Echo Z-Score and Normal ECG Project

Echocardiography (echo) is crucial for the evaluation of children with congenital and acquired heart diseases. It is the primary imaging modality for establishing diagnoses, determining treatment options, monitoring disease progression, and assessing the effects of intervention. The sizes of cardiovascular structures are frequently affected by the abnormal hemodynamics of a disease state, particularly in children. Because treatment decisions rely on an accurate determination of cardiovascular size, quantification is an important component of the echo study. Reliable, accurate, and generalizable normal reference values must be readily available for clinicians to distinguish a normal finding from an abnormal one and to determine if the size of a structure is adequate, too small, or too large to function effectively. In addition to their importance in the clinical setting, reference values are also used to define the population of interest and as primary or secondary outcomes in research studies designed to evaluate medical or surgical therapies.

Previous studies suggest that measurements in normal children are confounded by body size, age, gender, and race. Adjustments must be made for these factors before the effect of a disease state on the cardiovascular system can be assessed. Echo Z-scores have gained widespread acceptance in both clinical practice and research in pediatric cardiology. Similar to growth charts, they allow comparisons of cardiac measurements obtained for an individual child with measurements obtained from a normal population adjusted for the effects of body size, age, gender, and/or race.

The Z-score tells us how many standard deviations (SDs) the observation is from the mean, and it is positive or negative according to whether the observation lies above or below the mean. It can be calculated from various subpopulations based on a particular set of parameters (including body size, age, gender, and race).

The objectives of this study were:
  • To establish a Z-score database for common echo measurements based on a uniformly defined and racially diverse population of normal children from multiple centers over a wide geographic area
  • To collect electrocardiograms (ECG) from the same population of normal children for future establishment of ECG reference values

Who was in the study?

Patients less than or equal to 18 years of age with echo images in DICOM (Digital Imaging and Communications in Medicine) format from studies performed after January 1, 2008. Patient records must also have documented height, weight, gender, and race.

What happened during the study?

This study was based on a retrospective chart review.
This is what was done in the study:
  • We calculated the mean and standard deviation (SD) for common echo measurements adjusted for body size, age, gender, and race for a large group of normal children using retrospective data from multiple centers.
  • We also evaluated the effects of ethnicity on echo measurements.
  • We developed a repository of ECGs obtained from normal children that will be used to establish pediatric ECG reference values.

What were the results of the study?

  • Age, sex, and race do not have a clinically significant effect on the relationship between the sizes of cardiovascular structures and body size; in other words, age, sex, and race do not have a clinically significant effect on the Z-scores of common pediatric echo measurements
  • Transformed BSA (BSA raised to a specific exponential power) is a good parameter to use for evaluating the effect of body size on the sizes of cardiovascular structures
  • Z-scores for common pediatric echo measurements based on BSA alone are available in an online Z-score calculator

How do I calculate echo z-scores?

There are currently two options for calculating the Z-scores for common echo measurements using the PHN models: (1) with a table listing the exponential power to be used with body surface area (BSA), the mean value of the indexed measurement, and the standard deviation (SD) of the indexed measurement; or (2) with an online Z-score calculator.
  1. Calculating echo z-scores using the table of exponential power for BSA, mean, and SD:
    From the table below, one would take the values listed for the parameter of interest and use them along with the measurement value of the parameter and the BSA in the following formula to calculate the Z-score:


     Parameter  α Mean  SD 
      MVAP 0.50  2.31 0.24
      MVLAT 0.50 2.23 0.22
      MVA 1.00 4.06 0.68
      TVAP 0.50 2.36 0.28
      TVLAT 0.50 2.36 0.29
      TVA 1.00 4.39 0.83
      ANN 0.50 1.48 0.14
      ROOT 0.50 2.06 0.18
      STJ 0.50 1.69 0.16
      AAO 0.50 1.79 0.18
      ARCHPROX 0.50 1.53 0.23
      ARCHDIST 0.50 1.36 0.19
      ISTH  0.50  1.25 0.18
      LMCA 0.45 2.95 0.57
      LAD  0.45 1.90  0.34
      RCA 0.45 2.32 0.55
      PVSAX 0.50 1.91 0.24
      PVLAX 0.50 2.01 0.28
      MPA 0.50 1.82 0.24
      RPA  0.50 1.07 0.18
      LPA 0.50 1.10 0.18
      LVEDD 0.45 3.89 0.33
      LVPWT 0.40 0.57 0.09
      LVST 0.40 0.58 0.09
      LVEDL 0.45 6.31 0.46
      LVEDLEPI 0.45 6.87 0.45
      LVEDA 0.90 11.91 1.89
      LVEDAEPI 0.90 20.00 2.59
      LVEDV 1.30 62.02 11.94
      LVEDVEPI 1.30 113.14 17.85
      LVM 1.25 53.02 9.06
      LVMTV 0 0.88 0.16
      LVTTD 0 0.15 0.03
      LVSI 0 1.63 0.17

    Parameter Abbreviations: AAO=ascending aortic diameter; ANN=aortic annular diameter; ARCHDIST=distal transverse arch diameter; ARCHPROX=proximal transverse arch diameter; ISTH=aortic isthmus diameter; LAD=proximal left anterior descending coronary artery diameter; LMCA=left main coronary artery diameter; LPA=left pulmonary artery diameter; LVEDA=LV (LV) short-axis end-diastolic endocardial area; LVEDAEPI=LV short-axis end-diastolic epicardial area; LVEDD=LV short-axis end-diastolic endocardial diameter; LVEDL=LV long-axis end-diastolic endocardial length; LVEDLEPI=LV long-axis end-diastolic epicardial length; LVEDV=LV short-axis end-diastolic endocardial volume; LVEDVEPI=LV short-axis end-diastolic epicardial volume; LVM=LV mass; LVMTV=LV mass-to-volume ratio; LVPWT=LV short-axis end-diastolic posterior wall thickness; LVSI=LV sphericity index; LVST=LV short-axis end-diastolic septal thickness; LVTTD=LV thickness-to-dimension ratio; MPA=main pulmonary artery diameter; MVAP=mitral anteroposterior diameter; MVLAT=mitral lateral diameter; MVA=mitral area; PVLAX=pulmonary annular long-axis diameter; PVSAX=pulmonary annular short-axis diameter; RCA=proximal right coronary artery diameter; ROOT=aortic root diameter; RPA=right pulmonary artery diameter; STJ=sinotubular junction; TVAP=tricuspid anteroposterior diameter; TVLAT=tricuspid lateral diameter; TVA= tricuspid area.
  2. Calculating echo z-scores using an online calculator:
    The results from the PHN Echo Z-Score Project are available as a Z-score calculator on the ParameterZ website using the following link: http://www.parameterz.com/refs/lopez-circimaging-2017. In the future, the PHN website will have a Z-score calculator that can be easily accessed.