Loftis, Patrick

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Marquette Universitye-Publications@MarquettePhysician Assistant Studies Faculty Research andPublications Physician Assistant Studies, Department7-15-2010Determining Axis and Axis Deviation on an ECGPatrick LoftisMarquette University, patrick.loftis@marquette.eduAccepted version. Journal of the American Academy of Physician Assistants ( July 2010). ahref="http://journals.lww.com/jaapa/pages/default.aspx" target="_blank".Publisher link.. © 2010,American Academy of Physician Assistants and Haymarket Media Inc. Reproduced with permission.The published version of the journal article is also available here.   << Return to Determining axis and axis deviation on an ECGPatrick Loftis, PA-C, MPAS, RNJuly 15 2010Each lead shown on an ECG represents electrical changes in the heart that are detected by two electrodes. Awave of depolarization that is moving toward a positive electrode creates a positive deflection on the ECG, whilea wave of depolarization that is moving away from a positive electrode creates a negative deflection. Aperpendicular wave will create a biphasic deflection that contains both positive and negative components.The average direction of electrical flow detected by a lead at any point in time is the lead's orientation angle. Itcan be represented by an arrow, or vector. The electrical axis is the average direction, or the mean vector, of allthe waves of depolarization in the heart at a given point in time. Deviations from a normal axis may or may notsuggest hypertrophy of a chamber. Increased electrical activity within the enlarged muscle can cause a change inthe axis.In order to determine the axis, the electrocardiograph makesone electrode positive and one or more electrodes negative.The instrument then calculates the direction ofdepolarization, or the vector, for each lead and averagesthose results to yield the heart's axis. Interpretation of a12-lead ECG depends on precise placement of the variouselectrodes. The placement of electrodes for the limb leads (I,II, III, aVR, aVL, and aVF) is based on a universallyaccepted schematic that uses a circle representing the areabetween the chin and the pubic symphysis (Figure 1). Thelimb leads track the electrical changes in the frontal plane.Recording of an ECG begins with placement of one electrodeon each extremity. Lead I is formed by making the electrodeon the left arm positive and making the electrode on the rightarm negative. This lead looks at the electrical changesbetween the negative right electrode and the positive leftDetermining axis and axis deviation on an ECG - Print Article - JAAPA http://www.jaapa.com/determining-axis-and-axis-deviation-on-an-ecg/pri...1 of 4 10/14/2010 10:22 AMelectrode. In addition to creating an orientation angle of 0°for this lead in the normal heart, the line divides the circleinto two hemispheres of 180° each. Angles around the circle are read from the point representing the leftelectrode and are positive in the lower hemisphere and negative in the upper hemisphere.Lead II is formed by making the leg leads positive and an electrode on the right arm negative. Theelectrocardiograph views the electrical changes from the perspective of the negative right arm electrode alongthe line to the positive electrode and forms an orientation angle of +60°. Lead III is formed by making the legleads positive and the left arm negative, creating an orientation angle of +120°. Leads aVL, aVR, and aVF areaugmented leads. In an augmented lead, a single electrode is made positive and all the other electrodes are madenegative, with their average serving as the negative electrode. Lead aVL is created by making the electrode onthe left arm positive, resulting in a -30° angle of orientation. Lead aVR is created by making the right armpositive, resulting in a -150° angle of orientation. Lead aVF is created by making both legs positive, resulting in a+90° angle of orientation.In leads V1-V6, the four electrodes on the extremities are averaged and used as a single negative electrode at thecenter of the body. The six positive electrodes are placed horizontally across the precordium, and theelectrocardiograph views the electrical activity from the “central” negative electrode to each positive electrode.These leads measure the wave of depolarization on the horizontal plane.The clinical relevance of vectors and the axis is based on the fact that the left ventricle is the most muscularportion of the normal heart. The mass of the left ventricle and its orientation in the body results in a wave ofdepolarization for the heart as a whole that generally points down and left. The lead that best portrays the axis ofa “normal” heart is lead II, with its downward and leftward orientation. A normal axis that points down and tothe left is between 0° and +90°. An axis outside this range is considered deviated.Determining the exact degree of the axis is much lessimportant clinically than determining whether it isnormal or deviated. We determine whether an axis isnormal or deviated to the left or right by looking at theQRS complex of leads I and aVF because these are theleads located at 0° and 90°, respectively. An upwarddeflection is positive and a downward deflection isnegative (Table 1).Deviation of the axis suggests a change in the meanvector of electrical flow within the heart. This change can be the result of anomalies in the heart that cause thedirection of depolarization to change. Improperly positioned electrodes can also cause an axis shift. The mostlikely cause of axis deviation is hypertrophy. Hypertrophy of either ventricle can shift the axis toward the areathat is hypertrophied. Hypertrophy of the left ventricle is most commonly caused by hypertension; other lesscommon causes include hypertrophic cardiomyopathy, extreme exercise, and aortic disease. Hypertrophy of theright ventricle is most commonly caused by severe lung disease; other causes are pulmonary embolus andpulmonary valve disease.Figure 2 is the ECG of a 66-year-old man with long-standing hypertension who presented to his primary careclinician for a physical examination prior to undergoing foot surgery.Determining axis and axis deviation on an ECG - Print Article - JAAPA http://www.jaapa.com/determining-axis-and-axis-deviation-on-an-ecg/pri...2 of 4 10/14/2010 10:22 AM Using the stepwise approach presented in “Electrocardiography: Understanding the basics,” we need to checkthe following:(1)  Is this ECG regular? Yes, the QRS complexes march out.(2)  What is the heart rate? Find a QRS complex on or near a dark line.Method A: Counting the large boxes, we see that the span between QRS complexes is just slightly more thanfour large boxes. Four boxes would put the heart rate at 75 beats per minute, so we can estimate it at just lessthan that, or 70 beats per minute.Method B: The number of QRS complexes in 6 seconds (30 large boxes) is seven. Multiplying that number by10 gives us the number of beats in 60 seconds or a heart rate of 70 beats per minute.Method C: Dividing 300 by the number of large boxes (four) between QRS complexes gives us an estimatedheart rate of 75 beats per minute.(3)  There is a P wave for every QRS complex, and each looks the same as the next.(4)  The PR interval is approximately three small boxes, which is 0.12 seconds. This means the patient has asinus rhythm.(5)  The QRS complex spans less than three small boxes, which is normal.(6)  The ST segments are slightly elevated. This is consistent with an early repolarization abnormality, which is anormal variant.(7)   The T waves are inverted in leads V3-V6. This is a nonspecific finding that can represent early ischemia andmay need to be investigated.(8)  U waves are absent.Determine the axis by looking at leads I and aVF. Lead I shows a QRS complex with a positive deflection, whileDetermining axis and axis deviation on an ECG - Print Article - JAAPA http://www.jaapa.com/determining-axis-and-axis-deviation-on-an-ecg/pri...3 of 4 10/14/2010 10:22 AMthe aVF lead shows a biphasic QRS that is more negative than positive. According to Table 1, the combination ofa positive deflection in lead I and a negative deflection in lead aVF indicates left axis deviation.By itself, left axis deviation is associated with no specific symptoms. If the deviation is due to left ventricularhypertrophy (LVH) and the hypertrophy has resulted in heart failure, then the patient may demonstrate suchsymptoms of heart failure as dyspnea on exertion or leg edema.As noted earlier, axis deviation is most commonly a result of ventricular hypertrophy. The most common causeof LVH is hypertension, which is present in this patient.There is no specific treatment for left axis deviation, although its presence may suggest the need for furtherevaluation of the ventricles. JAAPASuggested readingThaler M. The Only EKG Book You'll Ever Need. 6th ed. New York, NY: Lippincott Williams & Wilkins;2010:65-74.Patrick Loftis practices emergency medicine and is clinical assistant professor, Department of PhysicianAssistant Studies, Marquette University, Milwaukee, Wisconsin. The author has no relationships to discloserelating to the contents of this article.Determining axis and axis deviation on an ECG - Print Article - JAAPA http://www.jaapa.com/determining-axis-and-axis-deviation-on-an-ecg/pri...4 of 4 10/14/2010 10:22 AM

Determining Axis and Axis Deviation on an ECG

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Determining Axis and Axis Deviation on an ECG

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