Nitric oxide delivery by ultrasonic cracking: some limitations

Area of Science:

• Biomedical Engineering
• Acoustic Medicine
• Cardiovascular Research

Background:

• Nitric oxide (NO) is crucial for smooth muscle relaxation and widely used in cardiology.
• Hemoglobin effectively scavenges NO, necessitating local application or high doses.
• Microbubbles are proposed as a vehicle for targeted NO delivery via ultrasonic cracking.

Purpose of the Study:

• To evaluate the feasibility of using ultrasonic cracking of microbubbles for nitric oxide (NO) delivery in small vessels.
• To quantify the amount of NO released from microbubbles using ultrasonic cracking.
• To assess the dissolution kinetics of released NO in relation to its half-life in blood.

Main Methods:

• High-speed optical recordings of stiff-shelled microbubbles subjected to ultrasonic cracking (0.5 or 1.7 MHz, MI>0.6).
• Quantification of gas released per microbubble.
• Simulation of NO dissolution times and comparison with NO half-life in blood and extravascularly.
• Analysis of hydrostatic pressure effects on microbubble dissolution and release.

Main Results:

• A mean quantity of 1.7 fmol of gas was released per microbubble, exceeding NO production in small vessels.
• Simulated NO dissolution time matched the half-life in whole blood (1.8 ms) but was shorter than the extravascular half-life (>90 ms).
• Higher hydrostatic pressures reduced microbubble release but increased gas dissolution speed, with cracking primarily occurring during the expansion phase.

Conclusions:

• Ultrasonic cracking can release sufficient gas for NO delivery, but efficacy depends on release location, ideally in the plasma near the endothelium.
• The rapid scavenging of NO by hemoglobin presents a significant challenge for in vivo applications.
• Further research is needed to optimize ultrasonic cracking parameters for effective in vivo NO delivery, considering blood pressure variations.