Noninvasive determination of aortic input impedance and external left ventricular power output: a validation and repeatability study of a new technique.

OBJECTIVE: The study was designed to test whether aortic input impedance and left ventricular power output can be accurately assessed noninvasively.
BACKGROUND: Aortic input impedance describes both the pulsatile and nonpulsatile artery load encountered by the left ventricle. Until now, this measure of afterload has only been determined by invasive techniques.
METHODS: The aortic pressure wave was estimated by recording the calibrated carotid artery pressure wave noninvasively with use of a micromanometer-tipped probe by the technique of applanation tonometry. Flow was determined with pulsed wave Doppler measurement of ascending aortic velocity profile and aortic diameter. In 18 subjects undergoing cardiac catheterization, invasive measurements were taken to assess the accuracy of noninvasive data. In 17 other subjects noninvasive measurements were taken on different days to assess the reproducibility of results.
RESULTS: Noninvasive pressure measurements correlated well with invasive data: systolic pressure (mm Hg), noninvasive 126 +/- 28 versus invasive 127 +/- 28, r = 0.96, p less than 0.001; diastolic pressure (mm Hg), noninvasive 71 +/- 10 versus invasive 66 +/- 7, r = 0.60, p less than 0.02; augmentation index (%), noninvasive 23.9 +/- 9.3 versus invasive 30.7 +/- 11.9, r = 0.87, p less than 0.001. Doppler-measured cardiac output was closely correlated with invasively measured flow (liters/min): Doppler, 5.3 +/- 1.2 versus invasive, 5.5 +/- 1.3, r = 0.98, p less than 0.001. Impedance and left ventricular power variables calculated from noninvasive and invasive techniques were also closely related: systemic vascular resistance (dynes.s.cm-5), noninvasive 1,479 +/- 488 versus invasive 1,502 +/- 498, r = 0.91, p less than 0.001; characteristic impedance (dynes.s.cm-5), noninvasive 137 +/- 52 versus invasive 136 +/- 79, r = 0.92, p less than 0.001; pulsatile power (mW), noninvasive 249 +/- 94 versus invasive 291 +/- 103, r = 0.91, p less than 0.001; mean power (mW), noninvasive 1,107 +/- 319 versus invasive 1,144 +/- 266, r = 0.93, p less than 0.001. Repeated measures of impedance variables and power output showed coefficients of variation of less than 9%.
CONCLUSIONS: Measurement of noninvasive impedance by this technique provides an accurate and repeatable assessment of mean and pulsatile cardiac load.