection in another lead. For example, if the isoelectric limb lead is lead II, which has a positive end directed at +60 degrees, then the electrical axis is directed approximately 90 degrees from +60 degrees in either direction. Therefore, the axis can lie at around +150° (RAD) or -30 degrees (borderline LAD). If lead I, with a positive pole at 0 degrees, has a net positive QRS deflection, then the axis will be closer to -30 degrees (LAD); and, if lead I has a net negative QRS deflection, then the axis will be closer to +150 degrees (RAD).

Finally, it’s important to note that these three methods determine the electrical axis in the frontal plane. There is also a horizontal plane with a horizontal axis. The axis along this plane can be determined by viewing the heart under the diaphragm. The axis can be considered to have a clockwise or counter-clockwise rotation depending on when the transition from mostly negative QRS complexes to mostly positive QRS complexes occurs along the precordial leads (V1-V6). Ideally, this would be the isoelectric precordial lead. Normally, this transition occurs between leads V3 and V4. If it occurs earlier, it is considered to a counterclockwise rotation and an early transition. This would indicate that the left ventricular forces are directed more anteriorly. On the other hand, if it occurs later in which there is poor R wave progression, then it is considered a clockwise rotation and a late transition. This would indicate that the left ventricular forces are directed more posteriorly.

Clinical Significance

Causes of LAD include:

  • Normal variation (physiologic, often age-related change)
  • Left ventricular hypertrophy
  • Conduction defects: left bundle branch block, left anterior fascicular block
  • Inferior wall myocardial infarction
  • Preexcitation syndromes (e.g., Wolff-Parkinson-White syndrome)
  • Ventricular ectopic rhythms (e.g., ventricular tachycardia)
  • Congenital heart disease (e.g., primum atrial septal defect, endocardial cushion defect)
  • Hyperkalemia
  • Emphysema
  • Mechanical shift, such as with expiration or raised diaphragm (e.g., pregnancy, ascites, abdominal tumor, organomegaly)
  • Pacemaker-generated rhythm or paced rhythm

Causes of RAD include:

  • Normal variation (e.g., children, young adults)
  • Limb-lead reversal (left- and right-arm electrodes)
  • Right ventricular overload syndromes (acute or chronic)
  • Right ventricular hypertrophy
  • Conduction defects: left posterior fascicular block, right bundle branch block
  • Lateral wall myocardial infarction
  • Preexcitation syndromes (e.g., Wolff-Parkinson-White syndrome)
  • Ventricular ectopic rhythms (e.g., ventricular tachycardia)
  • Congenital heart disease (e.g., secundum atrial septal defect)
  • Dextrocardia
  • Left pneumothorax
  • Mechanical shift, such as with inspiration or emphysema
  • Conditions that cause right-ventricular strain (e.g., pulmonary embolism, pulmonary stenosis, pulmonary hypertension, chronic lung disease, and resultant cor pulmonale)

Other Issues

Ventricular (QRS) Axis in Bundle Branch Blocks 

Determining ventricular (QRS) axis in the setting of a bundle branch block is controversial. With right bundle branch block (RBBB), RAD or LAD may indicate a bifascicular block. Thus, knowing that the terminal portion of the QRS complex reflects the delay in right ventricular activation with RBBB, one approach to estimating the frontal plane axis is by using the initial 80 to 100 millisecond (ms) of QRS deflection, which primarily reflects left ventricular activation. Similarly, with left bundle branch block (LBBB) and other intraventricular conduction delays, the initial 80-100 ms of the QRS deflection or the entire QRS complex can be used to determine the axis.[4][5][6]

Enhancing Healthcare Team Outcomes

Clinicians including nurses who look after patients with heart and lung pathology should be familiar with the interpretation of the ECG. Being able to interpret the electrical axis may offer insight into the cardiac pathology and help avoid unnecessary imaging tests. For those who are not well versed in ECG interpretation, a consult with a cardiologist is highly recommended.


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    Contributed by Anthony H. Kashou, www.theekgguy.com
Attributed To: Contributed by Anthony H. Kashou, www.theekgguy.com

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Electrical Axis (Normal, Right Axis Deviation, and Left Axis Deviation) - Questions

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A 55-year-old male presents to the office for a regular follow up. He feels fine most of the time except for mild to moderate breathlessness on exertion. Currently, he has no other complaint. His previous EKG show left axis deviation. Which of the following is the most likely cause for this left axis deviation?



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A 65-year-old male is brought to the hospital after a car crash. The patient sustained multiple injuries including a femur fracture and broken ribs. He is unconscious on arrival. Vitals are heart rate: 110 beats/min, blood pressure: 90/60 mmHg, respiratory rate: 9 breaths/min and temperature of 98 Fahrenheit. Intravenous fluids are given through large bore cannula and an emergent EKG is done which shows right axis deviation. Which of the following is the most likely cause of right axis deviation in this patient?



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A 16-year-old girl with an altered mental status is brought to the hospital. She is a known case of diabetes with poor compliance with insulin. On arrival, her random blood glucose is 350 mg/dl. Her ECG shows tall tented T-waves and left axis deviation. Which of the following findings is most consistent with left axis deviation?



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A 66-year-old male presents to the hospital with severe chest pain for one hour. He describes the pain as crushing in quality and radiating towards his left arm. EKG reveals ST-segment elevation in lead V5, V6, and avL with right axis deviation. Which of the following findings is consistent with right axis deviation?



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A 38-years-old woman presents to the hospital with complaints of breathlessness and severe chest pain for the last 2 hours. She is a lecturer at a local university and recently returned from an overseas educational conference. The patient is obese and regularly takes oral contraceptive pills. Examination and radiologic findings are consistent with pulmonary embolism. ECG findings show sinus tachycardia, prominent S wave in the lead I, with Q wave and T wave inversion in the lead III and right axis deviation. Which of the following is most likely associated with right axis deviation?



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Electrical Axis (Normal, Right Axis Deviation, and Left Axis Deviation) - References

References

Storkås HS,Hansen TF,Tahri JB,Lauridsen TK,Olsen FJ,Borgquist R,Vinther M,Lindhardt TB,Bruun NE,Søgaard P,Risum N, Left axis deviation in patients with left bundle branch block is a marker of myocardial disease associated with poor response to cardiac resynchronization therapy. Journal of electrocardiology. 2019 Apr 5;     [PubMed]
Lazović B,Svenda MZ,Mazić S,Stajić Z,Delić M, Analysis of electrocardiogram in chronic obstructive pulmonary disease patients. Medicinski pregled. 2013 Mar-Apr;     [PubMed]
Kronborg MB,Nielsen JC,Mortensen PT, Electrocardiographic patterns and long-term clinical outcome in cardiac resynchronization therapy. Europace : European pacing, arrhythmias, and cardiac electrophysiology : journal of the working groups on cardiac pacing, arrhythmias, and cardiac cellular electrophysiology of the European Society of Cardiology. 2010 Feb;     [PubMed]
Bertaglia E,Michieletto M,Spedicato L,Pascotto P, Right bundle branch block, intermittent ST segment elevation and inducible ventricular tachycardia in an asymptomatic patient: an unusual presentation of the Brugada syndrome? Giornale italiano di cardiologia. 1998 Aug;     [PubMed]
Lacombe P,Lévy S,Metge M,Cointe R,Bru P,Gérard R, Electrocardiographic characteristics of the escape rhythm in transient complete atrioventricular block induced by transcatheter electrical ablation of the atrioventricular junction. Pacing and clinical electrophysiology : PACE. 1988 Feb;     [PubMed]
Sohi GS,Flowers NC,Horan LG,Sridharan MR,Johnson JC, Comparison of total body surface map depolarization patterns of left bundle branch block and normal axis with left bundle branch block and left-axis deviation. Circulation. 1983 Mar;     [PubMed]

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