The QT interval is a measurement derived from an electrocardiogram (EKG or ECG), a common test used to assess the heart’s electrical activity. This specific interval represents the total time it takes for the heart’s lower chambers, known as the ventricles, to electrically activate and then fully reset after each heartbeat. It reflects the duration of ventricular depolarization and subsequent repolarization. The QT interval’s duration is an important indicator for assessing overall heart health. Abnormalities in this interval, whether it is too long or too short, can suggest underlying cardiac issues. While it does not diagnose specific conditions on its own, its value helps healthcare professionals evaluate the heart’s electrical stability. This measurement plays a role in identifying potential risks related to heart rhythm.
Identifying the Key EKG Landmarks
Measuring the QT interval begins with identifying specific points on the EKG. The measurement starts at the beginning of the QRS complex, which represents the electrical activation of the ventricles. On an EKG, the Q wave is the first downward deflection of the QRS complex, followed by the R wave as an upward deflection, and then the S wave as a downward deflection after the R wave. The onset of the Q wave marks the starting point for the QT interval measurement.
The measurement concludes at the end of the T wave, which signifies the electrical resetting or repolarization of the ventricles. A normal T wave typically appears as a smooth, rounded, and slightly asymmetrical upward curve. The challenge in measurement often lies in pinpointing the exact end of the T wave, as it can sometimes merge gradually with the baseline.
Occasionally, a small wave called a U wave may appear immediately after the T wave. For accurate QT interval measurement, U waves should generally be excluded unless they are large and clearly fused with the T wave, making it impossible to distinguish the T wave’s true end.
The Measurement Process
Once the starting and ending points are identified, the raw QT interval can be measured. This is commonly done on EKG paper, which is a grid with standardized squares. Each small square on the horizontal axis represents 0.04 seconds, and five small squares make up one large square, representing 0.20 seconds.
To manually measure the QT interval, a clinician counts the number of small squares between the beginning of the QRS complex and the end of the T wave. This count is then multiplied by 0.04 seconds to obtain the QT interval duration in milliseconds. Modern EKG machines frequently provide an automated QT interval measurement. For consistent and accurate readings, clinicians often prefer to measure the QT interval in specific leads, with Lead II or Lead V5-6 being commonly used due to their clear visibility of the T wave. The lead showing the longest QT interval is often selected for the most representative measurement.
Correcting for Heart Rate
The raw QT interval changes with heart rate, shortening when the heart beats faster and lengthening when it beats slower. This variability makes direct comparisons of QT intervals between individuals or over time difficult if their heart rates differ. To account for this natural phenomenon and allow for a standardized comparison, the QT interval is corrected for heart rate, resulting in the corrected QT interval, or QTc.
The most widely used method for this correction is Bazett’s formula. The formula is expressed as QTc = QT / √RR interval, where QT is the measured QT interval in seconds, and RR is the interval between the peaks of two consecutive R waves, also in seconds. Bazett’s formula aims to estimate what the QT interval would be if the heart rate were 60 beats per minute. While widely adopted for its simplicity, Bazett’s formula can sometimes overcorrect at very fast heart rates and undercorrect at very slow heart rates. Despite these limitations, it remains a common tool for standardizing QT interval measurements.
Interpreting the QTc Value
Understanding the QTc value is important for assessing heart health. For adult males, a normal QTc is less than 430 to 450 milliseconds (ms). For adult females, the normal range is slightly longer, less than 450 to 470 ms.
A QTc is prolonged if it exceeds these established thresholds, for example, over 450 ms in men and over 460 ms in women. A QTc greater than 500 ms is clinically significant and is associated with a greater risk of heart rhythm problems. Conversely, an abnormally short QTc is less than 350 ms.
An abnormal QTc, whether prolonged or short, suggests electrical instability in the heart’s ventricles. A prolonged QTc indicates an increased risk for dangerous heart rhythm disturbances, such as Torsades de Pointes, which can lead to sudden cardiac death. Short QTc also carries an increased risk for certain arrhythmias.