Electromechanical coupling in patients with the short QT syndrome: further insights into the mechanoelectrical hypothesis of the U wave

Abstract

Background: Patients with a short QT syndrome (SQTS) are at risk of sudden cardiac death (SCD). It is not known whether abbreviation of cardiac repolarization alters mechanical function in SQTS. Controversies persist regarding whether the U wave is a purely electrical or mechanoelectrical phenomenon.

Objective: The present study uses echocardiographic measurements to discriminate between the hypotheses for the origin of the U wave.

Methods: Diagnostic work-up including echocardiography and electrocardiogram was performed in 5 SQTS patients (39 +/- 19 years old) from 2 unrelated families with a history of SCD and 5 age-matched and gender-matched control subjects.

Results: QT intervals were 268 +/- 18 ms (QTc 285 +/- 28 ms) in SQTS versus 386 +/- 20 ms (QTc 420 +/- 22 ms) in control subjects (P < .005). In SQTS patients, the end of the T wave preceded aortic valve closure by 111 +/- 30 ms versus -12 +/- 11 ms in control subjects (P < .005). The interval from aortic valve closure to the beginning of the U wave was 8 +/- 4 ms in patients and 15 +/- 11 ms in control subjects (P = .25). Thus, the inscription of the U wave in SQTS patients coincided with aortic valve closure and isovolumic relaxation, supporting the hypothesis that the U wave is related to mechanical stretch.

Conclusion: Our data show for the first time a significant dissociation between the ventricular repolarization and the end of mechanical systole in SQTS patients. Coincidence of the U wave with termination of mechanical systole provides support for the mechanoelectrical hypothesis for the origin of the U wave.

Figure 1

Echocardiography of control and SQTS patient with simultaneous single-lead ECG recording (bottom line, panel B and C, paper speed 50 mm/sec). A: Echocardiographic sector of the parasternal long-axis view with alignment of the M-mode cursor through the aortic valve before application of M-mode echocardiography. The right ventricle (top of each panel) is close to the transducer probe, whereas the left atrium is the most distant structure (white dotted line = M-mode cursor). B: M-mode echocardiography of the aortic valve in a proband. Opening and closure of the aortic valve is visible by the rhombus-like echogenic reflections generated by abrupt movements of the anterior cusp and posterior cusps toward and away from the transducer. The termination of electrical repolarization (end of T wave) coincided with the end of mechanical systole (closure of aortic valve, arrow). C: M-mode echocardiography of the aortic valve in a patient with SQTS. Termination of electrical repolarization (end of T wave) preceded end of mechanical systole (closure of aortic valve) by 115 ms (arrow). AV = aortic valve; ECG = electrocardiogram; LA = left atrium; RV = right ventricle; SQTS = short QT syndrome.

Figure 2

Precordial ECG recordings from a control and a SQTS patient. Inscription of the U wave is indicated by arrow. A: Control: QT 360 ms, QTc 397 ms, difference of the end of the T wave to the inscription of the U wave 10 ms. B: SQTS patient: QT 270 ms, QTc 392 ms, difference of the end of the T wave to the inscription of the U wave 110 ms (paper speed 50 mm/sec, amplitude gain 20 mm/mV). ECG = electrocardiogram; SQTS = short QT syndrome.

Figure 3

Tissue Doppler echocardiography recording in a patient with SQTS. Tissue velocity (yellow line, right) with sample volume positioned in the basal interventricular septum in the apical 4-chamber view (yellow circle, left lower view without and left upper view with color Doppler imaging). AVC (vertical green dotted lines) and beginning of isovolumic relaxation is correlating with beginning of the U wave. Left upper corner timings with corresponding blue dotted markers in ECG (bottom right): 1: RR interval (831 ms); 2. interval from Q wave to beginning of the U wave (398 ms, transferred timing from 12-lead ECG); 3. interval from Q wave to aortic valve closure (381 ms). Bottom right single-lead ECG (speed 50 mm/sec). AVC = aortic valve closure; ECG = electrocardiogram; SQTS = short QT syndrome.