TO FULLY ASSESS A PATIENT'S ECG, you'll need to understand the concept of an axis, or the?general direction of the electrical signal as it travels through the heart. Understanding the axis can help you know if the patient's heart is depolarizing normally. If it's not, the axis may provide clues to the underlying pathology. In this article, I'll review what you need to know about the QRS axis and its deviations.
The heart's compass
The QRS axis represents the general direction of the electrical signal depolarizing the ventricles. When the?direction of depolarization changes, as in conduction defects or a myocardial infarction (MI), the axis also changes.
Graphically, the axis is a vector, or an arrow with a specific size and direction. The size (or amplitude) of the arrow represents the strength of the signal, and the arrow points in the direction of the average signal. In the ventricles, the electrical signal travels in all directions at the same time; in a normal heart, both ventricles depolarize at the same time.
In normal conduction (See Conducting impulses), the signal travels down the left and right bundle branches and reaches the left and right ventricles at nearly the same time. This results in two major vectors, one toward the left ventricle and one toward the right ventricle. When these are added together, the average leans toward the left ventricle because it's much larger than the right ventricle.
You don't need higher mathematics to understand the concept of adding and averaging vectors. Just imagine throwing a ball to a friend. When you throw the ball you give it a force upward as well as out toward your friend. The direction the ball travels is the average vector—a combination of the vertical and horizontal forces that you applied when you threw the ball. If you throw the ball high in the air, the vertical arrow will be larger than the horizontal arrow, and the average vector will appear mostly vertical. Conversely, if you throw the ball directly to your friend, the vertical arrow would be smaller than the horizontal arrow, and the average vector would be mostly horizontal.
The same thing occurs in the heart. Some of the electrical force travels to the right ventricle, some to the left ventricle. The average will be more in the direction of the stronger force, normally the left ventricle.
Determining the QRS axis
You can use several methods to determine the QRS axis; the quadrant and the hexaxial methods are the most popular.
• The quadrant method uses leads I and aVF, and is quick, but tells you only which quadrant the patient's axis is in. However, it's easy to perform in a progressive care unit or in situations when only a few leads can be monitored at once.
The normal QRS axis points toward the bottom of the heart and slightly to the left. This quadrant (0 to +90 degrees) is called the left lower quadrant in reference to its location in the body.
The horizontal line in the quadrant diagram represents lead I, which measures the heart signal as it travels across the chest from left to right. The vertical line represents lead aVF, which measures the signal as it travels from the top to bottom of the heart. Using these two leads, you can determine which quadrant the patient's axis falls in.
For example, in the quadrant method, if the QRS complex in lead I is mostly positive, the axis must lie in one of the two left quadrants. If the QRS complex in lead I is negative, then the QRS axis must lie somewhere in the two right quadrants because the signal is traveling from the left side of the heart to the right.
You can determine the up-and-down direction of the signal from lead aVF, which looks at the bottom of the heart. If a signal is traveling toward the bottom of the heart, lead aVF will be positive. If the signal is traveling toward the top of the heart, lead aVF will be negative. Putting this information together, we can form a table that gives the QRS axis quadrant based on the deflection of these two leads (See Using the quadrant method).
The normal range for the QRS axis is from 0 to +90 degrees, but in some cases, left or right axis deviation is considered normal. Obesity, for example, pushes the abdominal contents up, displacing the contents of the chest cavity. This causes the heart to tilt more to the left than normal, and the QRS axis will shift to the left also. A QRS axis greater than +90 degrees may not be abnormal. In a child or thin adult, the heart is more vertical, so the axis shifts slightly to the right—or an axis of about +120 degrees for infants and +110 for thin adults.
So if your patient exhibits left or right axis deviation, you'll need to evaluate a little further to determine whether it's still in the normal range. If the patient exhibits left axis deviation, look at the QRS complex in?lead II: If it's more positive than negative, the QRS is?normal. If it's more negative than positive, abnormal left axis deviation is present.
Because the quadrant method isn't precise enough for this type of axis determination—it just gives you the axis within 90 degrees—we'll turn to the hexaxial method, which can narrow the range down to within 30 degrees.
• The hexaxial method (See Making sense of the hexaxial reference system) uses six leads: the standard limb leads (I, II, and III) and the augmented leads (aVR, aVL, and aVF). Using this method to determine the axis takes more time, but it's more accurate. Use this method with a 12-lead ECG or when multiple leads can be printed out for a monitored patient.
To determine the axis using the hexaxial method, find the smallest or most equiphasic QRS complex from among leads I, II, III, aVL, aVR, and aVF. Frequently, you can just eyeball this, but if necessary, count the number of small boxes in the positive and negative portions of the QRS complex in a lead. Find the difference; the lead with the number closest to 0 is the most equiphasic, with positive and negative deflections almost equal in size. (This indicates that the electrical current is traveling toward the lead, then away from it, or vice versa.)
Now turn to the hexaxial diagram and find the lead?that's at a 90-degree angle (perpendicular) to the lead with the smallest or equiphasic QRS complex. Look closely at the QRS complex in the perpendicular lead: If the electrical activity is moving toward the lead's positive pole, the QRS will have a positive deflection. If the electrical activity is moving away from the lead's positive pole, the QRS complex will have a negative deflection.
For example, say you determine that lead aVL contains the most equiphasic QRS complex. Lead II is at a 90-degree angle to lead aVL, so the QRS axis is either 60 degrees (if the QRS complex in lead II is positive) or -120 degrees (if the QRS complex in lead II is negative).
Axis deviation can help you identify some bundle-branch blocks and provides subtle clues to support or suggest other cardiac and noncardiac conditions. Let's look at some conditions that cause deviations.
Left axis deviation
The axis shifts to point toward hypertrophied tissue: More mass means more signal, which means a larger vector, and thus the average vector will shift somewhat toward the chamber with the hypertrophy. For example, left axis deviation is often present in, but isn't caused by, left ventricular hypertrophy.
Conversely, the axis shifts away from infarcted or dead tissue because this tissue doesn't conduct the electrical signal. Also, if one ventricle has some dead tissue, then the relative size of the other ventricle is increased, leading to a relative hypertrophy of the noninfarcted chamber. For example, in an inferior wall MI, the average QRS vector would be shifted away from the infarcted tissue, resulting in a left axis deviation.
Other causes of left axis deviation include left anterior hemiblock, artificial cardiac pacing, Wolff-Parkinson-White syndrome with a right-sided accessory pathway, tricuspid atresia, ostium primum atrial septal defect, emphysema, and hyperkalemia.
Right axis deviation
Right ventricular hypertrophy and anterolateral wall MI are two causes of right axis deviation. Other causes include left posterior hemiblock, pulmonary embolism, Wolff-Parkinson-White syndrome with a left-sided accessory pathway, atrial and ventricular septal defects, and chronic lung disease (with or without pulmonary hypertension.) Right axis deviation is normal in children and tall, thin adults.
Extreme right axis deviation
Extreme right axis deviation is an average QRS axis that's in the right upper quadrant of the heart, toward the right atrium. Often referred to as a northwest axis or no-man's-land, this deviation frequently is present in ventricular tachycardia and artificial pacemaker rhythms. It also may be seen in emphysema and hyperkalemia and can indicate lead transposition.
By understanding axis deviation, you'll be better able to interpret your patient's ECG and help him get appropriate therapy.