Measurement of cardiac output by impedance cardiography under various conditions.

The suitability of impedance cardiography as a method to follow changes in cardiac output (CO) was investigated by comparing it with thermodilution and by measurements in conditions with predictable effects on CO. The correlation between absolute CO values obtained by impedance and thermodilution techniques was moderate (r = 0.61), but the percentage changes showed a good correlation (r = 0.84). Head-up tilting decreased CO by 25% in 5 normal subjects. Bloodletting in three subjects caused a 20% decrease, reinfusion in one subject a 16% increase. Occlusion of artificial large flow arteriovenous shunts in 6 dialysis patients caused a 12% decrease in CO, whereas the effect was less with moderate and absent with small flow shunts. Ultrafiltration caused a 28% decrease in two dialysis patients. In 20 hypertensive patients CO dropped 25% after salt depletion; propranolol administered intravenously gave a 15% decrease (n = 13), diazoxide intravenously a 30% increase (n = 6). It is concluded that, provided conditions are strictly defined, impedance cardiography is a reproducible method and suitable for the measurement of intrapatient changes in cardiac output.