Derivation of Indices of Left Ventricular Contractility in the Setting of Continuous-Flow Left Ventricular Assist Device Support

It is important to accurately monitor residual cardiac function in patients under long-term continuous-flow left ventricular assist device (cfLVAD) support. Two new measures of left ventricular (LV) chamber contractility in the cfLVAD-unloaded ventricle include I-Q, a regression coefficient between maximum flow acceleration and flow pulsatility at different pump speeds; and K, a logarithmic relationship between volumes moved in systole and diastole. We sought to optimize these indices. We also propose RIQ, a ratio between maximum flow acceleration and flow pulsatility at baseline pump speed, as an alternative to I-Q. Eleven patients (mean age 49 +/- 11 years) were studied. The K index was derived at baseline pump speed by defining systolic and diastolic onset as time points at which maximum and minimum volumes move through the pump. I-Q across the full range of pump speeds was markedly different between patients. It was unreliable in three patients with underlying atrial fibrillation (coefficient of determination R-2 range: 0.38-0.74) and also when calculated without pump speed manipulation (R-2 range: 0.01-0.74). The K index was within physiological ranges, but poorly correlated to both I-Q (P=0.42) and RIQ (P=0.92). In four patients there was excellent correspondence between RIQ and I-Q, while four other patients showed a poor relationship between these indices. As RIQ does not require pump speed changes, it may be a more clinically appropriate measure. Further studies are required to determine the validity of these indices.