A Left Ventricular Assist Device (LVAD) is a mechanical pump designed to support a failing heart by moving blood from the left ventricle into the aorta. This action helps restore circulation to the body, offering a life-changing option for people with advanced heart failure. The device’s controller constantly monitors and displays a set of numbers necessary for managing the patient’s health and the pump’s performance. Understanding these values allows both patients and clinicians to ensure the device is working correctly and the patient is receiving adequate circulatory support.
Defining the Three Key Parameters of LVAD Operation
The LVAD controller screen displays three main parameters: Rotational Speed, Estimated Flow, and Power.
Rotational Speed (RPM) is the rate at which the pump’s internal rotor spins. This is the only parameter intentionally set by the clinical team, usually ranging from 8,600 to 9,800 RPM. This speed is generally constant unless a clinician makes an adjustment or a safety mechanism is triggered.
Estimated Flow, measured in Liters per Minute (L/min), represents the volume of blood the pump is moving through the circulatory system. This value is an estimation of the cardiac output provided by the device, typically ranging from 4 to 6 L/min. The flow is not directly measured but is calculated by the device’s internal computer using the set speed and the measured power consumption.
Power, measured in Watts (W), indicates the electrical energy the pump requires to maintain its set speed and move the blood. A typical operating power range is between 4 and 7 Watts.
Understanding the Clinical Meaning of Flow Rate
The Estimated Flow rate is considered the most clinically relevant number because it reflects the patient’s immediate circulatory status. Unlike the Rotational Speed, which is fixed by the care team, the Flow rate is dynamic and constantly changes based on physiological conditions. For instance, during physical activity or excitement, the body’s demand for oxygenated blood increases, causing the native heart’s output to rise, which in turn leads to a higher LVAD Flow reading.
Flow fluctuations relate to the volume of blood available to the pump, known as preload. If a patient is dehydrated, the left ventricle may not fill adequately, causing the pump to draw less blood and the Estimated Flow to decrease. Conversely, fluid overload can lead to increased filling pressure in the left ventricle and a higher flow rate.
A drop in Estimated Flow can signal a “suction event.” This occurs when the pump’s inflow cannula sucks against the ventricular wall or septum due to low blood volume, such as from hypovolemia or dehydration. The flow drops because the pump has nothing to pull, and some LVAD models automatically reduce speed temporarily to resolve the issue.
The flow rate is also sensitive to blood pressure, which represents the afterload the pump must work against. A high mean arterial pressure (MAP), generally above 90 mm Hg, increases the resistance for the pump to push blood into the aorta. This increased resistance causes the Estimated Flow to decrease, highlighting why blood pressure management is important for LVAD patients.
Power Consumption, Alarms, and Troubleshooting
The Power number is a diagnostic indicator of the pump’s mechanical health. Stable power consumption suggests the pump motor is running efficiently. Any sustained change in the power required to maintain the set speed warrants investigation.
A gradual increase in Power over several weeks can indicate the formation of a thrombus (blood clot) within the pump. As a clot forms, the motor must work harder against the obstruction to maintain the set RPM, leading to a higher power draw. Conversely, a gradual decrease in power may suggest an obstruction in the inflow or outflow of the pump.
The LVAD controller uses alarms triggered by abnormal power or flow readings. A “low flow” alarm, which sounds if flow drops below a pre-set limit (around 2.5 L/min), requires immediate attention as it suggests inadequate circulatory support. Patients must contact their LVAD coordinator immediately if this alarm sounds.
Other common alarms relate to the power source, such as a “low battery” warning, signaling less than 15 minutes of power remains. For a power alarm, the patient must connect the device to a working power source. Understanding these warnings ensures continuous operation of the device.