Ozone levels linked to cardiac arrest

Cardiac arrests are more likely when levels of air pollution - especially soot-like particles and ozone - have been high in recent days or even hours, according to a large study from Texas.

Evidence already links airborne particles with heart disease and lung problems but the new findings are the first to show that high ozone may immediately raise the risk that a person’s heart will stop beating.

“Heart patients should consider when there are high ozone levels that they should take extra care of themselves,” lead author Katherine Ensor of Rice University in Houston told Reuters Health.

About 300,000 Americans experience cardiac arrest - when the heart abruptly stops and therefore can’t get blood to the rest of the body - outside of hospitals each year and less than 10 percent survive. Cardiac arrest can be caused by electrical problems in the heart muscle, sudden trauma or longstanding disease.

Previous studies have found that living in polluted cities or near highways for many years can raise the risk of heart disease in general, but they mainly point the finger at small airborne particles. Ozone is more often associated with short-term worsening of asthma and other lung diseases.

To see whether various air pollutants have any direct effect on cardiac arrest rates, Ensor and her colleagues compared a database of cardiac arrests that took place outside of hospitals in Houston with air quality records for the city between 2004 and 2011.

A cardiac arrest, also known as cardiopulmonary arrest or circulatory arrest, is the abrupt cessation of normal circulation of the blood due to failure of the heart to contract effectively during systole.

A cardiac arrest is different from (but may be caused by) a heart attack (myocardial infarction), where blood flow to the still-beating heart is interrupted (as in cardiogenic shock).

Arrested blood circulation prevents delivery of oxygen to ‘‘all’’ parts of the body. Cerebral hypoxia, or lack of oxygen supply to the brain, causes victims to lose consciousness and to stop normal breathing, although agonal breathing may still occur. Brain injury is likely if cardiac arrest is untreated for more than five minutes, although new treatments such as induced hypothermia have begun to extend this time. To improve survival and neurological recovery immediate response is paramount.

Cardiac arrest is a medical emergency that, in certain groups of patients, is potentially reversible if treated early enough. When unexpected cardiac arrest leads to death this is called sudden cardiac death (SCD).

Among the more than 11,000 cardiac arrests without an obvious cause (such as a traumatic injury), researchers found a slight rise when ozone levels where higher than usual.

Cardiac arrest risk went up by 4.4 percent for every 20 parts per billion of ozone above average within the previous three hours, according to the results published in the journal Circulation.

A difference of 20 ppb in ozone would be significant, according to Ensor. “In general I think people would notice, the air would feel thick,” she said.

Summer ozone levels typically hover between 50 and 60 ppb in the U.S., according to a 10-year study by the U.S. Environmental Protection Agency (EPA). But lung function in healthy people can start to suffer at about 70 ppb, which is still within the U.S. National Ambient Air Quality Standard.

Ensor’s team found a similar rise in cardiac arrest risk with elevated small-particle pollution. For every increase by 6 micrograms of fine particulates per cubic meter of air in the prior two days, cardiac arrests rose by 4.6 percent.

What is the difference between a heart attack and a cardiac arrest?

We often hear in news reports that some well-known person has died of a “heart attack” or a “cardiac arrest.” Or worse, after the sudden death of a loved one, a doctor might use one of these two terms to describe the cause of death. Unfortunately, these terms often seem to be used almost interchangeably by news reporters (and even, disturbingly, by some doctors). They are two different things.

A heart attack, or myocardial infarction, occurs when a coronary artery (one of the arteries that supplies blood to the heart muscle) becomes suddenly blocked. The sudden blockage robs a portion of the heart muscle of its vital blood supply, and the muscle dies. So a heart attack is the death of a part of the heart muscle.

The sudden blockage of a coronary artery is usually caused by the rupture of a plaque in the artery. Plaque rupture can produce a variety of clinical conditions - including heart attacks and unstable angina - that are lumped together under the name acute coronary syndrome (ACS). All forms of ACS are medical emergencies, and they generally need to be treated with medications, angioplasty and stenting, or surgery to restore normal blood flow through the blocked artery. If blood flow can be restored within a few hours, permanent damage to the heart muscle can usually be minimized.

A cardiac arrest, in contrast, is caused by a sudden heart arrhythmia called ventricular fibrillation. In ventricular fibrillation, the electrical signals within the heart suddenly become completely chaotic. Because these electrical signals control the timing and the organization of the heartbeat, when those signals degenerate to total chaos, the heart suddenly stops beating. That is, it goes into “cardiac arrest.” The most common outcome of a cardiac arrest is sudden death.

A 2010 study from New York City found comparable effects for particulate pollution: cardiac arrests rose by between four and 10 percent with every 10-microgram-per-cubic-meter increase in fine particulates.

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