Background: Aortic pulse pressure can be partitioned into the height of the first systolic shoulder (P1) and augmentation pressure AP. P1 is thought to be determined by an outgoing pressure wave generated by ventricular contraction and AP by a backward wave “reflected” from the distal circulation. Augmentation index (AIx = AP / cPP) is commonly used to quantify wave reflection. Nitroglycerin (NTG) has a powerful effect to reduce AIx which has been attributed to a reduction in wave reflection. The objectives of this study were to examine the contribution of forward and backward waves to AIx at rest and after administration of NTG.

Methods: A ComboWire 9500 catheter (VolcanoCorp, USA) with a Doppler probe and a pressure sensor at the tip was placed in the aortic root in 21 subjects (11 men, aged 45–81). Simultaneous measurements of aortic blood flow velocity and blood pressure were made at baseline and after the admission of sublingual NTG (400 μg). Using wave decomposition, AIx was expressed as the summation of forward and backward components, F_AIx and B_AIx respectively.

Results: AIx decreased by 17.3% (from 39.5±3.6 to 22.2±5.0%, P<0.001) after NTG. The decrease in AIx was attributable to a similar decrease in both forward and backward components (decreases in F_AIx and B_AIx by 8.7% and 8.6% respectively (P<0.05).

Conclusions: These results suggest the forward wave is a major determinant of AIx and that the role of reflection in mediating effects of NTG may be less than previously thought.