By Paul Mazurek
As promised, we continue our previous discussion with respect to basic operation and troubleshooting of the Abiomed® ventricular assist device. Recall that our patient is a 58-year-old male who underwent a 3-vessel coronary artery bypass graft (CABG). The patient was unable to wean from the cardiopulmonary bypass circuit and a BVS system was placed to assist the left ventricle.
Upon arrival to the bedside, the crew finds the patient on a ventilator with settings that are providing good chest wall compliance, adequate oxygenation and ventilation per clinical assessment parameters and blood gas readings (although his last lactate level was 6.4 mmol/liter). He has two mediastinal chest tubes. He has received four units of type-specific packed red blood cells (PRBCs) since arriving from the operating room.
The Abiomed console reads flows of 2.1 liters per minute and a rate of 52 beats per minute. His vital signs include an ECG heart rate of 116 beats per minute (sinus), a right radial art line blood pressure of 82/42 (mean arterial pressure of 55 mm Hg) with a rate of 52 beats per minute and an oxygen saturation of 91 percent (FiO2 of 1.0). He has a right internal jugular triple lumen catheter. His last central venous pressure (CVP) reading was 4 mm Hg and his last central venous oxygenation (ScvO2) reading was 59 percent. Is this a “transport as-is” or “troubleshoot and stabilize” situation?
Qualifying and quantifying
The complexity of this patient transport requires a high degree of clinical expertise and an extremely competent critical care transport crew. This team may be one in the same. However, the transport team may realize that a transport such as this might be out of their scope of practice from the perspective of routine operation, logistics and troubleshooting of the ventricular assist device. Eliciting the assistance of an “expert” such as a perfusionist or VAD (Ventricular Assist Device) credentialed Registered Nurse would probably be the best option. Understanding one’s limitations while ensuring patient and crew safety during transport is what signifies a true professional in the critical care transport environment, regardless of degree of training or scope of clinical practice.
Stabilizing our patient prior to transport
Two things about this device and how it interfaces with the patient need to be understood in order to answer the question of whether or not this patient is stable for transport. First, it is important to understand that venous return for the Abiomed is entirely preload dependent, which implies that the patient may need special positioning in flight and fluids may be needed to increase preload. The transport team needs to take this in to consideration and must have appropriate fluid (i.e., packed red blood cells or 5 percent albumin preferably) and in adequate amounts based upon patient needs and transport time.
Second, optimizing cardiac output with the assistance of the Abiomed requires certain “target hemodynamics” being met. At minimum, the following parameters should be maintained:
• MAP 70-80 mm Hg
• CVP 12-16 mm Hg
• Cardiac Index > 2 liters/min/m2
• Or Cardiac Output > 4 liters/min
Additional parameters, should they be available, to follow and maintain include:
• Left Atrial Pressure / Pulmonary Capillary Wedge Pressure (PCWP)
• Systemic Vascular Resistance (SVR)
• Pulmonary Vascular Resistance (PVR)
Back to our patient
As you will recall, our patient’s current cardiac output (CO) is 2.1 liters/minute (i.e., abiomed flow), his CVP is four mm Hg (recall target hemodynamics), and his ScvO2 is 59 percent (normal is 70-90 percent; refer to our resuscitation endpoints discussion several months ago for a refresher). Based upon this, our patient needs a “tune up” prior to transport.
So how do we do this? It is important to understand how the ventricle or blood pump performs its job:
• Diastole is the duration of time necessary to fill the ventricular chamber. As previously stated, the pump adjusts to changes in preload (i.e., volume). Therefore, time spent in diastole shortens with increasing preload.
• Systole is the period of time necessary to eject about 80 ml of blood. The pump adjusts to changes in afterload (i.e., changes in SVR). Therefore, systole lengthens with increasing afterload.
With the understanding of this, we now know that we must manipulate preload with volume and afterload with medications affecting SVR. In our current patient situation, increasing preload with volume to increase our CVP in to our target range is our first course of action. Keep in mind that this VAD was placed to assist a failing left ventricle (LV). In all such cases, there is significant and ongoing danger of right ventricular (RV) failure. Overzealous fluid administration could easily lead to RV failure. Without the benefit of a pulmonary artery catheter (PAC), the only indicator we have of impending RV failure is the CVP. When CVP values approach or exceed 20, it is reasonable to suspect RV failure as the cause. The need to be careful and gradual with volume administration cannot be overemphasized. RV failure is a catastrophic occurrence, especially in the presence of LV failure.
After volume replenishment, we will examine the afterload situation and consider optimizing SVR if flows remain inadequate. ScvO2 should normalize accordingly once target hemodynamics are established.
In this particular patient scenario, one liter of 5 percent albumin are administered followed by two units of type-specific packed red blood cells (PRBC) to attain a CVP of 14 mm Hg. Pump flow increases to 4.5 liters per minute. Mean arterial pressure (MAP) is now 68 mm Hg. The next course of action was to calculate the SVR which was 960 dynes x sec / cm-5
With the knowledge of MAP, CVP and Cardiac Output (CO), SVR is calculated utilizing the following equation:
SVR = (MAP – CVP) X 80
CO
In this particular situation, SVR is normal. Were the SVR to be significantly elevated, flows would more than likely be low. In many patients, especially where high SVRs are accompanied by increased MAPs, use of vasodilating agents is warranted to improve flow. Appropriate vasodilators range from nitroglycerine for situations where SVR is mildly elevated to nitroprusside or nicardipine infusions when SVR is markedly elevated. When considering the physiology of flow versus pressure, it is important to remember that flow is more closely related to end organ perfusion than is pressure. Ordinarily, we are unable to measure LV outflow. The Abiomed technology, unlike many other VAD units, provides the operator with measured (not calculated) flows. Given the ability, as we have here, to measure blood flow, the usefulness of Mean Arterial Pressure is of considerably less value. This is an important hemodynamic concept and one well illustrated here.
Hence, in the setting of hypotension that persists after restoration of adequate fluid volume status, where we might ordinarily reach for a pressor agent, caution is warranted for two reasons. Firstly, flow values are reflecting adequate perfusion. We can confirm this by improved ScVO2 values. This suggests that the MAP, while lower than normal, is in fact adequate. Secondly, VAD units operate on preload and gravity. Any resistance, including increases in afterload induced pharmacologically (by initiation of a pressor infusion), will decrease VAD flows. Pressors nearly always tend to be counterproductive to improving perfusion in VAD patients. The air medical team transports the patient to the receiving center with a MAP of 65 mm Hg and a CVP of 16 mm Hg. ScvO2 upon arrival to the receiving ICU is 66 percent. The patient is further stabilized in the ICU and is switched in the operating room to a more permanent ventricular assist device which will allow him to go home after several weeks.
Conclusion
With the ever increasing complexity and acuity of patients needing transport from point A to point B, it is imperative to send the appropriate team and crew configuration for the job. Equal to logistical and operational knowledge is a strong handle on pathophysiology. The abiomed is merely one device used in the battle against the failing heart.
References
1. Abiomed Clinical Reference Manual: Circulatory Support Systems. Abiomed Inc. Danvers, MA. 2006
2. University of Michigan. CVCICU guidelines for managing Abiomeds. UMHS Health System. Last revised 2007.
3. University of Michigan: Survival Flight Medical Protocols. Abiomed Transports. UMHS Health System. Last revised September 2008.
