From the Guidelines
During inspiration, intrathoracic pressure decreases, creating a subatmospheric pressure within the chest that draws air into the lungs through the airways. This decrease in intrathoracic pressure is essential for breathing and is achieved by the contraction of the diaphragm and external intercostal muscles, which increases the volume of the thoracic cavity 1. The negative pressure typically drops from about -5 cmH2O at rest to approximately -8 cmH2O during normal inspiration.
Key Points
- The decrease in intrathoracic pressure during inspiration creates a pressure gradient that draws air into the lungs through the airways.
- The magnitude of the pressure change varies with the depth of inspiration, becoming more negative with deeper breaths.
- This pressure change is reversed during expiration when the thoracic cavity volume decreases and intrathoracic pressure increases, facilitating the outflow of air from the lungs.
- The use of accessory muscles of respiration, such as the intercostal, sternocleidomastoid, and scalene muscles, can produce additional signs of severe respiratory distress, including head nodding, tracheal tugging, and intercostal recessions 1.
Clinical Implications
The understanding of intrathoracic pressure changes during inspiration is crucial in clinical practice, particularly in the management of respiratory diseases. For example, in patients with pneumonia, the need to generate more negative intrapleural pressures during inspiration can lead to chest indrawing, a diagnostic sign used by the World Health Organization 1. Additionally, the measurement of intrathoracic pressure can be useful in assessing respiratory muscle function and endurance, as described in the ATS/ERS statement on respiratory muscle testing 2.
From the Research
Effect of Inspiration on Intrathoracic Pressure
- The effect of inspiration on intrathoracic pressure is a decrease in pressure, with large reductions (>20 cmH2O) potentially causing adverse hemodynamic effects 3.
- A "sweet spot" for generation of negative intrathoracic pressure during inspiration can be maintained at or less than 10 cmH2O below normal inspiration, resulting in beneficial physiological effects such as greater cardiac filling and output, lower intracranial pressure, and increased cerebral blood flow oscillatory patterns 3.
- The use of inspiratory resistance, such as an impedance threshold device, can create a pressure difference of 7 cm H2O, resulting in multiple physiological benefits including enhanced venous return and cardiac stroke volume, lower intracranial pressure, and resetting of the cardiac baroreflex 4.
Mechanisms of Inspiration
- The respiratory pump can improve cardiovascular functions by harnessing the natural mechanisms associated with inspiration to enhance circulation to vital organs 3, 4.
- The generation of negative intrathoracic pressure during inspiration can increase circulation to the heart and brain, especially in the setting of physiological stress 4.
- The decrease in intrathoracic pressure during inspiration can also result in an increase in LV stroke volume, influenced by factors such as respiratory variations in RV stroke volume, variations in functional residual capacity or tidal volume, and an inspiratory decrease in RV volume 5, 6.
Hemodynamic Effects
- The hemodynamic effects of inspiration are complex and can result in both increases and decreases in cardiac output, depending on factors such as the level of intrathoracic pressure, tidal volume, and cardiac contractility 5, 6, 7.
- The decrease in intrathoracic pressure during inspiration can result in a net decrease in left ventricular output, despite an increase in mean transmural left atrial pressure, due to an increase in left ventricular outflow impedance 7.