Cardiac Arrest Management: Part 1
You've just finished renewing your CPR card at the firehouse, and now you're sitting in the day room with your classmates. One of them is frustrated that the guidelines keep changing. "First it's five compressions to one breath," he says. "Then it's 15 to two, and now 30 to two. And what about shocking first? They just can't seem to make up their minds." You liked the class, but you can't help thinking about all the cardiac arrests you've been on over the years. For all the changes in CPR and the new toys and drugs paramedics use now, very few patients seem to get pulses back and walk out of the hospital. You wonder if things will change with the new guidelines.
Despite advances in care over the years, survival from out-of-hospital cardiac arrest remains low, averaging about 6% worldwide.1 It's easy to get cynical about all the changes recommended by the American Heart Association (AHA) and wonder if the latest ones will make any difference. However, there are several things that make this revision of the AHA's Basic Life Support (BLS) and Advanced Cardiac Life Support (ACLS) guidelines different.
First, the science behind cardiac arrests is more robust and the pathophysiology is better understood now. An international committee of experts met and closely scrutinized studies of cardiac arrests to see what works, what might work and what doesn't.1 Second, there is new insight into what happens to patients in cardiac arrest, which helps determine what treatment options are most likely to work.2
Looking at what has worked in the past and understanding what's happening to the body during cardiac arrest, it is clear that EMS must rethink its approach to managing these cases. The key is to prioritize our interventions and perfect the ones that are most important.
THE THREE PHASES OF CARDIAC ARRESTS
The vast majority of sudden cardiac arrests begin with ventricular fibrillation (v-fib), where electrical impulses chaotically fire from all parts of the heart instead of the normal pathways. This causes the heart to "quiver," and no blood can be pumped from it. An AED will deliver a shock if this rhythm (Figure 1) is detected. The heart may also have electrical activity conducted through normal pathways and appear normal on a monitor when it's really not pumping in response to the electrical impulses. This is known as pulseless electrical activity, or PEA (Figure 2). Finally, there may be no electrical activity in the heart at all, and subsequently no pumping. This is known as asystole, and appears as a flat line on a monitor.
Previous cardiac arrest treatment guidelines focused only on the rhythm displayed. Upon detection of v-fib, shocks were delivered immediately. Now it appears that simply defibrillating anytime v-fib is seen may be not be helpful. A three-phase model of the body's changes during cardiac arrest has been proposed, and the appropriate treatment now depends on the patient's time of collapse or which phase they're in.2
I. Electrical phase (0–5 minutes)—For the first five minutes of a cardiac arrest with v-fib, the best treatment is immediate defibrillation. Because the patient's time down usually isn't known, the only patients who should be defibrillated immediately are those whose arrests were witnessed by EMS.2,3
II. Circulatory phase (5–10 minutes)—If a patient has been in v-fib longer than five minutes, defibrillation is more likely to be successful after a period of CPR. Defibrillating without doing CPR first is more likely to convert the rhythm to asystole or PEA, making the chances of survival even smaller. For most patients in arrest on EMS arrival, it is safe to assume that more than five minutes have passed since collapse.2,3
III. Metabolic phase (after 10 minutes)—The metabolic phase, beginning approximately 10 minutes after the collapse, is the least understood phase. At this point toxins have begun to circulate throughout the body, and systemic cell death has occurred. So far few interventions appear to be effective to help this, and medications currently used for cardiac arrests may actually make cell death worse. The most promising intervention to slow the process of cell death is induced hypothermia, which is currently being tested in some EMS systems.2,3
CPR, DEFIBRILLATION AND VENTILATION
In addition to a better understanding of what happens to the body in cardiac arrest, the latest AHA guidelines reflect a better understanding about how CPR works.
When done perfectly, CPR will circulate a small amount of oxygenated blood to the heart and brain.1 Chest compressions create pressure changes that allow blood to flow through the heart's chambers. With this, blood enters the coronary arteries to perfuse heart muscle. The amount of blood perfusing heart muscle is known as the coronary perfusion pressure (CPP, or "good" pressure). CPP increases with each compression, and we want to get it as high as possible. Each chest compression builds on the last, and any break will rapidly lower CPP. Several compressions will then be needed to get it back to where it was. This is why there's new emphasis on continuous CPR.
Conventional wisdom says that breathing is as important as a heartbeat. While oxygenation is important, it appears that many patients are ventilated too often and too forcefully during CPR. This is very harmful.
First, many patients still breathe on their own for a period after their hearts stop, which appears as gasping respirations.1,3 These are known as agonal respirations, and the guidelines emphasize the need to recognize cardiac arrest in patients who have them. These patients need chest compressions to help their hearts much more than ventilation to help their breathing.3,4 Also, chest compressions create pressure changes that draw air into and out of the lungs.
Remember that less blood is circulated though the lungs with CPR than when the heart is beating on its own, and the goal of CPR is to get oxygenated blood to the heart and brain. This requires much less oxygen than a person who is alive and has oxygen delivered to all their organs.1 The air exchange produced from gasping respirations and chest compressions may be adequate in the beginning of the arrest.
Many problems have been associated with positive-pressure ventilation delivered during CPR. First, the break in chest compressions to ventilate lowers CPP. Second, forcing air into the lungs increases intrathoracic ("bad") pressure—pressure within the chest that the heart must pump against.
That said, patients in whom cardiac arrest was caused by a respiratory arrest, such as drowning, will benefit more from positive-pressure ventilation than those whose arrests were caused by cardiac events. The AHA has recommended a compromise for this by increasing the compression-to-ventilation ratio to 30:2 and minimizing breaks in compressions to ventilate. It also stresses ventilating the patient for only one second, to limit the increase of harmful pressure in the chest.
A higher compression-to-ventilation ratio is an attempt to balance delivering enough oxygen to the lungs with keeping intrathoracic pressure as low as possible.1
Defibrillation is the most effective way to stop the electrical chaos going on in the heart during v-fib. It works by depolarizing all myocardial cells, which stops all electrical activity in the heart. This gives the heart's natural pacemaker the opportunity to reset an organized rhythm. Defibrillation is considered successful when it stops ventricular fibrillation for five seconds or more, even though there may not be a perfusing rhythm after v-fib is stopped.
What has changed is when we believe the best time to defibrillate is, and how many shocks should be delivered. Again, if the arrest is witnessed by EMS, it is best to defibrillate immediately. If the down time is not known or more than five minutes have passed, it is best to perform two minutes or five 30:2 cycles of CPR before defibrillation. To limit interruptions in chest compressions, deliver one shock instead of stacked shocks.
There are two types of defibrillators used today: biphasic and monophasic. Virtually all those manufactured today are biphasic. These deliver shocks from two directions and have been shown to terminate ventricular fibrillation with less energy than their monophasic counterparts. The optimal biphasic energy setting is still being studied, and each manufacturer will provide a recommendation for its model.
Monophasic defibrillators can still be found on many ambulances, though they are being phased out. They deliver energy from one direction, and the recommended setting is 360 joules on all models.
The AHA now recommends two minutes of CPR immediately after defibrillation. Resist the urge to see what rhythm the patient is in or check for a pulse; in this case it's OK to do CPR for a short time even if they have one. Unless the patient wakes up after defibrillation or has been down for a short time, an organized rhythm and pulse immediately after defibrillation aren't likely to last long without some help. It is also likely that the patient will be hypotensive and can be helped by two more minutes of CPR.
ALTERNATIVES TO CPR
The role of artificial ventilation in cardiac arrests is controversial. The AHA acknowledges that the ideal compression-to-ventilation ratio is not known, and determining it is subject to more research. Dr. Gordon Ewy, of the CPR Research Group at the University of Arizona's Sarver Heart Center, advocates an alternative method of CPR known as cardio-cerebral resuscitation. Here the breathing portion of CPR has been eliminated, and people are taught to deliver chest compressions alone at a rate of 100 a minute. Support for this approach comes from studies done on pigs in a laboratory setting and one done with EMS systems in rural Wisconsin. The EMS study showed significantly improved long-term survival in humans receiving compression-only CPR.4
CPR doesn't seem difficult—it's just pumping and blowing. However, studies have shown that many healthcare providers, both in and out of hospital, perform CPR poorly.1 On average, as many as 40% of compressions aren't deep enough, performance is subject to long pauses, and patients are hyperventilated.1 That has led to several points of emphasis in the latest guidelines.
To perform chest compressions most effectively, the rescuer must place the heel of one hand on the lower half of the sternum (between the nipples on males) and the heel of the other hand over it (Figure 3). The chest should be compressed 1½–2 inches. There is a new emphasis on allowing the chest to completely recoil, or return to its normal position, between compressions. The compression and recoil times should be equal, which allows blood to fill the heart before being pushed out.1
While it's important to deliver oxygen to the lungs during CPR, many problems have been found with use of positive-pressure ventilation (usually done with a bag-valve mask connected to 100% oxygen). The goal of positive-pressure ventilation is to get oxygen to the lungs and keep it out of the stomach. This is best accomplished by maintaining the head-tilt/chin-lift position and using oral and nasal airway adjuncts. The mask should be held tight on the patient's face so that no air leaks out the side. This is so important that one rescuer should be dedicated solely to holding the head in position and maintaining a good mask seal. The bag should be squeezed just enough to see a visible chest rise (about 500–600 ml).1 Each breath should be delivered over one second, so count "one-one thousand" to yourself as you squeeze.
Before an advanced airway is placed, ventilation must be coordinated with breaks in compressions. Ideally, compressions should be stopped for no more than four seconds to deliver two ventilations. When an advanced airway is placed, ventilation should be delivered 8–10 times a minute with no break in chest compressions. Do not hyperventilate!
Everyone knows that the rate of chest compressions is 100 a minute, but it can be challenging to keep track of that in the excitement of working an arrest. The last time I recertified with my agency, we all did compressions much too fast. Devices are available that monitor compression and ventilation rates, and metronomes can be purchased and set to 100 beats per minute. Another method of pacing yourself is to choose a song that has a bass line of 100 beats per minute and sing it to yourself as you do compressions. Ironically, two such songs are the Bee Gees' "Stayin' Alive" and "Another One Bites the Dust" by Queen.
An important issue in CPR is rescuer fatigue. It is believed that rescuers begin to get fatigued after one minute of chest compressions, though they may not feel it until several minutes have passed.1 When fatigue sets in, the relaxation phase of CPR is compromised first. Because there must be a pause every two minutes to analyze the rhythm, it's a good idea to use this pause to change positions, even if the rescuer doing compressions doesn't feel tired.
A great deal of research has demonstrated the importance of high-quality BLS care for cardiac arrests. Like many procedures in EMS, performing high-quality CPR requires teamwork, attention to detail and lots of practice.
Next Month: The Role of Advanced Life Support
1. American Heart Association. 2005 American Heart Association guidelines for cardiopulmonary resuscitation and emergency cardiovascular care. Circ 112 (suppl): IV1–IV211, 2005.
2. Weisfeldt M, Becker L. Resuscitation after cardiac arrest: A 3-phase time-sensitive model. JAMA 288: 3,035–38, 2002.
3. Ewy G. Cardiocerebral resuscitation: The new cardiopulmonary resuscitation. Circ 111: 2,134–42, 2005.
4. Kellum W, Dennedy K, Ewy G. Cardiocerebral resuscitation improves survival of patient with out-of-hospital cardiac arrest. Am J Med 119: 335–40, 2006.
Robert E. Sullivan, NREMT-P, is a paramedic with New Castle County (DE) EMS and is a CPR and ACLS instructor. Contact him at firstname.lastname@example.org.