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HeartMate II Anatomy

 

  • Thoratec corporation

  • continuous axial flow device, titanium rotor with helical blades

  • electrically powered motor within the pump housing generates a magnetic field that induces rotary motion and torque

  • the inflow and outflow conduits are coated with a textured material to encourage the deposition of pseudoneointima

  • HMII has a low incidence of thromboembolism; however anticoagulation is required

  • percutaneous lead is coated with woven polyester designed to encourage skin ingrowth

  • can generate upto 10L/min

  • inflow cannula: attached to the left ventricular apex; directs blood to the pump

  • pump — located in the abdomen 

  • outflow graft: flexible conduit that directs blood from the pump to the ascending aorta

  • driveline: subcutaneously tunneled from the epigastrium or RUQ to the pump; contains wires that provide power and operating details to the pump from the controller

  • controller — monitors pump function (flow, speed, power), controls pump speed and manages power supply, records pump data and alarms; displays battery life and function; data can be downloaded for analysis

HeartMate II Physiology

 

Pressure sensitive (output varies depending on pressure differential between pump inflow and outflow); therefore, a change in either preload or afterload can change LVAD pump flow. In certain situations, when a significant change in preload or afterload occurs, excessive suction of the left ventricle can take place. This event will result in decreased flow through the LVAD and can potentially induce ventricular arrhythmias.

 

HMII has two modes of operation:

  • fixed RPM (6,000-15,000)

  • emergency "power-saver" (8,000 RPM or fixed speed if less than 8,000)

 

Flow is a calculated number

  • RPM

  • pump power input

  • hematocrit / blood viscosity

  • except for very low and high pump speeds, there is a linear relationship between flow and power at a given speed

 

RPM

  • pump speed (usually between 8,000-10,000)

  • should be set according to ECHO results

 

Power

  • the amount of power required to power the motor

  • between 0-25.5 watts

  • averaged over cardiac cycle

 

Pulsatility Index (PI)

  • quantifies the continuous flow augmented with each contraction of the native ventricle, creating a variation in flow (flow pulse) 

  • native heart pulsatility contributing to cardiac output

  • dimensionless number between 1-10 (usually between 3-4) and should not vary greatly

  • determined by preload, myocardial contractility and the amount of assistance provided by the device

  • higher PI occurs with ventricular filling (Frank Starling stretch) or the device is providing a lower level of support

  • lower PI occurs with underfilled ventricles or the device is providing a high level of support 

  • "PI event" anything which causes the PI to change by more than 40% within a set time frame (may not necessarily be a suction event)

HeartMate II Pathophysiology

 

  • Right Ventricular Failure

    • common problem in LVAD recipients

    • right ventricular function might improve after LVAD insertion secondary to reduced RV afterload; not always the case:

      • fixed peripheral vascular resistance (as a result of end-stage HF) that does not return to normal when the left ventricle is decompressed

      • LVAD activation may cause a surge in right ventricular preload that is poorly tolerated

      • right ventricle depends on loaded left ventricle to generate adequate contractile force, so decompression of the left ventricle may result in right ventricular dysfunction 

      • RV depressed by the effects of CPB and massive blood transfusions

 

Left ventricular function significantly affects right ventricular systolic function. Experimental studies have shown that about 20% to 40% of the right ventricular systolic pressure and volume outflow result from left ventricular contraction. This dependency of the right ventricle on the left ventricle helps to explain the right ventricular response to volume overload, pressure overload, and myocardial ischemia. The septum and its position are not the sole mechanism for ventricular interdependence. Ventricular interdependence causes overall ventricular deformation, and is probably best explained by the balance of forces at the interventricular sulcus, the material properties, and cardiac dimensions.

HeartWare

Complications

  • infection

  • neuroembolic events (thrombosis)

    • sintered ridged titanium materials promote pseudointimal layer formation

    • axial flow mechanics

    • elevated LDH

    • Treatment:

      • bival​

      • Integrilin (Eptifibatide)

      • tPA + heparin 

      • pump exchange

  • bleeding

  • secondary organ failure

    • pulmonary

    • renal

  • aortic valve fusion

  • thrombosis of coronary sinus

  • arteriovenous malformations / gastrointestinal bleeding

HeartWare

  • RPM usualy 2400-2600 (with RVAD 3000 because less likely bowing of septum)

… And I ask, as the lungs are so close at hand, and in continual motion, and the vessel that supplies them is of such dimensions, what is the use or meaning of this pulse of the right ventricle? And why was nature reduced to the necessity of adding another ventricle for the sole purpose of nourishing the lungs?— —

 

William Harvey, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628

RV Failure

  • RV failure after LVAD has poor prognosis; the degree of pulmonary hypertension (eg PAP does not correlate with symptoms or survival, whereas RV mass and size and right atrial pressure reflect functional status and are strong predictors of survival

  • 3 determinants to RV function:

    • preload

      • decrease RV preload if elevated

    • contractility

      • RV weakens during surgery

      • ventricular septum is responsible for most of the RV contraction

      • make sure LVAD is optimized

    • afterload (PHTN = mPAP > 25mmHg)

  • RV is quite sensitive to acute changes in pulmonary pressures (afterload) (Abel)

    • RV afterload usually decreases after LVAD placement, so this is not it

  • TAPSE does not account for transverse shortening (which accounts for the outcomes)

  • RV function is truely measured with P-V analysis

    • end systolic elastance

    • effective arterial elastance (afterload)

      • systolic pulmonary artery pressure / stroke volume

    • ratio of elastance

LVAD

last updated: February 16, 2016

 

Ventricular assist devices (VAD) aid cardiac function by offloading part or all of the pumping responsibilities from the ventricle. VADs may be placed in the left, right or both ventricles (biventricular).  

 

Indications

No consensus criteria exist for placement of a VAD - indications vary among institutions. However, there are guidelines that exist to optimize patient selection:

  • CI < 2.0 L/min/m2

  • MAP < 60mmHg

  • Cardiac filling pressure (right or left atrium) > 20mmHg

  • Persistent inotropic use

  • VO2 < 12mL/kg/min  despite maximal medical therapy

  • life threatening ventricular arrhythmias

 

Long-Term options

  • bridge to recovery — temporizing measure to optimize ventricular function while awaiting native ventricular function to return (e.g. unable to wean off bypass, transient cardiomyopathy)

  • bridge to heart transplantation

  • bridge to decision — temporizing measure until a decision can be made on one of the above

  • destination therapy if not eligible for cardiac transplantation 

 

Contraindications (Absolute and Relative)

  • Irreversible hepatic or renal failure not due to poor cardiac output

  • coexisting terminal condition

    • metastatic cancer

  • cerebral accident with neurological deficits

  • abdominal aortic aneurysm >5cm

  • active systemic infection

  • severe pulmonary dysfunction 

    • FEV1<1L

    • fixed pulmonary hypertension

  • active coagulopathy

  • inability to tolerate anticoagulation

  • psychosocial inability to manage the device

  • age > 65 years

  • chronic kidney disease

  • severe chronic malnutrition

  • morbid obesity

  • uncorrected aortic regurgitation

  • uncorrected mitral regurgitation

  • uncorrected mitral stenosis

 

2 main types of pump currently in use:

  • pulsatile

    • roller pump - require continuous vigilance and cause more trauma to blood cells; rarely used in VADs

    • rotary pump - no significant hemolysis

      • axial (screwlike)

      • radial (centrifugal)

  • nonpulsatile (continuous)

 

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RVAD placement

LVAD are designed to offload some or all of the work from the left ventricle. It can increase the cardiac output which in turn increases the venous return to the right side of the heart. This increase in right ventricular preload and the continuous emptying of the LV will lead to:

  • right ventricular distention

  • worsening RV contractility

  • increase tricuspid regurgitation

In addition, perioperative cytokine release can cause pulmonary vasoconstriction which placed further stress on the RV.

LVAD Assessment

 

  • Most patients have tag located on their controller around their waist indicating what type of device it is, what institution put it in, and a number to call.

  • pulse may or may not be palpable (impalpable with continous LVAD (DBP increases)

  • always auscultate for LVAD hum 

  • palpate over LVAD to assess temperature (indicator if over working)

  • obtain blood pressure (typically aim for MAP 60-90 mmHg)

    • may be difficult or impossible with standard non-invasive blood pressure measurement

    • intra-arterial line is ideal

    • can use brachial artery doppler to to detect loud contiuous noise during non-invasive blood pressure measure, this corresponds to the MAP

  • check battery life and performance parameters on controller

  • obtain bedside echocardiography 

  • obtain EKG — VAD pateints may tolerate otherwise lethal dysrhythmias (unless RV dysfunction occurs)

  • assess for sepsis (including exit site swabs)

  • call LVAD coordinator

References

  1. http://lifeinthefastlane.com/ccc/ventricular-assist-device/

  2.  

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