Total vertebral artery flow volume, the 200 mL/min threshold and stroke after isolated coronary artery bypass grafting: a retrospective cohort study

Before the heart is opened for surgery, doctors often look at the blood vessels in the neck to make sure the brain will stay safe. They use a painless scan called duplex ultrasonography, which acts like a sonar for blood vessels. This machine does two things: it measures how narrow a vessel has become due to plaque, and it measures how much blood is actually flowing through it. For decades, surgeons have focused almost entirely on the width of the arteries, specifically the carotid arteries that run up the front of the neck. If a vessel is too narrow, it is considered a risk. However, there is a second pair of arteries, the vertebral arteries, that run up the back of the neck to feed the back part of the brain. These vessels are often smaller, sometimes missing, or shaped differently from person to person. While the machines measure the blood flow in these rear arteries, that number has historically been ignored in the final decision-making process. Surgeons have wondered if a low flow of blood to the back of the brain might increase the risk of a stroke during heart surgery, but no one had tested this idea with hard data.

A team of researchers in Istanbul decided to settle this question by looking back at the records of nearly a thousand patients who had undergone a specific type of heart surgery called coronary artery bypass grafting. In this procedure, surgeons take a healthy vein or artery from elsewhere in the body and use it to create a new path for blood to flow around a blocked heart vessel. The team gathered data from patients who had surgery between January and August 2022. Before their operations, every patient had received a duplex scan. The researchers focused on the total amount of blood flowing through both vertebral arteries combined. They compared this total flow against a specific number that their hospital had been using for years: 200 milliliters per minute. This threshold was meant to act as a warning sign, suggesting that a patient with less than this amount of flow might be at higher risk for a stroke. The researchers wanted to see if the patients who had strokes actually had lower blood flow numbers than those who did not.

The results were clear and surprising. Out of the 915 patients in the study, 19 suffered a stroke after their surgery. When the researchers compared the blood flow numbers of the patients who had strokes with those who did not, they found no difference at all. The patients who had strokes had a median blood flow of 227 milliliters per minute, while those who remained stroke-free had a median flow of 220 milliliters per minute. These numbers are so close that the difference is statistically meaningless. Even when the researchers looked specifically at the 210 patients whose flow was below the 200 milliliter warning line, they found no increase in stroke risk. Only four of those patients had a stroke, which was the same rate as the patients with higher flow. The study showed that the 200 milliliter threshold, which had been used to flag patients as "abnormal," was actually identifying healthy people who were at no extra risk. In fact, the average flow in this group of patients was higher than the average found in general reference populations, meaning the warning line was set too high to catch anyone with a genuine problem.

While the flow in the back of the neck turned out to be unrelated to the outcome, the study confirmed that the condition of the front neck arteries still matters. The researchers found that patients with significant narrowing in their carotid arteries were indeed more likely to have a stroke. This proved that the scans were working correctly to detect vascular risk, but that the risk came from the front arteries, not the flow in the back. Furthermore, when the researchers looked at where the strokes occurred, they found that the back part of the brain was involved in nearly half of the cases. This was important because it showed that the study was looking at the right kind of events. If the strokes had all happened in the front of the brain, the lack of connection to the back-artery flow would have been expected. Instead, the strokes happened in the back, yet the blood flow numbers in the back arteries did not predict them.

The researchers explained that this result makes sense given how strokes happen during heart surgery. Most strokes in these cases are caused by tiny clots or debris that travel from the heart or the large aorta vessel and block a small artery in the brain. This is an embolic event, like a sudden clog in a pipe, rather than a slow starvation of blood due to low flow. A chronically low flow of blood to the back of the brain does not seem to make a patient more likely to suffer from this type of sudden blockage. The study also noted a limitation: there were very few patients in the group who had truly, severely low blood flow. Only a tiny fraction of the patients had flow levels that were genuinely below the normal range, and none of them had a stroke. Because the study did not include enough people with genuinely poor circulation, it cannot say for certain what would happen to a patient with extremely low flow. However, for the vast majority of patients, the data suggests that the current practice of using a 200 milliliter cutoff to change surgical plans is unnecessary.

In the end, the study concludes that measuring the total blood flow in the vertebral arteries does not help predict who will have a stroke after this type of heart surgery. The 200 milliliter threshold used by the hospital was found to be higher than the average flow of a normal population, meaning it labeled many healthy people as risky without any evidence to support it. A value below this line is not, by itself, a reason to change the surgical plan, order more tests, or delay the operation. The findings suggest that surgeons should continue to focus on the narrowing of the front neck arteries, which remains a proven risk factor, while letting go of the idea that the total volume of blood in the back arteries is a useful warning sign for stroke in this specific group of patients.