The Pre-brief
There are several parameters available on the patient controller module for left ventricular assist devices (LVADs). Rotations per minute (RPM), flow, and power are all rather ubiquitous and straightforward concepts. The 4th common parameter reported is pulsatility index (PI). This corresponds to the magnitude of flow pulse through the LVAD pump. When the left ventricle contracts, the increase in ventricular pressure causes an increase in pump flow during systole. In the Heartmate III LVAD, these flow pulses are measured and averaged over 15 second intervals and reported on the patient controller1.
PI values typically range from 1 to 10. The magnitude of the PI is related to the amount of contractility provided by the native heart as well as the preload entering the LVAD. Higher values represent more ventricular filling and contractility (the LVAD is thus providing less support to the left ventricle). Lower values indicate lower filling pressures and lower contractility (the LVAD is providing greater support and unloading the left ventricle).
The PI should be relatively stable under resting conditions. A drop in PI is either secondary to a decrease in circulating blood volume2 or a marked decrease in native LV contractility3. Given that folks with LVADs have relatively poor contractility (hence the LVAD), it is far more common for a PI drop to be a marker of hypovolemia. This should prompt evaluation of intravascular volume and possible bleeding given the need for anticoagulation in this patient population.
Of note, one single pump parameter is not diagnostic of any particular pathology, and the entire patient presentation and relevant data must be considered as part of diagnosis and management.
The Debrief
The pulsatility index of LVADs represents the magnitude of flow pulse through the LVAD. It is a marker of circulating blood volume and native LV contractility. A fall in PI should prompt evaluation of volume status and native LV function.
References
- Heartmate 3 Left Ventricular Assist System Instructions for Use. Page 1-22.
- Alhama-Belotto, M., Pandey, A., Mohan, R., Heywood, J. T., & Srivastava, A. (2018). Relationship between Pulsatility Index and Volume Status in Patients with Left Ventricular Assist Devices. Journal of Cardiac Failure, 24(8), S112-S113. PMID: 32921219.
- Sen, A., Larson, J. S., Kashani, K. B., Libricz, S. L., Patel, B. M., Guru, P. K., … & Farmer, J. C. (2016). Mechanical circulatory assist devices: a primer for critical care and emergency physicians. Critical care, 20(1), 153. PMID: 27342573.
Finally an article that gives a brief and understandable explanation of the PI.