Unidad de Perinatología, Universidad de Carabobo. Ciudad Hospitalaria “Dr. Enrique Tejera”, Valencia, Estado Carabobo, Venezuela.
To demonstrate that the impedance ratio of ductus arteriosus and pulmonary artery could be used as a diagnostic test for preterm, term and post-term pregnancies.Methods
We included 134 pregnant patients, 52 preterm, 65 term and 17 post-term pregnancies. Ductus arteriosus and pulmonary artery blood flow velocity waveforms were investigated in all pregnancies by pulsated-wave Doppler ultrasound.
The study was conducted in Perinatology Unit, Hospitalary City “Dr. Enrique Tejera”, Valencia, Carabobo State, Venezuela between August 1999 and January 2000.Methodology
In a sagittal or axial plane we first obtained the pulsed Doppler wave of the ductus arteriosus flow and measured the acceleration time and ejection time in milliseconds obtaining an impedance index (TA/TE). The same measurement was done for the pulsed wave of the pulmonary artery. The ratio between these two values was counted, the ductus arteriosus impedance index was divided by impedance index of the pulmonary artery. The result was multiplied by 1000 and was called IRIDAPta/te. (Impedance Relative Index between Ductus Arteriosus and Pulmonary artery using acceleration time / ejection time) This procedure which uses a noninvasive tool, reveals a redirection of blood flow from the ductus arteriosus to the pulmonary area as the pregnancy progresses.
This method has been used in our perinatology unit for past eleven years. Since implementation of this method we have not performed amniocentesis to assess lung maturity.Results
The values of relative impedance index between ductus arteriosus and pulmonary artery in the preterm pregnancies were 1.678 (0.407 SD - standard deviation), in term pregnancies were 1.034 (0.326 SD) and post term pregnancies were 0.882 (0.104 SD). The cutoff point equal or below 1.1 (likelihood ratio of 12.5). All cases (100%) with 42 or more weeks of gestation showed values under 1.0. Chi square value was 30.9 with p< 0.05.Conclusion
Along the normal pregnancy, ductus arteriosus and pulmonary artery impedance ratio values show opposite tendency patterns. Ductus arteriosus impedance increases towards term as pulmonary impedance decreases. At term, the relative impedance index values are equal or below 1,1 suggesting a circulatory lung hemodynamic changes in accordance with lung maturity. In post-term patients the impedance index values are below 1.0. We recommend the use of this index as a diagnostic tool in the functional assessment of the gestational age.
Images 1, 2
: PA pulmonary artery, DA ductus arteriosus, RV right ventricle, Ao aorta, AI aortic isthmus, SA subclavia artery. The red circles show the area of Doppler measurement. Image 1 is a sagittal plane of fetal thorax. Image 2 is the short axis of the pulmonary artery.