Toxicology for Writers: Cardiotoxins

Cardiotoxins: How The Heart Works

Welcome back to toxicology for writers, covering cardiotoxins. The heart is vital, if it is not beating, we are dead. It works away, pumping blood around our bodies to deliver oxygen and nutrition to every cell. The heart is surprisingly resilient and autonomous: it has its own internal pacemaker that works even with no input from the brain.

• Mechanisms of toxicity
• Cardiotoxic Medications
• Local Anaesthetics
• Digitalis
• Other Toxins
• Snake Venom
• End

There are three kinds of muscle in the body, skeletal muscle, smooth muscle and cardiac muscle. Nerves tell muscles to contract via electrical impulses, and the heart is no different.

The brain controls the heart rate, largely via the brainstem. This is why people who have suffered severe damage to the cerebrum can still be ‘alive’ while being ‘brain-dead’. There’s a massively complex system of nerves, hormones and feedback that controls heart rate and blood pressure. This system relies on electrolyte balance, oxygen supply and function of the autonomic nervous system. The sinoatrial node and the atrioventricular node ensure the signal is sent to the heart to beat and pump blood. These nodes also act as internal pacemakers independent of the nervous system. A valve system ensures the blood flows in the correct direction. The coronary arteries and veins provide blood to the heart walls to ensure the cells keep working.

Cardiotoxins: Mechanisms of Toxicity

So you can see how easy it is to damage the heart and potentially kill someone. With this in mind, we can group toxins by their mechanism of action. Arrhythmic agents disrupt electrical signaling, leading to abnormal heart rhythms which can be fatal if they prevent proper flow of blood. Anything that damages the myocardium, the muscle of the heart, will result in weakened function and eventually heart failure. Electrolyte imbalances (salt) can have a serious impact on heart function as the muscles and nerves need a certain level of sodium, calcium and potassium ions to function.

A lot of medications that are used to treat certain heart diseases can also cause problems. Dose control and monitoring is therefore essential. On a related note, when you have someone in cardiac arrest (which is not a heart attack), and their ECG/EKG is flatlining, a defibrillator will do nothing. Only some abnormal rhythms are ‘shockable’ and if they are flatlining, you need chest compressions. I will cover cardiac arrest and CPR in more detail in a separate section of the site.

Let’s move on to some specific agents that are cardiotoxic. Like with neurotoxins, a lot of these are medications or illegal drugs. The list also includes certain snake venoms and even plants. One of which is a pretty and common wildflower in the UK.

Return to the top here.

Cardiotoxic Medications

We know that chemotherapy drugs can have very unpleasant side effects. There are a number that specifically have cardiac side effects, such as doxorubicin and cyclophosphamide. Often a full cardiac work up will be done prior to someone starting these drugs. A pre-existing cardiac problem would increase the risk of these side effects.

A few antipsychotics and antidepressants affect the electrical profile of the heartbeat, by prolonging the QT interval. This reduces the efficiency of the heart function and will exacerbate underlying heart issues and affect the safety of anaesthesia. Examples include: amitriptyline (tricyclic antidepressant), haloperidol (antipsychotic) and erythromycin (antibiotic).

Return to the top here.

Local Anaesthetics

This includes drugs used to numb areas of the body for procedures or surgery such as lidocaine and bupivicaine. Cocaine, which is illegal, is also in the same class. Cocaine is derived from a plant and was once used medically before better drugs were developed. Lidocaine is used to treat abnormal heart rhythms, but if given directly into the bloodstream it can also cause abnormal heart rhythms. This can be fatal if overdosed, but it requires IV injection.

Cocaine used recreationally will increase the risk of a heart attack, stroke, or sudden cardiac death. It has been known for young healthy people to die of cardiac arrest due to using this drug recreationally. 0 out of 10, do not recommend this. A reminder that this is just for information to aid in fiction writing. You should never break the law and you should never use illegal substances like this. Ever. But it is something to be aware of.

Return to the top here.

Digitalis Toxin

A pretty flower, isn’t it? This is the Foxglove, a member of the Digitalis genus of plants. Beautiful, and very common, certainly in the UK where it grows wild. This plant is toxic however, as it produces a substance called a ‘cardiac glycoside’, known as digoxin. This substance slows the heart rate (bradycardia) and can cause abnormal heart rhythms including heart block.

This means it can be deadly to consume part of this plant, for both humans and animals. However, digoxin is used as a medication to treat abnormal heart rhythms. Remember, the dose makes the poison! I will keep repeating this point. Digoxin has a very narrow therapeutic index, meaning the safe and effective dose range is narrow, and it’s easy to overdose. Certainly a plant derived toxin like this one would be found in many fantasy worlds. The plants are native across Europe, Western Asia and North Western Africa.

Return to the top here.

Other Toxins

Heavy metals and pesticides, as discussed in neurotoxins, also damage the heart and are considered cardiotoxic. Diphtheria toxin targets the heart as well. Radiation can induce heart disease as well, which may be relevant in cases of nuclear war or nuclear accidents. An overdose of potassium will cause abnormal heart rhythm if given intravenously. Conversely low sodium, potassium and/or calcium, will also cause abnormal rhythms and even death. Low calcium can be caused by vitamin D deficiencies or other diseases.

Low sodium and potassium caused by water toxicity is also fatal. This requires drinking a huge amount of water in a short timeframe. This dilutes the electrolytes and overwhelms the kidneys. When sodium and potassium is too low, known as hyponatraemia and hypokalaemia in medical terms, cardiac arrhythmias such as ventricular tachycardia can develop. Calcium and magnesium are also essential for normal cardiac function.

Remember that the dose makes the poison. This is true for potassium especially. If it is too low, affects the heart, but if it is too high, it also affects the heart. In a medical setting, overcorrecting low potassium or giving it too quickly will risk cardiac arrest. This is something that can be difficult to identify during post-mortem as after death, electrolyte contents within the body change quickly. Again, this information is to inform writers of fiction.

But it can be caused accidentally if the dose calculation is wrong. Other medicines can affect electrolyte levels too, including diuretics like spironolactone and furosemide. Over-supplementation with electrolytes and salts is a risk as well. With regards to calcium, over-supplementation with vitamin D or vitamin A is a common cause. These vitamins are essential, but they are also toxic if overdosed.

Return to the top here.

Snake Venom Cardiotoxins

Some snake venoms are known cardiotoxins, particularly from cobra species and mambas. A well studied example is the ‘cardiotoxin III’. This is an example of a toxic ‘polypeptide’ or protein. The molecule binds to certain types of cells and causes lysis, essentially causing the cells to burst. This molecule binds to myocardial cells (heart muscle cells) and human T-cell lymphocytes (a type of white blood cell involved in the immune system) and induces apoptosis, cell death. This venom is being studied for potential use in cancer treatments if it can target specific cell types.

Return to the top here.

Thanks For Reading

I hope this information was interesting and useful. As always, a reminder that this information is provided for research purposes only. I am providing information on toxicology for writers, this page is on cardiotoxins. The aim is to give some general pointers for fictional world building and writing, and should not be used in real life. It is also not intended as a replacement for appropriate medical advice. If you are concerned a person or animal has ingested a toxin, you must seek qualified medical or veterinary advice immediately.

That said, next up we will be covering haemotoxins, toxins that affect the blood cells. I spell it the UK English way, so you may find it written as hemotoxin elsewhere, which is the US English way. Both are correct, but I am from the UK and studied veterinary medicine in the UK so I use those spellings.

I’ve covered neurotoxins already, which can be found here. Return to the top here.