MEDICAL MATTERS:Chemotherapy and radiotherapy affect more than the tumour they are aimed at, writes MUIRIS HOUSTON
CANCER AND heart disease, which together account for two-thirds of all deaths in the Republic, vie with each other for healthcare funding, charitable donations and, indeed, public attention. Although mortality from heart disease has dropped dramatically in the past decade, the Irish Heart Foundation recently felt obliged to assert that cardiovascular disease is the number one cause of death in the Republic (Letters, December 10th).
But deaths from cancer continue to increase and already outnumber deaths from heart disease and stroke here. Taken with ministerial priority for cancer control and a dated national cardiovascular strategy, it helps explain the current prominence of cancer in comparison to cardiovascular disease.
Both are quite distinct illnesses, albeit with common contributors in the form of obesity, smoking and diet. And when it comes to specific treatments, there is limited overlap between the two specialties. So I was interested to see a recent review article in the Journal of the National Cancer Institute which looked at the cardiotoxicity of anticancer drugs.
Due to our ageing population it is increasingly probable that a person may have both cancer and cardiovascular disease during their lifetime. A patient with a tumour who undergoes cancer treatment or chemoprevention is now at a substantial risk for the deterioration of his or her cardiovascular health. In the past, this risk was less evident because the life span of a patient with widespread disease was often too short to make the cardiovascular complications a major matter of concern.
However, now that progress has been made in terms of early diagnosis and survival, targeted drugs and combinations of two or three different agents have emerged. As a result, cardiotoxicity – drug toxicity that affects the heart – has moved to centre stage.
Not surprisingly, given their powerful nature, chemotherapy drugs affect more than just the tumour they are aimed at. A combination of drugs often amplifies cardiotoxicity, and radiotherapy can also cause heart problems, particularly when combined with chemotherapy. Oncologists must be fully aware of cardiovascular risks to avoid adverse cardiovascular effects, and cardiologists must help by performing evaluations relevant to the choice of therapy.
Acute cardiac effects include the sudden onset of heart rhythm disturbances, acute blockages in the coronary arteries and inflammation of the heart muscle. Chronic cardiotoxicity may cause an asymptomatic dysfunction of the left side of the heart that can lead to severe congestive heart failure. Many drugs can induce heart problems.
The drug 5-Fluorouracil has been linked to a reduced supply of oxygen to heart muscle, while cyclophosphamide may cause mild valve problems. And newer agents such as monoclonal antibodies are associated with heart failure and high blood pressure.
It’s not all bad news. Tamoxifen, given to hundreds of thousands of women to help prevent breast cancer recurrence, helps reduce total cholesterol levels. There is some evidence it reduces the incidence of heart attack. However, there is no persistent association between taking the drug and the prevention of heart disease. Indeed, tamoxifen may have a detrimental effect, with a substantially increased risk of venous clots, pulmonary embolism, and stroke due to its effect on oestrogen receptors.
What are the mechanisms by which cancer drugs damage the heart? Some drugs directly damage cardiac muscle cells. Others affect the coagulation system and can promote blood clotting in the vessels, leading to a lack of oxygen for the heart and brain. Damage to peripheral vessels can impair circulation and predispose to thrombosis.
Many cancer drugs work by preventing the formation of new blood vessels around tumours and are associated with changes in vessel structure and possibly with bleeding and haemorrhage. Hypertension, also a side effect of this type of drug, causes thickening of heart muscle. Atrial fibrillation, a rhythm disturbance common in older people, can be exacerbated by cancer treatments.
In the past, cancer specialists weren’t aware of cardiac complications for the simple reason that patients did not survive as long after a diagnosis of cancer. Even now, in cases with a very poor prognosis, cardiovascular risks are less important. But for those cancers with high probability of long-term survival, such as breast and prostate cancers, it is very important to consider cardiovascular risks. Most cancer patients should be offered a baseline cardiac assessment to help prevent adverse effects from chemotherapy agents.