Management of Cardiogenic Shock

Abstract

A 52 year old man with no known past medical history presents to the emergency department with 24 hours of chest pain, nausea and vomiting. An initial EKG shows a right bundle branch block (RBBB) and ST elevations in anterolateral leads (Fig. 9.1). He is given intravenous heparin, prasugrel and aspirin and taken directly to the cardiac catheterization lab. Coronary angiography reveals chronic appearing 100% right coronary artery (RCA) occlusion with left to right collateralization and 100% left anterior descending (LAD) artery occlusion. Two drug-eluting stents are placed into the LAD with restoration of TIMI 3 flow (see Table 9.1 for definition). Left ventriculogram reveals diffuse hypokinesis and a left ventricular end-diastolic pressure (LVEDP) of 30–35 mmHg. The patient is confused and not following commands. An intra-aortic balloon pump is placed. Arrangements are then made to transfer to tertiary care center for further management.

Appendix

9.1.1 IABP Counterpulsation

Intra-aortic balloon pump counterpulsation is the most common temporary mechanical support device and have been in use since since 1967 [23]. Intra-aortic balloon pumps (IABPs) are made up of two components: a double-lumen balloon positioned in the thoracic aorta via percutaneous femoral artery access, and a console that inflates and deflates the balloon with 30–60 cc of helium in synchrony with the cardiac cycle.

The devices were designed to address the central therapeutic dilemma of cardiogenic shock, namely maintaining perfusion to coronary arteries comes at the expense of increased afterload, which only increases stress and myocardial demand for a failing left ventricle. An intra-aortic balloon pump (IABP) effectively solves this issue by decoupling afterload and coronary perfusion. By inflating a balloon in the aorta during diastole, the balloon augments diastolic filling pressure to the coronary arteries. Furthermore by deflating during systole, the intra-aortic balloon is a able to decrease afterload and mechanically offload the left ventricle, decreasing myocardial demand.

Evidence

Based largely on compelling physiologic principles and the lack of other alternatives, intra-aortic balloon pumps were widely adopted in the use of cardiogenic shock, especially in case of acute myocardial infarction. Based on favorable observational studies that followed the ACC/AHA gave a Class I indication for IABP in the treatment of cardiogenic shock secondary to acute myocardial infarction [24]. However, as mentioned above there is growing skepticism regarding the role of IABP even in cardiogenic shock. Much of this has been driven by the SHOCK II trial [25], which randomized 600 patients with acute myocardial infarction with cardiogenic shock to IABP support or medical management. Using an intention to treat analysis, there was no observed statistically significant differences in 30 day all-cause mortality (39.7% and 41.3%; p = 0.69) or in multiple secondary end points. The authors of the trial and several commenters have raised limitations to the study, including the lower than expected mortality rate leading to underpowering, a significant number of cross-overs to IABP therapy, and lack of long term followup [9]. Based on the SHOCK-II trial and several other smaller randomized control trials which also did not show clear benefit, the ACC/AHA revised their recommendation a Class IIa indication in patients who develop cardiogenic shock following acute MI despite medical therapy (level of evidence B) [8]. The ESC/EACTS Guidelines on Myocardial Revascularization went further and downgraded IABP use in AMI complicated by cardiogenic shock to a Class III recommendation indicating that routine use of IABP in patients with cardiogenic shock is not recommended [22]. Regardless of the equivocal current supportive data and the need for further study, IABPs remain the most common form of temporary mechanical circulatory support, and knowledge of their uses and limitations is needed for patient care.

Complications

The two most common types of complications that require troubleshooting with IABPs is ensuring proper anatomical position and proper timing of inflation and deflation. In each case, improper use cannot not only limit the effectiveness of the support but cause significant harm.

The ideal positioning of the IABP is 1–4 cm below the aortic arch, typically within 2nd rib space just above the left main bronchus and usually 2–3 cm distal take off of the subclavian artery. The tip of the IABP is radio-opaque allowing for fluoroscopic and radiographic confirmation of correct placement. An improperly positioned IABP risks occlusion of the subclavian artery proximally and renal arteries distally. With this in mind, urine output and a careful vascular exam should be monitored regularly.

Just as anatomical balloon location should be evaluated with any major change in patient position or clinical status, the console function should also be examined regularly via inspection of aortic pressure waveforms to assess for problems related to timing of inflation/deflation and under/over-filling of the balloon.

IABP inflation is triggered either by sensing cardiac electrical activity or aortic pressure sensing . In normal function, inflation is initiated when aortic valve closes and continues until the initiation of the next ventricular contraction, encompassing all of diastole. (See Fig. 9.3) Common timing malfunctions include early and late activation in either inflation or deflation. The most concerning malfunctions exist when the balloon is inflated during systole as this impairs cardiac output by increasing afterload. This can be seen in early inflation or late deflation. Late inflation and early deflation are not dangerous and merely reduce the physiologic benefit of ventricular support. (See Figs. 9.3, 9.4, 9.5, 9.6, and 9.7)

Figure 9.3

Analysis of IABP waveforms requires the balloon pump to be set to a 1:2 setting, which times balloon inflation to every other ventricular beat. This allows the clinician to observe both a normal and a balloon assisted cardiac cycle. An example of typical unassisted cardiac cycle followed by balloon assisted cycle is included above. The dotted horizontal lines mark unassisted systolic and diastolic blood pressures. The dotted red line is included to demonstrate a non-assisted cycle for comparison. The green box is included to illustrate timing of balloon inflation. In this example the black line represents an initial systolic waveform tracing followed by a pump generated waveform, which generates a sharp “v” appearance with increased diastolic pressures during balloon inflation) and then the systolic waveform seen following a balloon waveform with reduced systolic pressure

Figure 9.4

Earl Deflation: Optimal function of the IABP involves inflation of the IABP for the entire diastolic cycle and without inflation during the sytole. The most worrisome timing complication involves inflation of the balloon during systole as this leads impairs cardiac output. Early deflation is worrisome in that it results in the loss of afterload reduction which is the a goal of IABP therapy. Notice the sharp drop following diastolic inflation. Diastolic augmentation also becomes sub-optimal. This results in sub-optimal coronary perfusion, potential for retrograde coronary and carotid blood flow, suboptimal afterload reduction and increased myocardial oxygen demand

Figure 9.5

Early Inflation: This may result in premature closure of aortic valve, incomplete LV emptying, aortic insufficiency, increase in LVEDV and LVEDP, increased afterload, and increased myocardial oxygen demand. This requires immediate attention

Figure 9.6

Late Deflation: This represents one of the most significant functional complications of intra-aortic balloon pumps. During late balloon inflation the left ventricle contracts increased afterload from an inflated balloon. This can result in increased mycoardial demand and reduced cardiac output

Figure 9.7

Late Inflation: This results in decreased augmented coronary perfusion, decreased O2 supply to coronaries and reduced overall benefit of the IABP

Other well described complications of IABPs include anemia and thrombocytopenia from mechanical shearing, vascular injury from insertion site leading to limb ischemia, dissection, pseudo-aneurysm formation. Finally, while rare, the balloon can rupture leading to a gas embolus. IABP are currently programmed to detect ruptures, and will attempt to aspirate helium from the aorta.

Duration of use

There is no established recommendation on how long these devices can be used. Observational data has shown that these devices can be used for as long as 20 days, though the most common duration of use is 2 days [26, 27]. These studies all show that there is a correlation between complications and duration of support. With this in mind, weaning of mechanical support should be attempted if the patient has reached a clinical stable state.

Contraindications

With the means of functioning and complications in mind, contraindications to IABP use are not difficult to surmise. First, the basic physiology of IABP assumes normal aortic valve function, specifically the absence of aortic regurgitation. If significant aortic regurgitation is present, it will be worsened by the diastolic inflation of the balloon pump and thus may cause vastly more harm than benefit. Other contra-indications include preexisting large arterial pathologies such as aortic dissections or aneurysms that would be worsened by balloon inflation.

9.1.2 Temporary Ventricular Assist Devices

In patients with cardiogenic shock and need for mechanical circulatory support, consideration should also be given to percutaneous ventricular assist device (VAD) support. The most common of this type are the Impella, Tandem Heart and Centrimag devices. It is important to be able to distinguish between these devices by their level of circulatory support, location of vascular cannulation, and durability of support.

To address the subject of evidence broadly, it is important to note that while all these devices have been approved for use as ventricular support devices, they have been approved based on demonstrating hemodynamic improvement and not based on improved survival [28,29,30].

9.1.3 Impella

The Impella 2.5 and Impella 5.0 are percutaneous, catheter-based rotary pumps that continuously pull blood from the left ventricle into the aorta via an Archimede’s screw mechanism at a maximal rate of 2.5–5.0 l/min, respectively. Like the IABP, the Impella 2.5 is typically placed by interventional cardiologists into a femoral artery. It is then and guided up the aorta until it crosses the aortic valve at which point it is able to properly function. The Impella 5.0, while providing greater circulatory support, is larger and requires a vascular cut down into the femoral artery or requires open surgical access to place directly into thoracic aorta. Just as the Impella devices have different means of cannulation they also have different recommendations on level of support. The Impella 2.5 is approved for less than 6 h of circulatory support, while the Impella 5.0 has been approved for expected durations less than 6 days [31].

The Impella devices have been clinically demonstrated in several small trials to improve several important hemodynamic indices, such as increased cardiac output, elevated mean arterial pressures counterpulsation [28, 29]. The primary way these device achieve these improved hemodynamic indices is simply by providing a higher degree of hemodynamic support. Whereas an intra-aortic balloon pump may be able to augment cardiac indice by up to 0.11 L/min/m², an Impella 2.5 can offer greater than 0.49 L/min/m²²⁸ In addition to offering a higher level of circulatory support, Impella devices also offer the advantage of decompressing the left ventricle, which in theory lessens wall stress and myocardial oxygen demand.

Contraindications to Impella use include left ventricular and aortic valve pathologies which would predispose to calcific or thrombo-embolism, specifically, aorta stenosis (aortic valve area less than 0.6 cm²) or LV wall thrombus. Additionally, it use to not recommended in cases of aortic regurgitation or mechanical aortic valve.

9.1.4 Tandem Heart

The Tandem Heart is a percutaneous support device that bypasses the left ventricle by taking oxygenated blood directly from the left atrium and shunts it to the systemic arterial circulation. It does this by using to cannulas. The inflow cannula is inserted through the venous system into the right atrium and then in the left atrium via a trans-septal puncture. Blood is then pumped from the left atrium into a magnetically driven centrifugal pump that drives blood into the systemic circulation via a femoral artery cannula.

The first data regarding the Tandem Heart was published in 2001, where its use was associated with improvements in cardiac index, MAP and concurrent reduction in markers of congestion (CVP and PWCP) [32, 33]. A randomized trial was then conducted which recruited 41 patients with cardiogenic shock following acute myocardial infarction, which found no statistically significant difference in 30 day mortality when compared to IABP counterpulsation [30]. Additionally while hemodynamic indices were improved over IABP, the Tandem Heart cohort had more episodes of severe bleeding and acute limb ischemia.

Randomized trials and observational studies have shown that duration of Tandem Heart use tends to be around 4 days though is typically recommended for as little as 6 h to as many as 30 days [30, 33].

9.1.5 Centrimag

The CentriMag ventricular assist device is most commonly known for being the circulatory support mechanism in very common extracorporeal membrane oxygenation (ECMO) system. The CentriMag along with a Maquet Quadrox are together able to provide full cardiopulmonary support (both hemodynamic and oxygenation of venous blood). However when pulmonary support is not needed the CentriMag device alone can be used for short term mechanical circulatory support. If pulmonary support is then needed, the oxygenator can be added to the support circuit without another procedure. The inflow and outflow cannulas for the CentriMag system can be arranged a number of combination. In cases of left ventricular failure, the inflow cannula can be surgically placed via thoracotomy in to the left ventrical and the outflow into the aorta or femoral artery. The CentriMag also affords the possibility of right ventricular support with placement of an inflow catheter into the right ventricle and outflow catheter into the pulmonary artery.

Data on the CentriMag support system is largely observational. But studies have thus far shown an acceptable safety profile with limited patient complications and device failures [34].