Findings on artery plaque

                                            

Tuesday 10 August 2010

Medical Research
Findings on artery plaque

Haim Shmilovich and his fellow researchers from Heart Institute, Cedars-Sinai Medical Centre, Los Angeles, California found that, besides the degree of blockage, the composition of the plaque causing the blockage has significant impact on blood flow through coronary artery, writes K S Parthasarathy

If a person is diagnosed as suffering from heart disease, the current medical practice is to carry out an invasive procedure such as angioplasty or stent-placement or open heart surgery as appropriate. A study presented at the 57th Annual Meeting of the Society of Nuclear Medicine in June at Salt Lake City, USA is challenging this practice.

People with similarly blocked arteries can experience vastly different symptoms. Nobody knows why.

Dr Haim Shmilovich and his fellow researchers from Heart Institute, Cedars-Sinai Medical Centre, Los Angeles, California found that, besides the degree of blockage, the composition of the plaque causing the blockage has significant impact on blood flow through coronary artery. May be this is why two people with similarly blocked arteries can experience vastly different symptoms.

According to Dr Mason W Freeman, Associate Professor, Harvard Medical School coronary artery disease (CAD) begins when plaques build up in the arteries.

Plaque deposit hardens later

He vividly described the process. Initially, the plaque deposit may be soft and mushy; later it hardens, narrowing the artery. Blood flow to the heart decreases; at the beginning, this reduction is not severe enough to compromise heart muscle function or to produce any symptom.

Later on, the plaque may enlarge further reducing the blood flow. After a critical point, the heart muscle no longer gets adequate oxygen delivery when it is working vigorously. A prolonged or complete interruption will kill myocardial cells and cause a heart attack.
The plaques are not homogenous. They contain fat and other substances including calcium. Plaques vary in size. Even very young persons may have early stage plaques. When plaques cause blockade of the arteries, symptoms develop .When the blockade is 70% or more physicians carry out invasive procedures to reduce symptoms and potentially to prevent heart attacks. How can we determine the composition of the tiny deposits of fat in the blood vessel of a few millimetres diameter deep inside a living person? Researchers use coronary CT angiography (CCTA) for this and to find the degree of blockage it causes.

Use of an intravenous dye

The coronary CT angiography uses an intravenous dye which contains iodine and CT scanning to image the coronary arteries. Physicians measure the relative blood flow to different regions of the heart by myocardial perfusion imaging (MPI). During myocardial perfusion imaging, the physician administers a radio-pharmaceutical intravenously to depict the distribution of blood flow nourishing the myocardium- the middle of the three layers forming the wall of the heart.

Perfusion imaging identifies areas of reduced myocardial blood flow associated with ischemia or scar. Physicians can assess the relative regional distribution of perfusion at rest, cardiovascular stress, or both. They also perform imaging during chest pain of unknown etiology, such as in the coronary care unit or emergency department.

They found that clinicians can more accurately determine a patient’s risk of reduced blood flow to the heart muscle by identifying three plaque characteristics: the presence of a fatty core, signs of spotty calcifications and enlargement of the arterial wall from “positive remodelling”, which means that the body has responded to arterial damage by altering the structure of the artery. Researchers noted that either individually or combined, the presence of these characteristics can predict reduced blood flow to the heart muscle, which could lead to symptoms including heart attack.

They imaged 34 patients without known coronary artery disease using CCTA and MPI to determine the presence of adverse plaque characteristic and blood flow. All patients had severe (70 to 89 per cent) blockage in the beginning or middle section of a major coronary artery on CCTA.

A third-party expert evaluated adverse plaque characteristics on CCTA; an automated computer -based analysis carried out the myocardial perfusion imaging.

When imaged with MPI, over 38 per cent had significantly limited blood flow to the heart muscle.

In the arteries with plaques with a fatty core, significant ischemia (condition in which blood flow and hence oxygen supply is reduced) of the heart muscle occurred at a much higher and statistically significant frequency (80 per cent) than those without fatty core (21per cent).

When specialists found multiple adverse plaque characteristics in a plaque, that was associated with higher degrees of significant ischemia.

Findings could redefine treatment

The researchers claimed that if they can determine certain characteristics of the coronary artery plaque, they can predict whether a patient’s symptoms are due to limitation of blood flow to the heart.

They believe that with more studies, their findings may change treatment planning for patients with severe but stable coronary artery disease by helping them determine which patients could be treated just as effectively with medications and life style changes, thereby avoiding unnecessary invasive angioplasty and bypass surgery. The study is promising. A major problem with all medical imaging modalities is that the technology advances with unbelievably astonishing speed; the clinical use lags disappointingly behind.

(The writer is Raja Ramanna Fellow, Department of Atomic Energy.)