Final answer:
In a normal Doppler study of fasting mesenteric arteries, a low resistance waveform is expected. Blood speed is greatest at the constriction in a partially constricted artery due to the need to maintain flow, with higher resistance caused by either a high cardiac output and low peripheral resistance, or low cardiac output and high peripheral resistance.
Step-by-step explanation:
The question refers to what type of waveform is expected during a Doppler ultrasound study of the mesenteric arteries in a fasting patient. In a normal study of a fasting patient, the mesenteric arteries, which supply blood to the intestines, exhibit a low resistance waveform. This is because in the fasting state, the digestive tract does not require as much blood flow, thus the mesenteric arteries are not as dilated, and resistance to flow is lower.
When performing a Doppler ultrasound, if there is a partial constriction of an artery, blood speed is expected to be greatest at the constriction, as the blood must move faster through the narrowed area to maintain flow, similar to how water moves faster through a narrow section of a hose. The two distinct causes of higher resistance in the constriction are a high cardiac output and low peripheral resistance before the constriction, or a low cardiac output and high peripheral resistance within the constriction itself.