HCM patients were enrolled from those who visited the echocardiography laboratory of the Peking University Third Hospital between November 2010 and March 2011. Exclusion criteria were: 1) Patients who had a history of left ventricular outflow tract obstruction (LVOTO) diagnosis; 2) left ventricular ejection fraction (LVEF) <50%; 3) atrial or ventricular arrhythmia; 4) valvular disease of moderate or greater severity; or 5) pericardial diseases.

Thirty-one age- and gender-matched healthy volunteers were enrolled as controls. The protocol was approved by Peking University Institutional Review Board. All subjects provided a written informed consent before participation.

Standard 2-dimensional measurements (left ventricle diastolic and systolic dimensions, intraventricular septum (IVS), posterior wall thickness (LVPW), left atrial volume, and left ventricle outflow tract) were obtained with the patient in the supine position, before and after exercise. LV mass was calculated using the Devereux formula [22].

After obtaining the resting images from the standard parasternal and apical views, all subjects were submitted to symptoms-limited treadmill exercise (modified Bruce protocol [23]). Right after the subjects stopped exercising, echocardiography was performed in the supine position, within one minute of exercise end, using an ultrasound system (Vivid I, GE Healthcare, Waukesha, WI, USA) with a 2.5-MHz transducer. From the apical window, a 2-mm pulsed Doppler sample volume was placed at the mitral valve tip, and mitral flow velocities of 3 cardiac cycles were recorded, obtaining peak velocities of the early diastolic transmitral flow (E), of the late diastolic transmitral flow (A), and of the early diastolic lateral mitral annulus velocity (Em_lateral) were measured by TDI using the pulsed wave Doppler mode. The filter was set to exclude high frequency signals, and the Nyquist limit was adjusted to a range of 15 to 20 cm/s. Gain and sample volume were minimized to allow for a clear tissue signal with minimal background noise. Em_lateral was measured from the apical 4-chamber view with a 2 mm sample volume placed at the lateral corner of the mitral annulus. These measurements were made at baseline and during recovery, in the same sequence. Measurements were recorded with simultaneous electrocardiography. All data were digitally stored.

Each subject was submitted to a symptoms-limited modified Bruce protocol [23] treadmill CPET under the supervision of a qualified exercise physiologist and a physician. Expired gases were collected continuously throughout exercise and analyzed for ventilatory volume (VE), and for oxygen (O₂) and carbon dioxide (CO₂) content by a professional analyzer. Expired gases were reported every 30 seconds, and were reported as peak oxygen consumption (VO_2max, ml/kg/min), peak respiratory exchange ratio (the ratio of CO₂ production to O₂ consumption at peak effort), and VE/VCO₂ slope (the slope of the increase in peak ventilation/increase in CO₂ production throughout exercise).

Heart monitoring consisted of continuous 12-lead electrocardiography, automatic blood pressure measurements, and heart rate (HR) recordings at every stage via the electrocardiogram. Test termination criteria were: 1) patient’s request; 2) ventricular tachycardia; 3) 2 mm or more of horizontal or down sloping ST-segment depression; or 4) a drop in systolic blood pressure (SBP) of 20 mm Hg or more.

Blood samples for analysis of NTproBNP were obtained before the start of exercise and at maximal exercise. Serum NT-proBNP levels were determined using an electro-chemiluminescence immunoassay according to the manufacturer's instructions performed on a Cobas E601 (Roche Diagnostics, Basel, Switzerland).

Normally distributed continuous variables are expressed as means ± SD. Non-normally distributed continuous variables were log-transformed to normalize their distribution for analysis. Categorical data are expressed as percentages. Variables between the study groups were compared by the Student’s t-test and by one-way analysis of variance (ANOVA). Non-parametric tests were used in cases of unequal variances. Variables at rest and peak stress within each group were compared by the Student’s t-test and categorical data were compared by the pearsonchi-square test. Differences were considered significant when the p-value was <0.05. All analyses were performed using SPSS 17.0 (SPSS Inc., Chicago, IL, USA).

Two hundred and sixty-eight patients were screened; 103 patients suffered from atrial fibrillation, 43 refused to participate, 39 patients could not be contacted, 12 patients suffered from left ventricular outflow tract obstruction, 35 suffered from severe mitral valve regurgitation, 5 had a hydropericardium, and 4 patients had limited physical capacities. Therefore, 27 patients were included. The clinical characteristics of HCM patients and controls are compared in Table 1. There were no differences in age, gender, body mass index (BMI), and comorbidities (hypertension and diabetes).

Hemodynamic and echocardiographic parameters are compared in Table 2. There were no statistical differences in resting and stress HR (p = 0.621 and 0.086, resepectively) and SBP (p = 0.955 and 0.615, respectively) between the two groups. Standard echocardiography showed that there were no differences in LVEF, LVESD and LVEDD (all P > 0.05). However, the HCM patients displayed larger LAD, LAA, IVS, LVPW and LVMI (all p < 0.01) (Table 2).

The baseline peak velocity of E in the HCM group was lower than in controls (0.61 ± 0.16 vs. 0.72 ± 0.15, P < 0.05). There were no differences in other mitral inflow variables between the two groups.

Both baseline and stress Em_lateral of the HCM group were lower than in controls (baseline: 8.1 ± 2.6 vs. 11.3 ± 2.3, p < 0.001; 10.3 ± 3.2 vs. 13.3 ± 2.8 p <0.01). There was no difference in baseline Sm_lateral between the two groups (baseline: 9.1 ± 2.3 vs. 9.7 ± 2.0, p > 0.05). But stress Sm_lateral of the HCM group were lower than in controls (14.3 ± 4.1vs. 16.3 ± 3.0 p <0.05) (Table 2). Both baseline and exercise E/Em_lateral were higher in HCM patients (baseline: 8.0 ± 2.5 vs. 6.5 ± 1.5, P < 0.01; exercise: 9.1 ± 3.0 vs. 7.1 ± 1.3, P < 0.01). E/Em_lateral increased after exercise in HCM patients (from 8.0 ± 2.5 to 9.1 ± 3.0, P < 0.01), but not in controls (from 6.5 ± 1.5 to 7.1 ± 1.3, P = 0.085) (Table 3).

For HCM patients, VO_2max was lower (24.3 ± 5.2 vs. 27.6 ± 3.9, P < 0.01) and VE/VCO₂ slope was higher (28.8 ± 4.0 vs. 26.9 ± 2.7, P < 0.05) than in controls (Table 4). No LVOTO was observed in the HCM patients after CPET.

In HCM patients, both baseline and exercise NT-proBNP were higher than in controls (baseline: 884 vs. 72 pg/ml, P < 0.001; exercise: 1019 vs. 79 pg/ml, P < 0.001) (Table 4).

According to an E/Em_lateral above or below 10 before and after exercise, the HCM group could be divided into 3 subgroups: group A (n = 17, baseline and stress E/Em_lateral < 10), group B (n = 5, baseline E/Em_lateral < 10, and stress E/Em_lateral > 10), and group C (n = 5, baseline and stress E/Em_lateral > 10) (Table 5). In group B, the E/Em_lateral ratio was increased in all patients (P < 0.05) (Figure 1). There were no differences in age, BMI, LVEDD, LVESD and LVEF between the controls and the three HCM subgroups. The VE/VCO₂ slope in group B was similar to that of group C (p > 0.05), but was higher compared with group A (p < 0.05) and controls (p < 0.05) (Figure 2). NT-proBNP in group B was similar to that of group C (p > 0.05), but was higher compared with group A (p < 0.05) and controls (p < 0.01) (Figures 3 and 4).