Air Care Series: Man Meets Machine
/Cases
You are called to transport a 59-year-old male with a history of congestive heart failure on coumadin with a trip, fall, and intracranial hemorrhage. The patient presented to an outside hospital, a 15-minute flight from your level 1 trauma facility, with a GCS of 7 (E1 V2 M4) and a non-contrast head CT demonstrating an intracranial hemorrhage. He was intubated for airway protection and he had no other injuries noted. His INR was 3.0. When asked why the patient is anti-coagulated on coumadin, you are told that the patient has a left ventricular assist device and receives his care at your facility…
You are called to transport a 52-year-old female who suffered a cardiac arrest at the outside hospital. This case is unique, however, because the outside hospital emergency department needs your assistance in assessing the patient’s left ventricular assist device…
Ventricular Assist Device Background
Ventricular assist devices (VAD) are one modality used to manage patients with congestive heart failure and fall into the category of mechanical circulatory support. Despite advanced medical therapy, the diagnosis of heart failure still carries a mortality of 39% at one year and 50% at five years. (1)
In 1966, DeBakey reported the first successful use of a ventricular assist device for post-cardiotomy support. (2) Over 9,000 continuous flow left ventricular assist devices (LVADs) were implanted between 2006 and 2013. (3) Patients may have these devices placed as a bridge to heart transplantation (BTT), recovery of cardiac function, or as destination therapy (DT). In fact, the REMATCH trial demonstrated patients on VAD support had increased survival at one year by 50%. (4) VADs as a form of permanent or destination therapy were approved by the FDA in 2003.
There are various types of devices that are classified based on the type of flow through the device and the location they are implanted. Originally, these devices had pulsatile flow but this limited the device life to approximately 18 months. Then, the concept of a continuous flow device was developed. The first human implant of the MicroMed DeBakey Noon VAD was in Germany in 1998, which was the first continuous flow VAD in man. (5) The Thoratec HeartMate II VAD was the first continuous axial flow pump to be approved by the FDA in 2008. The continuous flow devices have a longer pump life than the original pulsatile flow devices and have been approved for DT.
The focus of this presentation will be on LEFT ventricular assist devices (LVAD) as these patients are able to live independently out in the community, however, providers should be aware that bi-ventricular devices do exist but typically these patients are hospitalized at a mechanical circulatory support and transplant center.
LVAD Structure and Considerations
The specific operative technique of inserting LVADs are outside the scope of this discussion, however, a basic understanding of the technique of insertion can assist in understanding the device. Through a median sternotomy the outflow graft is anastomosed to the lateral wall of the ascending aorta after the patient is on cardiopulmonary bypass. The inflow cannula is placed in the left ventricle. The driveline is then tunneled and fixated so that it can be connected to the controller.
Figure 1: Individual components of the LVAD System
Let’s look at these components individually (Figure 1). Inflow and outflow terminology is based on their relationship to the PUMP not the patient. So, the inflow cannula takes blood from the left ventricle to the pump (IN to the pump) and then blood flows through the outflow cannula to the ascending aorta (OUT of the pump). The pump (Figure 2) houses the rotor which rotates at various speeds (RPM) and generates flow. The driveline is connected to the pump and is tunneled subcutaneously to the controller. The controller is external to the patient and monitors battery life, analytics, and displays alarms. Pressing and releasing the display button on the HeartMate II (Thoratec, Pleasanton, CA) allows for specific information about the functioning of the device as discussed further below. Data can also be downloaded from the controller. At least two lithium- ion batteries are carried on the patient at all times. The device can also recharge with standard power sources. In order to prevent pump thrombus, patients are maintained on warfarin.
Many patients with an LVAD will also have an over sewn aortic valve if they had significant aortic insufficiency at the time of implantation. Patients who are able to communicate should be able to tell you that they have an over sewn valve, however, without contacting a VAD coordinator, this information may not be readily available for patients in extremis. Patients with aortic insufficiency (AI) will likely require oversewing. (6) This is because as the LVAD pumps blood from the left ventricle to the ascending aorta, with severe AI this blood will backflow into the left ventricle again. This could lead to cardiogenic shock with decreased distal organ perfusion. It is important to note that patients with an over sewn aortic valve are completely LVAD dependent for cardiac output.
A patient with a malfunctioning aortic valve and an over sewn aortic valve cannot rely on any cardiac activity to provide forward flow- Even with some contractility, blood cannot move through the left ventricle into the ascending aorta. In the setting of cardiac arrest, if the patient has an overseen valve and the LVAD is not functioning, even with chest compressions, forward flow will not be established. However, when in doubt, in the setting of cardiac arrest and a non-functioning valve, external compressions should be performed.
Figure 2: The LVAD PUMP Components
LVAD Device
The following information can be determined from an LVAD:
Speed
Power
Flow
PI (pulsatility index)
Speed: Pump speed is measured in RPMs. Values typically vary from 8,600 to 9,600 RPMs and is patient specific. The speed is chosen in the operating room on initial placement where the septum appears neutral and the LV appears well offloaded. Speed is a measured value and is the ONLY adjustable parameter.
Power: Pump power is measured in watts. This is the exact amount of power required to achieve a designated rotor speed. Under normal conditions, power should be within a certain range for speed. Increasing speed should increase power and vice versa. Power is a measured value.
Flow: Flow is a CALCULATED value based on motor power and speed. It is measured in L/min. Importantly when looking at a flow value, it is derived from speed and power and the calculated value is only accurate under normal operating conditions.
Pulsatility Index: The PI is a measurement of LV function. A higher PI indicates more LV function. PI events (45% reduction from average PI) lead to an automatic reduction in speed to avoid a suction event and indicate LV collapse.
Table 1: Variables that lead to changes in the pulsatility index
Troubleshooting LVAD Complications
Pump Thrombus: Patients require anticoagulation to hopefully avoid this. Patients with subtherapeutic INR may require heparin infusions and close follow up with their coordinators. The origin of the thrombus can come from a poorly contracting LA or LV or from development on the rotor itself. A thrombus can increase the drag on the rotor and increase the power requirements to continue to turn at a constant speed. PI will also decrease because the power requirement of the pump will be much higher than the native generated contractility. These thrombi can embolize.
Suction Events: If the pump speed is set higher than the relative LV volume, the interventricular septum can collapse which results in flow obstruction and arrhythmias. When the system detects a suction event, the pump speed is decreased to the lowest speed limit of the device. LVAD patients are preload dependent and afterload sensitive so a suction event should lead to volume replacement in the setting of hypovolemia or pump speed adjustments if the patient appears euvolemic. Causes can include:
Hypovolemia
Poor cannula positioning
RV failure (preload)
Pulm HTN (preload)
Tamponade (preload)
Cannula Obstruction: In the setting of an inflow or outflow cannula obstruction, there is a higher pressure in the LV (since blood cannot move from the LV into the ascending aorta, especially in the setting of an over sewn aortic valve). There is decreased flow through the pump and thus decreased power and increased PI. Patients may have symptoms of heart failure and poor end organ perfusion. This can be assessed with an echo or CT with IV contrast. Axial flow devices (such as the Heartmate II) do not have a one-way valve so there will also be back flow and a LV pressure increase in an outflow obstruction as well.
Figure 2: Details Common Pump Problems and the factors differentiating them
Pump Stops: An audible alarm will be heard. This can be due to a hardware malfunction or if the pump has run out of battery (red heart flashes on controller). Batteries should never be changed simultaneously. Be sure to PLUG IN the LVAD power base unit to wall power.
GI Bleed: The most common complication is GI bleeding. GI bleeding occurs in 20 to 40% (7,8) and is thought to be due to acquired von Willebrand’s factor deficiency and mucosal arteriovenous malformations. Both of these complications are thought to originate from the altered physiology of continuous flow. Therapies include endoscopy and reversal of anticoagulation in severe cases. In more minor GI bleeding cases where the patient is hemodynamically stable and has a stable hemoglobin, the risks of reversal may outweigh the benefits. However, these decisions should be made in conjunction with the patient’s VAD center. In cases of an unstable patient with GI bleeding, the patient should be reversed.
Trauma or Spontaneous Bleeding: Given that LVAD patients are anticoagulated, they are at increased risk for hemorrhagic complications. In the unstable patient, the benefits of reversal outweigh the potential risks of pump thrombus and the patient should be reversed. When in doubt or if the patient is otherwise stable, reversal decisions can be made in conjunction with the patient’s VAD team.
Hemolysis: Patients with LVADs may also develop hemolysis which can be assessed with free hgb, total bilirubin, and LDH. Patients can have fatigue, dark urine, and scleral icterus and will also likely have a pump thrombus. Patients who are fully pump dependent may be in cardiogenic shock.
Driveline Infection: An aggressive cleaning regimen is provided to patients to minimize the risk of this infection as a pump exchange or urgent need for transplant can result. Surgical debridement and VAC placement are also utilized as well as antibiotics.
Right Ventricular Failure: Patients are screened for right ventricular failure (RVF) prior to VAD placement but can develop RVF after implantation. It is estimated to occur in 15-20% of patients. (9) Higher mortality is seen secondary to CHF. Patients may require inotrope or RVAD support.
Stroke: LVAD patients are at risk for stroke. Specific treatments will depend on the type, location, and severity of stroke as well as INR. The specific treatment of strokes in these patients will be a multi-disciplinary discussion between the stroke team and the VAD center.
Ventricular Fibrillation (VF) and Ventricular Tachycardia (VT): LVAD patients may develop shockable rhythms, however, given their mechanical circulatory support device may be awake, alert, and oriented. LVAD patients can be defibrillated but in the setting of a stable patient with a shockable rhythm, the VAD center should first be contacted. Many of these patients have implantable cardioverter-defibrillator (ICDs) as well. Heartmate II patients who are unresponsive can be defibrillated per standard protocol.
LVAD Patient Assessment
The initial assessment of the LVAD patient begins in the same way as the non LVAD patient- consciousness, airway, breathing, history, and physical10. In the setting of the unstable LVAD patient, history can be obtained from family or from the patient’s VAD center if this contact information is available. LVAD specific questions for the responsive patient or family can include any alarms, change in parameters already discussed above, and known infections. Bleeding is a common complication of LVAD patients so questions regarding source of bleeding or trauma (even minor) should be asked. Also, the patient may know their INR and a supratherapeutic value may indicate increased risk for spontaneous bleeding.
Hemodynamic assessment can be challenging with the continuous flow devices and an automated blood pressure cuff may be unreliable. Physical exam findings such as peripheral warmth and mental status can be used as a surrogate for a blood pressure number in the absence of an arterial line that provides a mean arterial pressure. (3) If the patient is unresponsive, a Doppler assisted manual blood pressure can be used to obtain a mean arterial pressure. ECGs can be performed to assess for arrhythmias.
In order to assess LVAD function, a good first test is to auscultate. If a mechanical humming is heard, the pump is running. (10) If the VAD is noted to be functioning, plug into a power source, Ensure the driveline cable is plugged into the controller, and provide CPR if the patient is unresponsive without a functioning VAD. If the VAD is functioning and the patient is unresponsive, external chest compressions do not need to be provided. The VAD center may be contacted with emergent questions. For services that have access to ultrasound in transport, an echo may be performed to assess for any native cardiac function. Alarms for each device may be slightly different and the patient or family may have an emergency card that summaries the major alarms noted.
Patients with VADs can be safely flown in rotor and fixed wing platforms as well as ground transport. It is important to note their battery life prior to disconnection from the power base unit prior to transport. Additional batteries may be needed and length of transport should be considered.
Summary:
Left ventricular assist devices provide flow from the left ventricle to the ascending aorta
Some patients will have an over sewn aortic valve
Speed and power are measured values while flow is a calculated value
LVAD patients are anti-coagulated. Unstable patients with bleeding can and should be reversed
When in doubt, external chest compressions may be performed, however, an unresponsive patient with a functioning LVAD do not require external chest compressions
Contact the VAD center early in the course of the evaluation of the stable and unstable LVAD patient to assist with management
Authored by LIZ POWELL, MD; PAIGE BARGER, NP; aDAM gOTTULA, md
Dr. Powell is an Emergency Medicine attending and Critical Care fellow at the University of Cincinnati. Ms. Barger is an Anesthesia NP with the Cardiology Consult service
Posted by Adam Gottula, MD
Dr. Gottula is a second-year resident at the University of Cincinnati
Faculty Editors SUZANNE BENNETT, MD
Dr. Bennett is an attending Anesthesiologist and critical care physician and fellowship director of Anesthesia Critical Care at the University of Cincinnati
References
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