ECG Challenges: Measurement and Rate Correction of the QT Interval

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ECG Challenges: Measurement and Rate Correction of the QT Interval

Gerard B. Hannibal RN, MSN, PCCN; Department Editor
Amer A. Hasanien RN, MSN
David Pickham RN, PhD

AACN Advanced Critical Care
March 2013
Volume 24 Number 1
Pages 90 - 96

The surface electrocardiogram (ECG) is an indirect measure of the heart's electrical activity and is one of the most commonly used diagnostic tools. Measuring the QT interval on the surface ECG is important because when it is long, it is associated with syncope, the arrhythmia torsade de pointes, and death.1-7 The QT interval is an indirect measure of ventricular repolarization and is measured in seconds (s) or milliseconds (ms) from the onset of the QRS complex to the end of the T wave. Including the QRS complex in the QT interval measurement is necessary because the start of a ventricular repolarization is buried somewhere within the QRS complex. Therefore, as the QRS complex is usually less than 120 ms and its onset is easily identifiable, the QRS complex is included. However a second step after the measurement of the QT interval is needed to determine the true ventricular repolarization time.

The QT interval is inversely related to heart rate. As heart rate increases, repolarization time decreases, and vice versa. The dependence of QT-interval duration on the underlying heart rate is adjusted by using an equation to derive a QT interval that is independent from heart rate. The derived value is called the heart rate corrected QT interval, or commonly the QTc interval, and represents the expected QT-interval duration when the underlying heart rate is 60/min. Correcting the QT interval for heart rate is necessary for monitoring, because the correction allows the comparison of multiple QT intervals over time and across varying heart rates.

The average QTc interval values for men and women differ.8-10 In terms of risk, the upper limits of normal for men and women are 470 ms and 480 ms, respectively11; however, a QTc value greater than 500 ms is considered alarmingly high and substantially increases the risk for the occurrence of adverse events, especially torsade de pointes, in both sexes.11

Nurses working in monitored acute and critical care units are required by the practice standards developed by the American Heart Association's Council on Cardiovascular Nursing, Clinical Cardiology, and Cardiovascular Disease in the Young 11 to be knowledgeable about the QT interval and its basic physiological concepts. This column discusses the major concerns related to precision in QT interval measurement and heart rate correction, which is important, especially when administering drugs that effect ventricular repolarization in at-risk patients.

Manual Versus Automatic Measurement

In clinical practice, multiple ways can be used to measure the QT interval. Traditionally, the QT interval was measured manually using a single beat (QRS-T complex) on a standard ECG. However, advances in technology have introduced 2 new methods: (1) a semiautomated method using digital onscreen calipers and (2) a fully automated method that can measure QT intervals at single time points or continuously over time.

In reviewing the simplest method, the QT interval can usually be determined manually by skilled ECG readers.12 Figure 1 explains the procedure for manually measuring and correcting the QT interval. With manual measurement, clinicians are advised to set the ECG recording speed at 25 mm/s and the gain at 10 mm/mV (standard settings). Higher recording speeds, such as 50 mm/s, distort low-amplitude waves such as the U wave and are considered to be inaccurate.13 Before acquiring the ECG for measurement of the QT interval, clinicians should follow standard procedures such as vigilant skin preparation, comfortable body positioning, and proper electrode placement. Next, measure both the QT interval and the preceding R-R interval. This is important as it most appropriately represents the heart rate associated with the QT interval of interest.

Figure 1:. Manual me... - Click to enlarge in new window

Figure 1:. Manual measurement of the QT interval.

Several drawbacks are associated with manual measurement: the method is time consuming, depends on the nurse's experience, and is inconsistently performed. A recent study of acute and critical care nurses demonstrated that the manual QT interval measurement was successfully completed by only 6% of nurses at baseline and was associated with significant errors, even after an intensive education intervention.14 Studies of other clinical professions demonstrate similar QT interval measurement competency.15-17 As such, the authors do not encourage manual QT interval measurement.

The second approach, a semiautomated method, uses the central station's monitoring software to allow the clinician to manipulate the ECG recording. By increasing or decreasing the on-screen waveform size and using digital calipers, nurses can easily identify and measure the QT and R-R intervals. These intervals are then applied to an automated QT interval correction, which can then be printed with the rhythm strip for documentation. This method limits the need for manual calculation, avoiding the errors that are associated with manual measurements. With skilled monitor operators, a semiautomated approach is much more precise than manual measurement alone. Hence, this method is recommended for use in clinical practice.

The fully automated method is the third approach to QT measurement. Almost all standard 12-lead ECG machines derive the QT interval and calculate the QTc interval automatically. Fully automated measurements have several advantages: they are instant, objective, consistent, and cost-effective. However, these measures may not reflect those derived manually or semiautomatically and need to be overread for accuracy.18 A simple method to overread an automated measure is to verify whether the heart rate and QT measures recorded on the top of the ECG are correct. If correct, the QTc interval calculation must be accurate and can be accepted. One caution to note with automatic measurements is that different ECG machines, even if they have been manufactured by the same company, might use different QT measurement algorithms or different generations of the same algorithm, resulting in slight measurement variation. When performing QT interval monitoring with serial 12-lead ECGs, nurses should ensure that they use the same recording equipment.

A variation of the automated measurement that has been developed is continuous QTc interval monitoring. Though complex, essentially this method measures every QT interval that is available from all monitoring leads and uses a representative heart rate for QT interval correction. The continuous QTc interval is then derived every 5 minutes for display, alarms, and trending.19 A recent study of continuous QTc interval monitoring in critical care settings reported that nearly one-quarter of all critical care patients had evidence of at least 1 episode of dangerous QT interval prolongation (>500 ms), which was associated with higher rates of all-cause mortality.20 This prevalence was higher than those of studies using noncontinuous 10-second ECGs.

The benefits of continuous QT interval monitoring are similar to other physiological parameters commonly used in cardiovascular monitoring; for example, alarm limits can be set, the QTc measure can be continuously tracked, and the QT measure is highly stable as it rejects outlier beats and extraneous noise. An important caveat to continuous monitoring is that QT interval knowledge, normal ranges, and associations with adverse outcomes have all been determined through studies using standard ECGs. Therefore, abnormal QTc values obtained with continuous QT interval monitoring should be confirmed using a standard 12-lead ECG.

Lead Selection

In the standard ECG, slight interlead variations are present in the length of the QT interval. These variations are called QT dispersions and can result in differences of up to 50 ms between one lead and another in healthy subjects.21 A QT dispersion was once thought to result from differences in regional ventricular repolarization22; however, currently, the clinical significance and the methodological relevance of QT dispersion have been challenged and are not included in routine ECG reports.18,23 Current evidence suggests that QT dispersion results primarily from differences in T-wave projection in the different leads of the surface ECG.23,24 Nevertheless, the challenge QT dispersion poses is in determining which ECG lead is best for QT interval monitoring.

When selecting a lead for QT interval monitoring, nurses should consider (1) the ability to identify the QRS onset and the T-wave offset clearly and (2) the longest QT interval duration. The leads with the longest QT interval duration often have the highest T-wave amplitudes 25 and are predominately V2, V3, and V4.22,25,26 The leads with the shortest QT interval durations are I, avL, and V1.25 Most important however, in lead selection is that the T-wave amplitude is of sufficient size (0.2 mV) to determine a clear T-wave offset.11

Determining QRS Onset and T-Wave Offset

Determining the QRS-complex onset is less challenging than determining the T-wave offset, because the QRS-complex deflection from the baseline is sharp, compared to the more gradual return of the waveform from the terminal portion of the T wave (Figure 2). The difference in the ECG waveform shift can be appreciated knowing that the speed in which depolarization occurs is 100 times faster than that of repolarization.27 As such, the QRS-complex onset can be determined with a high degree of accuracy.28

Figure 2:. Determini... - Click to enlarge in new window

Figure 2:. Determining QRS-complex onset and T-wave offset.

Determining the end of the T wave can be problematic as a result of the slow transition of the terminal portion of the T wave, noise in the ECG signal, and electrical interference. The task is even harder when the T wave is flat or biphasic (has 2 peaks) or when it coincides with the U wave or the following P wave. When the end of the T wave is difficult to identify, the QT interval should not be measured in that lead, as error is likely.18 Figure 3 illustrates one of the most commonly used manual techniques for determining the end of the T wave. Automated algorithms for QT interval measurement use more sophisticated techniques. An example of these is shown in Figure 4.

Figure 3:. Tangent m... - Click to enlarge in new window

Figure 3:. Tangent method for determining T-wave offset.

Figure 4:. Automated... - Click to enlarge in new window

Figure 4:. Automated methods for QT interval measurement. (A) Threshold method, in which the T-wave offset is determined when the downsloping segment of the T wave exceeds a predefined threshold (ie, less than a critical point). The threshold may be set at 0.05 or 0.10, the amplitude of a T wave. (B) The area method, in which the T-wave offset is determined at a point over the baseline where the calculated area under the T wave from T-wave onset to that point represents a predefined proportion. The proportion may be set at 94% or 98% of the total area under the T wave. (C) Maximum distance method, in which the T-wave offset is determined by drawing a line from the peak of the T wave to a heart rate adjusted point after the T wave. Vertical lines are drawn from each point sampled from the ECG waveform to the previously drawn line. The vertical line with the maximum distance is selected to determine the T-wave offset.

The presence of a U wave complicates the process of locating the end of the T wave. Although the exact origin of U waves is not yet clear, scientists believe that it is most probably an electromechanical phenomenon occurring after the end of ventricular repolarization and consequently should not be considered part of the QT interval.6,26,29,30 When measuring the QT interval in the presence of U waves, the clinician should choose a lead without the presence of a U wave or a lead with clear distinction between the end of the T wave and the start of the U wave.18,25 Discrete U waves are usually best seen in the lateral precordial leads,13 are absent in aVR and aVL, and are typically less than 0.2 mV in amplitude.18,30 If the waveform is a biphasic T wave and not 2 distinct waves, then the suggested T-wave offset is the intersection between the descending limb of the T wave and the baseline.13

QT Measurement With Multiple Leads

In certain circumstances, expert clinicians may decide to measure the QT interval from a particular lead for reasons other than accuracy. For example, a clinician who is interested in the change in the QT interval from baseline compared with the absolute QT interval might select lead II considering that it is not the most accurate lead. The rationale for this choice is that the QT interval from lead II is convenient and more importantly has high reproducibility, an important factor in the clinical setting.31

Whenever more than a single lead is available, the lead with the longest QT interval should be selected for the QT interval measurement. Selecting the lead with the longest QT interval is more accurate than averaging the QT value from different leads. Clinicians should also be consistent when deciding which lead to select for the QT measurement.11 The lead being selected and the QT interval baseline value should be documented and repeated for each measure.

Fully automated QT monitoring programs factor interlead differences in the QT interval length into their measurement algorithms. The QRS-complex onset and the T-wave offset, and consequently the length of the QT interval in any lead are primarily affected by the spatial orientation of that particular ECG lead relative to the cardiac vector.32 For example, when a particular lead is perpendicular to the cardiac vector during late ventricular repolarization, the T wave's return to the isoelectric line is earlier. Therefore, the total repolarization time is best determined by the earliest QRS-complex onset in any lead to the latest T-wave offset in any lead, and it cannot be performed manually. Computer algorithms superimpose all available leads19,33 and align them using a fiduciary marker such as the QRS-onset to provide a QT interval that is referred to as a global QT interval (Figure 5). The global QT interval is considered the most accurate representation of the total ventricular repolarization time.31 The global QT measurement will result in a QT interval that is longer than the one measured from any single lead, as a single lead underestimates the overall ventricular repolarization time.

Figure 5:. Superimpo... - Click to enlarge in new window

Figure 5:. Superimposed electrocardiographic leads demonstrating the global QT interval.

Measuring the QT Interval in Special Situations

Wide QRS Complex

Wide QRS complexes may result from bundle branch block, ventricular pacing, preexcitation, and certain drugs. In these situations, depolarization, not repolarization, is responsible for prolonging the QT interval. Automated measures do not distinguish wide QRS complexes when deriving the QTc interval. Therefore, manual or semiautomated methods are preferred in these instances. The American Heart Association and the American College of Cardiology recommend correcting the QT interval for a wide QRS complex either by including the QRS complex as a covariate in a linear QT correction formula in addition to the R-R interval or by measuring the JT interval.18

The JT interval is the measured distance from the J point to the T-wave offset (Figure 6). In a study of 20 687 healthy adults and 2865 adults with ventricular conduction delay, Zhou et al32 determined that the QRS duration accounted for approximately 16% of the overall QT interval but had negligible effects in the presence of conduction delay. They proposed a JT correction formula [JT interval = JT (heart rate + 100)/518], with a JT interval greater than 112 ms representing prolonged repolarization.32 However, as normal JT interval values have not been determined, the JT measure is not commonly used in clinical practice. If a clinician wants to use it, it is used similarly to QT monitoring, with serial measures performed to identify changes in duration from a baseline measure, typically after administration of a proarrhythmic drug. Consultation is needed to establish the parameters indicating likely proarrhythmia and guidelines for cessation of the offending drug.

Figure 6:. The JT an... - Click to enlarge in new window

Figure 6:. The JT and the RT

Atrial Fibrillation

Measuring and correcting the QT interval when the underlying rhythm is atrial fibrillation is challenging because of the substantial R-R variability. Automated algorithms do not perform reliably in atrial fibrillation; therefore, manual or semiautomated approaches are recommended. Currently, a consensus is lacking as to how to resolve this issue.33 Some suggest not to attempt correction at all,18 although several methods do exist.

The first suggested method to provide a representative QTc interval is to average the longest and shortest QTc intervals: (QTc_short + QTc_long)/2. The second suggested method is to measure the RT_peak interval (from the R-wave onset to peak of the T wave; see Figure 6). If this is more than half of the R-R interval, then the QTc interval is likely to be above the critical threshold of 500 ms.11 The third method is to measure multiple corresponding QTc intervals (up to 10) and determine their average by the number of measurements made.33 The fourth proposed method is to identify an R-R interval that is of exactly 1-second duration (1000 ms or 5 large boxes on standardized ECG paper). When 1 second is entered into the Bazett or Fridericia correction formula, the QTc interval will essentially be the measured QT interval; the square or cubed root of 1 is 1. Therefore, with a heart rate of 60/min, the QTc interval will equal the QT interval. However, with beat-to-beat variability in atrial fibrillation, this final method is not recommended because it is questionable whether a single measure represents the underlying repolarization duration. For patients with atrial fibrillation, the authors recommend the third method, semimanually averaging multiple QTc interval estimates.