The Assessing Value Of Cardiac Function With Six-minute Walking Test In The Mid-term Postoperative Children With Congenital Heart Disease

Background and objectivesThe six-minute walking test (6MWT) is a safe, convenient and sub-maximal exercise test, which reflects body's cardiopulmonary function by measuring the walking distance in six minutes.It has been proved that 6MWT has a better correlation with NewYork Heart Association functional calss (NYHA calsss) and peak oxygen uptake (PVO2) of cardiopulmonary exercise test (CPET). In the last 20 years, it has been widely used in evaluating the cardiopulmonary function in adult patients with chronic heart failure (CHF) and chronic lung diseases and their rehabilitation trainining. Recently, few reports on 6MWT in healthy children and children with congenital lung disease were shown, but studies about 6MWT in evaluating the cardiac function in postoperative children with congenital heart disease (CHD) haven't been reported.In this study, we aimed to explore the assessing value of cardiac function in postoperative children with CHD by 6MWT, and to analyse the correlations between the results of treadmill exercise test, echocardiography and the electrocar-dio-parameters in 6MWT.Methods1.Subjects1.1 CHD group:89 children diagnosed with CHD and received heart surgical treat-ments during 1991 to 2008 in Children's Hospital of Fudan University cardiovascular center. All cases were devided into 3-5 age-group (39 cases, aged 4.6±0.8 years old), 6-8 age-group (27 cases, aged 6.9±0.8 years old),9-12 age-group (11 cases, aged 10.4±0.9 years old) and 12-15 age-group (12 cases, aged 13.8±1.5 years old); or clas-sified into the complex postoperative CHD group (29) and the simple postoperative CHD group (60).1.2 Control group:133 health children confirmed by routine physical examination, electrocardiogram, echocardiography and pulmonary function test during July,2009 to February,2010 in the same hospital. They were also devided into 3-5 age-group(26 cases, aged 4.3±0.7 years old),6-8 age-group(39 cases, aged 7.2±0.9 years old),9-12 age-group(39 cases, aged 10.1±0.8 years old) and 12-15 age-group(29 cases, aged 12.8±0.7 years old).2. Test methods2.1 Process of 6MWT Test was conducted on a flat and straight corridor of 30.5m, and subject was asked to walk as far as he or she could in 6 minutes. The six-minute walking distance (6MWD), respiratory rate, heart rate, blood pressure, percutaneous oxygen saturation (SPO2) and ECG were recorded during the test. When children ap-peared fatigue, shortness of breath, cold sweat, chest pain, and so on, the test need stop.2.2 Treadmill exercise test (TET) TET was carried out according to the modified Bruce protocol. The heart rate, blood pressure and METs were documented. The car-diac chronotropic dysfunction was analysed with one of the standards as follows:①maximal heart rate less than 85% age-predicted heart rate (220-age) beat/min;②heart rate reserve less than 80%;③cardiac chronotropic index less than 0.8.Statiscical analysisAll the results were analyzed on SPSS 13.0 software. Values were expressed as mean±sd or ratio. The t test, Chi-square test, analysis of variance, Pearman's correla-tion analysis were used and P value less than 0.05 was considered significant.Results1. Feasibility of 6MWT 89 children in CHD group and 133 children in control group easily finished 6MWT. No accident happened, such as dyspnoea, chest pain, syncope and so on.2. Outcomes of 6MWT in healthy children2.16MWD The difference in 6MWD showed in 3-5,6-8 and 9-12 age-groups (404.4±59.0m vs 497.2±72.0m vs 557.6±66.8m, P<0.01), but no difference was found between 9-12 and 12-15 age-groups (P>0.05).2.2 Physiological parameters in 6MWT In each group, after 6MWT the respiratory rate and heart rate got much faster, and as the same the systolic blood pressure (SBP) increased, while the diastolic blood pressure (DBP) and SPO2 changed, but no differ-ence was found.2.3 Electrocardio-parameters in 6MWT Before 6MWT the Pwd and QRS duration and QTd in healthy children were 0.019±0.067s,0.073±0.016s and 0.016±0.011s respectively; after 6MWT they became a litter shorter than before (0.013±0.008s, 0.071±0.0129s and 0.015±0.012s) respectively, but no difference was found (P>0.05).3. Outcomes of 6MWT and TET in post-operative children and healthy children3.1 6MWD The 6MWD in CHD group aged 3-5 and 6-8 years old were signifi-cantly shorter than that in control groups respectively (383.7±52.8m vs 404.4±59.0m, P=0.044; 442.2±58.8m vs 497.2±72.0m, P=0.002), but no differ-ence was found between the 9-12 and 12-15 age CHD group and control group.3.2 Physiological parameters of 6MWT in CHD and control group There was no difference showed in respiratory rate, heart rate, SBP and SPO2 during 6MWT in 3-5 age CHD and control groups. The basic respiratory rate in CHD group aged 6-8 years old was faster (25±3 beats/min vs 24±3 beats/min, P= 0.026) but the heart rate was lower (97±12 beats/min vs 101±15 beats/min, P=0.021) than that in the control group, and after 6MWT the respiratory rate and heart rate increased in both groups, but no difference was found. The basic heart rate in CHD group aged 9-12 and 12-15 years old were lower than that in the control groups, but after 6MWT no difference was found. The respiratory rate and the heart rate in simple and complex CHD group showed no difference during 6MWT.3.3 Electrocardio-parameters of 6MWT in CHD and control group The Pwd showed no statistical difference during 6MWT in each group. The QTd in 3-5 and 9-12 age CHD groups were longer than that in control groups before 6MWT (0.025±0.018s vs 0.012±0.011s, P=0.004;0.029±0.014s vs 0.019±0.012s, P=0.019), but no statistical difference was found after 6MWT. Compared with the control groups, the QRS dura-tion in 6-8 and 12-15 age CHD group were longer (P<0.01). Compared with the simple CHD group, the QRS duration became wider in complex CHD group before (0.104±0.026 vs 0.073±0.017, P=0.001) and after (0.105±0.027 vs 0.080±0.020, P=0.003) 6MWT.3.4 TET in CHD and control group The maximum heart rate, METs and heart rate reserve in 3-5 and 6-8 and 9-12 age group with CHD were lower than that in the paired control groups, while there were no difference in 12-15 age group.In complex CHD group, the maximum heart rate and heart rate reserve were lower than that in the simple CHD group (0.61±0.19 vs 0.72±0.18, P=0.025), while the METs, and the cardiac chronotropic were found no statistically difference.4. Influence factor of 6MWT In healthy children, age, height and weight had obvious positive correlation with 6MWD (r=0.650,0.717 and 0.520, P<0.01),and also the METs and heart rate reserve (r=0.543,0.370, P<0.01), except the body mass index (BMI) (r=0.084, P=0.338). In CHD group, the follow-up time, heart rate reserve and METs also had a positive cor-relation with 6MWD (r=0.661,0.383,0.486, P<0.01), while the ejection fraction (EF), the left ventricular fractional shortening (FS) and the age at operation had no correla-tion with 6MWD (r=0.136,0.180,0.100, P>0.05).Conclusions1.6MWT can be used as a safe and convenient sub-maximal exercise test in chli-dren.2.6MWD had obvious positive correlation with METs, so 6MWT can be used as an effective method in assessing the cardiac function and sub-maximal exercise capacity in children with CHD.3. The widened QRS duration and increased QT dispersion in 6MWT in postopera-tive patients with CHD suggested that their heart electric activity is still unstable.