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In this chapter you will learn:
1 | what happens to a wave on the EKG when an atrium enlarges or a ventricle hypertrophies
2 | the meaning of electrical axis and its importance in diagnosing hypertrophy and enlargement
3 | the criteria for the EKG diagnosis of right and left atrial enlargement
4 | the criteria for the EKG diagnosis of right and left ventricular hypertrophy
5 | about the cases of Mildred W. and Tom L., which will test your ability to recognize what happens to the EKG with hypertrophy and enlargement, and why this matters
A Few Introductory Remarks
The EKG can diagnose many important and urgent problems—things that can really get your heart pumping! Hypertrophy and enlargement, alas, are—with a few exceptions—not among them. Don’t misunderstand—recognizing atrial enlargement or ventricular hypertrophy can have important clinical implications for your patients (you’ll encounter some of these in this chapter), but for genuine soul-stirring excitement it does not compare to diagnosing an evolving heart attack or a potentially lethal rhythm disturbance.
So why start here? First, because hypertrophy and enlargement are easy to understand. Second, their EKG manifestations build logically from what we have discussed so far. And third, any good book should have an appealing narrative arc, building slowly to a thrilling climax. Starting high and finishing low would not leave you at the end with that shiver down the spine, that feeling that you can’t wait to get out there into the real world and save some lives!
So, here we go—our first foray into how we can use the EKG to diagnose abnormalities of the heart.
Definitions
The terms hypertrophy and enlargement are often used interchangeably, but they are not really the same thing. Hypertrophy refers to an increase in muscle mass. The wall of a hypertrophied ventricle is thick and powerful. Most hypertrophy is caused by pressure overload, in which the heart is forced to pump blood against an increased resistance, as in patients with systemic hypertension or aortic stenosis. Just as weight lifters develop powerful pectoral muscles as they bench-press progressively heavier and heavier weights, so the heart muscle grows thicker and stronger (at least for a while) as it is called on to eject blood against increasing resistance.
Enlargement refers to dilatation of a particular chamber. An enlarged ventricle can hold more blood than a normal ventricle. Enlargement is typically caused by volume overload; the chamber dilates to accommodate an increased amount of blood. Enlargement is most often seen with certain valvular diseases. Aortic insufficiency, for example, may cause left ventricular enlargement, and mitral insufficiency may result in left atrial enlargement.
Enlargement and hypertrophy frequently coexist. This is not surprising, because both represent ways in which the heart tries to increase its cardiac output.
The EKG is not very good at distinguishing between hypertrophy and enlargement. However, it is traditional to speak of atrial enlargement and ventricular hypertrophy when reading EKGs.
The term atrial enlargement has been supplanted in the minds of some by the term atrial abnormalities. This change in terminology reflects the recognition that a variety of electrical abnormalities can cause the changes on the EKG characteristically associated with atrial enlargement. However, we will continue to use the term atrial enlargement in this book, both because the term is more rooted in tradition (and traditional values still matter as we race headlong through the 21st century) and because the vast majority of cases of P-wave changes are due to enlargement of the atria.
Because the P wave represents atrial depolarization, we look at the P wave to assess atrial enlargement. Similarly, we examine the QRS complex to determine whether there is ventricular hypertrophy.
Hypertrophy and enlargement can represent healthy and helpful adaptations to stressful situations, but because they often reflect serious underlying disorders affecting the heart, it is important to learn how to recognize them on the EKG. In addition, over time, the increase in muscular thickness and/or size can compromise the heart’s ability to adequately pump blood to the rest of the body, causing heart failure. Hypertrophied myocardium demands more blood supply for the overgrown heart muscle, but it has a reduced density of capillaries and is therefore more susceptible to ischemia than is normal myocardium.
How the EKG Can Change
Three things can happen to a wave on the EKG when a chamber hypertrophies or enlarges:
- The chamber can take longer to depolarize. The EKG wave may therefore increase in duration.
- The chamber can generate more current and thus a larger voltage. The wave may therefore increase in amplitude.
- A larger percentage of the total electrical current can move through the expanded chamber. The mean electrical vector, or what we call the electrical axis, of the EKG wave may therefore shift.
Because the concept of axis is so important for diagnosing hypertrophy and enlargement, we need to digress for just a moment to elaborate on this idea.
An increase in amplitude is the most dramatic change that occurs when a chamber enlarges, and is critical to all criteria for diagnosing enlargement and hypertrophy as you will shortly see. However, be aware that very thin people, particularly those with pectus excavatum, a common congenital deformity of the anterior thoracic wall, can have abnormally large EKG waves in the precordial leads simply because the chest electrodes are so much closer to the heart and not dampened by overlying tissue.
Axis
Earlier, we discussed how the EKG records the instantaneous vector of electrical forces at any given moment. Using this idea, we can represent the complete depolarization (or repolarization) of a chamber by drawing a series of sequential vectors, each vector representing the sum of all the electrical forces at a given moment.
Because it is easier to visualize, let’s first look at ventricular depolarization (the QRS complex) before turning to atrial depolarization (the P wave) and ventricular repolarization (the T wave).
The first vector represents septal depolarization, and each successive vector represents progressive depolarization of the ventricles. The vectors swing progressively leftward because the electrical activity of the much larger left ventricle increasingly dominates the EKG.
The average vector of all of the instantaneous vectors is called the mean vector.
The direction of the mean vector is called the mean electrical axis.
The mean QRS vector points leftward and inferiorly, representing the average direction of current flow during the entirety of ventricular depolarization. The normal QRS axis—the direction of this mean vector—thus lies between +90° and 0°. (Actually, most cardiologists extend the range of normal from +90° to −30°. In time, as you become more comfortable with the concept of axis, you should add this refinement to your electrical analysis, but for now, +90° to 0° is satisfactory.)
We can quickly determine whether the QRS axis on any EKG is normal by looking only at leads I and aVF. If the QRS complex is predominantly positive in leads I and aVF, then the QRS axis must be normal.
Why is this?
Determining Whether the QRS Axis Is Normal
We have already discussed how any lead will record a positive deflection if the wave of depolarization is moving toward it. Lead I is oriented at 0°. Thus, if the mean QRS vector is directed anywhere between −90° and +90°, lead I will record a predominantly positive QRS complex.
Lead aVF is oriented at +90°. Therefore, if the mean QRS vector is directed anywhere between 0° and 180°, lead aVF will record a predominantly positive QRS complex.
You see where this is going: If the QRS complex is predominantly positive in both lead I and lead aVF, then the QRS axis must lie in the quadrant where both are positive, that is, between 0° and +90°. This is the normal QRS axis.
Another way to look at this is to take the converse approach: If the QRS complex in either lead I or lead aVF is not predominantly positive, then the QRS axis does not lie between 0° and +90°, and it is not normal.
Many electrocardiographers believe that a normal axis can extend from +90° all the way to −30°. Using this criterion, QRS complexes that are predominantly negative in lead aVF can still be normal if the QRS complexes in lead I and lead II are positive. If you can’t intuitively see this, take a colored pencil and shade in the various quadrants as we did in the preceding figure, and you’ll soon see that this criterion extends the range of a normal axis out to −30°. In point of fact, however, very rarely does a clinical decision turn on a variation of a few degrees of axis, so if you are more comfortable with the simpler definition, you are in good company and should not feel at all embarrassed. We’re going to stick with the simpler definition from here on out, just to show you that we can be good sports about it.
Defining the Axis Precisely
Although it is usually sufficient to note whether the axis is normal or not, it is possible to be more rigorous and to define the actual angle of the axis with fair precision. All you need to do is look for the limb lead in which the QRS complex is most nearly biphasic, that is, with positive and negative deflections extended equally on both sides of the baseline (sometimes, the deflections are so small that the wave appears flat, or isoelectric). The axis must then be oriented approximately perpendicular to this lead because an electrode oriented perpendicularly to the mean direction of current flow records a biphasic wave.
Thus, for example, if the QRS complex in lead III (orientation, +120°) is biphasic, then the axis must be oriented at right angles (90°) to this lead, at either +30° or −150°. And, if we already know that the axis is normal—that is, if the QRS complex is positive in leads I and aVF—then the axis cannot be −150°, but must be +30°.
Axis Deviation: Getting More Specific About Defining Abnormal Axes
The normal QRS axis is between 0° and 90°. If the axis lies between 90° and 180°, we speak of right axis deviation. Will the QRS complex in leads I and aVF be positive or negative in a patient with right axis deviation?
The QRS complex in lead aVF will still be positive, but it will be negative in lead I.
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