Increased Aortic Characteristic Impedance Explains Relations Between Urinary Na+/K+ and Pulse or Systolic Blood Pressure.

Abstract

Alterations in sodium (Na+) relative to potassium (K+) intake increase systolic blood pressure, effects in-part attributed to enhanced pulsatile loads (pulse pressure) beyond steady-state pressures (mean arterial pressure). Whether this effect is through reversible changes (increases in blood volume and hence aortic flow [Q] or wave reflection [Pb]), or potentially irreversible structural changes in the proximal aorta, is unknown. In 581 black South Africans, we determined 24-hour urinary Na+ and K+ excretion and aortic function from central aortic pressure (radial pulse wave analysis [SphygmoCor software]), velocity, and diameter measurements. Proximal aortic function was assessed from characteristic impedance (Zc). Beyond mean arterial pressure and additional confounders, urinary Na+/K+ was independently associated with Zc (P<0.005) but not peak aortic Q (P=0.30) or alternative aspects of Q or ejection volume. Although age was strongly associated with proximal aortic diameter, no independent relations between urinary Na+/K+ and aortic diameter were noted (P=0.17). Relations between urinary Na+/K+ and Zc translated into independent relations with early systolic compression wave pressures (QxZc [PQxZc]) and aortic forward wave pressures but not Pb. Moreover, neither reflected wave magnitude (P=0.92) nor aortic pulse wave velocity were independently associated with urinary Na+/K+. In product of coefficient mediation analysis, the independent relations between urinary Na+/K+ and peak aortic or brachial pulse pressure or systolic blood pressure were accounted for by Zc and PQxZc. In conclusion, abnormalities in Na+/K+ intake determine pulse pressure or systolic blood pressure beyond mean arterial pressure mainly through potentially irreversible impacts on proximal aortic impedance rather than readily modifiable increases in aortic flow (blood volume) or wave reflection.