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More than half of coronavirus patients discharged from hospital may endure heart damage, research suggests.
The infection is thought to be mild in four out of five cases, however, it can trigger a disease called COVID-19.
Initially considered a respiratory virus, the infection is now known to affect many parts of the body, with heart complications being flagged early in the pandemic.
To learn more, scientists from University College London analysed 148 severely-ill coronavirus patients from six hospitals in the capital.
All of the patients had elevated troponin levels while being treated. Troponin is a protein released into the blood when the heart muscle is injured; like in response to inflammation, scarring or a restricted blood supply.
Results of the study reveal just over half (54%) of the patients showed signs of scarring or injury to their cardiovascular muscle at least one month after being discharged.
Although unclear, the immune system may over-react to the coronavirus in severe cases, triggering damaging inflammation.
"We found evidence of high rates of heart muscle injury that could be seen on the scans a month or two after discharge," said lead author Professor Marianna Fontana.
"Whilst some of this may have been pre-existing, MRI scanning shows some were new and likely caused by COVID-19.
"Importantly, the pattern of damage to the heart was variable, suggesting the heart is at risk of different types of injury.
Read more: One in 10 COVID patients has cardiac arrest
"While we detected only a small amount of ongoing injury, we saw injury to the heart that was present even when the heart's pumping function was not impaired and might not have been picked up by other techniques.
"In the most severe cases, there are concerns this injury may increase the risks of heart failure in the future, but more work is needed to investigate this further."
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The scientists investigated severely-ill coronavirus patients, average age 64, who were discharged up to June 2020.
The patients were offered an MRI scan at least one month post-discharge, with the results being compared against individuals who were hospitalised for reasons other than the coronavirus, as well as against healthy volunteers.
"The recovering COVID-19 patients had been very ill," said Professor Fontana. "All required hospitalisation and all had troponin elevation, with around one in three [32%] having been on a ventilator in the intensive care unit."
Raised troponin levels can occur when an artery becomes blocked or the heart inflamed.
Many severely-ill coronavirus patients develop elevated troponin levels during the critical phase of their condition, perhaps due to their immune system over-reacting to the infection.
"Raised troponin levels are associated with worse outcomes in COVID-19 patients," said Professor Fontana.
"Patients with severe COVID-19 disease often have pre-existing heart-related health problems including diabetes, raised blood pressure and obesity.
"During severe COVID-19 infection, however, the heart may also be directly affected.
"Unpicking how the heart can become damaged is difficult, but MRI scans of the heart can identify different patterns of injury, which may enable us to make more accurate diagnoses and to target treatments more effectively."
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The results of the study – published in the European Heart Journal – show the heart's left ventricle, the chamber that pumps oxygen-rich blood around the body, was normal in nearly nine in 10 (89%) of the 148 patients.
Scarring or injury to the cardiovascular muscle was detected in more than half (54%) of the patients, however.
Tissue scarring or injury that could be linked to inflammation was identified in just over a quarter (26%) of the patients, while ischaemic heart disease – when the organ receives insufficient oxygen – affected 22% of the study's participants.
More than one in 20 (6%) of the patients had both of the above conditions.
Around one month post-discharge, 8% of the patients showed signs of ongoing heart inflammation.
"Injury relating to inflammation and scarring of the heart is common in COVID-19 patients with troponin elevation discharged from hospital, but is of limited extent and has little consequence for the heart's function," said Professor Fontana.
"These findings give us two opportunities: firstly, to find ways of preventing the injury in the first place, and from some of the patterns we have seen, blood clotting may be playing a role, for which we have potential treatments."
Scientists from the UK and US recently reported blood-thinning drugs could cut the risk of death by more than a third (34%) among people hospitalised with the coronavirus.
"Secondly, detecting the consequences of injury during convalescence may identify subjects who would benefit from specific supporting drug treatments to protect heart function over time," said Professor Fontana.
The scientists stressed they only analysed patients who were ill enough to be admitted to hospital with the coronavirus, but survived.
"The patients in this study had severe COVID-19 disease and our results say nothing about what happens to people who are not hospitalised with COVID, or those who are hospitalised but without elevated troponin," said Professor Fontana.
"The findings indicate potential ways to identify patients at higher or lower risk and suggest potential strategies that may improve outcomes.
"More work is needed and MRI scans of the heart have shown how useful it is in investigating patients with troponin elevation."
Co-author Professor Eike Nagel from the German Centre for Cardiovascular Research in Frankfurt has reported ongoing heart problems affected just under four in five (78%) coronavirus patients who were not ill enough to be admitted to hospital.
Dr Sonya Babu-Narayan from the British Heart Foundation said: "The people who took part in the research were survivors of severe COVID-19. Whilst in hospital, they all had an abnormal troponin blood test, which is a sensitive signal of heart injury.
"Half then went on to have abnormal heart MRI scans after they went home.
"Some people in the study may have had heart damage they did not know about before they caught the virus.
"People who had heart damage thought to be caused by the virus often had subtle injuries, which did not stop the heart pumping normally.
"More research looking at the long-term effects of severe illness is needed so that we can learn how to prevent and treat any damage that COVID-19 does to the heart."