Following on from our previous discussions about cardiopulmonary resuscitation (CPR), it is important to understand the clinical evidence and arguments for changing the technique in sudden cardiac arrest (SCA) cases. This third article examines the reasoning and early studies in modifying resuscitation for witnessed cardiac arrest. In fact, it is these very data that prompted the AHA to issue a science advisory enhancing the 2005 CPR guidelines.
CPR is a lifesaving intervention that requires only two willing hands. It is widely taught and often used to maintain an SCA victim’s life force until a defibrillator can shock the victim’s heart back to normal rhythm. There is a growing body of evidence that suggests we can improve survival outcomes by rapidly starting compressions and not interrupting them until we are ready to defibrillate. This means there are no ventilations; no checking for a pulse; and no checking of the airway. Just call 9-1-1 and start pushing down hard and fast. Don’t stop until the EMTs arrive.
Why the change? There are a number of reasons, but foremost is the knowledge that interruptions to the compressions have a serious and damaging effect on the perfusion of the heart muscle and brain. Stopping for only 10 to 15 seconds can cause the already marginal blood pressure and circulation to stop completely. It then takes another 10 seconds or so to bring those levels up to the very minimum to sustain the cells and tissue. This implies that for nearly half a minute the organs do not receive sufficient blood flow. If you then consider that this can occur as often as every minute (30 compressions at 100 compressions per minute), it becomes apparent there is blood flowing only half the time.
In addition, studies have shown that ventilations create a positive pressure in the thorax (the thoracic cavity includes the throat and lungs, separated from the abdomen by the diaphragm) that can prevent the natural return of blood to the heart. Thus the dangerous drop in blood flow caused by interrupting the compressions is compounded by the decrease in venous return.
Gordon Ewy, M.D., Professor and Chief of Cardiology at the University of Arizona Sarver Heart Center, has developed a new technique called cardiocerebral resuscitation that eliminates mouth-to-mouth ventilation for bystander-initiated CPR. There are a number of EMS organizations using the technique, which aims to dramatically reduce the incidence of positive pressure ventilation by EMS responders, and directs them to perform chest compressions prior to and immediately after a single shock for cardiac arrests not witnessed by EMTs.
“There are seven reasons to perform continuous chest compressions (CCC),” Dr. Ewy said. “The first is that bystanders are much more willing to do it, the second is that [an SCA victim’s] lungs will be full of air, the entire arterial system would be full of oxygenated blood…” He went on to say the arterial blood gas (ABG level) would be normal, but as the blood is not circulating, it is not being used by the organs. The third reason is that “it takes 16 seconds to do the two quick breaths” and even highly trained and experienced people performing CPR interrupt the compressions for at least 12 seconds, which Dr. Ewy says is “still too long” for the brain to be deprived of blood flow. “The fourth reason you don’t want assisted ventilation… is because every time I take a breath I generate a vacuum inside my chest, and that’s why air goes in. That same vacuum returns blood from the lower part of my body to my heart. But if you breathe for me… you increase the thoracic pressure and decrease venous return and make things worse.” He explained that whether you use a mask, intubation or just mouth-to-mouth, a positive thoracic pressure results. “The fifth reason is… that you blow so hard that a lot of the air goes into the stomach, and that then causes regurgitation.” Dr. Ewy said research shows that 50% of the time, vomitus is found when mouth-to-mouth has been performed. The sixth reason is gasping or agonal breathing. “Mammals are the only ones that gasp when we’re born and gasp when we’re dying. It is a very good form of respiration because it drops inter-thoracic resistance, so air goes in and blood comes back to the heart. So gasping is a sign of marginal but adequate blood flow.” The last reason relates to the fallibility of relying on animal studies into CPR technique, where the porcine subjects are sedated and thus do not gasp.
Another perspective on whether ventilations are important is to understand the three phases of cardiac arrest. The first, electrical phase lasts about five minutes and defibrillation is the most important intervention. The second phase, circulatory, varies in duration from the fifth minute through to the 15th minute. During this time it is perfusion pressure that is most critical, and defibrillation in this phase often results in a pulseless rhythm called pulseless electrical activity (PEA). The metabolic third phase occurs after 15 minutes of arrest due to VF, and it is at this point that resuscitation is least effective. Generally EMTs arrive after the first phase, when defibrillation has the best chance. Any interruption to circulation during the second phase is detrimental, such as during rescue breathing, especially the delays associated with the AED analyzing and delivering “stacked shocks”.
It is apparent that more research is required, especially longer-term larger sample studies to confirm these initial findings. But in the meantime, should the minimally interrupted compression resuscitation, continuous chest compressions resuscitation, cardiocerebral resuscitation and chest-compression-only CPR be substituted for the current 2005 Guidelines?
In conclusion, it is important for us to appreciate that these advanced techniques are recommended for trained and certified EMS responders, and not designed for layperson or bystander-initiated CPR. That is not to say compression-only is in any way inferior, or that the current AHA guidelines shouldn’t be followed—just that survivability may be improved.
As always, do something. By taking action, you can only help.
- Jeremy Whitehead