This first picture is a normal vertebral artery waveform. Compared to the next one (which is abnormal) it is easy the see a difference. In the abnormal one, the flow dips down to baseline immediately after systole. The pattern created is called a “bunny waveform” for resembling a crouched rabbit as viewed from the side.
Normal vertebral artery waveform. Sharp upstroke and a good amount of diastolic flow typical for an artery feeding a low resistance vascular bed (the brain).
After the systolic peak, there is a severe drop in flow velocity. The resultant waveform resembles a crouched rabbit from the side – head to the right.
How does this happen? The left vertebral artery takes off from the left subclavian artery just after its origin. A stenosis in the origin of the subclavian artery is the culprit. A stenosis (narrowing) accelerates blood flow. In order to keep the same amount of blood flowing through a smaller opening everything has to move faster. By Bernoulli’s principle, higher velocity flow has a lower pressure and as we have discussed in prior posts, blood always flows toward the low pressure/low resistance vascular beds. To bring it all together, just at the peak of systole, blood flow is fastest through subclavian stenosis and pressure is lowest – low enough in fact to cause blood to flow backward from the vertebral artery into the low pressure zone. In other words, blood is stolen from brain into the arm.
In mild subclavian stenosis, the pressure differential is only great enough at peak systole, when velocity is highest, to elicit a dip in flow in the vertebral artery. The post systolic dip, as I call it, or the “bunny” waveform as others call it is formed.
As subclavian stenosis worsens, the pressure differential grows larger and lasts longer through the cardiac cycle. Flow reversal becomes more and more prominent making forward flow during systole and reversed flow in diastole (so called “to and fro” flow). Eventually, there is constant reversed flow when the subclavian artery is occluded.
A more significant steal. Blood flow velocity drops and then reverses in the vertebral artery during most of systole.
What does all this mean for patients? First and foremost, as a clinical effect of atherosclerosis, it defines people at higher risk for things like stroke and heart attack. And, as discussed previously, those patients may benefit from treating their vascular risk factors of smoking, diabetes, cholesterol, blood pressure with medication, diet and exercise. We suspect that most patients with mild subclavian stenosis and bunny waveforms in the vertebral artery never notice it.
In uncommon situations the blood that is stolen from the brain through the vertebral artery can cause neurological symptoms such as vertigo, passing out, blurry vision or slurred speech. Usually for this to happen there has to be something else affecting blood flow to the brain such as an occluded carotid artery or very low blood pressure from shock, dehydration or sepsis.
Interestingly, I have seen patients with subclavian steal who developed neurological symptoms while on dialysis. Can anyone explain why?