Input impedance of the lower abdominal aorta in chronically instrumented fetal sheep

Five fetal sheep (gestational age, 118 to 125 days) were instrumented to measure impedances of the lower abdominal aorta. Four days after surgery, flow and pressure pulses were recorded during control conditions and during infusion of norepinephrine (0.3 to 3 micrograms.min-1.kg-1 body weight) or angiotensin II (0.2 to 3 micrograms.min-1.kg-1). The protocol was repeated after injection of hexamethonium (10 mg.kg-1). Moduli and phases of impedances for the first ten harmonics were calculated by fast Fourier transformation. During control, input resistance was 4300 +/- 940 dyne.s.cm-5 (mean +/- SD) at a mean blood flow of 13.3 +/- 2.4 cm3.s-1 and pressure of 54,960 +/- 6980 dyne.cm-2. Moduli fell to 50% of input resistance between 2 and 3 Hz and, declining continuously, reached a minimum of 20% near 10 to 12 Hz, then increased slightly to 30% at about 30 Hz. At the first three harmonics, flow was always leading pressure. Infusion of angiotensin or norepinephrine increased impedance moduli significantly. Resistance increase was largest with angiotensin (19,800 +/- 11,370 dyne.s.cm-5), but no difference was detectable between angiotensin and norepinephrine when related to the same increase of input resistance. The position of the minimum seemed to be unchanged at high resistance values, but relative impedance moduli were smaller than during control, and low-frequency phases were significantly more negative. An analog of inertance, compliance, and resistance to steady flow was used to simulate impedances, and the effect of resistance increases on flow waveforms in the fetal abdominal aorta was calculated.(ABSTRACT TRUNCATED AT 250 WORDS)