Aortic coarctation : Physiological and model studies

Abstract: In 22 patients, referred for the assessment of significant native- or re-coarctation, the systolic cuff blood pressure difference between arm and ankle at rest and after three different subrnaximal exercise tests and one maximal test was compared with invasive blood pressure. Reference values of cuff blood pressure were obtained from 19 healthy adult volunteers. The cuff blood pressure difference at rest correlated closely with the invasive pressure difference and the degree of constriction as assessed by angiography. The cuff blood pressure difference one minute after the different exercise tests varied with the load. A pressure difference of 50 mm Hg one minute after submaximal treadmill exercise identified all individuals with an invasive pressure difference exceeding 50 mm Hg during supine exercise. After maximal exercise, a large difference was seen between arm and ankle in healthy subjects as well as in patients.Theoretical analysis and pressure measurements across a constriction in a physical flow model showed a relation between mean pressure and flow that could be expressed as a power function. Pressure recovery was 0-4 mm Hg in the model and of similar magnitude in patients.Simulations in a computer model of the central circulation showed that the downstream pressure and flow depended strongly on the properties of the collaterals. The length and diameter of the collateral influenced the transmission of pressure and flow, while collateral wall stiffness did not. The resistance and wall stiffness in the upstream circulation exerted an important influence upon the upstream pressure.Twenty patients, of whom 16 had undergone coarctation surgery, were investigated with bi-plane transoesophageal echocardiography (TEE) as well as with continuous wave Doppler from the suprasternal notch and magnetic resonance imaging (MRI). Seventeen healthy volunteers were investigated with MRI to obtain reference values. MRI in the axial plane showed the largest coarctation diameter, mean difference between methods 1.4±3.5 mm. Coarctation systolic velocity was 0.23 m/s higher with Doppler than with MRI. MRI peak flow ratio between the descending and ascending aorta showed a linear correlation with Doppler velocity and is proposed as a new measure of obstruction to flow.Twenty-four-hour non-invasive ambulatory blood pressure monitoring, performed on the 20 patients mentioned above, showed a significant negative correlation between the systolic blood pressure level and coarctation diameter, suggesting a remaining influence of the coarctation on the blood pressure even in patients who had undergone surgery.