The ECG consists of waves and complexes plotted as voltage on the vertical axis and time on the horizontal axis (see Figure 19-2). The spaces between waves and complexes are called intervals and segments. The P wave (see Figure 19-2, A) is produced as a result of atrial depolarization. The mechanical event of atrial contraction normally follows a fraction of a second after the P wave appears.

The QRS complex, produced by ventricular depolarization, is composed of three separate waves: the Q, R, and S waves. The first small downward (negative) deflection from the baseline that occurs after the P wave is called the Q wave, the next tall upward (positive) deflection is the R wave, and the following small negative deflection is called the S wave. The QRS complex may not always possess Q or S waves, depending on the lead from which it is recorded. The QRS complex represents the electrical events associated with ventricular contraction (systole).

The T wave is produced by ventricular repolarization and is associated with ventricular relaxation (diastole). The atria also have a repolarization wave, but it is hidden by the QRS complex.

Height, or amplitude, on the ECG represents voltage. Because the ventricular muscle mass is much greater than the atrial muscle mass, ventricles generate a much greater voltage when they depolarize. (Notice the difference in height between the P wave and QRS complex.) The amplitude of waves and complexes is related to the muscle mass involved.

Intervals and Segments

Figure 19-2, B, illustrates various ECG time intervals and segments. The PR interval is the time required for the sinus node impulse to reach the ventricles. It is measured from the beginning of the P wave to the next deflection, whether it is a Q or an R wave. The average adult PR interval is between 0.12 second and 0.20 second; a PR interval greater than 0.20 second indicates abnormally slowed impulse conduction from atria to ventricles. The PR interval is normally shorter in fast heart rates than in slow heart rates.

The adult QRS complex lasts on average about 0.08 to 0.10 second. QRS width represents ventricular depolarization time and is measured from the point at which the tracing leaves the baseline to the point at which it returns to baseline (see Figure 19-2, B). This point of return is called the J point.

The ST segment extends from the J point to the beginning of the T wave and represents the early phase of ventricular repolarization. At its end, the normal ST segment curves slightly upward into the beginning of the T wave. The ST segment length varies according to the heart rate; fast heart rates have shorter ST segments than slow heart rates. The ST segment is normally flat, lying on the ECG baseline. However, this segment may be elevated 2 mm above the baseline or depressed 0.5 mm below the baseline and still be considered normal.² An ST segment that becomes depressed more than 0.5 mm during a stress exercise test signals myocardial ischemia. The drug digitalis produces the same effect. An elevated ST segment generally indicates myocardial tissue injury.³

The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT interval varies according to the heart rate but is usually less than 0.40 second.¹ It represents the general refractory period of the ventricles. During this time, the ventricles generally cannot accept another depolarizing stimulus. However, as repolarization progresses, some of the ventricular muscle fibers are repolarized, while other fibers are still depolarized. This represents a vulnerable period during which part of the heart can respond to an additional stimulus and part of it cannot. The most vulnerable period is located at about the peak of the T wave and is sometimes called the relative refractory period of the heart. A depolarizing stimulus during this vulnerable time can create electrical chaos in the heart, rendering it quivering (fibrillating) and unable to pump blood. A prolonged QT interval is associated with life-threatening fibrillation because it is associated with a longer vulnerable period.²

CONCEPT QUESTION 19-1

What does a high-amplitude P wave indicate about atrial muscle mass, and what condition might be the cause of this abnormality?

Positive and Negative Electrocardiogram Deflections

ECG leads consist either of two opposite polarity electrodes (one positive and the other negative) or one positive electrode and a reference (ground) electrode. Leads composed of two opposite polarity electrodes are known as bipolar leads. A single positive electrode and its reference point are known as a unipolar lead.

The heart depolarizes in a base-to-apex direction (i.e., from atria to ventricles); the electrical current generated by depolarization flows in the same direction. Current flowing toward a lead’s positive electrode generates a positive, or upward, deflection on the ECG. Conversely, current flowing away from a positive electrode generates a negative, or downward, deflection.

Lead Axis and Current Flow

An imaginary straight line between the positive and negative electrodes of a bipolar lead is defined as the lead axis. Similarly, an imaginary line between a unipolar lead’s positive electrode and the center of the heart is the unipolar lead’s axis. In Figure 19-3, a myocardial depolarization current flowing at right angles to a lead’s axis generates an equiphasic or isoelectric deflection on the ECG monitored by the unipolar lead. Current flowing parallel to a lead’s axis generates a strongly negative or positive ECG deflection, depending on whether the current flows toward or away from the positive electrode; this is illustrated by the bipolar lead in Figure 19-3. Figure 19-4 illustrates the range of ECG deflection possibilities, depending on the direction of current flow relative to the lead’s axis.

Figure 19-3 Lead axis and the average direction of current flow in the heart. The polarity of the QRS deflection is determined by the direction of current flow relative to the recording lead’s axis.

Figure 19-4 The range of QRS deflections depends on the heart’s average current flow direction (*arrows*) with respect to a lead’s axis. The *positive and negative circles* represent the electrodes of a bipolar lead, and the *straight, heavy line* between them represents the lead’s axis. The *arrows* represent seven possible current flow directions in the heart. (From Conover MB: *Understanding electrocardiography,* ed 7, St Louis, 1996, Mosby.)

The heart’s electrical axis refers to the average direction of current flow in the heart. The heavy arrow in the heart in Figure 19-3 is the heart’s electrical axis. The length of this arrow is proportional to the current’s intensity and is called a vector because it has direction and magnitude. For this reason, the average direction of current flow in the heart is commonly called the mean cardiac vector (MCV)

CONCEPT QUESTION 19-2

In what way would you expect an increased right ventricular muscle mass to affect the MCV?

Electrocardiogram Graph Paper

ECG graph paper is a grid, permitting time measurement along the horizontal lines and voltage measurement along the vertical lines (Figure 19-5). Dark and light intersecting lines form large and small squares. The light vertical lines that help form the small squares are 1 mm apart and represent 0.04 second at a standard sweep speed of 25 mm per second. The heavy vertical lines are 5 mm apart and represent 0.20 second at standard recording speed. (The large squares have five small squares on each side [see Figure 19-5]).

Figure 19-5 ECG graph paper. The horizontal axis represents time, and the vertical axis represents amplitude or voltage. (From Aehlert B: *ECGs made easy,* ed 4, St Louis, 2011, Mosby.)

Vertically, each small square represents 0.1 mV, which means each large square represents 0.5 mV (see Figure 19-5). By convention, ECG machines are adjusted, or standardized, so that a 1-mV electrical signal produces a 10-mm (two large squares) deflection. The amplitude of the ECG is discussed in terms of millivolts.

The hard copy (paper) ECG grid is convenient for determining the heart rate and uniformity of spacing between QRS complexes. Because each large square represents 0.2 second, five large squares represent 1.0 second. Five large squares are 25 mm, or 2.5 cm, long; thus, 1 inch of horizontal distance equals 1 second on ECG graph paper.

Short vertical markers spaced 3 seconds apart (15 large squares or 75 small squares) are usually placed at the upper edge of the ECG paper. These markers allow a 3-second or 6-second time interval to be easily selected for analysis. The 6-second “strip” is a common ECG paper length used for electrocardiographic analysis and interpretation. These strips may be placed in the hard copy medical records of patients undergoing cardiac evaluations.

Electrocardiogram Leads

Standard Bipolar Limb Leads: Einthoven’s Triangle

At the turn of the twentieth century, Einthoven invented the ECG machine and introduced the three bipolar limb leads.³ Figure 19-6 illustrates these three standard limb leads. In lead I, the negative electrode is placed on the right arm, and the positive electrode is placed on the left arm. The axis of lead I is a horizontal line running from shoulder to shoulder. In lead II, the negative electrode is on the right arm, and the positive electrode is on the left leg; the axis of lead II runs from the right arm to the left leg (see Figure 19-6). Lead III has a negative electrode on the left arm and a positive electrode on the left leg. The lead III axis runs from the left arm to the left leg.

Figure 19-6 The axes of leads I, II, and III form Einthoven’s triangle. (From Seidel JC, editor: *Basic electrocardiography: a modular approach,* St Louis, 1986, Mosby.)

The axes of these three leads form an equilateral triangle around the heart, called Einthoven’s triangle. Einthoven discovered that the voltage (amplitude) of the QRS complex recorded in lead II is always equal to the sum of the voltages in leads I and III (Einthoven’s law).

The ECG recorded from three different leads gives three different views of the heart’s electrical activity—similar to walking around an object to see it from different angles. Other bipolar lead combinations could be invented, but leads I, II, and III are the standard bipolar limb leads used in medical practice. In reality, three electrodes are needed to monitor the ECG from one standard limb lead: a positive electrode, a negative electrode, and a ground electrode. The ground electrode is usually placed on the right leg.

Standard Unipolar Limb Leads

The term unipolar refers to a single electrode of positive polarity. The three standard unipolar limb leads use similar

Clinical Focus 19-1 Methods for Assessing Heart Rate on the Electrocardiogram

At least two different ways exist to determine the heart rate from the ECG recording. One method works well for regular heart rhythms, and the other method is better for irregular rhythms.

Method 1

Look at Figure 19-5. Notice that five large squares equal 1 second (0.2 sec × 5 = 1.0 sec). Because 1 minute contains 60 seconds, 1 minute is represented by 300 large squares (5 squares/sec × 60 sec = 300 squares). If the heart rate is 300 beats per minute, the interval between two QRS complexes is one large square (i.e., a heartbeat [QRS complex] occurs every 0.2 second). When the heart rate is 150 beats per minute, the interval between two QRS complexes is two large squares. For determining the heart rate, the number 300 is divided by the number of large squares between two QRS complexes. This is summarized as follows:

Number of Large Squares between Two QRS Complexes	Rate (beats/min)
1	300 (300 ÷ 1)
2	150 (300 ÷ 2)
3	100 (300 ÷ 3)
4	75 (300 ÷ 4)
5	60 (300 ÷ 5)
6	50 (300 ÷ 6)

Memorization of the numbers in the right column (“300, 150, 100, then 75, 60, 50”) allows quick identification of the heart rate at the patient’s bedside. This method is appropriate only for regular heart rhythms in which QRS complexes are uniformly spaced.

Method 2

In Figure 19-5, markers identify 3-second intervals on the ECG paper. First, a 6-second interval is identified by using these markers (two 3-second intervals). Next, within this 6-second strip, the number of QRS complexes is counted. Because 1 minute contains 10 6-second intervals, the number of QRS complexes that were counted is multiplied by 10. For example, if eight QRS complexes are counted in a 6-second strip, the heart rate is 8 × 10 or 80 beats per minute. This method is best for averaging the heart rate when the rhythm is irregular (i.e., when the interval between QRS complexes varies).

electrode attachment points as the standard bipolar limb leads—that is, right arm, left arm, and left leg. The QRS complex recorded by a unipolar lead has a greater amplitude than the QRS complex recorded by bipolar leads; for this reason, unipolar leads are known as augmented limb leads. (Figure 19-7 illustrates the three unipolar augmented limb leads and their axes.)