568528 research-article2015
SCVXXX10.1177/1089253214568528Seminars in Cardiothoracic and Vascular AnesthesiaBlum et al
Article
Postoperative Management for Patients With Durable Mechanical Circulatory Support Devices
Seminars in Cardiothoracic and Vascular Anesthesia 2015, Vol. 19(4) 318–330 © The Author(s) 2015 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1089253214568528 scv.sagepub.com
Franziska Elisabeth Blum, MD1, Gregory Michael Weiss, MD2, Joseph C. Cleveland Jr, MD2, and Nathaen S. Weitzel, MD2
Abstract Mechanical circulatory support devices have been approved as bridge to transplantation, as bridge to recovery, or as destination therapy to treat end-stage heart failure. The perioperative challenges for the anesthesiologist and the intensivist caring for these patients include device-related complications, hemodynamic instability, arrhythmias, right ventricular failure, and coagulopathy. Perioperative management in this high-risk population has a significant impact on patient outcomes. This review focuses immediate postoperative intensive care unit management of device-related complications. Keywords left ventricular outflow obstruction, heart, heparin, intensive care unit, complications The use of implanted mechanical circulatory support has evolved from its initial function, as bridge to transplantation or bridge to recovery, to an increasingly more common-use destination therapy for end-stage heart failure in nearly 44% of cases (400 to 800 per year).1,2 Recent data from the Interagency Registry for Mechanically Assisted Circulatory Support suggest a significant increase in use for destination therapy, as this application was found in 18% of cases before 2001 and in 44% in 2012.2,3 The perioperative management of patients following implantation of a left ventricular assist device (LVAD) has many challenges. The average patient stay in the intensive care unit (ICU) is 9 days following ventricular assist device (VAD) implantation,4 and the most common complications are hemodynamic instability, arrhythmias, right ventricular (RV) failure, infections, inappropriate device settings, and bleeding.5,6 Reported mortality in the ICU is 10% to 32%, with multiorgan failure and sepsis as the most common causes of mortality.6,7 Klotz et al published a predictive scoring system for mortality (Table 1), where a score of 30 points, a high mortality rate of 65.2%.6 Despite immediate risks associated with VAD implantation, the newer continuous flow pumps allow for improved long-term survival, with 80% survival for continuous flow pumps at 1 year and 70% at 2 years.2 This review aims to provide a management overview based on the most critical challenges faced in caring for these patients, including
device-related complications, hemodynamic instability, arrhythmias, RV failure, coagulopathy, and surgery for noncardiac procedures.8-10
Basic Review: VAD Types and the Basics of Operation This discussion is focused on patients receiving the HeartMate II (HMII; Thoratec Corp, Pleasanton, California) or the HeartWare HVAD (HVAD; HeartWare HVAD Inc, Miramar, Florida), as these are the most common devices currently in use. Basic VAD physiology includes an altered blood flow pattern, with blood flowing from the ventricle to the VAD via the inflow cannula and then being pumped to the body and various organs via an outflow graft sewn to the ascending aorta in most cases.11 The first-generation pulsatile flow pumps have been replaced by improved second-generation continuous flow designs in the HMII (Figure 1) and so-called third-generation devices, such as the HVAD (Figure 2). 1
Weiss Memorial Hospital, Affiliate of the University of Illinois at Chicago, Chicago, IL, USA 2 University of Colorado Denver, Aurora, CO, USA Corresponding Author: Franziska Elisabeth Blum, Weiss Memorial Hospital, Affiliate of the University of Illinois at Chicago, 4646 N Marine Drive, Chicago, IL 60640, USA. Email: [email protected]
Downloaded from scv.sagepub.com at UNIV OF VIRGINIA on December 10, 2015
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Blum et al Table 1. Risk Score for Post–Ventricular Assist Device Implant Mortality.6,a Parameters: Cutoff
Risk Score
Transfusion Heart laboratory Lactate dehydrogenase > 500 U/L Creatine kinase > 200 U/L Lactate > 3 mg/dL Inotropic support: epinephrine and/or norepinephrine and/or dobutamine Infectious laboratory C-reactive protein > 8 mg/dL White blood cell count > 13 000/µL Mechanical support Extracorporeal membrane oxygenation support Intra-aortic balloon pump Redo operative procedures Emergent implant Emergency implant Implant postcardiotomy Ventilation Renal impairment Renal replacement therapy Creatinine > 1.5 mg/dL BUN > 40 mg/dL Ischemic cardiomyopathy Preoperative resuscitation Platelets < 100 000/µL Heart rate > 100/min Blood laboratory Hemoglobin < 12 mg/dL Hematocrit < 35% Age > 50 y
6 5 5 5 4 4 4 3 3 3 2 2 1 1 1 1
a Maximum score, 50 points: low-risk score < 15 (mortality, 15.8%); medium-risk score = 16 to 30 (mortality, 48.2%); high-risk score > 30 (mortality, 65.2%).
Both the HMII and HVAD devices are implanted into the beating heart using cardiopulmonary bypass to facilitate optimal surgical conditions, while a side-biting aortic clamp eliminates the need for cardioplegia. Despite the same general approach, there are significant differences between the 2 devices. The HVAD is smaller in size (weighing about 145 g) and can therefore be implanted into smaller patients when compared to the HMII. The HVAD consists of an integrated inflow cannula inserted into the apex of the left ventricle, while the HMII utilizes a synthetic graft as a conduit to the sewing ring. Both devices connect the pump with the aorta via the outflow cannula. A driveline then connects the pump to an external controller, which is a processor unit that sends power and operating signals to the blood pump and simultaneously collects information from it.12,13
Figure 1. HeartMate II LVAD with permission from © 2013 Thoratec Corporation.
The HMII and the HVAD devices generate continuous flow; thus, patients have a greatly reduced, if not absent, pulse.13,14 The HMII is an axial flow pump that generates flow via “pushing” or “propelling” blood, while the HVAD is a centrifugal flow pump that generates flow by “throwing” blood. These 2 concepts are important, as both devices demonstrate slightly different flow characteristics due to these inherent differences and can demonstrate various flow pulsatility patterns, specifically in the postoperative period during periods of volume shifting. An advantage of centrifugal devices is their ability to operate over a range of flows for a small change in delta P (the pressure difference between the left ventricle and aorta), while flow in axial devices increases and decreases with small changes in delta P. This effect causes the axial flow device to increase the negative pressure in the ventricle during low flow, potentially sucking the ventricular wall into the inlet cannula and further limiting flow and increasing the risk for arrhythmias. The centrifugal demonstrates greater variation of pulsatility under conditions of elevated and normal blood pressure.15 In addition, HVAD and centrifugal pumps in general may provide more accurate flow estimation than the axial flow devices, especially at lower speed.16
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320
Seminars in Cardiothoracic and Vascular Anesthesia 19(4) parameters vary based on the set speed, intrinsic cardiac function, volume status, vascular tone, and viscoelastic properties of the blood itself. The optimal operating speed of the device is selected through determining the minimum and maximum speed of the device in revolutions per minute (RPM). The optimal range of VAD speed is determined by monitoring left ventricular (LV) size, the position of the septum, and the degree of aortic valve opening, typically via echocardiography.17 For the HMII, the normal operating range tends to be between 8000 and 10 000 RPM, with flow estimation becoming increasingly inaccurate at or below 8000 RPM.18 For the HVAD, normal clinical operation occurs with RPM between 2000 and 4000. Both the function of the HMII and HVAD devices and their dynamic interaction with the patient’s physiology can be assessed and tracked by observing the provided parameters: flow, power, and pulsatility index (PI) for the HMII.6 To appreciate the contribution of these values, it is imperative to understand how flow is generated and how different physiologic states affect device function. Key to the function of these devices is the concept of differential pressure.
Differential Pressure Figure 2. HeartWare HVAD with permission from HeartWare HVAD Inc. Miramar, Florida.
General Concepts in Postoperative VAD Management The immediate postoperative course for patients following VAD implantation is highly variable, and as such, management must be tailored to the rapidly changing hemodynamic state of the patient. For the hemodynamically stable patient, the standard fast-track approach for weaning from the ventilator should be considered. However, hemodynamic stability depends on 4 key variables, including level of vasodilation or afterload, degree of coagulopathy and ongoing blood loss, preload conditions, and the status of the right ventricle. Each of these variables affects the overall balance of hemodynamics with regard to the VAD and, thus, the decision on how aggressively to wean from respiratory support. RV function, volume, and geometry should be maximized prior to removal from mechanical ventilation, and afterload should be stable, since suspending positive pressure ventilation will increase the already-higher-than-preoperative blood return to the right ventricle. VAD pump management becomes a key point starting at separation from cardiopulmonary bypass, until the patient achieves a stable hemodynamic balance. The only parameter within the control of the intensivist and the only variable that can be changed is the speed. All other
Differential pressure is the difference in pressure between the inflow and outflow cannulas or the difference between LV pressure and aortic pressure: differential pressure = (aortic pressure – LV pressure) + pressure loss across the pump. ∆P =
( Pa −
PLV ) + ∆Ppump
Pressure drop is related to flow but is typically inconsequential. Aortic pressure is typically in the normal range, and as such, the dynamic variable in determining differential pressure is LV pressure or its determinants: volume status or preload and contractile state. The lower the differential pressure is, the higher the flow becomes. In other words, higher LV pressures due to increased contractility and/or increased preload increase pump flow at any given speed. Based on the concept of differential flow, it is possible to understand how changes in physiology, volume status, and heart function alter the parameters displayed on the control unit. Once these parameters are defined, interpretation and intervention can be performed. Parameters displayed on the console for HMII and HVAD differ; the HMII console displays pump flow, pump speed, pulse index, and power, while the HVAD console displays mainly pump flow, pump speed, and power to the user. In the following section, we define parameters for both devices and explain their meaning for device and heart function.
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321
Blum et al Power. Power is simply defined using Watt’s law (power = voltage × current); it is directly measured; and it is equivalent to the amount of work that the device must perform at a given speed. When operated within the recommended speed range (eg, for HVAD, 2400 to 3200 RPM), power ranges between 2.5 and 8.5 watts. When confronted by increases in power, the operator should consider complications that make it harder for the pump to spin: thrombus formation in the device, valid high flows related to vasodilation from any cause, and hypervolemia. Speed. Speed is user defined and adjusted based on hemodynamic and echo findings. Flow. Flow is estimated based on the power, as both parameters normally retain a linear relationship at a given speed; furthermore, the HVAD takes the hematocrit into consideration for estimation of pump flow. However, the flow value becomes imprecise at high and low regions of the power-flow curve; therefore, it is recommended to maintain a minimum of 2 to 4 L/min for pump flow. For example, an increase in power related to thrombus formation in the pump head that is clearly not related to an increase in flow will produce an erroneously high flow. At the other extreme, a partial occlusion of the outflow cannula will reduce flow significantly as well as power, since less blood is moving through the device and less work is being done. Low flows can result from hypovolemia either relative or secondary to bleeding, arrhythmias, or any cause of inflow or outflow obstruction. Pulse Index. PI applies to HMII only. PI is defined as flow magnitude averaged over 15 seconds, typically ranging from 1 to 10. The degree of flow pulsatility is inversely related to the degree of LV unloading and is proportional to the strength of LV contraction; hence, PI can be used to measure LV function.17 An increase of pump support leads to less ventricular filling and a decrease in PI in an otherwise stable patient, reflecting a decrease in circulating blood volume or worsening LV function. A decrease of PI may furthermore indicate hypovolemia or suction event. An increase in PI reflects more ventricular filling and a decrease in differential pressure either due to decreased afterload with preserved blood return to the LV or due to improving LV function. The concept of LV volume and pressure dictating the flow characteristics of the LVAD can be useful in troubleshooting postoperative device malfunction in the ICU. Even with severely depressed function, the LV will still contribute a pressure pulse, lowering differential pressure and increasing flow and pulsatility. Only a heart rendered completely flaccid or one in fibrillation will create a truly nonpulsatile flow pattern. This is the mechanism by which hypovolemia and excessive pump speed reduce flow,
either by not delivering enough blood to the LV or by taking too much away, thus increasing differential pressure. The pressure flow curve demonstrates that relatively small increases in differential pressure cause significant reductions in flow that is, excessive vasoconstriction and severely depressed LV function (Figure 4). Suction events are easily explained based on these principles. Furthermore, centrifugal pumps operate over a wide range of flow with a small pressure change across the pump (Figure 3). Should the LV pressure be sufficiently negative to collapse the ventricle walls (hypovolemia, excessive pump speed, or malpositioned inflow cannula), an effective occlusion of the inflow cannula occurs; power and flow decrease; and the device is designed to “ramp down” its speed until the suction is relieved, at which time it returns to its programmed speed. The converse is a flow state that significantly decreases the differential pressure, such as vasodilation with high pump flow and high return to the LV. The LV pressure may exceed aortic pressure, causing the aortic valve to open and the differential pressure to effectively equal net cannula pressure loss, causing maximum flow and high pump speeds and power values. In addition to the parameters mentioned above, echocardiography plays an important role in troubleshooting for postoperative device dysfunction. Echocardiography is mainly used to evaluate surgical results after LVAD implantation and, more important, to evaluate the reason for postoperative hemodynamic compromise, caused by, for example, thrombosis of the in- and/or outflow cannula, hypovolemia, acute RV dysfunction, cardiac tamponade, and pulmonary embolism. For example, the combination of rightward deviation of the interventricular and interatrial septum, mitral valve regurgitation, aortic valve opening, and a decrease of LVAD flow is indicative of LVAD dysfunction.19 Figure 4 shows a summary of the diagnostic pathway to troubleshoot device problems, as well as potential alarm and treatment options.
Hemodynamic Optimization Hemodynamics following VAD implantation change frequently in the initial postoperative period. VAD speed settings are typically determined in the operating room using transesophageal echocardiography guidance, and attempts should be made to maintain these settings. Hemodynamic instability should be approached via management of preload and afterload conditions first, followed by parameters such as VAD pump speed, cardiac output, filling pressure, and echocardiographic assessment of right and left heart function.5 Dehydration, ongoing bleeding, vasoplegia, sepsis, aortic valve regurgitation, and vasodilating agents may cause hypotension,5, 20 whereas fluid overload, pain, and agents causing vasoconstriction may lead to hypertension.21 The optimal adjustment of the patient’s hemodynamic
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Seminars in Cardiothoracic and Vascular Anesthesia 19(4)
high
Alarm Type Poten al Causes Treatment
low
-Medium
-High -Medium
high
-Fluid Bolus -Transfuse -Address arrhythmias
low
-High -Medium
-Pump thrombosis
-Vasodila on -Hypovolemia -Sepsis -Bleeding -Hypervolemia -Arrhythmias -Suc on event -Thrombus -Hold vasodilators -Pressors -Treat underlying cause -Diur e cs
Speed
Power
Flow
-Consider thrombolysis -Consider devicee exchange -Consider an coagula on
-High -Medium
increase
-Medium
-Pump stop -Lead disconnec on -Low ba ery -System controller in back up mode -Check power supply and device components.
-Cannula Kinking or complete thrombosis
decrease
-Medium
-Suc on event -Controller/p ump dysfunc on
-Consider -Ensure thrombolysis adequate -Consider preload device exchange. ge.
-Treat arrhythmias -Inves gate for VAD occlusion -A erload reduc on -Change controller
Legend • Alarm types -Critical/High: Symbol: None or flashing red Tone: Continuous or two-toned unable to mute Alarm display:VAD stopped, Controller failed, Critical Battery -Advisory/Medium: Symbol: Flashing yellow Tone: Intermittent beep-gradual increase in alarm volume over time if not muted, alarm can be muted for 5 minutes to 1 hour Alarm display: High Watts, Controller Fault, Low Flow, Electric Fault, Suction -Advisory/Low Symbol: Solid yellow Tone: Intermittent beep gradual increase in alarm volume if not muted, able to mute alarm for 5 minutes Alarm display: Low Battery, Power disconnect Figure 3. Troubleshooting device malfunction for HeartWare HVAD.51,52 Alarm types: Critical/high: symbol: none or flashing red; tone: continuous or 2-toned unable to mute; alarm display: ventricular assist device stopped, controller failed, critical battery. Advisory/medium: symbol: flashing yellow; tone: intermittent beep, gradual increase in alarm volume over time if not muted, alarm can be muted for 5 minutes to 1 hour; alarm display: high watts, controller fault, low flow, electric fault, suction. Advisory/low: symbol:solid yellow; tone:intermittent beep gradual increase in alarm volume if not muted, able to mute alarm for 5 minutes; alarm display:low battery, power disconnect.
values includes achieving a target mean arterial pressure (MAP) of 60 to 80 mm HG, keeping MAP
SCVXXX10.1177/1089253214568528Seminars in Cardiothoracic and Vascular AnesthesiaBlum et al
Article
Postoperative Management for Patients With Durable Mechanical Circulatory Support Devices
Seminars in Cardiothoracic and Vascular Anesthesia 2015, Vol. 19(4) 318–330 © The Author(s) 2015 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1089253214568528 scv.sagepub.com
Franziska Elisabeth Blum, MD1, Gregory Michael Weiss, MD2, Joseph C. Cleveland Jr, MD2, and Nathaen S. Weitzel, MD2
Abstract Mechanical circulatory support devices have been approved as bridge to transplantation, as bridge to recovery, or as destination therapy to treat end-stage heart failure. The perioperative challenges for the anesthesiologist and the intensivist caring for these patients include device-related complications, hemodynamic instability, arrhythmias, right ventricular failure, and coagulopathy. Perioperative management in this high-risk population has a significant impact on patient outcomes. This review focuses immediate postoperative intensive care unit management of device-related complications. Keywords left ventricular outflow obstruction, heart, heparin, intensive care unit, complications The use of implanted mechanical circulatory support has evolved from its initial function, as bridge to transplantation or bridge to recovery, to an increasingly more common-use destination therapy for end-stage heart failure in nearly 44% of cases (400 to 800 per year).1,2 Recent data from the Interagency Registry for Mechanically Assisted Circulatory Support suggest a significant increase in use for destination therapy, as this application was found in 18% of cases before 2001 and in 44% in 2012.2,3 The perioperative management of patients following implantation of a left ventricular assist device (LVAD) has many challenges. The average patient stay in the intensive care unit (ICU) is 9 days following ventricular assist device (VAD) implantation,4 and the most common complications are hemodynamic instability, arrhythmias, right ventricular (RV) failure, infections, inappropriate device settings, and bleeding.5,6 Reported mortality in the ICU is 10% to 32%, with multiorgan failure and sepsis as the most common causes of mortality.6,7 Klotz et al published a predictive scoring system for mortality (Table 1), where a score of 30 points, a high mortality rate of 65.2%.6 Despite immediate risks associated with VAD implantation, the newer continuous flow pumps allow for improved long-term survival, with 80% survival for continuous flow pumps at 1 year and 70% at 2 years.2 This review aims to provide a management overview based on the most critical challenges faced in caring for these patients, including
device-related complications, hemodynamic instability, arrhythmias, RV failure, coagulopathy, and surgery for noncardiac procedures.8-10
Basic Review: VAD Types and the Basics of Operation This discussion is focused on patients receiving the HeartMate II (HMII; Thoratec Corp, Pleasanton, California) or the HeartWare HVAD (HVAD; HeartWare HVAD Inc, Miramar, Florida), as these are the most common devices currently in use. Basic VAD physiology includes an altered blood flow pattern, with blood flowing from the ventricle to the VAD via the inflow cannula and then being pumped to the body and various organs via an outflow graft sewn to the ascending aorta in most cases.11 The first-generation pulsatile flow pumps have been replaced by improved second-generation continuous flow designs in the HMII (Figure 1) and so-called third-generation devices, such as the HVAD (Figure 2). 1
Weiss Memorial Hospital, Affiliate of the University of Illinois at Chicago, Chicago, IL, USA 2 University of Colorado Denver, Aurora, CO, USA Corresponding Author: Franziska Elisabeth Blum, Weiss Memorial Hospital, Affiliate of the University of Illinois at Chicago, 4646 N Marine Drive, Chicago, IL 60640, USA. Email: [email protected]
Downloaded from scv.sagepub.com at UNIV OF VIRGINIA on December 10, 2015
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Blum et al Table 1. Risk Score for Post–Ventricular Assist Device Implant Mortality.6,a Parameters: Cutoff
Risk Score
Transfusion Heart laboratory Lactate dehydrogenase > 500 U/L Creatine kinase > 200 U/L Lactate > 3 mg/dL Inotropic support: epinephrine and/or norepinephrine and/or dobutamine Infectious laboratory C-reactive protein > 8 mg/dL White blood cell count > 13 000/µL Mechanical support Extracorporeal membrane oxygenation support Intra-aortic balloon pump Redo operative procedures Emergent implant Emergency implant Implant postcardiotomy Ventilation Renal impairment Renal replacement therapy Creatinine > 1.5 mg/dL BUN > 40 mg/dL Ischemic cardiomyopathy Preoperative resuscitation Platelets < 100 000/µL Heart rate > 100/min Blood laboratory Hemoglobin < 12 mg/dL Hematocrit < 35% Age > 50 y
6 5 5 5 4 4 4 3 3 3 2 2 1 1 1 1
a Maximum score, 50 points: low-risk score < 15 (mortality, 15.8%); medium-risk score = 16 to 30 (mortality, 48.2%); high-risk score > 30 (mortality, 65.2%).
Both the HMII and HVAD devices are implanted into the beating heart using cardiopulmonary bypass to facilitate optimal surgical conditions, while a side-biting aortic clamp eliminates the need for cardioplegia. Despite the same general approach, there are significant differences between the 2 devices. The HVAD is smaller in size (weighing about 145 g) and can therefore be implanted into smaller patients when compared to the HMII. The HVAD consists of an integrated inflow cannula inserted into the apex of the left ventricle, while the HMII utilizes a synthetic graft as a conduit to the sewing ring. Both devices connect the pump with the aorta via the outflow cannula. A driveline then connects the pump to an external controller, which is a processor unit that sends power and operating signals to the blood pump and simultaneously collects information from it.12,13
Figure 1. HeartMate II LVAD with permission from © 2013 Thoratec Corporation.
The HMII and the HVAD devices generate continuous flow; thus, patients have a greatly reduced, if not absent, pulse.13,14 The HMII is an axial flow pump that generates flow via “pushing” or “propelling” blood, while the HVAD is a centrifugal flow pump that generates flow by “throwing” blood. These 2 concepts are important, as both devices demonstrate slightly different flow characteristics due to these inherent differences and can demonstrate various flow pulsatility patterns, specifically in the postoperative period during periods of volume shifting. An advantage of centrifugal devices is their ability to operate over a range of flows for a small change in delta P (the pressure difference between the left ventricle and aorta), while flow in axial devices increases and decreases with small changes in delta P. This effect causes the axial flow device to increase the negative pressure in the ventricle during low flow, potentially sucking the ventricular wall into the inlet cannula and further limiting flow and increasing the risk for arrhythmias. The centrifugal demonstrates greater variation of pulsatility under conditions of elevated and normal blood pressure.15 In addition, HVAD and centrifugal pumps in general may provide more accurate flow estimation than the axial flow devices, especially at lower speed.16
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Seminars in Cardiothoracic and Vascular Anesthesia 19(4) parameters vary based on the set speed, intrinsic cardiac function, volume status, vascular tone, and viscoelastic properties of the blood itself. The optimal operating speed of the device is selected through determining the minimum and maximum speed of the device in revolutions per minute (RPM). The optimal range of VAD speed is determined by monitoring left ventricular (LV) size, the position of the septum, and the degree of aortic valve opening, typically via echocardiography.17 For the HMII, the normal operating range tends to be between 8000 and 10 000 RPM, with flow estimation becoming increasingly inaccurate at or below 8000 RPM.18 For the HVAD, normal clinical operation occurs with RPM between 2000 and 4000. Both the function of the HMII and HVAD devices and their dynamic interaction with the patient’s physiology can be assessed and tracked by observing the provided parameters: flow, power, and pulsatility index (PI) for the HMII.6 To appreciate the contribution of these values, it is imperative to understand how flow is generated and how different physiologic states affect device function. Key to the function of these devices is the concept of differential pressure.
Differential Pressure Figure 2. HeartWare HVAD with permission from HeartWare HVAD Inc. Miramar, Florida.
General Concepts in Postoperative VAD Management The immediate postoperative course for patients following VAD implantation is highly variable, and as such, management must be tailored to the rapidly changing hemodynamic state of the patient. For the hemodynamically stable patient, the standard fast-track approach for weaning from the ventilator should be considered. However, hemodynamic stability depends on 4 key variables, including level of vasodilation or afterload, degree of coagulopathy and ongoing blood loss, preload conditions, and the status of the right ventricle. Each of these variables affects the overall balance of hemodynamics with regard to the VAD and, thus, the decision on how aggressively to wean from respiratory support. RV function, volume, and geometry should be maximized prior to removal from mechanical ventilation, and afterload should be stable, since suspending positive pressure ventilation will increase the already-higher-than-preoperative blood return to the right ventricle. VAD pump management becomes a key point starting at separation from cardiopulmonary bypass, until the patient achieves a stable hemodynamic balance. The only parameter within the control of the intensivist and the only variable that can be changed is the speed. All other
Differential pressure is the difference in pressure between the inflow and outflow cannulas or the difference between LV pressure and aortic pressure: differential pressure = (aortic pressure – LV pressure) + pressure loss across the pump. ∆P =
( Pa −
PLV ) + ∆Ppump
Pressure drop is related to flow but is typically inconsequential. Aortic pressure is typically in the normal range, and as such, the dynamic variable in determining differential pressure is LV pressure or its determinants: volume status or preload and contractile state. The lower the differential pressure is, the higher the flow becomes. In other words, higher LV pressures due to increased contractility and/or increased preload increase pump flow at any given speed. Based on the concept of differential flow, it is possible to understand how changes in physiology, volume status, and heart function alter the parameters displayed on the control unit. Once these parameters are defined, interpretation and intervention can be performed. Parameters displayed on the console for HMII and HVAD differ; the HMII console displays pump flow, pump speed, pulse index, and power, while the HVAD console displays mainly pump flow, pump speed, and power to the user. In the following section, we define parameters for both devices and explain their meaning for device and heart function.
Downloaded from scv.sagepub.com at UNIV OF VIRGINIA on December 10, 2015
321
Blum et al Power. Power is simply defined using Watt’s law (power = voltage × current); it is directly measured; and it is equivalent to the amount of work that the device must perform at a given speed. When operated within the recommended speed range (eg, for HVAD, 2400 to 3200 RPM), power ranges between 2.5 and 8.5 watts. When confronted by increases in power, the operator should consider complications that make it harder for the pump to spin: thrombus formation in the device, valid high flows related to vasodilation from any cause, and hypervolemia. Speed. Speed is user defined and adjusted based on hemodynamic and echo findings. Flow. Flow is estimated based on the power, as both parameters normally retain a linear relationship at a given speed; furthermore, the HVAD takes the hematocrit into consideration for estimation of pump flow. However, the flow value becomes imprecise at high and low regions of the power-flow curve; therefore, it is recommended to maintain a minimum of 2 to 4 L/min for pump flow. For example, an increase in power related to thrombus formation in the pump head that is clearly not related to an increase in flow will produce an erroneously high flow. At the other extreme, a partial occlusion of the outflow cannula will reduce flow significantly as well as power, since less blood is moving through the device and less work is being done. Low flows can result from hypovolemia either relative or secondary to bleeding, arrhythmias, or any cause of inflow or outflow obstruction. Pulse Index. PI applies to HMII only. PI is defined as flow magnitude averaged over 15 seconds, typically ranging from 1 to 10. The degree of flow pulsatility is inversely related to the degree of LV unloading and is proportional to the strength of LV contraction; hence, PI can be used to measure LV function.17 An increase of pump support leads to less ventricular filling and a decrease in PI in an otherwise stable patient, reflecting a decrease in circulating blood volume or worsening LV function. A decrease of PI may furthermore indicate hypovolemia or suction event. An increase in PI reflects more ventricular filling and a decrease in differential pressure either due to decreased afterload with preserved blood return to the LV or due to improving LV function. The concept of LV volume and pressure dictating the flow characteristics of the LVAD can be useful in troubleshooting postoperative device malfunction in the ICU. Even with severely depressed function, the LV will still contribute a pressure pulse, lowering differential pressure and increasing flow and pulsatility. Only a heart rendered completely flaccid or one in fibrillation will create a truly nonpulsatile flow pattern. This is the mechanism by which hypovolemia and excessive pump speed reduce flow,
either by not delivering enough blood to the LV or by taking too much away, thus increasing differential pressure. The pressure flow curve demonstrates that relatively small increases in differential pressure cause significant reductions in flow that is, excessive vasoconstriction and severely depressed LV function (Figure 4). Suction events are easily explained based on these principles. Furthermore, centrifugal pumps operate over a wide range of flow with a small pressure change across the pump (Figure 3). Should the LV pressure be sufficiently negative to collapse the ventricle walls (hypovolemia, excessive pump speed, or malpositioned inflow cannula), an effective occlusion of the inflow cannula occurs; power and flow decrease; and the device is designed to “ramp down” its speed until the suction is relieved, at which time it returns to its programmed speed. The converse is a flow state that significantly decreases the differential pressure, such as vasodilation with high pump flow and high return to the LV. The LV pressure may exceed aortic pressure, causing the aortic valve to open and the differential pressure to effectively equal net cannula pressure loss, causing maximum flow and high pump speeds and power values. In addition to the parameters mentioned above, echocardiography plays an important role in troubleshooting for postoperative device dysfunction. Echocardiography is mainly used to evaluate surgical results after LVAD implantation and, more important, to evaluate the reason for postoperative hemodynamic compromise, caused by, for example, thrombosis of the in- and/or outflow cannula, hypovolemia, acute RV dysfunction, cardiac tamponade, and pulmonary embolism. For example, the combination of rightward deviation of the interventricular and interatrial septum, mitral valve regurgitation, aortic valve opening, and a decrease of LVAD flow is indicative of LVAD dysfunction.19 Figure 4 shows a summary of the diagnostic pathway to troubleshoot device problems, as well as potential alarm and treatment options.
Hemodynamic Optimization Hemodynamics following VAD implantation change frequently in the initial postoperative period. VAD speed settings are typically determined in the operating room using transesophageal echocardiography guidance, and attempts should be made to maintain these settings. Hemodynamic instability should be approached via management of preload and afterload conditions first, followed by parameters such as VAD pump speed, cardiac output, filling pressure, and echocardiographic assessment of right and left heart function.5 Dehydration, ongoing bleeding, vasoplegia, sepsis, aortic valve regurgitation, and vasodilating agents may cause hypotension,5, 20 whereas fluid overload, pain, and agents causing vasoconstriction may lead to hypertension.21 The optimal adjustment of the patient’s hemodynamic
Downloaded from scv.sagepub.com at UNIV OF VIRGINIA on December 10, 2015
322
Seminars in Cardiothoracic and Vascular Anesthesia 19(4)
high
Alarm Type Poten al Causes Treatment
low
-Medium
-High -Medium
high
-Fluid Bolus -Transfuse -Address arrhythmias
low
-High -Medium
-Pump thrombosis
-Vasodila on -Hypovolemia -Sepsis -Bleeding -Hypervolemia -Arrhythmias -Suc on event -Thrombus -Hold vasodilators -Pressors -Treat underlying cause -Diur e cs
Speed
Power
Flow
-Consider thrombolysis -Consider devicee exchange -Consider an coagula on
-High -Medium
increase
-Medium
-Pump stop -Lead disconnec on -Low ba ery -System controller in back up mode -Check power supply and device components.
-Cannula Kinking or complete thrombosis
decrease
-Medium
-Suc on event -Controller/p ump dysfunc on
-Consider -Ensure thrombolysis adequate -Consider preload device exchange. ge.
-Treat arrhythmias -Inves gate for VAD occlusion -A erload reduc on -Change controller
Legend • Alarm types -Critical/High: Symbol: None or flashing red Tone: Continuous or two-toned unable to mute Alarm display:VAD stopped, Controller failed, Critical Battery -Advisory/Medium: Symbol: Flashing yellow Tone: Intermittent beep-gradual increase in alarm volume over time if not muted, alarm can be muted for 5 minutes to 1 hour Alarm display: High Watts, Controller Fault, Low Flow, Electric Fault, Suction -Advisory/Low Symbol: Solid yellow Tone: Intermittent beep gradual increase in alarm volume if not muted, able to mute alarm for 5 minutes Alarm display: Low Battery, Power disconnect Figure 3. Troubleshooting device malfunction for HeartWare HVAD.51,52 Alarm types: Critical/high: symbol: none or flashing red; tone: continuous or 2-toned unable to mute; alarm display: ventricular assist device stopped, controller failed, critical battery. Advisory/medium: symbol: flashing yellow; tone: intermittent beep, gradual increase in alarm volume over time if not muted, alarm can be muted for 5 minutes to 1 hour; alarm display: high watts, controller fault, low flow, electric fault, suction. Advisory/low: symbol:solid yellow; tone:intermittent beep gradual increase in alarm volume if not muted, able to mute alarm for 5 minutes; alarm display:low battery, power disconnect.
values includes achieving a target mean arterial pressure (MAP) of 60 to 80 mm HG, keeping MAP
