More than 1,000 people suffer out-of-hospital cardiac arrest every day in the United States. After more than 50 years, survival remains dismal in most communities, with less than 8% of people, on average, surviving neurologically intact to hospital discharge. While the use of active compression-decompression cardiopulmonary resuscitation (ACD-CPR) has shown mixed results in improving outcomes, the combination of ACD-CPR with an impedence threshold device (ITD) has been shown to improve overall survival to one year by 49% in adults with non-traumatic cardiac arrest of cardiac etiology.1 We’ll discuss how the device combination works and review the data supporting its use in resuscitation.
Physiology of CPR
Understanding the physiology of CPR helps us understand how ACD-CPR with an ITD works. There are two theories on the mechanism of CPR. The first is the Cardiac Pump Theory. This theory states that when the chest is depressed (compression) the heart is compressed between the sternum and spine, which squeezes the blood out of the heart and into the systemic vasculature. Blood naturally goes back into the heart and the process starts all over again with the next compression. People are most familiar with this theory because it’s been established for quite some time.
The second, newer theory is the Thoracic Pump Theory. This theory states that with each compression of the chest, a resultant positive pressure is created in the chest. This positive pressure is transmitted to the blood inside the heart, and that blood then moves from the relative higher pressure inside the heart to the relative lower pressure of the systemic vasculature. Because compression of the chest increases the pressure to all the structures inside the chest, including the lungs, air in the lungs is expelled. Following chest compression, the chest passively recoils. (See Figure 1 below.)
Compression & decompression phases of CPR
Figure 1: During the compression phase of CPR, positive pressure pushes blood out of the heart (a). During decompression, a negative pressure is created that pulls more blood back to the heart for preload (b).
The Thoracic Pump Theory states that this recoiling of the chest, or decompression, creates a small but very important vacuum (negative pressure), which sucks blood back into the heart, thereby providing preload. This negative pressure also lowers intracranial pressure (ICP) by promoting venous and cerebral spinal fluid drainage from the brain. The lowered ICP results in less resistance to blood flow in the brain, which promotes cerebral perfusion. This alternating positive and negative pressure (modulation of intrathoracic pressure) helps circulate blood until the heart can be restarted.
Unfortunately, studies have shown that even when performed correctly, conventional CPR delivers less than 25% of normal blood flow to the heart and brain.2 Understanding the reasons for this can help us improve the effectiveness of CPR.
Research has helped us to better understand potential issues with CPR. First, an inherent inefficiency of CPR contributes to suboptimal blood flow. When the chest wall recoils, air is drawn in through an open airway, eliminating the vacuum responsible for creating preload. The heart stops filling as soon as the vacuum is neutralized.
Second, poor CPR quality can impact outcomes. Chest compression quality (rate and depth) have been shown to directly impact survival.3 Also, preload is dependent upon passive recoil during the chest decompression phase. If the chest doesn’t recoil completely, the vacuum doesn’t develop and preload is compromised. Inadequate chest wall recoil can be caused by broken ribs, poor chest wall compliance in old age or the weight of fatigued rescuers on a patient’s chest.
The use of ACD-CPR with an ITD during CPR can address both of these inefficiencies.
Active Compression-Decompression CPR
Active compression-decompression CPR (ACD-CPR) was first described by Rudolf Eisenmenger in 1903 when he published an article on a device called the Biomotor, which applied suction and pressure on the abdomen and lower chest to promote breathing and circulation.4 Interest in the Thoracic Pump Theory was rekindled in 1990 when Keith Lurie, MD, and colleagues published the case of a man whose family members resuscitated him by performing CPR with a household toilet plunger.5 This led to the development of an ACD-CPR device called the CardioPump outside the U.S. (See Figure 2 below.)
Figure 2: The ResQPOD ITD and the CardioPump (not available in the United States) ACD-CPR device.
This ACD-CPR device is a handheld device with a suction cup that’s placed on the chest during CPR. As with manual CPR, it’s used to compress the chest to a depth of 2 inches, but instead of relying on the chest wall to recoil passively, rescuers pull up on the handle to provide active decompression of the chest.
This ensures that the chest wall recoils to at least neutral, and even beyond neutral, thereby enhancing the vacuum and improving the amount of blood that’s returned to the heart. The handle contains a force gauge to guide compression and lifting forces, and a metronome to guide compression rate.
Early studies showed that in addition to doing a good job compressing and lifting the chest, ACD-CPR also moved more air in and out of the chest during CPR; in fact, tidal volumes are two to four times that of conventional CPR.6 This was initially seen as a way to both compress and decompress the chest, as well as potentially providing an alternative to having to perform mouth-to-mouth ventilation.
It wasn’t until years later that researchers realized that the movement of air into the chest while pulling up effectively eliminated the vacuum created by chest decompression and, therefore negated the vacuum we were trying so hard to promote. Early studies comparing ACD-CPR alone to conventional standard CPR showed mixed results, with little effect on the enhancement of the intrathoracic vacuum and no significant effect on long-term survival.7 Researchers realized they needed to find a way to move more blood, not more air.
ACD-CPR with an ITD
While studying the ACD-CPR device, researchers discovered that if air was impeded from moving into the chest during the decompression phase, the resultant vacuum was much larger and was sustained for a much longer period of time. This finding led to the development of the impedance threshold device (ResQPOD ITD), a check valve that’s placed between the facemask or advanced airway device, and the ventilation bag or ventilator. It selectively prevents air from being drawn into the chest during the chest decompression phase of CPR. The ITD does not restrict air movement during exhalation and ventilation, but checks during decompression. Preventing air from being drawn down the trachea and into the chest during decompression markedly improves the filling of the heart (preload) and, thus, markedly improves cardiac output during the next compression phase. Further, this increase in negative pressure also significantly lowers ICP, and also promotes significant increase in coronary circulation (which occurs during the decompression phase).
The combination of the ITD with ACD-CPR has shown significant improvements in both hemodynamics and ultimate survival in cardiac arrest.8
The two devices work synergistically to enhance the intrathoracic vacuum during the decompression phase. (See Figure 3 below.) Again, this has three effects:
1. Markedly lowers ICP, which lowers the resistance to forward blood flow, thus improving cerebral perfusion pressure.9
2. Increases preload, which leads to increased cardiac output on the subsequent compression.
3. Improves coronary circulation.
Intrathoracic pressures during CPR
Figure 3: During conventional CPR, a small negative pressure is created during chest wall recoil (a). When ACD-CPR with an ITD is used during CPR, negative pressure is enhanced during chest wall recoil, increasing preload (b).
A carefully conducted, large study showed that all of these improvements in circulation result in an increased likelihood of survival in patients with non-traumatic cardiac arrest. In addition to human outcome trials, animal studies have shown that vital organ blood flow with the combination of ACD-CPR and the ITD is significantly better than when either device is used individually, and can result in normal blood flow to the brain.10–12 (See Figure 4 below.)
Blood flow during CPR
Figure 4: In a porcine model of v fib, ACD-CPR with an ITD resulted in almost quadrupled blood flow to the heart and near normal blood flow to the brain. Note that pre-clinical data may not be indicative of clinical results.
In human trials, Plaisance et al demonstrated that negative intrathoracic pressure could be significantly enhanced with both a facemask and an endotracheal tube during ACD-CPR with an ITD.13 Other human CPR trials have shown that the device combination:
- Provided near-normal systolic and diastolic blood pressures.14,15 (See Figure 5 below.)
- Improved 24-hour survival in witnessed arrest by 78% compared to standard CPR.16
- Improved 24-hour survival by 45% compared to ACD-CPR alone.17
Systolic and diastolic blood pressure in humans
Figure 5: Results from a human study show that near normal blood pressures were achieved with ACD-CPR + ITD.
The largest randomized, controlled trial of ACD-CPR with an ITD to date, called the ResQTrial, showed that improved long-term survival can be a reality.8 This multicenter study compared conventional standard CPR to ACD-CPR with an ITD in more than 1,600 patients who had suffered a cardiac arrest of presumed cardiac etiology. Interestingly, rates of prehospital return of spontaneous circulation and survival to hospital admission were statistically similar; however, survival was 26% higher at hospital discharge, and 49% higher at one year in the group that received the device combination, despite receiving similar hospital care. In the larger cohort of patients who arrested from all non-traumatic causes, survival was 33% better in the group who received ACD-CPR with an ITD at one year.18 The neurological outcome in these patients appeared to be no worse than the control (standard CPR).
The intervention group did experience a higher rate of pulmonary edema, but all other complication rates between groups were similar. Interestingly, the presence of pulmonary edema was actually associated with a more than two-fold increase in survival to hospital discharge with good neurologic function.19 Thus, the ResQTrial clearly showed that ACD-CPR with an ITD is as safe as standard CPR across the board.
Although the four-year trial was prospective and randomized, there were a couple key limitations. First, it wasn’t possible to blind rescuers to the CPR method, though follow-up after resuscitation and out to one year was blinded. Secondly, the quality of CPR wasn’t measured in either group, though the device group had the availability of visual and audible metronomes to guide compression and ventilation rates.
In addition to overall survival, neurologically intact survival was measured during the ResQTrial. Although some of this data is presented below, conclusions and inferences regarding neurologic outcomes cannot be drawn from the ResQTrial due to interpretability issues related to the neurologic component of the data.
Neurologically intact survival was 52% higher at hospital discharge for patients with presumed cardiac etiology who received ACD-CPR + ITD compared to patients who received standard CPR. Similar improvements were measured in the larger cohort of patients who arrested from all non-traumatic causes, both at hospital discharge and one year, as shown in Figure 6 (See below).
Survival in the ResQTrial
Figure 6 shows results from the ResQTrial. Adult patients with cardiac etiology who received ACD-CPR + ITD had a 49% better chance of survival to one year than those who received standard CPR. Adult patients with non-traumatic cardiac arrest showed a 38% increase in survival to one year when they received ACD-CPR + ITD vs. standard CPR.
The trial showed that ACD-CPR with an ITD exhibited cardio- and neuro-protective qualities when used with or without therapeutic hypothermia. When using therapeutic hypothermia and ACD-CPR with an ITD, there was a six-fold improvement (11.1% to 69.2%) in the percentage of patients who improved from poor neurologic status at hospital discharge to favorable neurologic status at 90 days, compared to those patients who received standard CPR with therapeutic hypothermia.20 Conversely, for those patients who didn’t receive therapeutic hypothermia, use of the device combination was independently associated with a nearly two-fold increase in the number of survivors with favorable neurological function at the time of hospital discharge and 90 days after the cardiac arrest.21
Case Study from the ResQTrial
Professor Steve Dunn was working out at his gym in Oshkosh, Wis., when he began experiencing chest pain. He attempted to drive himself to the hospital but went into cardiac arrest and crashed his vehicle in the hospital’s parking lot. EMS crews arrived on scene and began performing CPR using an ACD-CPR device and ITD according to study protocol.
Steve Dunn with his sons
To the medics’ surprise, Dr. Dunn became conscious during CPR and asked them to stop. When they complied, Dunn lost consciousness. Once again they began ACD-CPR with the ITD, and once again Dunn regained consciousness.
Dunn ultimately survived with no neurologic deficits and, thankfully, no memory of the event. Signs of improved levels of consciousness (e.g., gasping, gagging, eye and limb movement) were reported during the ResQTrial and are indicative of how well the brain was being perfused.
While the FDA has not yet approved an ACD-CPR with an ITD System (ResQPUMP ACD-CPR device and ResQPOD ITD) for use in the United States, we hope that the device combination will soon be made available. Use of ACD-CPR with an ITD has been shown to have no increased risk, while at the same time resulting in a marked increase in survival over standard CPR in adult patients with non-traumatic cardiac arrest. With the potential of improved one-year survival of 49% in non-traumatic arrests of cardiac etiology, widespread adoption of the device combination could result in thousands more lives saved each year in the U.S. alone.
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