Aortic Reservoir Pressure Corresponds to Cyclic Changes in Aortic Volume

OBJECTIVE: Aortic reservoir pressure indices independently predict cardiovascular events and mortality. Despite this, there has never been a study in humans to determine whether the theoretical principles of the mathematically derived aortic reservoir pressure (RP(derived)) and excess pressure (XP(derived)) model have a real physiological basis. This study aimed to directly measure the aortic reservoir (AR(direct); by cyclic change in aortic volume) and determine its relationship with RP(derived), XP(derived), and aortic blood pressure (BP). APPROACH AND RESULTS: Ascending aortic BP and Doppler flow velocity were recorded via intra-arterial wire in 10 men (aged 62 ± 12 years) during coronary artery bypass surgery. Simultaneous ascending aortic transesophageal echocardiography was used to measure AR(direct). Published mathematical formulae were used to determine RP(derived) and XP(derived). AR(direct) was strongly and linearly related to RP(derived) during systole (r=0.988; P<0.001) and diastole (r=0.985; P<0.001). Peak cross-correlation (r=0.98) occurred at a phase lag of 0.004 s into the cardiac cycle, suggesting close temporal agreement between waveforms. The relationship between aortic BP and AR(direct) was qualitatively similar to the cyclic relationship between aortic BP and RP(derived), with peak cross-correlations occurring at identical phase lags (AR(direct) versus aortic BP, r=0.96 at 0.06 s; RP(derived) versus aortic BP, r=0.98 at 0.06 s). CONCLUSIONS: RP(derived) is highly correlated with changes in proximal aortic volume, consistent with its physiological interpretation as corresponding to the instantaneous volume of blood stored in the aorta. Thus, aortic reservoir pressure should be considered in the interpretation of the central BP waveform.