Negative and positive pressure ventilation are two types of mechanical ventilation. Mechanical ventilation is the process by which physical devices or machines assist or replace the spontaneous respiration of the patient.
Negative pressure ventilation
Negative pressure ventilation applies negative pressure to the patient during inspiration. With the advancement in technologies, positive pressure ventilation has replaced negative pressure ventilation.
- The patient can talk and eat
- Cannot provide much pressure gradient
- It May not be useful for patients who cannot undergo spontaneous respiration
- A lower value of FiO2
The working mechanism of negative pressure ventilation
The entire thoracic area is enclosed in a chamber while the mouth and nose are exposed to the atmosphere.
The negative pressure of about -10 cm H2O is applied inside the chamber. The low pressure inside the chamber causes the expansion of the chest walls. This will cause an increase in the volume of the alveolar sac. When volume increases pressure decreases and vice versa. So, intra-alveolar pressure decreases from 0 cm H2O to -7 cm H2O at mid inspiration. Due to the pressure gradient between the atmosphere and the alveoli, the air moves into the lungs. After mid-inspiration, the intra-alveolar pressure increases due to the pressure exerted by the air onto the walls of the lungs. Thus, at the end of inspiration intra-alveolar pressure comes back to 0 cm H2O.
Atmospheric pressure (0 cm H2O) is applied to the thoracic
cavity. The elastic recoil causes the lungs to contract. Intra-alveolar pressure increases from 0 cm H2O to 7 cm H2O at the mid- expiration. Due to the pressure gradient between the lungs and the atmosphere, the air moves out of the lungs. After the mid- expiration, the intra-alveolar pressure decreases due to the lack of air in the lungs. Finally, at the end of the expiration, the intra-alveolar pressure becomes equal to the atmospheric pressure.