Editor’s note:The ROC PRIMED clinical trial (partly involving the ITD) has ended its enrollment period early, and preliminary data suggested neither strategy under investigation significantly improved survival. Read the press release and follow-up from the JEMS editors.
This clinical review feature article is presented in conjunction with the Department of Emergency Medicine Education at the University of Texas Southwestern Medical Center, Dallas.
- Briefly describe the heart and thoracic pump theories as they relate to CPR and the production of forward blood flow.
- Describe how inadequate CPR techniques affect circulation and perfusion to the heart and brain.
- List factors that increase and decrease the effectiveness of CPR.
- Describe how impedance threshold devices (ITDs) work to improve blood circulation and increase the volume of forward blood flow,
Heart pump theory:The theory that forward blood flow during CPR is the result of direct compression of theheart between the sternum and spinal column.
Mueller’s maneuver: A forced inspiration against a closed airway or glottis that decreases intrathoracic and intrapulmonary pressures. During this diagnostic maneuver, patients attempt to breathe while the mouth is closed and the nostrils are pinched or plugged shut.”ž
Sham device: In experimental studies, this refers to a device that looks like a real device but doesn_t function as one.
Thoracic pump theory: The theory that forward blood flow during CPR occurs when increased intrathoracic pressure created by chest compressions moves blood out of the thorax, through open arteries, and into major organs.
According to 2004 statistics, more than 300,000 Americans suffer an out-of-hospital cardiac arrest each year. About one-fourth of those patients receive bystander CPR, and a tiny fraction receive bystander defibrillation.
Although the first reports of closed-chest cardiac compressions appeared in the mid-1800s, it would be more than 100 years before the procedure would become a widely recognized therapy. Simultaneous with the development of modern CPR, researchers reported long-term survival rates for in-patient cardiac arrest to be about 28%. Despite the fact that CPR has become more widespread, and experts have periodically updated treatment algorithms, there have been no appreciable increases in cardiac arrest survival rates since then.
One characteristic that appears to influence the success of resuscitation efforts is the amount of blood flow achieved during CPR. Some studies suggest that resuscitation techniques that create greater volumes of forward blood flow may improve survival. On the other hand, when perfusion is reduced, survival is subsequently reduced. In this article, we will discuss the theories behind forward blood flow along with the theoretical improvements offered through the use of the impedance threshold device (ITD).
The Physiology of Blood Flow During CPR
The goal of CPR is to move adequate volumes of oxygenated blood through an incapacitated cardiovascular system. Currently, two theories attempt to explain how external chest compression produces forward blood flow.
The heart pump theory is the first and oldest theory. This theory holds that direct compression of the heart between the sternum and the spinal column forces blood out of the ventricles. However, using echocardiography, researchers demonstrated that direct compression does not significantly change the size of the left ventricular chamber and therefore does not significantly affect stroke volume. In addition, the mitral valve appears to remain open during cardiac arrest, suggesting that the heart acts less as a pump and more like a conduit for blood flow.
In light of this evidence, researchers have proposed an alternative explanation for blood flow during CPR. The thoracic pump theory suggests that chest compression transmits pressure equally to all the intrathoracic structures, including the heart, lungs and blood vessels. When intrathoracic pressure is higher than extrathoracic pressure, blood flows out of the thorax through open arteries (especially under the influence of epinephrine) and to the major organs.
In actuality, neither theory completely and exclusively explains forward blood flow during CPR. Through direct compression, the heart pump model appears to be the major mechanism that forces blood from the right side of the heart to the lungs. Through increases in intrathoracic pressure, the thoracic pump model is likely responsible for most of the blood flow from the left heart.
Regardless of which theory predominates, both mechanisms cause blood to flow out of the heart and chest during the compression phase of CPR. Blood return to the heart, on the other hand, is accomplished during the relaxation phase of CPR. As the ribs and sternum rebound to their original position, negative pressure develops in the chest. This slight vacuum draws blood into the thorax and delivers it to the right heart.
Poor CPR technique leads to sub-maximal chest recoil, a reduced intrathoracic vacuum and decreased blood return to the heart. Animal studies demonstrate that a CPR cycle consisting of 50% compressions and 50% relaxation (decompressions) produces only about 30-60% of normal blood flow to the brain and only 5-20% of normal blood flow to the myocardium. Even perfectly performed CPR may not provide adequate perfusion to maintain organ system viability for very long. These poor perfusion statistics, especially to the heart muscle itself, in part explains the dismally low cardiac arrest survival rates.
Impediments to Effective CPR
The effectiveness of CPR is highly variable. Many factors in the prehospital environment can affect the ability of the rescue team to provide effective CPR. It’s reasonable to assume any factor that decreases the quality of CPR makes a successful resuscitation less likely.
Evaluations of the efficacy of chest compressions during EMS transport are not encouraging. During transport, EMS personnel deliver approximately half the number of chest compressions currently recommended by the American Heart Association. Stone and Thomas placed a training mannequin in a classroom (stationary group) and one on a stretcher in the back of a moving ambulance (moving group). EMTs performed CPR for five-minute sessions in each of the two study scenarios. Nearly 80% of the compressions delivered by the stationary group were correct (defined as proper hand placement and proper depth) compared with less than 50% of the moving compression group. These studies together suggest that a moving CPR environment, decreases the quality of resuscitation efforts.
Experts have also recently emphasized the importance of minimizing interruptions in chest compressions during the resuscitation attempt. CPR interruptions are common in the prehospital environment, resulting in EMS personnel delivering chest compressions for less than half of the time the patient is in cardiac arrest. Animal models demonstrate that these interruptions decrease both the effectiveness of defibrillation attempts and survivability. Human studies have also demonstrated decreased defibrillation success with frequent or prolonged interruptions in chest compressions.
Although EMS training programs have historically emphasized its importance, researchers have recently reconsidered the value of ventilation during resuscitation from cardiac arrest. Interrupting chest compressions to provide ventilation during CPR decreases coronary perfusion pressures and the number of effective chest compressions that can be delivered. Clinical studies have demonstrated that rescuers frequently over-ventilate cardiac arrest victims, and animal studies confirm the adverse effects on survival created by this excess positive pressure ventilation.
Impedance Threshold Device
As mentioned earlier, resuscitation techniques that increase the volume of forward blood flow generally improve survival rate. One of the most recent devices to promise improvements in blood flow is the impedance threshold device (ITD).
In our earlier discussion, we examined the effect of compression and decompression (recoil) on blood flow. Because air also flows into and out of the chest, the two phases of CPR similarly affect the movement of air. Compression of the chest increases the pressure in the lungs, which forces small puffs of air out of the patient’s open airway. As the chest recoils during the decompression phase of CPR, a slight vacuum sucks the small puff of air back into the airway in an effort to equalize the intra- and extrathoracic pressures.
Immediately after compression when exhalation releases the puff of air, suppose the airway was temporarily blocked or impeded. Chest recoil would proceed normally, but air could not rush into the airway to equalize the pressure. Without that airflow, the rapidly expanding intrathoracic space would pull blood into the thorax from the extrathoracic vessels, resulting in improved blood return to the heart. Greater blood delivery to the heart would ultimately result in greater volumes of blood squeezed out with the next compression.
It’s impractical to expect that rescuers could open the airway during the compression phase of CPR but close it during the relaxation phase. This is where the ITD helps. The device does not interfere with airflow during chest compression but temporarily prevents air from flowing back into the airway during the decompression (recoil) phase of CPR, thereby pulling more blood into the thorax and improving blood return to the heart.
Mueller’s maneuverprovides the model for the ITD. In this maneuver, patients attempt to inhale against a closed glottis, which decreases intrathoracic pressure, thereby delivering more blood to the right heart and enhancing the ability to appreciate right heart murmurs. For cardiac arrest patients incapable of inhalation, the ITD functions like a closed glottis, albeit during the decompression phase of CPR only.
The ITD performs well with a variety of airway control maneuvers, including supraglottic airways. The negative pressures generated when using the device with a facemask are similar to those seen with an endotracheal tube, making the ITD an important circulatory adjunct for EMS providers. Although the device will temporarily restrict airflow into the airway, the valve will open near the end of the decompression phase to allow the intrathoracic and extrathoracic pressures to equalize. The device will not interfere with positive pressure ventilation.
During a resuscitation attempt, rescuers frequently over-ventilate patients, resulting in positive intrathoracic pressure. Over time, this increased pressure reduces blood return to the heart, which creates a non-survivable perfusion deficit for the patient. Impedance of airflow with the ITD actually decreases the flow of respiratory gases into the airway, thereby decreasing pressure in the chest over the duration of the resuscitation effort.
Support for the ITD
In the first human study using ITDs, researchers in Paris randomized 21 asystolic patients to receive either a functional or a non-functional”ž(sham) devicein addition to their standard resuscitation protocol. Rescuers performed active compression-decompression CPR (ACD-CPR) for 30 minutes while administering epinephrine at five-minute intervals. ALS personnel intubated all patients on arrival. At the time of intubation, end-tidal carbon dioxide (EtCO) levels were similar between the two groups of patients. However, as the resuscitation effort progressed, EtCO levels rose faster in the functioning ITD group than in the sham group, suggesting greater blood delivery to the lungs (19 mmHg versus 13 mmHg, p < 0.001). In addition, coronary perfusion pressure improved (43 mmHg versus 25 mmHg, p < 0.001), diastolic blood pressure improved (56 mmHg versus 36 mmHg, p < 0.001), and return of spontaneous circulation almost doubled (36% versus 20%, p < 0.05).
Adding an ITD to the respiratory circuit during conventional prehospital resuscitation doubled the systolic arterial pressure and improved short-term survival for patients experiencing pulseless electrical activity. Results from an English implementation trial utilizing the device during ACD-CPR demonstrated a 50% increase in overall admissions to the emergency department compared with historical controls (34% versus 22%, p < 0.01). Subgroup analysis of that data demonstrated survival rates for asystolic patients treated with the ITD tripled (34% versus 11%, p < 0.001).
Despite the fact that the studies to date are small or inadequately controlled, and no investigations demonstrate long-term survival associated with the addition of ITDs to the respiratory circuit during conventional resuscitation, the AHA has given the device a class IIa rating, which means it’s considered safe, acceptable and probably beneficial for victims of cardiac arrest.
Whether the ITD is efficacious in pediatric patients is unproven. One animal trial evaluated the device in 8à12 kg piglets (roughly equivalent to the average six- to 12-month-old human child). The ITD used with a variation of closed-chest cardiac compression significantly increased coronary perfusion pressure and vital organ blood flow after prolonged ventricular fibrillation.
Two important clinical trials involving the ITD are currently underway. The first is a multi-center trial called the Resuscitation Outcomes Consortium: Prehospital Resuscitation Impedance Valve and Early vs. Delayed Analysis (ROC PRIVED) Trial. This study involves multiple locations across the U.S. and Canada and randomizes patients to receive functional ITDs or non-functional sham devices. Paramedics also analyze the ECG and deliver a countershock in a randomized fashion, employing either an analyze-early (30 seconds of CPR before analyzing) or analyze-later (three minutes of CPR before analyzing) strategy.
The second trial is known as the ResQ Trial and is being conducted at various sites in the U.S. Paramedics in the participating systems are combining the ITD with the ResQPump, a handheld device that assists in providing ACD-CPR. Researchers hope to determine whether ACD-CPR with the ITD improves survival to hospital discharge and neurological recovery compared with conventional CPR. The trial is currently enrolling patients, and data collection may be completed as early as summer 2010.
Use of the ITD augments the negative intrathoracic pressure associated with the decompression phase of CPR. This results in increased blood return to the heart and greater cardiac output with each subsequent chest compression. These actions combine to generate greater blood flow to the vital organs and improve short-term survival rates for victims of cardiac arrest. Despite the absence of data demonstrating long-term benefits, the AHA considers use of the device as “probably useful” and could represent a new standard of care in the near future. JEMS
Kenneth Navarro, M.Ed, is an assistant professor in the emergency medicine education department at the University of Texas Southwestern Medical Center at Dallas. He coordinates all continuing education activities and assists in medical oversight for the BioTel system, a multi-jurisdictional EMS system composed of 14 fire-rescue agencies and more than 1,600 paramedics.
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