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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Sudden Cardiac Arrest Part II: Heart Attack or Cardiac Arrest

By Jeff Danner Posted November 17, 2013 at 5:34 pm

Last week in Part I of this series, I reviewed the structure and function of the heart and let you know that my father suffered a sudden cardiac arrest in April of 2013.  Dad is fine and helping me write this series. This week, I’ll explain the difference between a heart attack and a cardiac arrest, a rather important difference that not many people fully understand.

Your heart has a network of arteries and veins which deliver and withdraw blood as part of your normal circulation; it does not absorb the oxygen it needs for itself from the blood it pumps. Rather, oxygenated blood is provided to the heart through the coronary arteries. Depending on genetic make-up, diet, exercise habits and other factors, many people develop a build-up on the inner walls of their coronary arteries known as plaque, which is composed of white blood cells, cholesterol and fat. The build-up of this plaque is known as coronary artery disease.

In certain situations, particularly during exertion, plaque in the coronary arteries can be deformed or dislodged such that it completely plugs one of the coronary arteries, preventing blood from reaching part of the heart. This is a heart attack. During a heart attack, the portion of the heart which is deprived of oxygen is damaged. This damage can be partially repaired with time, but scar tissue in the affected area will remain. We will return to the importance of scar tissue below.

Before we proceed, I need to give you a little more of my father’s health history. April 2013 is not the beginning of the story. In 1978, Dad was changing planes at the Pittsburgh airport and, at the age of 38, he had a heart attack. Dad was not overweight, was in good physical condition as a frequent squash player, and did not have a history of health issues. So, as you can imagine, this came as quite a shock to us. A coronary artery supplying blood to his left ventricle was blocked, resulting in damage to his heart in this area. After 7 days in the hospital, Dad came home and began what would become a very successful rehabilitation. Over the years, his body rebuilt blood vessels to the affected area so that a recent test revealed no significant loss of blood flow. However, the blood ejection fraction from his heart, a measure of pumping efficiency, has been somewhat below normal, indicating that the damage from the 1978 heart attack continued to have an effect.

As I reviewed last week, your heart rate is controlled by a bundle of nerves known as the sinoatrial node. The sinoatrial node generates electrical pulses which are conveyed via conductive fibers to the four chambers of your heart, causing them to contract at the appropriate time.  A cardiac arrest occurs when there is a disruption to the heart’s electrical system, causing the flow of blood to either cease entirely or to drop to a dangerously low rate.

There are too many potential causes for a cardiac arrest for me to cover them all in this column, so I’ll restrict this discussion to the most common one. Approximately 60-70% of cardiac arrests are caused by damage from a previous heart attack. The heart attack could have occurred either years ago, as in my father’s circumstance, or immediately prior to the arrest. If we return to the analogy of the heart being controlled by electric circuits, any scar tissue from a heart attack introduces extra resistance in the circuits. The presence of this additional resistance can have a dramatic impact, resulting in a nearly instantaneous change in heart function.

In my father’s case, he was playing squash with his friend Joe, who, as I will explain next week, has secured a permanent place among my collection of favorite people. They had only been playing for a few minutes, so Dad’s heart rate was likely around 120 beats per minute. Without warning, the influence of the scar tissue from his long-ago heart attack combined with some age-related changes in heart function to immediately (hence the term “sudden” in sudden cardiac arrest) elevate his heart rhythm to over 200 beats per minute, with each contraction being very shallow. This type of heart rhythm, often referred to as “quivering,” is not strong enough to circulate blood through the body. As soon as Dad’s blood stopped circulating, he collapsed and lost consciousness(1)

At this point, Dad’s situation was extremely serious.  Every day, approximately 800 Americans are struck by sudden cardiac arrest and only 7% survive. Next week in Part III, I’ll review the remarkable story of Dad’s survival and recovery, and what lessons it teaches us about how we can improve these dismal statistics. Next week’s column will quite literally teach you how you can save a life, so don’t miss it.

Have a comment or question?  Use the comment interface below or send me an email to commonscience@chapelboro.com.

(1)       I will be reviewing this point as part of next week’s column in detail, but felt compelled to mention it here as well.  There are a variety of reasons why people either faint or lose consciousness.  Many times, such as in a cardiac arrest, this is part of your body’s defense mechanism.  It’s trying to get you to fall down so that blood can reach your brain more easily.  If you encounter someone who is fainting or otherwise losing consciousness, do not hold them upright. Assist them to the ground and, if possible, elevate their legs.

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