Suddenly, Your Heart Stops and…?
The danger of asymptomatic arrhythmia’s in seemingly healthy people and how genetics can help.
March 17, 2012. Bolton and Tottenham Hotspur meet at White Hart Lane football stadium to compete in the FA Cup quarter finals. 43 minutes into the match Fabrice Muamba, a Bolton midfielder, collapsed on the pitch. Out of nowhere he “fell like a tree trunk. He didn’t put his arms out to break his fall, or anything, he just dropped.” His heart stopped for 78 minutes. For 1 hour and 18 minutes he was dead. It’s only after he reached the hospital it was uncovered he had suffered cardiac arrest from ventricular fibrillation. Muamba is one of the lucky ones, he received CPR seconds after he collapsed and was given repetitive defibrillator shocks on the pitch and in the ambulance; he went on to make a full recovery. There are many other people, not just athletes, that die from arrhythmias and this could be prevented using genetics.
An arrhythmia is irregular heartbeat. Individuals who experience it become lightheaded and dizzy along with having heart palpitations. It’s possible to experience harmless arrhythmias temporarily; everyone is familiar with the feeling of an extra or missed heartbeat. However, other arrhythmias are asymptomatic. These cases pose the most threat because they are usually diagnosed when the damage has been done and it’s too late.
Yes, arrhythmias are dangerous but why should we care about treating them if there are numerous other greater threats to the body? Sudden cardiac deaths and arrhythmia account for up to 20% of all deaths and it’s slowly becoming the most common cause of death worldwide.
Heart conduction consists of an electrical activity pattern that’s specific and structured to ensure an optimal mechanical pumping of the heart. The heart consists of four chambers: two atria and two ventricles, between the atria lies the sinoatrial node (SAN) and at the top of the ventricles is the atrioventricular node (AVN). An electrical signal passes through the SAN to allow the atria to contract first and then signals to the AVN for the ventricles to follow suit.
This process is a closed-circuit loop with no breaks, an arrhythmia is a disruption of this process. Imagine a train journey, the trains must stop at each station (the SAN and AVN) to allow passengers on and off (electrical signal), if there is a signal failure where the trains can’t communicate, it stops and disrupts the service of all other trains behind it — just as a disruption in the electrical signal prevents the heart from beating efficiently.
This electrical activity can actually be measured using an electrocardiogram (ECG), which reflects the conduction and produces a trace which doctors can use to pinpoint any possible problems. Each wave indicates a different part of the heart contracting or relaxing, an absence or extra wave and frequency highlights a problem and depending on which wave it is, corresponds to which part of the heart has the issue. ECGs are important because they are used as a diagnostic tool.
Diet and lifestyle are involved in causing arrhythmias — excessive alcohol consumption, obesity and smoking are risk factors. These are more manageable and have more apparent warning signs to healthcare professionals therefore easier to catch early. This doesn’t explain why a seemingly healthy person, would suffer a cardiac arrest from an arrhythmia; this is where your genes come into play.
Every person has their own unique set of genes from their parents which form their genetic makeup. Some variations of genes mean they can inherit a susceptibility to certain conditions such as arrhythmias. This means despite a conscious effort by an individual to lead a healthy life the genes they possess will still predispose them to have an arrhythmia. To combat this problem there are genetic tests which can identify the different variations of genes that will increase your risk of having an arrhythmia.
The only way scientists know which genes to look out for is thanks to previous research on families who have arrhythmias — testing for similar genetic variants helped to identify these risky genes. Over half of unexplained sudden cardiac deaths are found to be caused by a hereditary arrhythmia syndrome after the autopsy.
To permit genetic testing as a standard diagnostic method isn’t as straightforward as scientists would have liked. Not all variants which cause arrhythmia have been found so the tests are still not 100% guaranteed. There are also ethical issues that arise with owning the right to your genome versus the temptation companies would have in selling this information for a profit. The law in the UK states that analysis of DNA of a living person without consent is illegal under the Human Tissue Act (2004) which is another barrier doctors will have to jump through to be able incorporate genetic testing into regular checkups.
“A person commits an offence if he has any bodily material intending that any human DNA in the material be analysed without qualifying consent”
Even through prevention is the best treatment there are still ways to minimise arrhythmias with other interventions while we wait for genetic tests to become standard practice. Electrical cardioversion can restart the heart’s normal rhythm by delivering electrical shocks which reset the conduction pathway. Prescription of the anti-arrhythmic drug Amiodarone increases the time between the end of one cardiac action potential and the beginning of another to prevent electrical disruptions — it’s giving the train in front more time to completely leave the platform before another arrives at the station.
These are great temporary fixes but until more research is conducted to find all the genetic variants seemingly unexplainable deaths of fit individuals will keep happening. Genetics can help to improve the prediction of arrhythmic risk with the help of ECGs, and will, eventually.
References: Brooker, R. (2012). Concept of Genetics. 1st ed. New York: McGraw Hill.
Fu, D. (2015). Cardiac Arrhythmias: Diagnosis, Symptoms, and Treatments. Cell Biochemistry and Biophysics, 73(2), p.291–296.
