COVID-19's effect on blood flow and the heart appears to begin fading away after about nine months, according to a new Houston Methodist study, an encouraging finding about the disease's still poorly understood chronic phase.

The study also found that the most serious effect on cardiac function came from the beta and delta variants, which circulated in the pandemic's early waves and have been largely wiped out since then. The cardiac effect was significantly milder from omicron, the most recent variant.

"This is good news for individuals who had omicron and are concerned about long COVID-19," said Dr. Mouaz Al-Mallah, director of cardiovascular positron emission tomography (PET) at Houston Methodist DeBakey Heart & Vascular Center and the study's primary investigator. "And it's a pleasant surprise to see that long COVID-19 heart symptoms are probably self-limited and start to wean away after not too long."

Dr. Al-Mallah said the study's third main finding — that the sickest COVID-19 patients were significantly more likely to experience lasting heart issues — was more expected, although good news for those who had mild or no symptoms but were concerned that their positive test might be a reason for long-term worry.

The study, recently published online in the European Heart Journal – Cardiovascular Imaging, is the latest in the Houston Methodist research team's ongoing research into long COVID and the heart.

Unraveling the mysteries of long COVID

The team undertook the research because knowledge is limited about cardiac problems that linger long after the COVID-19 virus is gone from the patient's system.

More than 15 million American adults have long COVID, based on a Census Bureau survey conducted in June that found 11% of respondents report such symptoms. The survey didn't ask about heart symptoms, but one study has found that one in four long COVID patients have markers of cardiac injury.

The Houston Methodist team's research uses PET image data to analyze myocardial flow reserve (MFR), the capacity for blood flow to the heart under exertion compared to such flow at rest. MFR is considered a key indicator of coronary vascular health.

The team's previous study, published in JACC: Cardiovascular Imaging in 2022, was the first to link reduced MFR and long COVID, a departure from the previous focus on the heart muscle itself. The team launched the investigation after a number of studies found myocarditis was not the common cause of heart-related long COVID-19 it was originally thought to be.

(Related: Reduced Myocardial Blood Flow New Clue in COVID-19's Effect on Heart)

Dr. Al-Mallah said the new study is a response to questions the team received about the first one, questions that couldn't be answered because its sample size was too small. The first study included 101 patients with lingering symptoms well after a prior COVID-19 infection and 292 patients who'd never been diagnosed with the virus.

The new study included 1,086 patients, 271 who had post-COVID-19 symptoms and 815 who were uninfected, matched control patients. Patients, whose average age was 65 and most common symptoms were chest pain and shortness of breath, were followed for an average of 285 days.

Among the findings:

• Long COVID patients had impaired MFR by a nearly two-to-one ratio (50% vs. 27%) to the control group.
• In the majority of long COVID patients, impaired MFR peaked six to nine months after imaging, decreasing thereafter.
• COVID patients requiring ICU admission were most likely to have microvascular dysfunction; those with the mildest or no symptoms were the least likely to have microvascular dysfunction.
• Impaired MFR was most common among patients infected with the beta and delta variants; omicron had the lowest amount because most people infected did not require ICU admission or even hospitalization.

Because the patients were infected earlier in the pandemic, none had any of COVID-19's currently circulating XBB variants, which are subvariants of omicron. Dr. Al-Mallah said he thought they would follow the same pattern as omicron because they too are not causing a lot of disease.

Dr. Al-Mallah said the study's major takeaway continues to be the need for blood flow testing.

More long Covid research still needed

"If you have a patient who is continuing to have heart symptoms after COVID-19, that's what we've been emphasizing," said Dr. Al-Mallah. "Measuring blood flow allows you to gather more information on what's going on with their symptoms."

Dr. Al-Mallah added that more studies are needed to further assess the microvascular health of patients with prior COVID-19 and identify how these findings could influence patient care in the context of long COVID.

He also called for future study into whether H1N1 and other respiratory viruses cause similar blood flow and cardiac issues. He said his gut feeling is that to a lesser extent they probably have a similar effect, but currently there is no corroborating evidence.

The study was supported, in part, by grants from the National Institutes of Health. Additional collaborators included Drs. Ahmed Ibrahim Ahmed, Mahmoud Al Rifai, Fares Alahdab, Jean Michel Saad, Moath Said Alfawara, Malek Nayfeh, Maan Malahfji, Faisal Nabi, John J Mahmarian, John P. Cooke and William A Zoghbi and imaging research coordinator Yushui Han.