Evaluation of impedance based indices of cardiac contractility in dogs

Cardiac function can be monitored simply and safely by the impedance method. A number of parameters that reflect cardiac contractility can be derived from the impedance waveform. These include the systolic time ratio (STR), the index of contracticity (IC), the acceleration index (ACI) and the Heather index. This study evaluates their reliability.

In sixteen anaesthetized dogs an ultrasonic flow probe was placed on the ascending aorta using a left thoracotomy approach and catheter placed in the femoral artery to measure blood pressure. This allowed the reference measurement of cardiac contractility from blood flow (dF/dt(max)) and pressure (dP/dt(max)). Comparative 1-minute impedance based measurements were made by a RheoCardioMonitor (ACMA, Singapore), whilst contractility was increased 2 to 6 fold, over 113 (52 to 212) minutes, using dopamine and adrenaline infusions. The association between the reference and impedance measurements was determined by correlation. The correlation coefficients (r) were compared using paired t-tests. Results are presented as mean +/- SD.

The ACI (r = 0.76 +/- 0.13) and Heather index (r = 0.74 +/- 0.14) were more closely associated with the reference measurement (dF/dt(max)) than IC (r = 0.65 +/- 0.23) (p < 0.05) and STR (r = 0.33 +/- 24) (p < 0.01). Results for ACI and the Heather index were similar. STR was unrelated to the reference method in 10 out of 16 experiments. Correlation was better when using flow probe data (dF/ dt(max)) than arterial pressure data (dP/dt(max)) (p < 0.05).

ACI and the Heather index were the most reliable impedance derived indices of cardiac contractility.