There is a critical addendum to yesterday’s post, courtesy of Sam Schuberg. Patients with LVAD’s can go into dysrhythmias (VT, VF) despite the appearance of stability. Obtain an ECG on arrival in unstable patients with an LVAD. Thanks for reading the pearl, Sam.
Now, on with today’s #TRPearl
There are four parameters the LVAD monitor will provide you with: speed, flow, power, and pulsatility index (PI). Speed is reported in RPM’s, and is determined by the operator. The other values are dependent variables:
Power, measured in watts, represents how much work the LVAD has to do to produce a set number of RPM’s. Power and flow are directly related. However, if power increases (LVAD is doing more work to maintain the same RPM) and flow remains the same, think about pump thrombosis.
Flow, measured in L/min, is related to both the pump speed (RPM) and the pressure differential (“pressure head”) across the pump. For example, if the patient becomes septic and systemic vascular resistance drops, flow will increase without a change in the pump speed.
Pulsatility index (PI) is the most abstract of these parameters, but may be the most clinically relevant. To produce this value, the LVAD measures the flow pulses from native left ventricular contractions, and averages them over a 15 second interval. The PI demonstrates the relationship between the native LV function and the amount of ventricular unloading the pump does. If you increase the pump speed, the PI falls. If the native LV function falls, the PI will also fall. In pathologic states that make the native cardiac output fall such as decreased preload through hypovolemia/hemorrhage, the PI will also fall. Low PI (patient should know what their baseline PI is) should clue you into the possibility of hypovolemia, hemorrhage, or acute worsening in native LV function.
I’ll spend more time in part three going through how to apply these values clinically.
References:
Pratt, Alexandra K., Nimesh S. Shah, and Steven W. Boyce. “Left ventricular assist device management in the ICU.” Critical care medicine 42.1 (2014): 158-168.
Slaughter, Mark S., et al. “Clinical management of continuous-flow left ventricular assist devices in advanced heart failure.” The Journal of Heart and Lung Transplantation 29.4 (2010): S1-S39.
