The calculator below may be used to estimate the LHR  , the o/e LHR, and the  QLI

Quintero and colleagues have proposed, that mathematically, neither the LHR nor the observed/expected LHR are gestational age independent  and should therefore  be used with caution in predicting neonatal outcome. (3)

The Lung area to Head circumference Ratio (LHR),

The lung-to-head circumference  ratio (LHR) is a sonographic measure  proposed to identify fetuses with congenital diaphragmatic hernia (CDH) that have  a poor prognosis (4). The lung area contralateral to the CDH  was originally  obtained by taking  the product of the longest two perpendicular linear measurements of the lung  measured at the level of the 4-chamber view of the heart on a transverse scan of the fetal thorax.  The product is divided by the head circumference (HC) to obtain the LHR.       Lung area =  Length 1  X Length 2 The Lung area to Head circumference Ratio (LHR) =   Lung area / Head circumference

In a cross-sectional study of 650 normal singleton pregnancies Peralta and associates (5) found manual tracing of the lung to be a more reproducible way of measuring the lung area. In addition, the longest diameter method overestimated the right and left lung areas by about 45% compared to the tracing method. Using the tracing method the lung area contralateral to the CDH  is obtained by tracing of the limits of the lung. The Lung area to Head circumference Ratio (LHR) =  Lung area / Head circumference

Observed/expected lung-to-head ratio (o/e LHR)

Peralta and associates (5,7) also found that the left and right LHR increased  exponentially with gestation . Because the LHR is not gestational age independent, as previously assumed , Jani and colleagues (6) proposed the introduction of a new measurement the observed to expected (o/e) LHR, to correct for gestational age.

The observed LHR  may be expressed as a percentage of the expected mean for gestational age as the  Observed/expected lung-to-head ratio  O/E LHR (6,9).

o/e LHR = (Observed  LHR/ Expected LHR ) X 100

Where the Expected LHR using Longest Diameter Method is

Expected Right LHR  in left CDH
=– 3.4802 + (0.3995 X GA) - (0.0048 X GA ^2 ) (Jani)
=– 3.480 + (0.399 X GA) - (0.004 X GA ^2 )   (Dekoninck)

Expected  Left LHR  in right CDH
=- 2.5957 + (0.3043 X GA) - ( 0.0042 X GA ^2) (Jani)

GA = gestational age in weeks

Where the Expected LHR using the Tracing Method is

Expected Right LHR in left CDH 
-2.2481 + (0.2712 X GA)- (0.0033 × GA ^2)  (Perlata 2005)
-2.3271 + (0.27 X GA) -(.0032 X GA^2) (Jani)
-2.356+ (0.272 ×GA) -( 0.003 ×GA^ 2)  (Dekoninck)

Expected Left LHR in right CDH
-1.4815 + (0.1824 X GA)- (0.0023 × GA ^2) (Perlata 2005)
-1.4994 + (0.1778 X GA)−(0.0021 X GA^2)  (Jani)

GA = gestational age in weeks

The Quantitative Lung Index (QLI)

Quintero and colleagues  (3) proposed, mathematically, that neither the LHR nor the observed/expected LHR are gestational age independent. They derived  a new parameter to describe right lung growth expressed as the  quantitative lung index (QLI)  = lung area/(HC/10)^2. The 50th percentile of the QLI is constant at approximately 1.0 for the gestational age between 16-32 weeks. They defined a small lung (<1st percentile) as a QLI <0.6. Further studies are needed to assess the merits of this new parameter .

Acknowledgements

We would like to express our very great appreciation to Dr Ramen Chmait , director of the CHLA-USC Fetal Therapy Program for his valuable and constructive suggestions  in the development of this calculator.

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