During an average life span, the heart beats 2.5 billion times and pumps 1 million barrels of blood into circulation [1]. This amazing cardiac output is dependent on the heart rate and the stroke volume, which is the amount of blood ejected with each heartbeat, reflected in the familiar equation:
Cardiac output = heart rate x stroke volume
The stroke volume is determined by the amount of blood sitting in the left ventricle at rest (diastole) and how well the ventricle contracts (systole). The adult stroke volume ranges from 70 to 100 milliliters, depending on gender and age [2]. The stroke volume normally ejects 55 to 70 percent of the blood contained in the left ventricle. This ejection fraction is measured most commonly with a heart ultrasound called an echocardiogram. A low ejection fraction (<55%) is a sign of poor pump function [3].
Cardiac balloon pump function
If heart muscle is severely damaged or diseased it may be unable to generate an adequate ejection fraction. One example is a heart attack. A large myocardial infarct (MI) can result in enough dead, dying and injured cardiac muscle cells that the heart cannot maintain adequate pump action. This leads to an ejection fraction that is too low to maintain a normal perfusion pressure, and cardiogenic shock ensues.
Certain medications may improve perfusion pressure in this situation, such as blood vessel constricting vasopressors or drugs called inotropes that make the heart muscle squeeze harder and faster [4]. However, these medications also increase the workload of the heart and amplify oxygen debt and demand. Depending on the extent of muscle damage, these medications may not be helpful.
When cardiac demand outpaces supply
Muscle action requires oxygen. When muscle activity increases, so does its oxygen demand and consumption. For the heart, this is called myocardial oxygen consumption or myocardial oxygen demand, frequently noted as MVO2.
Our patient with the huge heart attack is not pumping on all cylinders. Both the damaged and undamaged myocardial muscle cells have an increased MVO2. The undamaged cells are carrying a higher than normal workload, and the damaged cells will die without adequate oxygen. This patient has been cathed and stented but has persistent hypotension due to a low ejection fraction. Vasopressors and inotropes have not been helpful. So what next?
Circulatory assist devices
Circulatory assist devices offer mechanical support for a failing heart and can be used in either the short or long term, depending on the type of machine and patient variables such as disease process, coexisting illnesses, age, or transplantation considerations.
Assist devices range from hospital-based machines that access the circulation through puncturing the skin (percutaneous access), to the surgically implanted left ventricular pump assist device that goes home with the patient.
Some machines function as an artificial heart and lung by removing carbon dioxide and adding oxygen as the blood passes through the apparatus and then gets pumped back into circulation. Others provide only mechanical assistance with external or internal pumps.
Intraaortic balloon pump (IABP)
The most commonly used circulatory assist device doesn’t actually “pump” blood. The intra-aortic balloon pump (IABP) targets those patients who need mechanical support over a short period of time to allow their heart to repair itself and resume normal function.
It is also used as a bridge to additional therapy for those needing surgery (bypass; valve replacement) or complicated medical therapy for severe heart failure, persistent angina, or difficult to control ventricular dysrhythmias.
The IABP was first used in 1968 for a patient with cardiogenic shock from a large myocardial infarct, kind of like our patient.
The balloon pump is a short term device, useful for periods of a few hours up to two weeks5. It decreases the workload of the heart, thus decreasing the oxygen demand or MVO2, and increases the circulation of the coronary arteries, thus increasing oxygen delivery to all cardiac cells.
For our MI patient, this will optimize the ability of the non-injured myocardial cells to meet the increased workload while allowing the injured cells to repair and increase their ability to pump as they heal. And it does this quite simply.
How Intraaortic balloon pumps work
The IABP is a long hollow tube with a four- to six-inch, hot dog-shaped balloon surrounding the distal end starting just below the tip. It is connected to a computerized control apparatus that determines when the balloon will inflate or deflate. The IABP is also termed a counter pulsation device, which simply means that when the heart is not pulsing (pumping), the balloon is inflated. When the heart pumps, the balloon deflates (not pulsing).
The device is inserted percutaneously, using the femoral artery as the entry point and the proximal descending aorta as the destination. The balloon tip sits just beneath the exit of the left subclavian artery.
The IABP senses the opening and closing of the aortic valve through input from the patient’s ECG leads and aortic pressure readings sent from the open tip of the hollow tube. The balloon inflates just as the aortic valve closes, which increases the blood pressure above the balloon long enough to push more blood through the coronary arteries, thus increasing delivery of needed oxygen and nutrients to the cardiac muscle.
The balloon deflates just prior to the aortic valve opening and creates a vacuum that siphons blood from the upper aorta and left ventricle, thus decreasing the work of the left ventricle, which decreases the MVO2.
The ease of insertion and the effectiveness of this relatively simple device have made it the number one circulatory assist device presently in use.
This nine-minute video provides a tour of how the IABP functions.
Critical care transport
Properly trained paramedics can safely transport IABP patients [6]. Of course, this requires a working knowledge of the particular pump model in use and how to troubleshoot any problems. Fortunately, pump difficulties during transport are uncommon; some reported examples include issues with the timing of inflation or deflation, loss of power, and cable malfunction [6,7]. You are more likely to have a transport complication with the patient’s underlying medical condition than with the IABP. And like a vent transfer, document your placement confirmation of the tube and balloon prior to transport, preferably in writing.
Although you may never care for a patient with an IABP – and hopefully will never have personal experience with one – you now grasp the basics of a device that can deliver a one-two punch that increases heart function long enough for a patient’s heart to resume internal life support function or obtain further life-sustaining treatment.
References
1. Nova, amazing heart facts. http://www.pbs.org/wgbh/nova/heart/heartfacts.html
2. Peter AC, Ragnhild A, Hedstrom E, Ugander M, Allansdotter-Johnsson A, Friberg P, Arheden H. Age and gender specific normal values of left ventricular mass, volume and function for gradient echo magnetic resonance imaging: a cross sectional study. BMC Medical Imaging 2009, 9:2 doi:10.1186/1471-2342-9-2
3. Ejection Fraction Heart Failure Measurement. http://www.heart.org/HEARTORG/Conditions/HeartFailure/SymptomsDiagnosisofHeartFailure/Ejection-Fraction-Heart-Failure-Measurement_UCM_306339_Article.jsp
4. Overgaard CB, Dzavík V. Inotropes and Vasopressors : Review of Physiology and Clinical Use in Cardiovascular Disease. Circulation. 2008;118:1047-1056. http://http://circ.ahajournals.org/content/118/10/1047.full
5. Kozik DJ, Plunkett MD. Mechanical circulatory support. Organogenesis 7:1, 50-63; January/February/March 2011. http://http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3082034/
6. MacDonald RD, Farquhar S. Transfer of Intra-Aortic Balloon Pump-Dependent Patients by Paramedics. PEC. 2005;9(4):449-453.
7. Sinclair TD, Werman HA. Transfer of patients dependent on an intra-aortic balloon pump using critical care services. Air Med J. 2009;28:40-46.
This article was originally posted Aug. 26, 2013. It has been updated.
