Circulatory Assist Devices

Published on 07/03/2015 by admin

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Chapter 19 Circulatory Assist Devices

Joseph L. Weidman, MD , Michael N. Andrawes, MD , Michael G. Fitzsimons, MD, FCCP

1 What is an intraaortic balloon pump (IABP)?

An IABP is a circulatory assist device that is intended to provide temporary support to a failing heart through synchronized actions focused on reducing afterload and augmenting diastolic pressure. The system consists of a 30- to 50-cc balloon attached to a catheter. The balloon rests within the lumen of the descending thoracic aorta, and the attached catheter snakes through the arterial vessels and out of the body. The catheter relays with an external console that monitors aortic pressure tracings and electrocardiogram (ECG) readings, inflating the internal balloon at the appropriate times. The balloon is inflated with either helium or CO2. Although CO2 has higher blood solubility and therefore a lower risk of embolization, helium is much more common because it is less dense and thus allows for faster filling and emptying times.

2 How is an IABP placed?

The most common site for IABP placement is through the femoral artery via the percutaneous Seldinger technique. Alternative insertion sites include the iliac, axillary, and subclavian arteries. IABP placement may even be accomplished via the ascending aorta, as is occasionally done after cardiac surgery. The ideal location for the tip of the balloon is just distal to the aortic arch, 1 to 2 cm beyond the left subclavian artery. The other end of the balloon should sit proximal to the takeoff of the celiac axis. Correct positioning of the balloon may be confirmed at the time of placement with either transesophageal echocardiography or fluoroscopy. Alternatively, confirmation may be performed after placement by chest radiograph.

3 What are the physiologic benefits of an IABP?

The IABP is designed to inflate during diastole and deflate during systole. It is beneficial in both phases of the cardiac cycle. During diastole, inflation of the balloon displaces blood proximally, increasing perfusion pressure to the coronary arteries and the critical vessels branching off the aortic arch. Perfusion and blood flow are equally improved in the aorta distal to the balloon through the vessels supplying blood to the mesentery and the lower extremities. In systole, deflation of the balloon creates a relative negative space within the aorta, which reduces afterload. As a consequence, not only is cardiac function improved because of lower systemic resistance, but the duration of isovolumic contraction, the most energy-demanding phase of the cardiac cycle, is also reduced.

4 How is IABP inflation-deflation timing coordinated?

Three possible triggers are routinely used to coordinate IABP inflation-deflation:

image An ECG tracing

image An arterial pressure waveform or

image A pacing device if one is being used

The ideal time for balloon inflation is at the onset of diastole, just after the closure of the aortic valve. This moment corresponds with the middle of the T wave on the ECG and with the dicrotic notch on the arterial pressure waveform. Deflation ideally occurs at the beginning of systole, which coincides with the peak of the R wave on the ECG and with the point just before the systolic upstroke on the arterial waveform. Alternatively, an option exists to run the IABP in an asynchronous mode with a preset rate, but it is not routinely used. Some recent work has also been done on real-time dicrotic notch detection and prediction, which may improve IABP timing during both regular and irregular rhythms.

5 What are the indications for IABP placement?

Clinical conditions that may warrant initiation of IABP therapy can be broken down into two categories: those conditions that would benefit primarily from increased diastolic coronary perfusion and those that would benefit mainly from afterload reduction. Balloon pumps may also be placed prophylactically in high-risk patients undergoing cardiac surgery (Box 19-1). The most common condition for which IABP therapy is started is cardiogenic shock, accounting for approximately 20% of placements.

Box 19-1 IABP Counterpulsation Indications

Increase Diastolic Coronary Perfusion

Persistent angina refractory to medical management

Infarction or ischemia associated with PCI

Cardiogenic shock after MI

Afterload Reduction

Acute ventricular septal rupture

Acute mitral regurgitation (chordae tendineae or papillary muscle rupture)

Cardiogenic shock refractory to medical management

Bridge to transplantation

Postbypass low cardiac output syndrome

Prophylactic Indications

Significant left main coronary artery disease before surgery

High-risk PCI

High-risk electrophysical intervention (i.e., ablation of ventricular fibrillation)

High-risk patient undergoing noncardiac surgery

MI, Myocardial infarction; PCI, percutaneous coronary intervention.

6 What are the contraindications for IABP placement?

The contraindications for IABP placement include the following:

image Anything more than mild aortic insufficiency

image Aortic aneurysm

image Aortic dissection

image Severe aortoiliac atheromatous disease

7 What are possible complications of IABP placement?

Complications related to the use of IABP fall into three categories: vascular, positional, and balloon related.

image Vascular complications include hematoma, perforation, dissection, pseudoaneurysm, and aneurysm. These complications are more common in patients with diabetes or peripheral vascular disease, smaller patients, and females.

image Complications resulting from poor positioning of the balloon occur because of the unintended obstruction of arterial branches of the aorta. This results in compromised perfusion and/or ischemia to the upper extremities and brain (if the balloon is too proximal) or to the mesentery (if it is too distal). Poor positioning may also affect the ability of the IABP to maximally augment.

image Balloon-related complications include balloon rupture, gas embolization, and traumatic thrombocytopenia. Of note, the reduction in platelet count caused by the balloon pump is predictable and generally stabilizes after 4 days. Platelet counts return to baseline quickly after removal of IABP.

8 Is anticoagulation necessary in a patient with an IABP?

The standard of care is that unfractionated heparin be used for anticoagulation during the IABP use, with a partial thromboplastin time goal from 50 to 70 seconds. Data are inconclusive as to whether this actually decreases the incidence of limb ischemia; however, in animal studies an immobile, deflated balloon is subject to thrombus after 20 minutes. For this reason, a nonfunctional balloon should be removed promptly, as soon as anticoagulation has been reversed. For elective removals, it is recommended that heparin be discontinued for at least 2 hours before taking out the device.

9 How is a patient weaned from an IABP?

The amount of aid a balloon pump provides can be quantified as a ratio of native beats to assisted beats. Full support is at a ratio of 1:1; that is, every beat is augmented by the IABP. Weaning occurs by gradually reducing the augmentation ratios to 1:2, 1:4, and then 1:8. Time intervals between step-downs vary depending on the clinical situation but are usually on the order of 1 to 6 hours. During weaning, cardiac output, blood pressure, mental status, kidney function, and distal perfusion should be monitored. It may be necessary to commence inotropic support as the patient is weaned. As the augmentation ratio is reduced, the risk of thrombus formation on the balloon pump increases. Weaning may also be accomplished by gradually reducing the balloon volume.

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