Morphine decreases peripheral vascular resistance and increases capacitance in man.

Area of Science:

• Cardiovascular Pharmacology
• Anesthesiology
• Pharmacology

Background:

• The effects of high-dose morphine on peripheral circulation, separate from cardiac and respiratory factors, remain unclear.
• Understanding these effects is crucial for managing patients undergoing procedures like cardiopulmonary bypass.

Purpose of the Study:

• To investigate the impact of large doses of morphine on peripheral vascular resistance (PVR) and capacitance.
• To determine the influence of promethazine and naloxone on morphine's circulatory effects.

Main Methods:

• Peripheral vascular resistance and capacitance were measured in 28 patients during cardiopulmonary bypass.
• Morphine (0.5 mg/kg or 1 mg/kg) was administered intra-arterially, alone or after promethazine or naloxone.
• Measurements were recorded for 15 minutes at a constant perfusion rate.

Main Results:

• Morphine (both doses) caused a 46% decrease in PVR within 2 minutes, returning to baseline by 9 minutes.
• Promethazine pretreatment reduced morphine's PVR-lowering effect to 25%.
• Naloxone did not alter morphine's effect on PVR. Morphine induced a sustained 600 ml increase in capacitance, unaffected by naloxone.

Conclusions:

• High-dose morphine acutely reduces peripheral vascular resistance and increases vascular capacitance.
• Promethazine antagonizes morphine-induced vasodilation, suggesting an interaction.
• Naloxone does not modulate morphine's direct effects on peripheral circulation.