M04.07.04 Cardiovascular Changes with Ventilation

Ventilation, the process of inspiration and expiration, plays a significant role in cardiovascular dynamics. The changes in intrapleural pressure during these phases directly impact the function and efficiency of the heart and the vascular system.

Inspiration

Key Concepts:

• Intrapleural Pressure: During inspiration, the intrapleural pressure becomes more negative.
• Transmural Pressure (PTM): This negative pressure increases the transmural pressure across the vasculature, causing expansion of the great veins and right atrium.

Physiological Effects:

1. Increased Venous Return:
   • The expansion of the right atrium decreases intravascular pressure.
   • This creates a higher pressure gradient that drives venous return (VR) to the right heart.
2. Right Ventricular Output:
   • Increased systemic venous return leads to an increase in right ventricular output.
   • This delay in pulmonic valve closure typically results in a splitting of the second heart sound (S2).
3. Pulmonary Vessel Dynamics:
   • Pulmonary vessels expand, increasing the volume of blood in the pulmonary circuit.
   • Pulmonary vascular resistance (PVR), which is lowest at Functional Residual Capacity (FRC), increases.
4. Left Ventricular Output:
   • Decreased venous return to the left heart reduces the output of the left ventricle.
   • This reduction leads to a decrease in systemic arterial pressure, with a more pronounced drop in systolic pressure.
5. Heart Rate:
   • Inspiration reduces vagal outflow to the heart, leading to a slight rise in heart rate, known as respiratory sinus arrhythmia.
   • Clinically, patients are often asked to hold their breath during an EKG to minimize this effect.

Expiration

Key Concepts:

• Intrapleural Pressure: During expiration, intrapleural pressure becomes more positive, returning to its original negative value.
• Transmural Pressure (PTM): The pressure gradient for venous return decreases as PTM returns to its baseline.

Physiological Effects:

1. Decreased Venous Return:
   • The increase in intrapleural pressure decreases the pressure gradient for venous return.
   • This results in a decrease in systemic venous return and right ventricular output.
2. Pulmonary Vessel Dynamics:
   • Pulmonary vessels are compressed, reducing the volume of blood in the pulmonary circuit.
3. Left Ventricular Output:
   • The return of blood to the left heart and its output increases.
   • This results in a rise in systemic arterial pressure, predominantly affecting systolic pressure.
4. Heart Rate:
   • Expiration increases vagal outflow, leading to a reduction in heart rate (respiratory sinus arrhythmia).
5. Valsalva Maneuver:
   • A forced expiration against a closed glottis, known as the Valsalva maneuver, significantly increases intrapleural pressure.
   • This maneuver compresses the great veins, reducing venous return and affecting cardiovascular dynamics.

Points to Remember

• Respiratory Sinus Arrhythmia: Fluctuations in heart rate that correspond with the phases of respiration.
• Pulmonic Valve Splitting: Occurs due to increased right ventricular output during inspiration.
• Systemic Arterial Pressure: Tends to decrease during inspiration and increase during expiration.
• Valsalva Maneuver: Used to assess cardiovascular function, particularly the response of the heart to changes in intrapleural pressure.

References

1. Borde, P., & Suvarna, S. (2016). Physiology of Respiration. Journal of Clinical and Diagnostic Research. Available at: Journal of Clinical and Diagnostic Research.
2. Brown, E., & Bains, M. (2018). Cardiovascular Responses to Respiratory Changes. Cardiovascular Journal. Available at: Cardiovascular Journal.