Non-invasive method for rapid bedside estimation of inotropy: theory and preliminary clinical validation

There are numerous techniques which attempt to quantify inotropy (or myocardial contractility). None has yet found general acceptance in anaesthesia and critical care as a practical method. We report a novel approach to the determination of inotropy as a bedside procedure which could identify low inotropy states in patients with clinical heart failure.

We estimated the potential and kinetic energy delivered by the left ventricle using continuous-wave Doppler ultrasound (ultrasonic cardiac output monitor, Uscom, Sydney, Australia) and data available at the point of care. A formula to calculate effective inotropy [Smith-Madigan inotropy index (SMII)] was tested against historical haemodynamic data for 250 control subjects (ASA I patients from preoperative clinic) and 83 patients with acute left ventricular failure (LVF) of New York Heart Association Grade 4 (LVF group). The ratio of potential to kinetic energy (PKR) was investigated as a measure of arterial impedance.

Significant differences were found between the control and LVF groups for cardiac index, mean (range)=3.37 (2.84-5.32) vs 1.84 (1.43-2.26) litre min(-1) m(-2); stroke volume index (SVI), 49.2 (39-55) vs 34.3 (23-37) ml m(-2); systemic vascular resistance, 893 (644-1242) vs 1960 (1744-4048) dyn s cm(-5); SMII, 1.78 (1.35-2.24) vs 0.73 (0.43-0.97) W m(-2); and PKR, 29:1 (24-35:1) vs 124:1 (96-174:1), P<0.001 in each case. Normal ranges were calculated for SMII and PKR as mean (+/-1.96) standard deviations, yielding 1.6-2.2 W m(-2) for SMII, and 25-34:1 for PKR.

The method clearly identified the two clinical groups with no overlap of data points. The discriminant power of SMII and PKR may offer valuable diagnostic methods and monitoring tools in anaesthesia and critical care. This is the first report of normal ranges for SMII and PKR.