A retrospective review of VFM in the Mitral Valve Repair program database in our hospital identified patients with a diagnosis of degenerative mitral regurgitation between June 2019 and May 2021. A total of 70 consecutive patients with degenerative mitral regurgitation because of prolapse degeneration of the mitral valve involving single or two leaflet scallops were enrolled. Patients who were lost to follow up, with insufficient quality of images, or with mitral valve replacement were excluded. The final analysis included 50 patients who underwent mitral valve repair by a single surgeon in our hospital. Based on mitral leaflet resection, the patients were divided into two subgroups: 29 patients with mitral leaflet resection (resection subgroup) and 21 patients without mitral leaflet resection (non-resection subgroup) (Fig. 1). All subjects underwent echocardiography, one week before and one month after operation. There was no significant difference in postoperative drug treatment between patients.

Fig. 1.

Study flow chart.

To compare patients with degenerative mitral regurgitation with controls of similar age and gender, 30 healthy volunteers were selected as control group during the same period. All included volunteers were confirmed to be free of abnormalities by physical examination, electrocardiogram, X-ray, echocardiography and laboratory tests in a physical examination center. Data on height, blood pressure and weight were collected.

Inclusion criteria were as follows. The subjects were in sinus rhythm, had left ventricular ejection fraction (LVEF) $>$50%, and the mitral valve regurgitation was caused by degeneration. The degree of mitral regurgitation was based on the American Society of Echocardiography (ASE) diagnostic criteria [14]: moderate and severe mitral regurgitation were defined as an effective regurgitant orifice area (EROA) of 0.30–0.39 cm${}^2$ and $\ge$0.40 cm${}^2$, respectively.

Patients with atrial fibrillation, rheumatic mitral valve, myocardial infarction, cardiomyopathy, other severe valve diseases, hypertension, diabetes, chronic kidney disease, and previous heart surgery history were excluded.

Only patients who underwent mitral valve repair were included. The procedure frequently involves leaflet resection, use of annular rings and neochordae to reshape the annulus and support leaflet repair.

The Aloka F75 color Doppler ultrasound system and the UST-52105 heart probe with a frequency of 1–5 MHz were used while the participant was in a left-side lying position, and breathed calmly. An electrocardiogram was simultaneously recorded. Height and weight were assessed to calculate the body surface area (BSA, unit m${}^2$). Routine echocardiography was conducted; left atrial dimension (LAD) was measured in the long axis view of the LV, and left ventricular end-diastolic dimension (LVEDD), left ventricular end-diastolic volume (LVEDV), left ventricular end-systolic dimension (LVESD), left ventricular end-systolic volume (LVESV), and LVEF were measured using the Simpson biplane method at the apical four-chamber and two-chamber view. Mean transmitral gradient was calculated using the ultrasound system which traces the peak velocity curve at the mitral valve.

Dynamic color Doppler blood flow images of the LV chamber were collected from the apical four-chamber view in VFM mode. The probe emission frequency was adjusted to clearly display the endocardium. The maximum velocity range of the color Doppler (Nykist limit) was set at 60–80 cm/s, and the color baseline was kept at 0 cm/s. Under these conditions, the image frame rate was increased as much as possible.

Three cardiac cycles were continuously collected and VFM data was stored on the mobile hard disk for offline analysis. The VFM image data were entered in the DAS-RSI workstation, the analysis interface was entered. The time-flow curve was used to define the time period, and the electrocardiogram (ECG) R-R wave apex was selected as a complete cardiac cycle. Based on the ECG and valve opening and closing conditions, a complete cardiac cycle was divided in three periods: fast filling period (P1), slow filling period (P2), and atrial systolic period (P3) (Fig. 2). The LV diastolic EL-AVE was measured in energy loss mode, and the average EL-AVE value of these three time periods was calculated. The differences in EL-AVE between groups was compared (Fig. 3).

Fig. 2.

The LV cardiac cycle time-flow curve. Each point of the curve corresponds to the frame rate of the ECG. The three periods of the diastole: P1—fast filling period, P2—slow filling period, and P3—atrial systolic period.

Fig. 3.

EL-AVE in patients who underwent mitral valve repair and controls. EL-AVE of patients after mitral valve repair was higher than that of controls. EL-AVE, average energy loss.

VFM uses blood flow velocity to determine energy loss caused by viscous friction [15]. Intracardiac energy loss is calculated using the following equation:

(1)$$\mathit{\text{Energy Loss}}=\int \mu \left\{2{\left(\frac{\partial u}{\partial x}\right)}^2+2{\left(\frac{\partial v}{\partial y}\right)}^2+{\left(\frac{\partial u}{\partial y}+\frac{\delta v}{\delta x}\right)}^2\right\}𝑑A,$$

In which $\mu$ is the viscosity of the blood, u and v are velocity components on the Cartesian axes (x and y), and A is the area of the unit of the grid.

As seen in the equation, energy loss is the total of squared differences between neighboring velocity vectors. It changes when the size and direction of velocity vector change.

Data were compared using SPSS version 22.0 (SPSS Inc., IBM, Chicago, IL, USA). Continuous variables were presented as mean $\pm$ standard deviation or median and interquartile range depending on the distribution. Categorical variables were presented as percentages. Results of two groups were compared using unpaired Student’s t-test and chi-squared test. One-way analysis of variance (or Kruskal-Wallis for non-normally distributed continuous variables) was used to examine the difference of variables across $>$two groups. Statistical significance was accepted at p 0.05.

The effect of postoperative EL-AVE one month after operation was investigated using muti-way analysis of variance, with subgroups of patients with leaflet resection and those without resection, patients with degeneration of mitral valve involving single or two leaflet scallops, and with or without neochordae. The patients in the subgroups were divided based on these factors.

Ten random individuals were selected for evaluation of intraobserver and interobserver agreement on EL-AVE using Bland-Altman analysis.

We screened 70 patients with degenerative mitral regurgitation and enrolled 50 patients in the study. The most common leaflet abnormality in the patients was prolapse involving the posterior mitral leaflet. The repair procedures included 50 patients with an O-shaped semi-rigid complete ring, 27 patients with neochordae, 29 patients with leaflet resection, and 21 patients with no leaflet resection. There were no statistically significant differences between patients who underwent mitral valve repair and the control group, including age (p = 0.64), sex distribution (p = 0.78), systolic blood pressure (p = 0.41), diastolic blood pressure (p = 0.35) and BSA (p = 0.38) (Table 1).

Compared with the control group, the left atrial and LV chamber were enlarged in patients one week before operation with statistically significant differences in LAD (p 0.001), LVEDD (p = 0.01), LVEDV (p 0.001), LVESD (p = 0.01), and LVESV (p = 0.01) (Table 2). Mean transmitral gradient was significantly increased after operation (p 0.001).

Compared with one week before operation, the left atrial and LV chamber were reduced one month after operation, and the difference in LAD (p 0.001), LVEDD (p 0.001), LVEDV (p 0.001), LVESD (p = 0.01), and LVESV (p = 0.01) were statistically significant. There was no statistically significant difference in LVEF (p = 0.08).

Compared with the control group, the EL-AVE before and after operation in the mitral valve repair group was significantly increased (p 0.001) (Table 2 and Figs. 3,4).

Fig. 4.

A statistically significant difference was observed when comparing EL-AVE of the LV during diastole in controls and patients before and after mitral valve repair. Green: EL-AVE of LV in controls. Orange: EL-AVE of LV in patients who underwent mitral valve repair one week before operation. Blue: EL-AVE of LV in patients who underwent mitral valve repair one month after operation. EL-AVE, average energy loss.

Compared with one week before operation, the EL-AVE after operation in the mitral valve repair group was significantly decreased (p 0.001).

The effect of mitral valve resection on EL-AVE one month after operation was significant (p 0.001), and no significance was observed in the effect of extension of leaflet degeneration and neochordae on EL-AVE one month after operation (p = 0.65, 0.20) in Table 3. There was no interaction of these factors on EL-AVE after operation (p = 0.98).

There were no statistically significant differences between the resection subgroup and the non-resection subgroup, including age (p = 0.67), sex distribution (p = 0.58), annuloplasty ring size (p = 0.39), systolic pressure (p = 0.37), diastolic pressure (p = 0.06) and BSA (p = 0.56).

Differences in the surgical procedure between the two subgroups were as follows. The most common leaflet abnormalities in the resection subgroup were prolapse involving single leaflet, posterior mitral leaflet and no use of neochordae. In the non-resection subgroup, prolapse involving anterior mitral leaflet and use of neochordae were the most common leaflet abnormalities. There was no significant difference in the non-resection subgroup involving single or two leaflet prolapse. Compared with the resection subgroup, the EL-AVE during diastole of the non-resection subgroup was significantly decreased one month after operation (p 0.001) (Table 4).

The Bland-Altman analysis for assessing the intraobserver (differences 4.62 $\pm$ 9.03, 95% CI –13.04~22.24) and interobserver (differences 3.11 $\pm$ 9.02, 95% CI –14.54~20.74) variability for EL-AVE one week before operation demonstrated excellent reliability (Fig. 5).

Fig. 5.

Bland-Altman plots for interobserver and intraobserver agreement on EL-AVE during diastole. EL-AVE, energy loss.

Of the fifty patients who underwent mitral valve repair, one patient experienced poor healing of the surgical incision in the resection subgroup, and one patient had hoarseness in the non-resection subgroup, and the rest of the patients recovered well after operation. Postoperative review by echocardiogram showed no significant abnormalities in mitral valve function according to the ASE guidelines [14] with definitions EROA 0.2 cm${}^2$ and mean pressure gradient 5 mm Hg.