Intrathoracic Pressure During Deep Inspiration
During deep inspiration in healthy adults, intrathoracic pressure becomes more negative (decreases below atmospheric pressure), typically reaching values of -10 cmH₂O or lower, which enhances venous return and cardiac filling. 1
Physiological Mechanism
Normal respiratory mechanics create predictable pressure changes:
- During inspiration, the diaphragm contracts and the chest wall expands, generating negative intrathoracic pressure relative to atmospheric pressure 2
- This negative pressure slightly reduces pulmonary arterial pressure relative to atmospheric pressure and increases venous return to the right heart 1
- With passive exhalation, intrathoracic pressure returns toward atmospheric pressure, reversing these hemodynamic effects 1
Magnitude of Pressure Changes
The degree of negative pressure varies with inspiratory effort:
- Normal quiet breathing generates modest negative intrathoracic pressure changes 2
- Deep inspiration without resistance maintains intrathoracic pressure changes that preserve normal cardiovascular function 3
- Forced deep inspiration can generate intrathoracic pressures of -20 to -30 cmH₂O or more negative 4, 3
Cardiovascular Effects of Deep Inspiration
The hemodynamic consequences depend on the magnitude of negative pressure generated:
- Negative intrathoracic pressure ≤-15 cmH₂O augments venous return without significantly affecting left ventricular stroke volume in healthy supine adults 3
- Negative intrathoracic pressure ≥-20 cmH₂O begins to reduce stroke volume due to direct ventricular interaction (septal flattening) and increased left ventricular afterload 5, 3
- At -30 cmH₂O, progressive end-diastolic septal flattening occurs with decreased left ventricular end-diastolic volume and stroke volume 5
The "sweet spot" for beneficial cardiovascular effects occurs when negative intrathoracic pressure during inspiration is maintained at ≤10 cmH₂O below normal inspiration, which enhances cardiac filling and output without adverse hemodynamic consequences 6
Clinical Measurement Considerations
For accurate pressure measurements during respiratory maneuvers:
- Pressures should be measured over 2-3 respiratory cycles at end-exhalation when intrathoracic pressure is closest to atmospheric 2, 1
- These effects become accentuated during assisted ventilation, in patients with lung disease, and during exercise or hyperventilation 2, 1
Pathophysiological Context
In disease states, the normal response may be altered:
- In asthmatic subjects, deep inspiration can paradoxically cause bronchoconstriction, potentially related to enhanced negative intrathoracic pressure increasing airway wall edema when vascular permeability is increased 4
- Large reductions in intrathoracic pressure (>20 cmH₂O) seen in heart failure or chronic obstructive lung disease can result in adverse hemodynamic effects 6
Key Clinical Pitfall
A common misconception is that all negative intrathoracic pressure is beneficial for cardiac function. However, excessive negative pressure (≥-20 cmH₂O) can actually impair left ventricular filling through direct ventricular interaction, where the interventricular septum flattens and shifts leftward, reducing left ventricular chamber size despite increased venous return 5, 3. This mechanism explains why patients with severe respiratory distress may develop circulatory compromise despite appearing to generate adequate respiratory effort.