Cardiovascular effects of airways obstruction

Area of Science:

• Cardiovascular Physiology
• Respiratory Physiology

Background:

• Airway obstruction causes significant decreases in intrathoracic pressure (ITP).
• ITP changes correlate with pulsus paradoxus, an inspiratory fall in arterial pressure.
• Several factors influence pulsus paradoxus, including venous return, ventricular afterload, and interdependence.

Purpose of the Study:

• To review and demonstrate the changing relative importance of factors influencing pulsus paradoxus under different circumstances.
• To discuss the role of transmural pressures in assessing ventricular performance.
• To explore potential pitfalls in estimating left ventricular (LV) surface pressure.

Main Methods:

• Review of existing literature on intrathoracic pressure, pulsus paradoxus, and cardiac mechanics.
• Analysis of factors affecting stroke volume during normal and loaded inspiration.
• Discussion of the impact of lung volume and respiratory maneuvers on cardiovascular dynamics.

Main Results:

• Decreased LV preload, due to RV-LV diastolic interdependence, is a primary cause of reduced inspiratory stroke volume during normal/loaded inspiration.
• LV afterload may be more critical during Mueller maneuvers or severe ITP decreases.
• Increased lung volume, as in asthma, elevates the importance of venous return in determining pulsus paradoxus.
• Coronary blood flow increases with inspiratory loading due to autonomic changes, not vagal mediation.

Conclusions:

• The mechanisms responsible for pulsus paradoxus vary with clinical context, including respiratory load, lung volume, and cardiac conditions like pericardial tamponade.
• Transmural pressure measurements are crucial for accurate ventricular performance assessment.
• Reflex-mediated effects during stressed breathing warrant further investigation.