From the Guidelines
Intrathoracic pressure decreases during inspiration, creating a negative pressure that facilitates air entry into the lungs and enhances venous return to the heart. This phenomenon is crucial for understanding respiratory physiology and is supported by the fact that lung compliance decreases during diseases like pneumonia, requiring greater inspiratory force to maintain adequate tidal volume respirations 1.
Key Points
- During inspiration, the diaphragm contracts and moves downward, while the external intercostal muscles contract to expand the ribcage, increasing the volume of the thoracic cavity.
- This increase in volume creates a negative pressure (subatmospheric pressure) within the chest, which can drop to approximately -8 cmH2O during normal quiet breathing and even lower during deep inspiration.
- The decrease in intrathoracic pressure is essential for proper respiratory function, creating a pressure gradient that draws air into the lungs through the airways.
- Additionally, this negative pressure affects venous return to the heart by increasing the pressure gradient between peripheral veins and the right atrium, enhancing blood flow back to the heart, as observed in conditions with low lung compliance, such as pneumonia 1.
Clinical Implications
- Understanding the changes in intrathoracic pressure during inspiration and expiration is vital for comprehending respiratory physiology and certain pathological conditions.
- Disruptions to normal intrathoracic pressure, such as those seen in tension pneumothorax, can severely impact both breathing and cardiovascular function.
- 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, as described in the context of pneumonia 1.
From the Research
Effect of Inspiration on Intrathoracic Pressure
- Inspiration causes a decrease in intrathoracic pressure, with the degree of decrease depending on the level of inspiratory effort and resistance 2, 3, 4.
- Large reductions in intrathoracic pressure (>20 cmH2O) can result in adverse hemodynamic effects, while smaller reductions (≤10 cmH2O) can have beneficial effects on cardiovascular function 2, 3.
- The decrease in intrathoracic pressure during inspiration can increase venous return, cardiac stroke volume, and cerebral blood flow, while also lowering intracranial pressure 2, 3, 4.
Factors Influencing Intrathoracic Pressure
- Tidal volume and chest compliance can influence the change in intrathoracic pressure during mechanical ventilation, with larger tidal volumes and decreased chest compliance resulting in greater changes in intrathoracic pressure 5.
- Inspiratory resistance can also affect intrathoracic pressure, with negative intrathoracic pressure (nITP) generated by the respiratory muscles during inspiration to overcome inspiratory resistance 4.
- The degree of airways obstruction can also impact intrathoracic pressure, with substantial decreases in inspiratory and mean intrathoracic pressure associated with airways obstruction 6.
Physiological Effects of Decreased Intrathoracic Pressure
- Decreased intrathoracic pressure can have various physiological effects, including increased cardiac output, lowered intracranial pressure, and improved cerebral blood flow 2, 3, 4.
- The effects of decreased intrathoracic pressure on systemic blood pressure, cerebral perfusion, and cerebrospinal fluid dynamics are complex and depend on the level of inspiratory resistance and other factors 4.
- Decreased intrathoracic pressure can also affect coronary blood flow and myocardial oxygen consumption, with potential implications for cardiovascular function 6.