### Introduction

### Methods

### 1. Study design and population

### 2. Sampling size and selection

### 3. Selection criteria

### 4. Ethics approval

### 5. Data collection

### 6. Data analysis

^{2}, which was then used to compute z scores for each echocardiographic measure:

*P*<0.05.

### Results

### 1. Anthropometric and physiologic characteristics of participants

*t*=1.522;

*P*=0.129), height (127.00±13.5 cm vs. 127.53±13.2 cm,

*t*=0.447;

*P*=0.655), and unit of BSA inserted (0.98±0.2 m

^{2}vs. 1.00±0.2 m

^{2},

*t*=1.085;

*P*=0.279), the girls were heavier than the boys (BMI: 17.24±2.6 kg/m

^{2}vs. 16.80±1.7 kg/m

^{2},

*t*=0.45;

*P*=0.03). The mean pulse rate was significantly higher in girls (93.75±7.7 bpm vs. 91.33±1.4 bpm,

*t*=3.076,

*P*=0.002) but there was no significant sex difference in their oxygen saturation (97.86%±0.7% vs. 97.85%±0.7%,

*t*=0.199;

*P*=0.843). Intraobserver variability test for each measurement computed using intraclass correlation coefficient showed that all measurements had excellent reliability of 0.968 (95% confidence interval [CI], 0.963–0.973;

*P*<0.001) for RVD1, 0.946 (95% CI, 0.941–0.957;

*P*<0.001) for RVD2, 0.976 (95% CI, 0.971–0.980;

*P*<0.001) for RVD3, 0.923 (95% CI, 0.910–0.934;

*P*< 0.001) for TAPSE, and 0.947 (95% CI, 0.938 –0.955;

*P*<0.001) for S’ with average measures.

### 2. Mean echocardiographic right ventricular dimensions and function of Nigerian school-aged children

*t*= 2.088;

*P*=0.037) but there were no significant sex difference in their RVD1 (32.74±4.0 vs. 33.16±4.3,

*t*=1.118;

*P*=0.264), RVD2 (25.79±3.5 vs. 25.94±3.6,

*t*=0.455;

*P*=0.649), RVD3 (54.59±7.5 vs. 54.55±7.6

*t*=0.058;

*P*=0.953), and S’ (18.43±2.2 vs. 18.05±2.2,

*t*=1.926;

*P*=0.055).

### 3. Anthropometric and age correlates of right ventricular dimension and function

*r*=0.099,

*P*=0.031). Of the 4 anthropometric variables, BSA had the highest correlation with each of RVD1, RVD2, and RVD3; height and weight had similarly the highest correlation with TAPSE. Age correlated significantly with all the indices of right ventricular size and systolic function.

### 5. Regression equations and z scores of echocardiographic measurements

_{0}+β

_{1}X+β

_{2}X

^{2}+β

_{3}X

^{3}where ‘Y’ is the predicted mean, ‘β

_{0}’ is the intercept or constant, ‘X’ is the BSA, and ‘β’ is the regression coefficient. Thus, for a particular BSA, the calculated or predicted value for each measurement can be derived by substituting the values from Table 5. For example,

*z*scores for all echocardiographic variables, using the regression coefficients shown in Table 5. Fig. 1 shows the scatter plots of the

*z*scores of the right ventricular measurements against BSA. The majority of the data points are within-3 and +3

*z*scores; only about 3.6%, 5%, 3.4%, 4.5%, and 4.5% of the population were above and below the ±2

*z*score for RVD1, RVD2, RVD3, TAPSE, and S’, respectively.

### Discussion

*z*scores. These reference values, perhaps the first from healthy Nigerian school-aged children, fill clinically useful gaps in objectively diagnosing, prognosticating, and monitoring Nigerian children with primary or secondary right ventricular disorders.

*z*scores for all measurements were between -3 and +3 using the cubic polynomial regression model, similar to the study by Pettersen et al. [30] depicting as appropriate for our sample With the use of z scores, the magnitude of an abnormality is easily appreciated [14]. This makes z score an important way of expressing cardiovascular measurements as implemented in our study. The z score-based scatter plots for right ventricular dimensions and systolic function showed 95% to 97.8% of the subjects within +2 and -2 z score. A normal range of z score for cardiac measures is defined as +2 and -2 with 0 as the mean [11]. The comparison of z score-based scatter plots for right ventricular dimensions and systolic function with other studies is limited. This is because only a few studies generated z score scatter plots [6,11] while others had charts and curves plotted for z score [7-10,24] using BSA and the measurements obtained. In addition, some of the studies obtained the z score based reference values using various regression models [6,7,9-11] while others did not use regression models [8,23,24]. In computing z scores, a regression model should be selected so that the fit is adequate across the whole population to avoid heteroscedasticity [31].