Study design and population

A total of 104 participants (mean age, 58 ± 21 years; 48 women) were referred to undergo IVRT measurements at our institution from January 2020 to November 2020. The participants were as follows: 42 healthy volunteers, 15 with abnormal electrocardiography (ECG), 14 individuals with hypertension, 9 with diabetes mellitus, 4 with dyslipidemia, 15 with ischemic heart disease, 3 with cardiomyopathy, 5 with valvular diseases, and 2 with hemodialysis. This study had been approved by clinical research ethics review committee in our hospital and a prospective study. Only participants could be obtained the consent for this study were included.

Transthoracic echocardiography (TTE) was performed using an ultrasound machine (LISENDO 880; FUJIFILM Healthcare Corporation, Tokyo, Japan). We used a cardiac microphone (MA-300; Fukuda Denshi, Tokyo, Japan). In all cases, the cardiac microphone was placed at the site of maximum IIs. All participants were in sinus rhythm and were hemodynamically stable during the examination.

Echocardiography

We measured IVRT using the Dual Gate Doppler method and seven other methods using the same machine at the same heart rate for all patients. These seven methods were described in the evaluation of left ventricular diastolic function of the ASE and EACVI guidelines [1] and in the reference literature of these guidelines [2,3,4,5,6]. The CW and PW methods were published in the ASE and EACVI guidelines for the IVRT measurement [1]. We combined phonocardiography (PCG), ECG, M-mode echocardiography, and Doppler echocardiography in the seven measurement methods of the IVRT as follows.

(1) Dual Gate Doppler method (Fig. 1-A): Each sample volume was placed at the left ventricular outflow tract and the tip of the mitral valve during mitral valve opening. The time interval from the end of left ventricular outflow to the beginning of mitral inflow was measured at the apical three-chamber view of the left ventricle (LV) by Dual Gate Doppler.

(2) CW method (Fig. 1-B): The cursor was placed on the line passing through the aortic mitral curtain in LV outflow tract to simultaneously displayed end of aortic ejection and onset of mitral inflow at the apical three chamber view of the LV. The time interval from the end of the left ventricular outflow to the beginning of the mitral inflow was measured using the CW method.

(3) PW method (Fig. 1-C): The sample volume was placed at the tip of the anterior mitral leaflet on the line passing through the aortic mitral curtain at the apical three-chamber view of the LV. The time interval from the end of the left ventricular outflow to the beginning of the mitral inflow was measured using the PW method.

(4) IIs-PW method (Fig. 1-D): IIs was recorded using PCG. The sample volume was placed between the tips of the anterior and posterior mitral leaflets. The time interval from the beginning of IIs to the beginning of mitral inflow was measured using the PW method.

(5) IIs-M-mode method (Fig. 1-E): IIs was recorded by PCG. M-mode echocardiography was performed in the parasternal long-axis view. The time interval from the beginning of IIs to the beginning of the mitral opening phase, determined by point D in the M-mode record, was measured in the same heartbeat.

(6) PW time difference method (Fig. 1-F): The first interval was between the time from the top of the R-wave on ECG to the end of left ventricular outflow by PW in the apical three-chamber view of the LV. The second interval was between the time from the top of the R-wave on ECG to the beginning of mitral valve inflow by PW. The time difference was calculated by subtracting the former time from the latter time.

(7) M-mode time difference method (Fig. 1-G): The first interval was between the time from the top of the R-wave on ECG to the end of left ventricular outflow by PW, as aforementioned. The second interval was between the time from the top of the R-wave on ECG to the D point of the mitral valve by M-mode echocardiography at the parasternal long-axis view, and the time difference was calculated by subtracting the former time from the latter time.

Fig. 1

A Dual-gate Doppler method. B Continuous wave Doppler (CW) method. C Pulsed Doppler (PW) method. D Second sound-pulsed Doppler (IIs-PW) method. E Second sound-M mode (IIs-M mode) method. F PW time difference method. G M-mode time difference method. The isovolumic relaxation time (IVRT) is obtained by measuring the time between the green lines in Fig. 1A, B, C, D, and E. The IVRT is calculated by measuring from the top of the R-wave on electrocardiography to the end of left ventricular outflow (orange arrow) and the beginning of left ventricular inflow (orange arrow) in Fig. 1F and G

In the CW, PW, and IIs-M-mode methods, the Dual Gate Doppler method and IIs-PW method, the IVRT was measured in the same heartbeat, whereas in the other methods, it was measured at two different heartbeats and subtracted. One professionally qualified technician recorded and performed the time-phase analysis at 200 mm/s, and each item was measured as the average of three heartbeats.

An additional study was performed to determine the cause of the difference in the IVRTs between the CW and PW methods. We compared the time from the R-wave on ECG to the aortic valve closure phase and from the R-wave on ECG to the mitral valve opening phase in 39 patients (mean age, 59 ± 22 years, 18 women) using the CW and PW methods, respectively.

Statistical analysis

Values are expressed as mean ± standard deviation. The Spearman correlation coefficient, Wilcoxon signed rank-sum test, intraclass correlation, and Bland–Altman analysis were used as appropriate. Statistical analysis was performed using EZR version 1.54. In all analyses, p-values < 0.05 were considered statistically significant.