What happens to the cardiac cycle in a typical adult patient with no underlying cardiac conditions during inspiration?

Cardiac Cycle Changes During Inspiration in Normal Adults

During normal spontaneous inspiration in healthy adults, pleural pressure decreases (becomes more negative), which increases right ventricular preload and venous return while simultaneously increasing left ventricular afterload and transiently decreasing left ventricular stroke volume. 1

Right Heart Effects

Increased Right Ventricular Filling

• The decrease in pleural pressure during spontaneous inspiration effectively lowers the pressure baseline of the heart relative to the rest of the body, creating an increased gradient for venous return to the right ventricle 1
• The inferior vena cava diameter decreases during inspiration as blood is drawn into the right atrium; in normal subjects, the IVC collapses >50% with inspiration when baseline diameter is <2.1 cm 1
• This transient inspiratory increase in RV filling is subsequently passed to the left ventricle over the next few cardiac cycles 1

Tricuspid Flow Changes

• Tricuspid valve flow increases during inspiration, while mitral flow decreases 1
• This reciprocal pattern reflects the enhanced venous return to the right heart and reduced left ventricular output during the inspiratory phase 1

Left Heart Effects

Increased Left Ventricular Afterload

• When pleural pressure decreases and intracavitary cardiac pressures are lowered relative to atmospheric pressure, left ventricular afterload increases because the heart requires more force to maintain normal arterial pressure against the unchanged systemic vascular resistance 1
• This afterload increase is a primary mechanism for the inspiratory decrease in left ventricular stroke volume, independent of changes in ventricular filling 2

Decreased Left Ventricular Stroke Volume

• Left ventricular stroke volume decreases during inspiration through multiple mechanisms: increased afterload, ventricular interdependence (the enlarged right ventricle shifts the interventricular septum and reduces left ventricular compliance), and transient sequestration of blood in the pulmonary vasculature 1, 2
• The transmural pressure of the superior vena cava decreases during inspiration, while right atrial transmural pressure remains relatively unchanged 3

Blood Pressure Variations

Normal Respiratory Variation

• Systolic blood pressure normally decreases slightly during inspiration (typically <10 mmHg) due to the reduced left ventricular stroke volume 1
• Diastolic blood pressure remains relatively unchanged during the respiratory cycle 1
• Release of inspiratory effort raises pleural pressure and produces a small expiratory rise in arterial pressure 1

Clinical Significance

Pulsus Paradoxus Definition

• An exaggerated inspiratory decrease in systolic blood pressure >10 mmHg defines pulsus paradoxus, which is pathological and suggests conditions like cardiac tamponade or severe airway obstruction 1
• In normal breathing, the inspiratory blood pressure drop should be <10 mmHg and is easily detected by palpating the pulse 1

Ventricular Interdependence

• The increased right ventricular volume during inspiration can decrease left ventricular diastolic compliance through septal shift, contributing to reduced left ventricular filling and stroke volume 1, 2
• This mechanism becomes clinically significant when inspiratory efforts generate large negative pleural pressure swings (>20 cmH₂O), which can occur in pathological states 4