Understanding Cardio-Pulmonary Resuscitation and cardiac arrest

Dr Lori Feldman, DVM
Henry Feldman, MA EMT-M, NREMT-B


Anatomy of the human heart

CPR, or Cardio-Pulmonary Resuscitation, is a set of techniques used to keep a patient, who has suffered a cardiac arrest, viable until more definitive treatment in the form of advanced cardiac life support (ACLS) can arrive. A common misconception is that CPR will actually restart a stopped heart; although this may occur very occasionally, this is not the expected outcome of CPR. To understand CPR, we need to understand what happens when the heart stops.

The heart's job is to circulate blood to the tissues of the body, and thereby deliver oxygen. The body combines this oxygen, with sugar (glucose) to burn as fuel. When the heart stops functioning effectively, breathing will stop shortly afterwards, and the body starts to deplete oxygen stored in the tissues; The body only stores a very small amount of oxygen, and when this runs out, the body reverts to an anaerobic energy supply. Unfortunately, this can only continue for a few minutes in the brain before brain cells start to die (typically 4-6 minutes), and brain damage results. After 10 minutes, under normal conditions, the patient is often neurologically unrevivable. So knowing this, and knowing that CPR won't restart the heart, why would we start CPR?

CPR counts on 2 things: that the heart when mechanically compressed, will pump blood without having a heartbeat of its own; second that exhaled air contains sufficient oxygen to sustain life. Let's examine each of these factors.

The primary problem in a cardiac arrest, is that the heart has stopped pumping blood; to keep the tissues oxygenated, we need to pump for the heart. Outside of the operating room we do this using closed chest compressions. This is performed by placing you hands on the lower half of the sternum (the breastbone) and compressing the chest approximately 1/3 of the body thickness. There is some debate on how this causes the heart to pump: one camp says that it is due to the heart being squeezed between the sternum and the spine, while the other group claims that it is due to increased interthoracic (in the chest) pressure. Regardless of the cause, the result is to force some blood through the lungs and into circulation. While this circulation is insufficient to actually sustain life, it is sufficient to slow the cell death process.

The second part of a cardiac arrest is the respiratory arrest, which is the lack of breathing. A common question is how can exhaled 'waste' air, support life? Air contains approximately 20% oxygen, and under normal conditions the body only uses only 1/4 of this in each breath. This means that the exhaled air contains 15% oxygen, which is sufficient to breath. We know that we can survive breathing 15% oxygenated air, because we can breathe standing on top of tall mountains, where the air is much thinner.

So how effective is CPR at simulating normal circulation? When performed perfectly, CPR only provides 20-40% of normal circulation. How can we improve this? Well, changing the oxygen concentration help some (I won't go into dissociation curves here); for this we use 100% oxygen in place of air, which helps the flow of oxygen into the blood from the lungs. This is done using mechanical respiratory devices, such as a Bag-Valve Mask Resuscitator (often referred to as an Ambu-Bag, after the original brand name) or a positive pressure resuscitator. Another technique that is coming into fashion is mechanical chest compression, such as the "Thumper". All of these techniques and adjuncts are used by medical personnel and are not part of lay-person CPR.

Why doesn't CPR restart the heart? To understand this we need to understand why the heart has stopped. In general (I will be leaving out choking, trauma, etc...) the heart stops in response to an Acute Myocardial Infarction (a heart attack). An AMI is caused by an interruption of blood flow through the coronary arteries to the muscle of the heart; the heart muscle is very oxygen demanding, and even brief interruptions of flow can cause tissue death. The flow is typically interrupted due to a narrowing of the artery, which has become clogged by fatty plaques (athersclerosis or Coronary Artery Disease). This is what the workout, healthy diet and healthy lifestyle that the American Heart Association promotes is designed to prevent.

The heart is electrically controlled by a small group of cells which form a natural pacemaker (the SA node) which then sends a signal through special heart cells designed to transmit this electrical impulse. When heart muscle dies, it can interrupt or distort this flow of electricity, and cause false signals or no signals to reach the rest of the heart. This results in changes in cardiac rhythms, including life threatening arrhythmias such as Ventricular Fibrillation and asystole. In 'V-Fib', the heart muscles are completely out of sync with each other, and the heart quivers. The treatment for this is an electrical shock from a device called a defibrillator, which resets the heart muscle and pacemaker; this is when they grab for the paddles on 'ER'. V-Fib can be recognized on an EKG as random, jagged lines with no regular rhythm. Asystole, or lack of rhythm, is seen as a flat-line on the EKG. This is treated with drugs, and correction of the underlying condition. Without correction of these electrical problems, CPR will not be able to restart the heart; to use a car analogy, if I disconnect the spark plugs, I can turn the starting motor as much as I want, and the engine won't start.

Does CPR work? In communities where a large fraction knows CPR, and have rapid access to ACLS, CPR appears to have around a 50% survival rate. In Seattle, WA where a major fraction of the population know CPR, and where the EMS system is designed to provide rapid ACLS support, we see these save rates. In communities, where there is little lay public CPR training, and little or no rapid ACLS response, the save rates can drop below 15%. We must remember that CPR, simply buys time for the ACLS crew to get there.

In conclusion if you don't know CPR, contact the American Heart Association, Red Cross or National Safety Council to inquire about training in your community. You never know when you might need to use it to save a life of a friend, family member or coworker.