Analysis of Aortic Energetics from Pulse Wave Examination in a Porcine Study of Septic Shock

Aortic pressure (Pao) can be separated into two components representing the wave phenomena of flow, called excess pressure (Pex), and the storage capacity of the elastic arteries, called reservoir pressure (Pwk). Subject-specific aortic models, identified from measurements from a porcine study on septic shock, were used to calculate the hydraulic work associated with the excess and reservoir pressures (Wex, Wwk). Changes in these energies were compared to a metrics derived from left ventricular pressure-volume analysis. Total aortic work (Wao=Wex+Wwk) compared well to clinically assessed left ventricular work (R2=0.88). However, only a weak relationship of R2 = 0.24 was found between ventricular arterial coupling (Ees/Ea) and Wex/Wwk. Although a strong relationship (R2 = 0.76) was noticed between the inverse of afterload (1/Ea) and Wex. As septic shock progressed, a drop in Wwk was seen, indicating the arterial system loses its ability to store the stoke volume (SV) from the ventricle for release during diastole, resulting in a flattening of the diastolic pressure. These results indicate that one of the main reasons left ventricular afterload decreases during septic shock is because arterial system loses its ability to act as a storage reservoir.