Article review: “Effects of Isoflurane on Coronary Blood Flow Velocity in Young, Old, and ApoE−/− Mice Measured by Doppler Ultrasound”
The heart needs to constantly adapt to changing metabolic conditions and physiological stressors, such as exercising and aging. Within that same principle, coronary arteries that supply the heart with blood to maintain normal cardiac functioning must adapt, mainly by changing arterial vessel dimensions and subsequent blood flow to the heart. For example, when exercising, the heart needs increased blood flow to itself or cardiomyocyte ischemia can occur1. This maximal blood flow to the heart is called coronary flow reserve (CFR). When needed, this protective mechanism allows the heart to respond to increased blood demand by increasing the blood flow, called a hyperemic state.
Coronary flow is notoriously difficult to measure so many researchers have opted for implantable probes and catheters. However, coronary disease can also be hidden under baseline conditions in many pathological models and is only uncovered when the heart is under demand, shown through the hyperemic state2. For example, the resting flow of a narrowed coronary artery does not always change, but when demand on the heart is added, maximal blood flow through this narrowed artery and thus CFR is significantly reduced, showing clear underlying pathology3. Induction of a hyperemic state of a blood vessel requires another step, and researchers typically choose between challenging the animal with exercise or hypoxia or administering a vasodilator, like adenosine, usually through intravenous infusion or an osmotic pump4,5. With the vasodilator adenosine specifically, in a normal coronary artery, this results in approximately a fourfold increase in flow rate compared to baseline6. Although all of these techniques are effective for CFR analysis, they are very labor-intensive and are invasive research procedures, complicating longitudinal studies.
A paper published in Ultrasound in Medicine and Biology by researchers from the Baylor College of Medicine and The Methodist Hospital showed a simplified technique that acquires CFR in mice using a common anesthetic with secondary dose-dependent vasodilator properties, Isoflurane7. Using the Doppler Flow Velocity System, the researchers measured coronary flow velocity at both a hyperemic and a baseline state. They stimulated hyperemic conditions by increasing the concentration of isoflurane gas to 2.5% and for baseline conditions isoflurane gas was lowered to 1%. This technique allows future researchers to test CFR reliably, longitudinally, and quickly. Since this is a non-invasive procedure using Doppler, this can easily be added to longitudinal studies to check if there may be narrowing, blockage, or bulging of the coronary arteries in multiple pathological models. Figure 1 shows representative baseline and hyperemic coronary doppler signals from a two-year-old WT mouse and a two-year-old ApoE−/− mouse. At both baseline and hyperemic states, the ApoE−/− have increased velocity, indicative of coronary lesions, which is likely in this atherosclerosis-prone model. Recently, these coronary flow reserve measurements made by Doppler technology have been established as feasible and accurate8–13. This current research and others have shown the niche research applications that Doppler technology can extend to, especially in simplifying complex protocols.
Figure 1: Coronary Doppler signals from A. an Old WT mouse and B. an ApoE−/− mouse with presumed stenosis (estimated to be 75% by area) of the left main coronary artery. Baseline velocity was taken at low (1%), and hyperemic velocity was taken at high (2.5%) levels of isoflurane gas. The horizontal lines show the average peak diastolic velocity (PDV) at baseline or during hyperemia in each panel. (*Figures from Hartley, C. J. et al. Ultrasound Med Biol vol. 33 (2007).
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