Impulse-response function of splanchnic circulation with model-independent constraints: theory and experimental...

Area of Science:

• Physiological modeling
• Tracer kinetic methods
• Hepatic and gut physiology

Background:

• Accurate modeling of physiological processes requires understanding tracer concentration in blood supplying organs.
• Direct measurement of hepatic dual-input function is impossible in humans due to portal vein inaccessibility.

Purpose of the Study:

• To develop a method for predicting the portal venous time-activity curve from arterial time-activity curve measurements.
• To validate an impulse-response function for predicting tracer kinetics in the gut and liver.

Main Methods:

• Developed an impulse-response function based on a continuous distribution of washout constants.
• Conducted experiments in anesthetized pigs using simultaneous aortic and portal vein blood sampling.
• Utilized intravascular [15O]CO and diffusible 3-O-[11C]methylglucose (MG) tracers.

Main Results:

• Estimated mean transit times from aorta to portal vein in pigs: T = 0.35 ± 0.05 min for CO and 1.7 ± 0.1 min for MG.
• Demonstrated that constraining regression fits improves portal venous time-activity curve prediction.
• Showed physiological relevance of the impulse-response function, including a power law relationship.

Conclusions:

• The developed impulse-response function is physiologically relevant and can predict portal venous tracer kinetics.
• Parameter estimates in pigs are similar to those in humans, suggesting the model's adaptability for human studies.
• This method offers a viable solution for non-invasive liver function assessment using tracer kinetics.