Statistical analysis
Epi Info version 7.1.1 software was used for the analysis of the data. Continuous data were presented as mean with standard deviation and compared by t-test. Continuous data of more than two groups were compared by ANOVA test. Categorical data were presented as frequency and percentage and compared using a chi-square test. P value <0.05 was considered as significant. To investigate for inter-observer variability for LA strain, analysis of 10 random subjects was done by two independent investigators who were blinded to the clinical data. For intra-observer variability, repeat offline LA strain estimation was done at 5 ± 2 days later in 10 randomly selected patients. The interclass correlation coefficients (ICCs) were calculated with point estimates and 95% confidence intervals (CIs) being reported.
Results
We enrolled 120 subjects including 80 cases and 40 controls in our study. There were 57 females and 23 males amongst cases while among control population there were 15 females and 25 males. Table 1 shows the baseline characteristics and various echocardiographic parameters among study subjects. The mean LA size among cases was 4.67 ± 0.65 cm and mean MVA was 0.93 ± 0.21 cm2. Severe MS (MVA 1-1.5 cm2) was seen in 44 (55%) subjects while very severe MS (MVA <1 cm2) was seen in 36 (45%) subjects. The mean RVSP in cases was 60.01 ± 19.88 mm Hg suggesting moderate to severe pulmonary hypertension. All three STE derived LA strain [reservoir strain (LASr), conduit strain (LAScd) and contractile strain (LASct)] parameters were significantly reduced among cases (p <0.001) with mean values of LASr(14.73 ± 8.59%), LAScd (-7.61 ± 4.47%) and LASct (-7.16 ± 5.15%) when compared to controls where the mean values were 44.11 ± 10.44%, -32.45 ± 7.63%, -11.85 ± 6.77% respectively. The interclass correlation coefficient for LA strain measurement was 0.95 (95% CI: 0.84-0.98) for inter-observer agreement and 0.97 (95% CI: 0.94-0.99) for intra-observer agreement, indicating good inter-observer and intra-observer correlations.
The NYHA class III dyspnea was present in 37 (46.25%) subjects, while thirty-eight (47.5%) subjects had NYHA class II dyspnea followed by 5 (6.25%) subjects who had NYHA class I dyspnea. The descriptive statistics associated with patients in different NYHA classes were analyzed both between the groups and within group by ANOVA (Table 2). There was no significant correlation between any echocardiography derived parameters as well as LA strain parameters between various NYHA classes except for MVA which was significantly lower in NYHA class III (0.87 ± 0.21 cm2) when compared to NYHA class II (0.97 ± 0.2 cm2) and I (1.18 ± 0.23 cm2) [p= 0.004 between all groups] and RVSP which was significantly higher in NYHA class III (66.32 ± 19.74 mmHg) when compared to class II (55.03 ± 18.47) [p= 0.01 between NYHA class II and III]. All the three mean LA strain values (reservoir, conduit, contractile strain) were numerically lower in NYHA class III when compared to NYHA class I patients but did not reach statistically significant difference.
We divided cases into groups based on increasing mean diastolic transmitral gradient (four groups), increasing peak diastolic transmitral gradient (three groups), increasing left atrial size (four groups), increasing severity of pulmonary hypertension (four groups) and decreasing mitral valve area (two groups) to study their correlation with LA strain (Table 3). The numerical value of all three STE derived LA strain parameters showed a trend towards decline with decrease in MVA, increase in LA size and increase in severity of PH, increase in MG and PG. But none of the above correlations achieved statistical significance (Table 4).